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Transcriber's notes:

In this transcription, italic text is denoted by _underscores_ and bold
text by =equal signs=. Superscripts are indicated by ^ (e.g. Fig. 509,
_I^a_).

The text contains numerous inconsistencies of hyphenation. A few have
been adjusted where there was clear evidence of a preferred style (e.g.
meso-colic-->mesocolic and meso-duodenum-->mesoduodenum) but most have
been left in their original format.

A few spelling typos have been corrected silently (e.g.
improtant-->important, mecocolon-->mesocolon) and missing letters have
been inserted inside square brackets (e.g. junct[i]on, t[r]ansverse).
Some spelling inconsistencies probably represent contemporarily
acceptable spelling alternatives (e.g. coati/coaiti, mesal/mesial,
praecava/precava, hyaena/hyena).

A small number of punctuation inconsistencies have been corrected
silently by insertion of missing punctuation or deletion of redundant
punctuation.

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font.

Illustrations (not displayed in this transcription) were originally
grouped separately from the text. Their captions have been relocated
adjacent to their first mention in the text. Some illustrations
contained features labelled as X, A, B, 1, 2, etc. and when these are
cross referenced in the text they are sometimes inconsistently styled
(upper/lower case, roman/italic).

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within the TOC nor with the corresponding headings in the text, and some
TOC entries correspond to in-line text rather than to true headings. No
attempt has been made to remedy these inconsistencies. One missing TOC
entry has been inserted.

The index is shown as it originally appeared - with some entries not in
correct alphabetic sequence.

Footnotes are located below the paragraph of reference.




                              THE ANATOMY
                                 OF THE
                            HUMAN PERITONEUM
                                  AND
                            ABDOMINAL CAVITY

             CONSIDERED FROM THE STANDPOINT OF DEVELOPMENT
                        AND COMPARATIVE ANATOMY

                                   BY

                    GEORGE S. HUNTINGTON, M.A., M.D.

       PROFESSOR OF ANATOMY, COLLEGE OF PHYSICIANS AND SURGEONS,
                          COLUMBIA UNIVERSITY,
                             NEW YORK CITY

            ILLUSTRATED WITH 300 FULL-PAGE PLATES CONTAINING
                      582 FIGURES, MANY IN COLORS

                           LEA BROTHERS & CO.
                       PHILADELPHIA AND NEW YORK
                                  1903

     Entered according to the Act of Congress, in the year 1903, by

                          LEA BROTHERS & CO.,

    In the Office of the Librarian of Congress. All rights reserved




PREFACE.


In the following pages an attempt has been made to emphasize the value
of Embryology and Comparative Anatomy in elucidating the difficult and
often complicated morphological problems encountered in the study of
human adult anatomy.

Moreover, in addition to the direct advance in the method and scope of
anatomical teaching afforded by these aids, it is further hoped that the
broader interpretation, both of structure and function, obtained by
ontogenetic and phylogenetic comparison, will impart an interest to the
study of adult human morphology, such as the subject, considered solely
in the narrow field of its own limitations, could never arouse.

The book represents part of the course in visceral anatomy as developed
during the past fourteen years at Columbia University. The sections
dealing with the morphology of the vertebrate ileo-colic junction and
with the structural details of the human caecum and appendix are
considered somewhat more fully, as warranted by the extensive material
available. The illustrations are for the greater part taken from
preparations in the Morphological Museum of the University. Wherever
practicable the direct photographic reproduction of the actual
preparation is given. In the case of preparations not suitable for this
purpose, careful drawings have been made which offer in every instance a
faithful and correct interpretation of the conditions presented by the
actual object. A number of the embryonic illustrations are taken from
the standard text-books on the subject, due credit being given to their
source. I desire to express my sincere thanks to Dr. Edward Leaming, of
the Department of Photography and to Mr. M. Petersen, artist of the
Anatomical Department of the University, for their skilful and
thoroughly reliable work in the preparation of the illustrations.

                                             =George S. Huntington.=

  =Columbia University=, in the City of New York,
             _December, 1902_.




CONTENTS.

                                                                 PAGE.

  INTRODUCTION                                                      17

    Development of Vertebrate Ovum                                  19

    Development of Coelom and of Alimentary Canal                    21

    Development of Cloaca                                           24

    Development and Divisions of the Peritoneum                     32

    Derivatives of Entodermal Intestinal Canal                      34

    Divisions of Alimentary Canal                                   38


  PART I. ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY            39

    COMPARATIVE ANATOMY OF FOREGUT AND STOMACH                      42

    Morphological Types of Stomach                                  43

    Development of the Intestine                                    51

      I. Intestinal Rotation and Definition of Adult Segments of
        the Intestinal Canal                                        58

        Development of Aortal Arterial System                       63

      II. Demonstration of Intestinal Rotation in the Lower
        Mammalia                                                    67

        Peritoneal and Visceral Relations in the Infra-colic
        Compartment of the Abdominal Cavity in the Adult            74


  Part II. ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT
    OF THE ABDOMEN                                                  99

    1. STOMACH AND DORSAL MESOGASTRIUM                             100

      _a._ Changes in Position of Stomach                          102

      _b._ Changes in Direction and Extent of Dorsal Mesogastrium  103

      _c._ Development of Spleen and Pancreas in the Dorsal
        Mesogastrium and Changes in the Disposition of the Great
        Omentum                                                    108

        1. Development of Spleen                                   108

        2. Development of Pancreas                                 111

          Development of Pancreas in Lower Vertebrates             115

          Comparative Anatomy of Pancreas                          116

          Pyloric Caeca or Appendices                               119

          Peritoneal Relations of Pancreas                         122

          Comparison of Embryonal Stages during the Development of
          the Human Dorsal Mesogastrium, Spleen and Pancreas with
          the Permanent Adult Condition of the same Structures in
          Lower Mammalia                                           126

        1. Spleen, Pancreas and Great Omentum of Cat               127

        2. Relation of Great Omentum to Transverse Colon,
          Transverse Mesocolon and Third Part of Duodenum          129

    2. VENTRAL MESOGASTRIUM AND LIVER                              140

      I. _A._ Development of Liver                                 141

        _B._ Comparative Anatomy of Liver                          144

        _C._ Development of Vascular System of Liver               145

          Comparative Anatomy of the Hepatic Venous Circulation    154

      II. Ventral Mesogastrium                                     163

        Peritoneal Relations of Liver                              167

        Relation of Hepatic Peritoneum to the "Lesser Sac"         174

        Caudal Boundary of Foramen of Winslow                      178

        Pancreatico-gastric Folds                                  181


  PART III. LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION
    AND CAECUM                                                      189

    I. GENERAL REVIEW OF MORPHOLOGY AND PHYSIOLOGY OF THE
      VERTEBRATE INTESTINE                                         190

      I. Midgut or Small Intestine                                 192

        Intestinal Folds                                           193

        Divisions of Small Intestine                               194

        Structure of Small Intestine                               194

          1. Secretory Apparatus                                   194

          2. Absorbing Apparatus                                   195

        Valvulae Conniventes                                        196

      II. Endgut or Large Intestine                                198

    II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED
      STRUCTURES IN VERTEBRATES                                    200

      I. Fishes                                                    200

      II. Amphibia                                                 201

      III. Reptilia                                                201

      IV. Birds                                                    203

      V. Mammalia                                                  204

        Monotremata                                                204

        Marsupalia                                                 204

        Edentata                                                   206

        Sirenia                                                    208

        Cetacea                                                    209

        Ungulata                                                   209

        Rodentia                                                   211

        Carnivora                                                  212

        Cheiroptera                                                212

        Insectivora                                                213

        Primates                                                   213

    III. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION AND CAECUM
      IN THE VERTEBRATE SERIES                                     217

      1. Symmetrical Form of Ileo-colic Junction; Mid- and End-gut
        in Direct Linear Continuity                                221

      2. Asymmetrical Development of a Single Caecal Pouch, lateral
        to the Ileo-colic Junction, Mid- and End-gut Preserving
        their Linear Continuity                                    223

      3. Rectangular Ileo-colic Junction, with Direct Linear
        Continuity of Caecum and Colon                              225

    IV. STRUCTURE OF CAECAL APPARATUS AND SPECIALIZED MORPHOLOGICAL
      CHARACTERS OF COLON IN RODENTS AND UNGULATES                 229

      1. Caecum Proper                                              229

      2. Structural Modifications of Proximal Segment of Colon
        analogous in their Functional Significance to the Caecal
        Apparatus                                                  230

    V. CAECAL APPARATUS AND COLON IN HYRAX.                         234


  PART IV. MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX    237

    I. Development of the Caecum and Appendix                       237

    II. Changes in the Position of the Caecum and Appendix during
      normal Development, depending upon the Rotation of the
      Intestine and the subsequent Descent of the Caecum            239

    III. Variations of Adult Caecum and Appendix                    244

      _A._ Shape of Caecum and Origin of Appendix. Types and
        Variations of Adult Caecum and Appendix                     245

      _B._ Position and Peritoneal Relations of Appendix           250

      _C._ Ileo-Caecal Folds and Fossae                              260




INTRODUCTION.


In considering the anatomy of the human abdominal cavity and peritoneum
in the following pages the explanation of the adult conditions
encountered is based upon the development of the parts, and the
successive human embryonal stages are illustrated by the examination of
the lower vertebrates presenting permanent adult structural conditions
which appear as merely temporary embryonal stages in the development of
the higher mammalian alimentary tract.

For the sake of clearness and brevity all discussion of the _theories of
peritoneal development_ has been designedly omitted. The assumption of
peritoneal _adhesion_, and consequent obliteration of serous areas,
offers many advantages in considering the adult human abdominal cavity,
especially from the standpoint of comparative anatomy. The same has
consequently been adopted without reference to divergent views and
theories.

In studying the descriptive text and the diagrams the student should
remember that the volume offers in no sense a complete or detailed
account of the development of the abdominal cavity and its contents. The
purpose is not to present the embryology of this portion of the
vertebrate body, but to _utilize_ certain embryological facts in order
to _explain_ the complicated adult conditions encountered. To avoid
confusion, and to bring the salient points into strong relief, the
majority of the diagrams illustrating human embryonal stages are purely
schematic.

Moreover, in order to avoid confusing and unnecessary details it is
often desirable to disregard developmental chronology entirely. Many of
the diagrams combine several successive developmental stages, showing
different degrees of development in different portions of the same
drawing. Again it is frequently necessary, for the sake of brevity and
clearness, to actually depart from known embryological conditions. If,
for example, the stomach and liver are treated as if they were from
their inception abdominal organs, the student of systematic embryology
will recall the fact that this position is only _obtained after_ their
primitive differentiation by growth and migration.

Again the mesenteries are treated here as if they formed definite and
well-defined membranes from the beginning--without reference to the
abdominal organs with which they are associated. We speak of the liver
as growing into and between the layers of the ventral mesogastrium,
because this conception offers the opportunity of more clearly
explaining the adult condition. Actually, however, the membrane
develops, as a new structure, after the first differentiation of liver
and stomach, as these organs descend into the abdominal cavity.

Similar discrepancies between fact and schema are encountered
throughout. Consequently, while the purpose of the volume is to
facilitate the study and comprehension of the _adult_ peritoneal cavity
and its contents, the reader should guard against receiving the
developmental illustration as a correct successive and detailed account
of the _embryology_ of the parts concerned.

In like manner the comparative anatomical facts adduced form in no sense
even approximately a complete serial morphological account of the
vertebrate alimentary tract.

To the student of human anatomy the zoological position of the forms
which help him to understand complicated human structural conditions is
immaterial. He can draw on all the vertebrate classes independently of
their mutual relations. Hence neither ontogeny nor phylogeny are here
introduced, except as aids to the study of adult human anatomy. The
following pages offer neither an embryology nor a comparative anatomy of
the alimentary tract, but an attempt has been made in them to illustrate
the significance of the complicated anatomical details presented by the
adult human abdominal cavity by reference to the simpler antecedent
conditions encountered during the early developmental stages of the
higher forms and permanently in the structure of the lower vertebrates.

While, as just stated, a complete presentation of the development of the
abdominal cavity is not required, yet the student will find it of
advantage to rehearse the main facts of vertebrate embryology, for the
purpose of bringing a clear understanding of the manner in which the
vertebrate body is built up to bear upon the problems which the special
organs and structures of the body-cavity present for his consideration.
This purpose can be accomplished by a very brief and condensed
consideration of the cardinal facts.

The entire vertebrate body is the product of developmental changes
taking place after fertilization in a single primitive CELL, the EGG or
OVUM (Fig. 1).

[Illustration: FIG. 1.--Human ovum, from a mature follicle, a sphere of
about 0.2 mm. diameter. x 25. (Kollmann.)]

In structure the ovum corresponds to other animal cells. On account of
their special significance during development the different component
parts of the egg-cell have received special distinctive names. The
_cell-body_ is known as the _vitellus_ or _yolk_. It is composed of two
substances, the _protoplasm_ or formative yolk and the _deuteroplasm_ or
nutritive yolk, which vary in their relative proportions in the ova of
different animals.

The protoplasm represents the material from which in the course of
development the cells forming the body of the individual are derived,
while the deuteroplasm serves for the nutrition of the ovum during the
earliest stages of development.

The _nucleus_ of the egg-cell is distinguished as the _germinal
vesicle_, and its _nucleolus_ as the _germinal spot_.

The _cell-body_ or _vitellus_ is surrounded by a condensed portion of
the cell contents to which the name of the vitelline membrane has been
applied, which in turn is enclosed by a transparent and elastic cover,
the _zona pellucida_, presenting a radially striated appearance.

The ovum is contained in the cortical portion of the ovary, enclosed in
the _Graafian follicle_, a vesicle 4-8 mm. in diameter, whose fibrous
walls are lined by several layers of epithelial cells, which surround
the ovum, forming the _discus proligerus_.

After impregnation the egg-cell, by a process of repeated division or
cleavage, undergoes _segmentation_, the cell-body being divided
successively into two, four, eight, sixteen, thirty-two, etc., _cells_,
called _blastomeres_ (Figs. 2 and 3). The mass of cells finally
resulting from this process of segmentation forms the ground work of the
future body. A vertebrate ovum in this stage of complete segmentation is
called the _morula_ from its resemblance to a mulberry (Fig. 4).

[Illustration: FIG. 2.--Segmentation of mammalian ovum (bat). (After E.
von Beneden.) Two blastomeres, each with a nucleus, shown in lighter
color. The dark bodies are yolk-granules.]

[Illustration: FIG. 3.--Segmentation of mammalian ovum. Four
blastomeres. (After E. von Beneden.)]

[Illustration: FIG. 4.--Ovum of rabbit, from terminal portion of
oviduct. The zona pellucida appears thickened, and contains many
spermatozoa which failed to penetrate the ovum. (After Bischoff.)]

After segmentation is completed a cavity filled with fluid and
surrounded by the developing cells is gradually formed in the interior
of the mass. This cavity is known as the _segmentation-cavity_. The egg
is now called the _blastula_, _blastosphere_ or _blastodermic vesicle_
and the cellular membrane enclosing the segmentation-cavity forms the
_germinal membrane_ or _blastoderm_ (Figs. 5 and 6). The cells of the
blastoderm become aggregated at one point on the circumference of the
vesicle (dorsal pole of blastosphere) forming, when viewed from above, a
thickened biscuit or disk-shaped opaque area. This is known as the
_germinal area_, or _primitive blastoderm_ or _embryonic shield_ (Figs.
7 and 12).

[Illustration: FIG. 5.--Blastodermic vesicle of rabbit. (After E. von
Beneden.)]

[Illustration: FIG. 6.--Blastodermic vesicle of _Triton taeniatus_.
(Hertwig.)]

[Illustration: FIG. 7.--Embryonic area of rabbit embryo. (Heisler, after
E. von Beneden.) The primitive streak beginning in the
cell-proliferation known as the "node of Hensen."]

[Illustration: FIG. 12.--Oval embryonic area of rabbit's egg, detached
with part of wall of blastodermic vesicle. x 30. (Kollmann.)]

This is the first indication of the coming division of the entire
egg-cell into the _embryo proper_ and the _vitelline_ or _yolk-sac_
(Figs. 8 and 9). The entire future individual develops from the cells of
the germinal area. This area comprises both the embryo proper and the
region immediately surrounding it.

[Illustration: FIG. 8.--Blastodermic vesicle of mammal. (E. von
Beneden.) The layer of cells lining the interior of the vesicle next to
the zona pellucida forms Rauber's "Deckschichte" or prochorion. This is
not the true ectoderm, since it does not participate in the formation of
the embryo, which is entirely derived from the cells of the germinal
area.]

[Illustration: FIG. 9.--Human embryo with yolk-sac, amnion, and
belly-stalk of fifteen to eighteen days. (Heisler, after Coste.)]

The remainder of the ovum, serving temporary purposes of nutrition and
respiration, gradually becomes absorbed and disappears.

[Illustration: FIG. 10.--Embryonal area of sheep, composed of ectoderm
and entoderm. (After Bonnet.)]

[Illustration: FIG. 11.--Blastodermic vesicle of rabbit. Section through
embryonic area at caudal limit of node of Hensen. (Rabl.)]

Transverse sections at right angles to the long axis of the embryonic
area show that the single layer of cells composing the primitive
germinal membrane becomes differentiated first into two (Fig. 10) and
subsequently into three layers of cells (Fig. 11). At the margins of the
germinal area these layers are of course continuous with the rest of
yolk-sac wall. From their position in reference to the center of the
cell the three layers of the blastoderm are described as--

    1. The outer, Epiblast or Ectoderm.
    2. The middle, Mesoblast or Mesoderm.
    3. The inner, Hypoblast or Entoderm.

The central nervous system (brain and spinal cord) is derived from the
ectoderm by the development of a groove in the long axis of the
embryonic area (Figs. 13, 14, 16 and 17), and by the subsequent union in
the dorsal midline of the ridges bounding the groove to form a closed
tube (Fig. 18). (Medullary groove, plates and canal.)

[Illustration: FIG. 13.--Transverse section of embryonic area of ovum of
sheep of fourteen and a half days. (Heisler, after Bonnet.)]

[Illustration: FIG. 14.--Germinal area of rabbit's ovum. (Kollmann.)]

[Illustration: FIG. 15.--Surface-view of area pellucida of an
eighteen-hour chick-embryo. (Balfour.)]

[Illustration: FIG. 16.--Transverse section of human embryo before
development of protovertebrae or chorda dorsalis. (Keibel.)]

[Illustration: FIG. 17.--Transverse section of a sixteen and a half day
sheep embryo. (Heisler, after Bonnet.)]

[Illustration: FIG. 18.--Embryo of bird, at beginning of third day, with
four blastodermic layers, resulting from the division of the mesoderm
into parietal and visceral layers, separated by the coelom cavity.
Transverse section. x 170. (Kollmann.)]

The following changes in the ventral aspect lead to the formation of the
alimentary canal and body-cavity:

The developing embryo at first lies flat on the subjacent yolk-mass, and
subsequently becomes gradually separated more and more from the rest of
the blastoderm by grooves or furrows which develop along the sides and
at the cephalic and caudal extremity of the embryo. The folds resulting
from these furrows indent the yolk more and more as development proceeds
and tend to approach each other at a central point, the future
_umbilicus_.

In the meanwhile changes in the region of the mesoderm have led to
conditions which produce a differentiation of the ventral portion of the
embryo into two tubes or cylinders, the _alimentary_ or _intestinal
canal_ and the _general body-cavity_, the former being included within
the latter.

Early in the course of development a number of spaces appear in the
mesoderm on each side of the axial line of the embryo. These spaces soon
unite to form two large cavities, one on each side. Taken together these
cavities constitute the _coelom_ or _body-cavity_, which becomes
subdivided in the adult mammal into the pleural, pericardial and
abdominal cavities.

As these coelom cavities develop in the mesoderm the cells lining them
become distinctly epithelial. This mesodermic epithelium lining the
coelom is called the _mesothelium_.

The development of the coelom space divides the mesoderm on each side
into an outer leaf, the _somatic_ or _parietal mesoderm_, and an inner
leaf, the _splanchnic_ or _visceral mesoderm_ (Figs. 18 and 19). The
former is closely applied to the ectoderm, forming with it the
_somatopleure_ or _body-wall_. The latter, in close contact with the
entoderm, forms with it the _splanchnopleure_ or wall of the alimentary
canal. In the dorsal median line both somatic and splanchnic mesoderm
become continuous with each other and with the axial mesoderm (Fig. 20).

[Illustration: FIG. 19.--Transverse section of a seventeen and a half
day sheep embryo. (Bonnet.)]

[Illustration: FIG. 20.--Curves of blastodermic layers and division of
mesoderm in amniote embryo. (Kollmann.)]

The folds of the splanchnopleure, indenting the yolk-sac, form a gutter
directly connected with the yolk, the _primitive intestinal groove_ or
_furrow_, whose margins gradually approach each other (Fig. 20). In this
way the primitive alimentary canal becomes separated from the yolk. At
first this separation is ill-defined, and the channel of communication
between the primitive intestine and the yolk is wide (Figs. 13, 16, 17
and 19). The folding of the splanchnopleure completes, at an early
period, the dorsal and lateral walls of the embryonic gut, but
ventrally, toward the yolk, the tube is incomplete and widely open.

By union and coalescence of the splanchnopleural folds, proceeding from
the caudal and cephalic ends towards the center, this primitive wide
channel gradually becomes narrowed down, until the communication between
the yolk-sac and the intestine is reduced to a canal, the
_vitello-intestinal_ or _omphalo-mesenteric duct_. The intestinal gutter
is thus converted into a closed tube except at the point of implantation
of the vitelline duct during the persistence of this structure. In the
meanwhile the somatopleural folds forming the body-walls grow more and
more together from the sides, approaching the vitello-intestinal duct.
Finally touching each other they coalesce to form the ventral body wall,
in the same manner as the splanch[n]opleural folds met and united to
form the alimentary tube.

At the same time the vitello-intestinal duct and the remnant of the
yolk-sac, to which it was attached ("umbilical vesicle"), normally
become obliterated and disappear.

After the intestinal tube and the body cavity have thus become closed
the embryo straightens out and the alimentary canal appears as a nearly
straight cylindrical tube extending from the cephalic to the caudal end
of the embryo. This primitive alimentary tube at first terminates at its
cephalic extremity in a blind pouch, while at the caudal end in the
early stages the intestine is connected with the nerve-tube by a channel
called the _neuro-enteric canal_, forming in the earliest embryos a
communication between the ectoderm lining the bottom of the medullary
groove and the entoderm (Figs. 22 and 26). In man this stage is
encountered very early, in embryos of 2 mm. before the formation of
either heart or provertebrae.

[Illustration: FIG. 22.--Caudal half of human blastoderm measuring 3
mm., with open medullary groove. Dorsal view. x 30. (After Spee.)]

[Illustration: FIG. 26.--Neuro-enteric canal in section of human embryo
of 2 mm. (After Spee.)]

At the point where the canal develops the primitive groove presents a
thickened circumvallate spot, marking the beginning perforation of the
medullary plate from the ectoderm to the entoderm. The canal exists only
for a short period during the earliest stages of embryonal life. It
becomes rapidly closed, the neural and intestinal tubes henceforth
remaining permanently separated from each other.

The embryonal caudal end of the primitive alimentary canal is not the
final adult termination of the tube. When the anal aperture is formed in
a manner to be presently detailed, the opening is situated cephalad of
the portion connected with the nerve-tube by the neuro-enteric canal.
Hence this terminal portion of the early embryonic alimentary canal is
called the "post-anal gut" (Fig. 21).

[Illustration: FIG. 21.--Sagittal section of caudal extremity of cat
embryo of 6 mm. (Tourneux.)]

The post-anal gut and the neuro-enteric canal are better developed in
the embryos of the lower than in those of the higher vertebrates. But in
all vertebrates of the present day both of these structures undergo
regressive changes and finally disappear altogether. They serve to
recall conditions which existed in bygone ages, and, while they have a
long and significant phylogenetic history, they have lost among living
vertebrates all physiological importance.

After closure of the neuro-enteric canal and obliteration of the
post-anal gut the alimentary tube ends, during a short period, both
cephalad and caudad in a blind pouch. Very soon, however, the ectoderm
becomes invaginated at both extremities and finally perforates into the
lumen of the intestine, thus establishing the oral and anal
communications with the exterior. The anal ectodermal invagination
(proctodaeum) (Fig. 21), is smaller than the oral (stomadaeum) (Fig. 27),
but the intestinal tube forms an extensive pouch in the anal region
which descends to meet the ectodermal invagination of the proctodaeum.
The details of the embryonic processes leading to the final
establishment of the adult condition are of great interest on account of
the pathological importance of abnormal or arrested development in these
parts. Failure of the caudal intestinal pouch to establish a
communication with the anal invagination, or failure of development in
either anal invagination or intestinal pouch, leads to the condition
known as atresia ani or imperforate anus, of which there are several
varieties.

[Illustration: FIG. 27.--Median section through head of embryo rabbit of
6 mm. (Mihulkovics.)]

Before the anal opening forms the primitive caudal intestine receives
from above the stalk of the _allantois_, while the Wolffian duct, the
canal of the embryonic excretory apparatus, also opens into it. The
renal bud on the Wolffian duct in Fig. 28 indicates the beginning
development of the permanent kidney (metanephros), and the proximal
portion of the allantoic stalk is destined to form by a spindle-shaped
enlargement the future urinary bladder (Fig. 28). The caudal gut has as
yet no anal opening. Ventrad of the tail end of the embryo the ectoderm
presents at this time a depression (Fig. 21). The ectoderm lining the
bottom of this anal fossa or depression is separated by a little
mesoderm tissue from the entodermal lining of the blind pouch of the
caudal gut. Ectoderm and entoderm in this region with the intervening
mesodermal layer form the _cloacal membrane_ (Fig. 21).

[Illustration: FIG. 28.--Reconstruction of caudal end of human embryo of
11.5 mm. (four and a half weeks), showing pelvic structures. x 40.
(After Keibel.)]

=Development of Cloaca.=--The entodermal pouch or prolongation sent down
from the end-gut to meet the anal invagination enlarges and dilates to
form a short wide piece of the intestinal tube into which open on the
one hand the urinary and sexual ducts of the genito-urinary system,
while it receives on the other the termination of the _end-gut proper_
(Figs. 28 and 29).

[Illustration: FIG. 29.--Reconstruction of caudal end of human embryo of
14 mm. (five weeks). x 20 (After Keibel.)]

This is the permanent condition of the terminal openings of the
alimentary and genito-urinary tracts in the lower vertebrates. It is
found in certain fishes, in all amphibia, reptiles and birds, and occurs
also in one order of mammals, the monotremes. In man and mammals
generally the anal orifice is separated from the genito-urinary opening,
lying dorsad of the same and provided with special sphincters. Only in
the monotremes do the anus and the genito-urinary tract open into a
common cloaca surrounded by a sphincter common to the anal and
genito-urinary openings (sphincter cloacae). In birds, reptiles, amphibia
and many fishes (especially the Plagiostomata) this cloacal formation is
the rule. In many fishes, especially the Teleosts, the anus and the
genito-urinary openings are separate, as in mammals, but their position
is reversed, the anus being ventral, while the genito-urinary opening is
placed dorsally.

[Illustration: FIG. 23.--Genito-urinary tract and cloaca of _Iguana
tuberculata_, female. (Columbia University Museum, No. 1846.)]

Fig. 23 shows the cloaca in a female specimen of _Iguana tuberculata_.
The ventral wall of the cloaca has been divided to the left of the
median line and turned over to the right, carrying with it the cloacal
opening of the bladder. The termination of the alimentary canal opens
into the cloaca from above.

A transverse fold of the mucosa separates this upper compartment of the
cloaca (_coprodaeum_) from a lower space (_urodaeum_) which receives in
its dorsal wall the openings of the two oviducts and immediately above
them--upon two papillae--the openings of the ureters, while the ventral
wall contains the cloacal opening of the bladder.

The right ovary has been removed--to show the abdominal opening of the
right oviduct--by dividing the mesovarian peritoneal fold.

[Illustration: FIG. 24.--Genito-urinary tract and cloaca of the hen,
_Gallus bankiva_. (Columbia University Museum, No. 1208.)]

Fig. 24--taken from a preparation of the hen--shows the typical
arrangement of the female genito-urinary tract and cloaca in the birds.

The terminal portion of the alimentary canal, in entering the cloaca,
forms an expanded upper cloacal compartment for the accumulation of the
excreta, called the _coprodaeum_.

It is separated by a prominent mucous fold from the central compartment,
or _urodaeum_ which receives the terminations of the two ureters and of
the single (left) oviduct. A second fold forms the distal limit of the
urodaeum and separates it from the lowest cloacal compartment, the
_proctodaeum_.

[Illustration: FIG. 25.--Genito-urinary tract and cloaca of _Platypus
anatinus_, duck-billed platypus. (Columbia University Museum, No.
1802.)]

Fig. 25 shows the male genito-urinary tract and the cloaca in the
monotreme, _Platypus anatinus_. The cloaca is a spacious sac formed by
the confluence of the rectum and the genito-urinary sinus.

The penis, consisting of two large cavernous bodies, is contained in a
fibrous sac which arises from the junction of the genito-urinary sinus
and the cloaca, and is continued into the ventral wall of the cloaca
near its termination by an opening through which the penis can pass into
the cloaca and beyond the external cloacal aperture.

The semen enters the penis at its root through a narrow opening situated
close to the junction of genito-urinary sinus and cloaca.

For a short period, therefore, the human embryo and the embryos of the
higher mammalia present conditions which correspond to the permanent
structure of the parts in these lower vertebrates. In human embryos of
11.5 mm. cervico-coccygeal measure (32-33 days) (Fig. 28), the cloaca
appears as a short sac continuous dorsad with the intestine, ventrad
with the rudiment of the urinary bladder. The larger portion of the
caudal gut (postanal gut) has disappeared, having been reduced to a thin
epithelial strand which gradually becomes entirely absorbed. Only the
proximal portion of the end-gut is used for the development of the
cloaca, which, however, at first has no external opening (Fig. 28).

The tail end of the embryo becomes more extended and between it and the
umbilical cord an interval appears in which the genital protuberance
develops. Behind this point the ventral cloacal wall is formed by the
cloacal membrane.

A considerable interval also develops between the points of entrance
into the cloaca of the intestine proper and of the allantoic stalk
(urinary bladder). The growth of the mesoderm pushes the intestine
against the sacral vertebrae, while the stalk of the allantois with the
rudimentary urinary bladder is forced against the ventral abdominal
wall. These changes prepare the way for the first appearance of the
_genito-urinary sinus_. The neck of the embryonic bladder elongates and
receives the ducts of the urinary and genital glands (Fig. 29). In
embryos of 14 mm. cervico-coccygeal measure (36-37 days) (Figs. 29 and
30), the genito-urinary sinus perforates the cloacal membrane on the
ventral aspect of the genital protuberance, forming the _uro-genital
cleft_. The rectum remains closed for a few days longer. The perforation
is preceded by the formation of a transverse ectodermal reduplication,
producing a depression called the _transverse anal fissure_. This
depression increases in depth until a distinct anal invagination
results, known as the _proctodaeum_, which grows as a funnel-shaped fossa
toward the blind termination of the endgut. In embryos of 25 mm.
cervico-coccygeal measure (81/2-9 weeks) the intestine still ends in a
blind pouch. The anus is, therefore, independent of the end-gut in its
development. It is derived from the ectoderm and its production is
analogous to the formation of the oral cavity by means of the ectodermal
invagination called the _stomadaeum_.

[Illustration: FIG. 30.--Human female foetus, 3.4 cm. long,
vertex-coccygeal measure. The external perineal folds separate the anal
invagination from the uro-genital opening. (Kollmann.)]

Finally the cloaca is converted into a ventral tube from which part of
the urinary bladder, the urethra and genito-urinary sinus develop, and a
dorsal tube from which the _rectum_ is derived. This double disposition
of the cloaca is accomplished by gradual changes in the entoderm and
mesoderm. The entoderm proliferates until a partition is formed which
separates the two divisions of the cloacal tube from each other, and the
mesoderm likewise increases, surrounding the newly formed entodermal
tubes with tissue from which the muscles, connective tissue and blood
vessels of the parts are derived (Figs. 28 and 29).

This partition, the _septum uro-rectale_, develops symmetrically on each
side, appearing first as paired folds on the right and left sides called
the _internal perineal folds_ (Figs. 28 and 29). When these folds have
reached the cloacal membrane they complete the separation of the cloaca
into two adjacent canals. Each of these canals is still closed caudad by
its respective portion of the cloacal membrane, now divided into an
_anal_ and _uro-genital_ segment. These two portions of the original
cloacal membrane become perforated separately, the uro-genital before
the anal. Hence the external opening of the uro-genital sinus is the
first to appear, to be followed by the anal perforation. The internal
perineal folds are supplemented by the formation of similar external
folds, ridges of mesoderm tissue which surround the anal orifice in the
form of a low wall and thus deepen the anal ectodermal invagination into
the fossa of the proctodaeum.

These developmental stages in the formation of the end-gut are of
importance because they offer the explanation of the pathological
conditions which result from an arrest of development and from the
failure of either the uro-genital or anal opening to form in the usual
manner. These malformations must date back to an early stage, and
probably have their inception in disturbances occurring in the normal
development between the 15th and 23d day (embryos of 3-6 mm.). Perhaps
in some cases of atresia there may be a secondary obliteration of a
previously formed opening. In Fig. 31 the proctodaeum persists but the
perforation of the anal membrane into the end-gut has not occurred. The
ectoderm of the anal fossa and the intestinal entoderm remain separated
by a transverse mesodermal partition. Different degrees of this
malformation are observed. The layer separating the skin from the blind
end of the rectum may be so thin that the meconium contained in the
latter can be felt through it. On the other hand the rectum may
terminate high up in a blind pouch, which is separated from the skin by
a distance of several centimeters.

[Illustration: FIG. 31.--Section of pelvis of human foetus, showing
atresia recti. (Esmarch.)]

We may now briefly consider the genetic, histological and mechanical
conditions which the above-outlined course of development imposes on the
alimentary tract.

The ectoderm forms the superficial covering of the embryo and in the
dorsal axial line develops the medullary groove which subsequently
becomes converted into the cerebro-spinal axis by closure of the
medullary plates and inclusion of the neural tube within the surrounding
mesoblast (Fig. 18). The entoderm forms the epithelial lining of the
interior of the alimentary canal and its appendages and derivatives
(Fig. 19). The mesoderm furnishes the skeletal, muscular and vascular
systems. At first single, like the two remaining layers of the
blastoderm, the mesoderm splits early on each side of the chorda
dorsalis into two layers, including between them spaces which after
coalescence form the _primitive pleuro-peritoneal_ or _body-cavity_
(Fig. 20). One of these mesodermal layers bounding this space becomes
closely connected with the ectoderm, forming the _somatopleure_ or body
wall, while the other joins the entoderm to complete the wall of the
alimentary canal, forming the _splanchnopleure_. In the course of
further development the edges of these two layers approach each other
ventrally in the median line and finally fuse.

The products of this fusion are two epithelial tubes, one included
within the other, with walls reinforced by tissue derived from the two
layers of the mesoderm. The internal or entodermal tube is of much
smaller diameter than the outer or ectodermal tube, but much longer. The
walls of the two tubes are placed in contact with each other by their
mesodermal elements dorsally in the axial line, but elsewhere are
separated from each other by the body-cavity (except in the region of
the ventral mesogastrium).

The splanchnopleure is not so wide as the somatopleure. As it closes in
the ventral median line it includes the deepest or entodermal layer. It
now forms a tube whose walls are composed superficially of mesoderm
(splanchnopleure) while the lumen is lined by epithelium derived from
the entoderm. This tube is the _primitive enteric_ or _alimentary
canal_. The somatopleuric layers bounding the body cavity take a wider
sweep and after they have united ventrally in the median line they
embrace a much more extensive space, the _primitive body cavity_ or
_coelom_. The walls of this space are largely made up of the skeletal and
muscular elements developed from the mesoderm of the somatopleure,
covered superficially by the common ectodermal investment of the body.
It will be seen that the enteric tube thus becomes included within the
wider and more capacious coelom cavity.

Both the somatic and the splanchnic leaf of the mesoderm consist at
first solely of a layer of flattened epithelial cells, the mesothelium.
But very early this tissue is increased to form a massive layer by
direct development from the mesothelium. The new mesodermal cells thus
produced constitute the _mesenchyma_, which includes the whole of the
mesoderm of the embryo except the mesothelial lining of the coelom. The
cells of the mesenchyma, connected with each other and with the
mesothelial cells by protoplasmic processes, are not as close together
as in an epithelium and do not form a continuous membrane. By migration
and multiplication a large mass of mesodermal tissue is produced which
fills the entire space between the mesothelium and the primary germ
layers. The mesenchymal tissue between the mesothelium and the ectoderm
forms the mass of the skeletal, muscular and vascular systems. The
mesenchymal tissue between the mesothelium and the entoderm forms an
important constituent of the alimentary canal and of its appendages. The
entoderm furnishes the internal epithelial lining of the tube upon which
the performance of the specific physiological function of the entire
apparatus depends. This epithelial tube is covered from without by the
splanchnic mesoderm. The mesodermal elements thus added to the enteric
entodermal tube consist of connective tissue and muscular fibers. The
latter, arranged in the form of circular and longitudinal layers,
control the contractility of the tube and regulate the propulsion of the
contents. The connective tissue of the splanchnic mesoderm appears as an
intermediate layer uniting the epithelial lining and the muscular walls.
Situated thus between the mucous and muscular coats of the intestine
this layer is known as the _submucosa_. It contains, imbedded in its
tissue, the glandular elements of the intestine derived from the
entodermal epithelium, and the blood vessels, lymphatics and nerves. The
second chief function of the splanchnic and somatic mesoderm is the
production of the serous membrane investing the body cavity and its
contents from the mesothelium lining the primitive coelom. This
mesothelial tissue, differentiated as a layer of flattened cells, lines
the interior of the body cavity and covers the superficial aspect of the
enteric tube. By subsequent partition of the common coelom the great
serous membranes of the adult, the pleurae, pericardium and peritoneum,
are developed from it.

The entodermal enteric tube is, as already stated, closely attached at
an early period along its dorsal surface to the axial rod of mesoderm
containing the chorda dorsalis immediately ventrad of the neural canal.
In the earliest stages, just after the splanchnopleure and somatopleure
have closed to complete the alimentary tube and body cavity, the remnant
of these layers extends between the ventral abdominal wall and the
ventral surface of the intestine forming a partition which divides the
body into a right and left half. (Fig. 32, _A._) For the most part this
primitive connection between the ventral abdominal wall and the
intestinal tube is lost very early. The stomach, however, is always
connected by a ventral mesogastrium, from which the lesser omentum is
derived, to the ventral body wall. The disappearance of the ventral
mesentery caudad of this point establishes the condition indicated in
Fig. 32, _B._ The entodermal tube and the surrounding splanchnic
mesoderm forming the intestinal canal is attached along its dorsal
surface to the axial mesoderm of the dorsal mid-line. The primitive
mesothelial peritoneum is reflected along this line from the internal
surface of the body wall upon the ventral and lateral surfaces of the
intestine. The coelom of one side communicates ventrad of the intestine
with the coelom of the opposite side. Hence by the disappearance of the
ventral mesentery caudad of the stomach the paired body-cavities have
become fused into a single abdominal cavity--while cephalad the original
division into right and left halves is maintained by the portion of the
ventral mesentery which attaches the stomach to the ventral abdominal
wall. The mesodermal tissue which at this time attaches the alimentary
tube along its entire extent to the dorsal wall of the coelom carries the
primitive embryonic arterial vessel, the aorta. This vessel supplies a
series of small branches to the intestine, which reach the same by
passing ventrad imbedded in the mesoderm connecting the tube to the
dorsal body wall.

[Illustration: FIG. 32.--Schematic diagrams, illustrating the vertebral
mesentery. _A._ earlier; _B._ later condition. (Minot.)]

With the further development of the alimentary canal a gradual
elongation of this connecting band of mesoderm and of the contained
vessels is observed, the tube itself gradually receding from the
vertebral axis. The early broad attachment is replaced by a narrower
stalk into which the mesoderm is drawn out. With this narrowing in the
transverse and elongation in the sagittal direction the connecting
tissue assumes the character of a thin membrane with two free serous
surfaces, including the intestinal vessels imbedded between them.
Coincident with this elongation of the enteric attachment and its
narrowing in the transverse direction the primitive intestine becomes
more completely invested by the serous lining membrane of the coelom
cavity. In this stage we can speak of the double-layered membrane
attaching the tube to the dorsal body wall and carrying the intestinal
blood-vessels as the primitive dorsal mesentery. The intestinal canal
itself is invested by serous membrane except along a narrow strip of its
dorsal border where the mesentery is attached and where the vessels
reach the intestine. We can now distinguish the serous lining membrane
of the abdominal cavity, derived from the mesothelium of the splanchnic
and somatic mesoderm as the _peritoneum_. The membrane presents the
following topographical subdivisions:

1. _Parietal Peritoneum_, lining the inner surface of the abdominal
walls.

2. _Visceral Peritoneum_, investing the external surface of the
intestine and its derivatives.

3. _Mesenteric Peritoneum_, connecting these two, carrying the
intestinal blood vessels and lymphatics and acting as a suspensory
support to the alimentary canal.

The dorsal mesentery in fishes, amphibia and reptiles contains smooth
muscular fibers derived from the mesoderm. These bands of smooth muscle
fibers are also encountered, though less well developed, in the
mesentery of birds and mammals. The so-called "suspensory muscle of the
duodenum" belongs to this category. It consists of a few strands of
unstriped muscular and fibrous tissue which passes from the praeaortal
tissue around the origin of the superior mesenteric artery and coeliac
axis to the duodeno-jejunal angle. Fasciculi from this band may
penetrate into the root of the mesentery (Gegenbaur).

Similar muscular fasciculi have been observed in the peritoneal folds of
the ileo-caecal junction (Luschka) and in the mesorectum--forming in the
latter situation the recto-coccygeal muscles of Treitz, and in the
female the recto-uterine muscles.

In its earlier stages the primitive common mesentery forms a membrane
which carries the intestinal blood vessels between its two layers,
surrounds the embryonic alimentary canal and attaches the same to the
ventral aspect of the chorda dorsalis and aorta. This is the permanent
condition in many of the lower vertebrates in which the intestinal tube
is suspended by a simple dorsal mesentery, a condition which is repeated
by the embryos of man and the higher vertebrates. From this primitive
common mesentery are derived, by further development, displacement and
adhesion, all the other mesenteries, omenta and peritoneal folds of the
adult. The character and degree of these subsequent changes is
determined by the increase in length and change in position of the
intestine and the growth of large organs, like liver, spleen and
pancreas. Many portions of the intestinal canal, at first suspended by
the mesentery and freely movable within the abdominal cavity, become
later, by secondary adhesion, firmly connected with adjacent portions of
the tube or with the abdominal parietes.

In certain of the lower vertebrates (fishes) large sections of the
intestine lie entirely free within the abdomen, their only connection
with the parietes being afforded by the blood vessels. This condition
depends upon _absorption_ of the original mesentery. A similar process,
though much more circumscribed, is observed in the omenta of many
mammals, which appear perforated at several points.

=Derivatives of the Entodermal Intestinal Tube.=--The entodermal
epithelium is physiologically the characteristic element of the
alimentary canal. Besides lining the entire internal surface of the tube
it gives rise by budding and protrusion from the intestinal canal to a
series of organs which from the mode of their development must be
regarded as diverticular or derivatives of the alimentary canal (Figs.
33, 34, and 35). These organs, proceeding in order cephalo-caudad, are
the following:

    The salivary glands.
    Thymus and thyroid.
    The lungs.
    Pancreas.
    Liver.

[Illustration: FIG. 33.--Schema of alimentary canal and accessory
organs, derived from same. (After Bonnet.)]

[Illustration: FIG. 34.--Reconstruction of alimentary canal of human
embryo of 4.2 mm. x 24. (After His.)]

[Illustration: FIG. 35.--Reconstruction of alimentary canal of human
embryo of 7 mm. (twenty-eight days). x 12. (After His.)]

The epithelium of all these structures is derived from the primitive
entoderm of the intestinal tube, except the epithelium of the salivary
glands, which, being derived from the stomadaeal invagination, is
ectodermal in character. We have previously noted the general history
and appearance of the yolk-sac and its connection by means of the
vitello-intestinal duct with the intestine. In contradistinction to the
adult organs just noted the yolk-sac or umbilical vesicle is merely a
temporary embryonal appendage to the alimentary canal. It also differs
from them in the fact that it is not an extension or budding from the
completed intestinal tube, like the liver and pancreas, but indicates,
by the implantation of the duct (Fig. 21), the last point at which
closure of the intestinal canal takes place, when after obliteration of
the duct the separation of the intestine from the yolk-sac is completed.

The segment of the primitive alimentary canal cephalad of the attachment
of the vitello-intestinal duct gives rise to the pharynx, oesophagus,
stomach, proximal portion of small intestine proper and its derivatives,
the liver and pancreas.

The portion situated caudad of the duct produces the rest of the small
and all of the large intestine (Figs. 33 and 35). At times in man and
other mammals (cat) the vitello-intestinal duct does not become
absorbed, but persists and continues to develop as a part of the small
intestine, forming the blind pouch or appendage known as _Meckel's
diverticulum_ (Figs. 37 and 38). This diverticulum may vary in length
from 1.5 to 15 cm. It either projects freely into the abdominal cavity
as a pouch arising from the convex border of the small intestine
opposite to the mesenteric attachment, or else it reaches the abdominal
wall at the umbilicus and is attached to the same. In a few instances it
has not terminated in a blind pouch, but has remained open at the
umbilicus, in which case the aperture discharges intestinal contents.
Sometimes the process of obliteration which normally leads to the
absorption of the vitello-intestinal duct extends to the adjoining
segment of the small intestine, resulting in obliteration of the
intestinal lumen and consequent obstruction at this point.

[Illustration: FIG. 36.--Reconstruction of alimentary canal of human
embryo of thirty-five days (13.8 mm.). x 8. (After His.)]

[Illustration: FIG. 37.--Human adult ileum with Meckel's diverticulum.
Ileo-diverticular serous fold and persistent omphalo-mesenteric artery.
(Columbia University Museum, No. 1803.)]

[Illustration: FIG. 38.--Human adult ileum, with Meckel's diverticulum.
(Columbia University Museum, No. 745.)]

The intestinal opening of the diverticulum is situated at a varying
distance above the ileo-colic junction, ranging from 27.5 cm. to 290
cm., with an average of 107 cm.

While the obliteration and complete absorption of the duct is normal in
nearly all vertebrates, a remnant persists in some birds, in which a
short caecal pouch (_diverticulum caecum vitelli_) is found at about the
middle of the small intestine. A portion of the vitello-intestinal duct
thus persists throughout life in some wading and swimming birds. Figs.
39 and 40 show this condition in the small intestine of _Urinator lumme_
and _imber_, the red-throated loon and the great northern diver. In
other birds, however, such as birds of prey, song birds, etc., the duct
is absorbed and disappears completely.

[Illustration: FIG. 39.--Small intestine of the red-throated loon,
_Urinator lumme_, showing persistent caecal pouch, the remnant of the
vitelline duct. (Columbia University Museum, No. 997.)]

[Illustration: FIG. 40.--Small intestine of great northern diver,
_Urinator imber_, with caecal pouch, the remnant of the vitelline duct.
(Columbia University Museum, No. 77, 1578.)]

In order to complete the embryological history of the alimentary canal
it is necessary to take brief account of another structure derived from
it, namely the _allantois_. Its significance to the adult organism is
seen in connection with the genito-urinary tract, the urinary bladder
being formed by its persistent portion. In the embryo, however, it has
important nutritive and respiratory functions. In the embryos of the
higher vertebrates nutrition depends only in the earliest stages upon
the yolk-sac of the ovum, over which a vascular network extends.

Very soon the caudal portion of the primitive intestine develops a
vascular sac-like outgrowth (Figs. 21 and 41). This pouch forms the
_allantois_. It is intimately connected with embryonal respiration, and
probably also forms a reservoir which receives the secretion of the
primitive kidney. This foreshadows the final destiny of the proximal
intra-abdominal portion of the allantoic sac which persists and is
converted into the urinary bladder of the adult.

[Illustration: FIG. 41.--Diagram illustrating the later stages in the
formation of the mammalian foetal membranes. (Heisler, modified from
Roule.)]

The allantois is present in Amphibia but is very small. In Amniota[1] it
is large and grows around the embryo. In those of the higher vertebrates
which are developed within an egg (reptiles, birds and monotremes) the
sac of the allantois comes to lie beneath the egg-shell and acts as a
respiratory organ. In the higher mammalia, developed within the uterus,
the allantois becomes attached by vascular villi to the uterine wall and
establishes a vascular connection between the foetal and maternal blood
vessels. In this way the _allantoic placenta_ is formed (Fig. 41). The
placenta, as just stated, is absent in the monotremes and is only
slightly developed in marsupials, in which animals the foetus develops to
maturity in the marsupial pouch after leaving the uterus. These animals
are therefore distinguished as _Aplacentalia_ from the remaining higher
mammals in which the allantoic placenta develops and which are hence
called the _Placentalia_.

[1] In the embryos of reptiles, birds and mammals folds of the
somatopleure arise externally to the constricting furrows by means of
which the embryo is gradually separated from the yolk-sac, with the
resulting formation of the intestinal and abdominal walls. These folds,
situated at the head, tail and on the sides, grow upwards and finally
meet and unite to form a membranous sac called the _amnion_. Hence these
higher vertebrates (reptiles, birds and mammals) are called _Amniota_,
in contradistinction to fishes and amphibia who have no amnion and are
hence known as _Anamnia_.

=Summary.=--To recapitulate, therefore, the intestinal tube gives origin
to two kinds of appendages or derivatives:

1. Organs of the adult body, derived by budding from the alimentary
entodermal epithelium, in the form of pouch-like diverticula which
follow the glandular type of development and become secondarily
associated with mesodermal elements. These organs are again of two
kinds:

(_a_) _Organs which retain their original connection with the lumen of
the digestive canal:_

  The salivary glands,}
  The liver,          }  Connected by their ducts with the digestive
  The pancreas,       }  canal.
  The lungs,          }

which open by means of the trachea and the laryngeal aperture into the
pharyngeal cavum.

(_b_) _Organs which lose their primitive connection with the alimentary
canal._

Thymus and Thyroid Gland.

2. Embryonic appendages of the alimentary tract.

(_a_) The vitello-intestinal or omphalo-mesenteric duct and the yolk-sac
or umbilical vesicle. This structure does not form as an extension from
the intestinal tube after the same has been closed by coalescence of the
splanchnopleure in the ventral mid-line, but is the result of the
folding in of the layers of the embryonic germinal area, by means of
which the body-rudiment is constricted off from the yolk-sac. The
reduced channel of communication forms the vitello-intestinal duct. In
the vast majority of vertebrates this disappears completely by
absorption in the course of further development. It may persist in part
abnormally as Meckel's diverticulum. In a few birds its proximal portion
remains normally as a small blind pouch attached to the free border of
the small intestine.

(_b_) The allantois. This is a hollow outgrowth from the embryonic
intestinal canal of the higher vertebrates, performing important
functions in connection with the early nutrition of the embryo. In the
course of subsequent development its proximal portion, situated within
the abdominal cavity, becomes converted into the urinary bladder. In
mammals it loses its original connection with the intestinal canal and
is assigned entirely to the genito-urinary tract. In some of the lower
vertebrates, amphibia and reptiles it retains its connection with the
ventral wall of the cloaca throughout life. (See Fig. 42, genito-urinary
tract of _Iguana tuberculata_.)

[Illustration: FIG. 42.--Genito-urinary tract and cloaca of _Iguana
tuberculata_, female. (Columbia University Museum, No. 1846.)]

After the intestinal canal has become separated from the yolk-sac it
forms at first a straight tube, running cephalo-caudad beneath the
chorda dorsalis. In most forms, however, the intestine grows much more
rapidly in length than the body-cavity of the embryo in which it is
contained. Hence the intestine is forced to form coils or convolutions.

The entire alimentary canal, from the mouth to the anus, can be
separated into the following divisions and subdivisions:

=I. Foregut=, including

    1. The oral cavity.
    2. The pharynx.
    3. The oesophagus.
    4. The stomach.

=II. Midgut=, closely associated at its beginning with the liver and
pancreas.

It extends between the pyloric extremity of the stomach and the
beginning of the last segment, the endgut, frequently separated from
both by ring-like aggregations of the circular muscular fibers and
corresponding projections of the mucous membrane (pyloric and ileo-colic
valves).

The midgut is usually the longest portion of the intestinal tube.

=III. Endgut=, the last segment of the intestinal canal, courses through
the pelvic portion of the body cavity. From this short end-piece are
developed: (1) The colon, sigmoid flexure and rectum; (2) the cloaca
with the uro-genital sinus and the duct of the allantois.




PART I.

ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY.


For the purpose of studying the adult human peritoneum it is in the
first place absolutely necessary to obtain a correct appreciation of the
disposition of the chief viscera within the abdominal cavity and of
their mutual relations. In the second place the visceral vascular supply
of the abdomen must be carefully considered in order to correctly
appreciate certain important relations of the peritoneal membrane.

A review of the visceral contents of the abdomen shows that we have to
deal chiefly with the divisions of the alimentary tract below the
oesophagus and the structures directly derived from the same, as liver
and pancreas, or associated topographically with the alimentary canal,
as the spleen. Portions of the urinary and reproductive systems situated
within the abdominal and pelvic cavities will also require
consideration.

The digestive apparatus as a whole presents, in the first place, a
segment designed to convey the food to the stomach, the
oesophagus--supplemented in mammalia by the special apparatus of the
mouth and pharynx, in which the food is mechanically prepared for
digestion by chewing and mixed with the secretion of the salivary
glands.

The _digestive apparatus proper_, succeeding to the oesophagus, is
usually divisible into two sections differing in function and structure.

1. The STOMACH, a short sac-like dilatation, in which chiefly
nitrogenous material is digested.

2. The SMALL INTESTINE, a long and usually much convoluted narrow tube,
chiefly devoted to the digestion of starches, fats and sugars, and to
the absorption of the digested matters.

In some of the lower vertebrates, as the _Cyclostomata_ (Fig. 43),
_Esox_, _Belone_, etc., among fishes (Fig. 48), _Necturus_ and _Proteus_
among amphibians (Figs. 50 and 51), the separation of the digestive
portion of the alimentary tract into stomach and small intestine is not
clearly defined (vide infra, p. 43).

[Illustration: FIG. 43.--Entire alimentary canal of the lamprey,
_Petromyzon marinus_, below the pericardium. (Columbia University
Museum, No. 1575.)]

[Illustration: FIG. 48.--Alimentary canal of _Belone_, pickerel.
(Nuhn.)]

[Illustration: FIG. 50.--_Necturus maculatus_, mud-puppy. Alimentary
canal and appendages. (Columbia University Museum, No. 1454.)]

[Illustration: FIG. 51.--Alimentary canal of _Proteus anguineus_.
(Nuhn.)]

A distinct digestive segment may even be entirely wanting, owing to its
failure to differentiate from the oesophagus on the one hand and from the
endgut on the other. In such forms the entire digestive canal appears as
a tube of uniform caliber extending from mouth to anus. It is necessary
to begin with these simple structural conditions in order to obtain a
clear conception of the disposition of the viscera in the adult human
abdomen. Such simple arrangement of the alimentary tract is found in the
embryo of man and of the higher vertebrates, and similar rudimentary
types are encountered, as the permanent condition, in some of the lower
forms. These latter are especially valuable for purposes of study,
because they afford an opportunity of examining directly, as macroscopic
objects, structural conditions which are found only as temporary
embryonal stages during the development of the higher mammalia (Fig.
43).

In the early stages the alimentary tract of the mammalian embryo
consists of a straight tube of nearly uniform caliber (Fig. 44, _A_),
extending from the pharynx to the cloaca, along the median line in the
dorsal region of the body cavity, connected with the ventral aspect of
the axial mesoderm by a membranous fold forming the primitive common
dorsal mesentery. Subsequently differentiation of this simple tube into
successive segments takes place, marked by differences in shape and
caliber and in histological structure.

[Illustration: FIG. 44.--Schematic diagram representing three stages in
the differentiation of the mammalian digestive tract: A. Early
undifferentiated stage, in which the entire canal appears as a tube of
uniform calibre. B. Spindle-shaped gastric dilatation. C. Typical
mammalian gastric dilatation.]

[Illustration: FIG. 45.--Reconstruction of human embryo. 1, 2, 3, 4,
Gill-pouches. (After Fol.)]

The first indication of the future stomach appears early, in human
embryos of from 5-6 days (Figs. 44, _B_, and 45; for later embryonal
stomach forms compare also Figs. 33, 35 and 36), as a small
spindle-shaped dilatation of a portion of the primitive entodermal tube,
placed in the median plane, dorsad of the embryonic outgrowth of the
liver, between it and the oesophagus. The appearance of this
dilatation marks the separation of the proximal cephalic part (pharynx
and oesophagus) from the distal caudal (intestinal) portion of the
primitive alimentary canal.

Further growth of the stomach takes place chiefly along the dorsal
margin of the dilatation, rendering the same more convex. The ventral
border develops to a less degree and in the course of further and more
complete differentiation the dorsal margin of the future stomach assumes
even at this period the character of the greater curvature, while the
opposite ventral margin, the future lesser curvature, following the
dilatation of the tube dorsad, becomes in turn concave (Fig. 44, _C_).

The early spindle-shaped dilatation has therefore assumed the general
shape of the adult organ. This differentiation of greater and lesser
curvature begins to appear in embryos of 5 mm. (Fig. 46) and is very
well marked in embryos of 12.5 mm., Fig. 36, of an embryo of five weeks,
indicates the adult form of the stomach clearly.

[Illustration: FIG. 46.--Alimentary canal of human embryo of 5 mm. x 15.
(Reconstruction after His.)]

It will, however, be noted that the oesophageal entrance is still at the
cephalic extremity of the rudimentary stomach, while the pyloric
transition to the intestine occupies the distal caudal point, under
cover of the liver, and turns with a slight bend dorsad and to the right
to pass into the duodenum. The future greater curvature is directed
dorsad and a little to the left toward the vertebral column, while the
concave lesser curvature is turned ventrad and a little to the right
toward the ventral abdominal wall. At this time there is but little
indication of the subsequent extension of the organ to the left of the
oesophageal entrance to form the great cul-de-sac or fundus of the adult
stomach.

In this stage of its development the stomach therefore presents ventral
and dorsal borders, and right and left surfaces, while the continuity of
its lumen with the adjacent segments of the alimentary canal appears as
a proximal or cephalic oesophageal and a distal or caudal intestinal
opening.


COMPARATIVE ANATOMY OF FOREGUT AND STOMACH.

A serial review of this portion of the alimentary tract in vertebrates
forms one of the most interesting and instructive chapters in
comparative anatomy.

Not only is every embryonal stage in the development of the higher
mammalia represented permanently in the adult structure of some of the
lower types, but the far-reaching influence of function and of the
physiological demands on the structure of this portion of the digestive
tract is strikingly illustrated by the numerous and marked modifications
which are encountered.

The foregut, strictly speaking, is in mammals separated from the oral
cavity by the musculo-membranous fold of the soft palate and uvula. In
all other vertebrates except the crocodile, the oral cavity and foregut
pass into each other without sharp demarcation (Fig. 47). In some of the
lower vertebrates the alimentary canal never advances beyond the
condition of a simple straight tube of nearly uniform caliber. There is
no gastric dilatation and hence no differentiation of a stomach properly
speaking. Such for example is the case in some teleost fishes, as the
pickerel (Fig. 48). In these forms we have to deal with the persistence
of the early embryonic pregastric stage of the higher types, before the
simple alimentary tube is differentiated by the appearance of the
distinct gastric dilatation.

[Illustration: FIG. 47.--_Gallus canis_, dog-shark, male. Genito-urinary
tract and cloaca _in situ_. The foregut has been divided just caudad of
the communication with the oral cavity. (Columbia University Museum, No.
1694.)]

In the _Cyclostomata_ (Fig. 43) the intestinal canal passes through the
body in a perfectly straight line and the three segments (mid-, fore-
and hindgut) are not clearly differentiated.

In the _Ammocoetes_ the foregut begins behind the wide branchial basket,
dorsad of the heart, with a narrow entrance, which is succeeded by a
dilated segment. The entrance of the hepatic duct separates fore- and
midgut.

In _Amphioxus_ the branchial pouch passes with a slight constriction
directly into the gut which extends through the body-cavity in a
straight line.

The narrow segment is usually regarded as the "oesophagus." This is
followed by a slightly dilated segment, the "stomach," into which a
blind pouch enters. This caecal pouch is usually considered as a
_hepatic_ diverticulum (Fig. 49).

[Illustration: FIG. 49.--_Amphioxus_, dissected from the ventral side.
The relatively enormous pharynx occupies more than half the length of
the body. The walls are separated by the gill-clefts, and the parallel
gill-bars abut at the midventral line on the _endostyle_. (Willey, after
Rathke.)]

But even in these rudimentary forms the point where the liver develops
from the entodermal intestinal tube marks the separation of fore- and
midgut. The stomach, when it develops, is situated cephalad of the
entrance of the hepatic duct into the intestine. The section cephalad of
the duct opening may be very short, and the food digested further on in
the intestinal tube. Consequently a function which in these lower
vertebrates is assigned to the midgut becomes transferred in the higher
forms to a specialized segment of the foregut, situated cephalad of the
hepato-enteric duct. This segment is the


STOMACH.

The distribution of the vagus nerve finds its explanation in this
derivation of the stomach. The primitive foregut is formed by the
passage between the branchial cavity and the midgut, and is within the
area supplied by the vagus. Hence when the stomach develops from the
foregut, as a specialized segment of the same, it is supplied by vagus
branches. The vertebrate stomach varies greatly in size and shape.

The type-form is presented by a longitudinal spindle-shaped dilatation
of the foregut, which retains its foetal vertical position in the long
axis of the body. An example of this form, which is encountered among
fishes and amphibia, is presented by the alimentary tube of _Proteus
anguineus_ and _Necturus maculatus_ (Figs. 50 and 51). Since this
condition is common to all vertebrates in the earliest foetal period it
can be designated as the foetal or primitive stomach form. All others
appear as secondary derivatives from this typical early condition.

The influences which bring about such derivations and modifications may
be enumerated as follows:

1. The habitual amount of food required by the animal.

2. The volume and digestible character of the food.

3. The size and shape of the abdominal cavity in which the stomach is
contained.

4. Structural modifications designed to increase the action of the
gastric juice on the food contained in the stomach.

5. The assumption, on part of the stomach, of functions which are
usually relegated to other organs.

Most of the individual stomach forms encountered among vertebrates owe
their production to several of these influences acting in conjunction.

We may group the main types as follows:

=1. Stomach Forms Depending on the Influence exerted by the Habitual
Amount of Food required by the Animal.=--The greater the activity of
tissue changes is, the greater will be the amount of food required and
the more pronounced will be the gastric dilatation of the alimentary
canal. Hence in the higher vertebrates generally the stomach appears as
a large and more sac-like dilatation than in lower forms, such as fishes
and amphibia and some reptilia, in which the stomach is usually smaller
and foetal in shape, forming a slight longitudinal dilatation situated in
the long axis of the body. An example is seen in the stomach of _Coluber
natrix_ (Fig. 52). Frequently this slight dilatation is scarcely
differentiated from the oesophagus at the cephalic and from the small
intestine at the caudal end. Many batrachians and perennibranchiates
possess this form among the amphibia. It is also encountered in the
pickerels, the _Cyprini_, and in _Labrus_ among fishes, and in some
saurians and ophidia among reptiles. It constitutes a slight advance in
development over the earliest stage represented, as we have seen, by the
nearly uniform and undifferentiated alimentary tube of amphioxus and the
cyclostomata.

[Illustration: FIG. 52.--Alimentary canal of _Coluber natrix_. (Nuhn.)]

This transition of the foetal form to the more advanced secondary types
of the stomach is marked by the development of two important structural
features:

(_a_) The separation in the interior of the canal of the stomach from
the intestine by the appearance of a ring-shaped valve, the _pyloric
valve_. This is produced by an aggregation of the circular muscular
fibers of the intestine at this point, and causes a projection of the
mucous membrane into the lumen of the canal. It begins to appear in the
fishes (pickerel, sturgeon, etc.), is found in most amphibia and is
regularly present in the stomach of the higher vertebrates. (Figs. 54
and 55.) A good example of the ring-shaped plate of the pylorus with
central circular opening produced by the aggregation of the circular
muscular fibers is afforded by the view of the interior of the
cormorant's stomach given in Fig. 69. The opposite or oesophageal
extremity of the stomach is less well differentiated from the afferent
tube of the oesophagus.

[Illustration: FIG. 54.--Human adult. Pyloro-duodenal junction and
pyloric valve in section. (Columbia University Museum, No. 1842.)]

[Illustration: FIG. 55.--Series of sections showing human pyloric valve
and gastro-duodenal junction:

1. Stomach of foetus at term in section.

2. Adult pyloric valve, gastric surface.

3. Adult pyloric valve and gastro-duodenal junction in section.

4. Foetal gastro-duodenal junction in section. Entrance of biliary and
pancreatic ducts on summit of papilla of duodenum. (Columbia University
Museum, No. 1851.)]

There is no aggregation of muscular circular fibers in this situation
and no valve. Superficially the external longitudinal muscular fibers of
the oesophagus pass continuously and without demarcation into the
superficial gastric muscular layer. The separation between oesophagus and
stomach is, however, marked on the mucous surface by a well-defined line
along which the flat, smooth and glistening oesophageal tesselated
epithelium passes into the granular cuboidal epithelium of the gastric
mucous membrane. The oesophageo-gastric junction in the adult human
subject is shown in Fig. 53.

[Illustration: FIG. 53.--Human adult. Mucous surface of
oesophageo-gastric junction. (Columbia University Museum, No. 1842.)]

(_b_) The pyloric end of the stomach makes an angular bend, while the
rest of the organ remains in the original vertical position in the long
axis of the body. An example of this condition is presented by the
stomach of _Scincus ocellatus_ (Fig. 56; cf. also Fig. 202).

[Illustration: FIG. 56.--Alimentary canal of _Scincus ocellatus_.
Pyloric extremity of the slightly marked gastric dilatation presents an
angular bend. (Nuhn.)]

The purpose of both of these provisions is to retain the gastric
contents for a longer time within the stomach. Hence this form is
encountered especially in those fishes and amphibians in which the
nutritive demands require a more complete digestion of the food taken.
This is the case, for example, in _Gobius_ (Fig. 57), the plagiostomata
(Fig. 58), and many saurians. The same transitory stomach form is even
found in some mammals, as the seals. Fig. 59 shows the stomach in _Phoca
vitulina_, the harbor seal. With the further increase in the demand for
complete digestion of the food the entire stomach assumes a transverse
position to the long axis of the body. This may occur while the stomach
still retains its primitive tubular form, as in most chelonians (Fig.
60). In others the change in position occurs after the gastric
dilatation has assumed the sac-like form, as in many land-turtles,
crocodiles, some batrachians and all higher vertebrates (Figs. 61 and
62). This transverse position, at right angles to the long axis of the
body, forms the starting point for the derivation of all secondary types
of stomach.

[Illustration: FIG. 57.--Alimentary canal of _Gobius niger_. (Nuhn.)]

[Illustration: FIG. 58.--Alimentary canal of shark. (Nuhn.)]

[Illustration: FIG. 59.--Stomach of _Phoca vitulina_, harbor seal.
(Columbia University Museum, No. 600.)]

[Illustration: FIG. 60.--Stomach of _Pseudemys elegans_, pond turtle.
(Columbia University Museum, No. 1710.)]

[Illustration: FIG. 61.--Stomach of _Chelydra serpentina_, snapping
turtle. (Columbia University Museum, No. 1852.)]

[Illustration: FIG. 62.--Same in section.]

=2. Stomach Forms Depending on the Influence Exerted by the Volume and
Digestible Character of the Foods.=--Vegetable substances usually have a
large volume in proportion to the amount of nutritive material which
they contain. Meat, on the other hand, contains considerable nutriment
in a comparatively small bulk. Hence carnivora (Fig. 63) usually have a
smaller stomach than herbivora (Fig. 64).

[Illustration: FIG. 63.--Stomach of _Lutra vulgaris_, otter. (Nuhn.)]

[Illustration: FIG. 64.--Stomach of _Equus caballus_, horse. (Nuhn.)]

=3. Stomach Forms Influenced by Size and Shape of the Abdominal Cavity
in which they are Contained.=--In animals whose bodies are long and
slender, as in snakes (Fig. 52), most saurians (Fig. 56), many tailed
batrachians and perennibranchiates (Figs. 50 and 51), many teleosts
(Fig. 48), the stomach is likewise usually long and slender in shape,
unless special modifying conditions exist. When on the other hand the
body is broad and short, as in Lophius (Fig. 65), Pipa (Fig. 66), and
most higher vertebrates, the stomach is also broader and more sac-like.

[Illustration: FIG. 65.--Stomach of _Lophius piscatorius_, angler.
(Nuhn.)]

[Illustration: FIG. 66.--Stomach of _Pipa verucosa_. (Nuhn.)]

=4. Stomach Forms Depending on Structural Modifications Designed to
Increase the Action of the Gastric Juice on the Food.=--This purpose is
accomplished:

(_a_) By increasing the source of supply of the gastric juice.

(_b_) By increasing the length of time during which the food remains in
the stomach.

(_a_) The source of supply of the gastric juice is increased by adding
to the usual gastric glands of the stomach a special accessory glandular
compartment, either placed at the cardia, where the oesophagus enters, as
in _Myoxus_ or _Castor_ (Fig. 67) or attached to the body of the stomach
to the left of the cardia, as in the manatee (Fig. 68). The first
arrangement is similar to the universal position of the glandular
stomach of birds (Fig. 69). In birds, however, the glandular
proventriculus is the _only_ source of the gastric juice, while in the
above-mentioned mammalia (myoxus and beaver) the accessory glandular
stomach is merely an addition to the supply derived from the usual
gastric glands situated in the body of the organ.

[Illustration: FIG. 67.--Stomach of _Castor fiber_, beaver. (Nuhn.)]

[Illustration: FIG. 68.--Stomach of _Manatus americanus_, manatee.
(Nuhn.)]

[Illustration: FIG. 69.--Stomach of _Phalacrocorax dilophus_,
double-crested cormorant; section. (Columbia University Museum, No.
67/1804.)]

(_b_) The increase of the length of time during which the food remains
in the stomach subject to the action of the gastric juice can be
accomplished in one of several ways.

1. The stomach, while it retains its general tubular form increases
considerably in length and assumes the shape and structure found in the
human large intestine. It is partially subdivided by folds projecting
into the interior and separating compartments resembling the colic cells
of the human large intestine. The time required for the passage of food
through the stomach is thus increased and the action of the gastric
juice is prolonged and rendered more intense.

Such modifications of the structure of the stomach are encountered in
_Semnopithecus_ among the monkeys and in the kangaroo, among marsupials
(Figs. 70 and 71).

[Illustration: FIG. 70.--Stomach of _Halmaturus derbyanus_, rock
kangaroo. (Columbia University Museum, No. 582.)]

[Illustration: FIG. 71.--Stomach of _Semnopithecus entellus_, entellus
monkey. (Columbia University Museum, No. 62/1805.)]

2. The same purpose is accomplished by the development of diverticula
from the stomach, in which the food is retained and acted on by the
gastric juice for longer periods.

The herbivora, omnivora and such carnivora as live on animal food
difficult of digestion furnish examples of this type of stomach. The
same is also found in most teleosts. In the latter the caecal gastric
pouch lies in the long axis of the body, opposite the entrance of the
oesophagus. A marked example of this arrangement is seen in the stomach
of the eel, _Anguilla anguilla_ (Fig. 72).

[Illustration: FIG. 72.--Alimentary canal of _Anguilla anguilla_, eel.
(Columbia University Museum, No. 1271.)]

In other forms, and in the mammalia especially, the blind pouch is
developed from the portion of the stomach lying to the left of the
oesophageal entrance at the cardia, and is hence placed transversely to
the long axis of the body.

This difference in the position of the cul-de-sac is explained by the
small transverse measure of the body in teleosts, while the greater
amount of available space in the abdominal cavity of mammalia permits
of the transverse position of the entire stomach and of the development
of the diverticulum from its left extremity.

Most mammals have only a single pouch, whose size varies with the
digestibility of the food habitually taken. It is greater in herbivora
(Figs. 64 and 73) than in omnivora and carnivora (Figs. 74 and 75). In
some of the latter, as _Lutra_ (Fig. 63), the cul-de-sac is almost
wanting.

[Illustration: FIG. 73.--Stomach of _Lepus cuniculus_, rabbit. (Nuhn.)]

[Illustration: FIG. 74.--Stomach of _Nasua rufa_, coati. (Nuhn.)]

[Illustration: FIG. 75.--Stomach of _Felis leo_, lion. (Nuhn.)]

[Illustration: FIG. 76.--Stomach of _Erethizon dorsatus_, American
porcupine. (Columbia University Museum, No. 358.)]

[Illustration: FIG. 77.--Stomach of _Cercopithecus cephus_, moustache
monkey. (Columbia University Museum, No. 158.)]

In some forms, as the pig, the left extremity of the stomach carries a
caecal appendix with a spiral valve in the interior separating its lumen
from the general gastric cavity (Fig. 78). Others have two such caecal
appendices added to the left end of the stomach (Peccary, Fig. 79).
These caecal pouches may arise from the _body_ of the stomach, instead of
from the left extremity. An example of this condition is furnished by
the American manatee (Fig. 68).

[Illustration: FIG. 78.--Stomach of _Sus scrofa_, pig. The fundus of the
stomach carries a caecal appendage separated in the interior by a spiral
fold of the mucous membrane from the gastric cavity.]

[Illustration: FIG. 79.--Stomach of _Dicotyles torquatus_, peccary. The
fundus is a capacious pouch prolonged ventrally and dorsally into two
caecal appendages resembling the single appendage of the pig's stomach.
(Columbia University Museum, No. 1806.)]

=5. Variations in the Form of the Stomach Depending upon the Assumption
by the Stomach of Special Functions, which are Usually Relegated to
other Organs.=--These functions are the following:

(_a_) Storage of food in special receptacles or compartments for
subsequent use.

(_b_) Mastication of the food is in some animals accomplished only
partly or not at all in the mouth, and is then performed in the stomach.
A portion of the stomach is thus converted into an apparatus for
mastication.

(_c_) The provisions for these two accessory functions may be combined
in the same stomach.

(_a_) Many of the higher vertebrates possess in connection with the
alimentary tract additional reservoirs for the storage of food until
used. Such reservoirs are found in mammals and birds connected with the
oral cavity, as cheek-pouches, or with the oesophagus, such as the crop
of the birds (Fig. 88). Fig. 80 shows the development of the
cheek-pouches in one of the primates, _Macacus nemestrinus_.

[Illustration: FIG. 80.--_Macacus nemestrinus_, pig-tail macaque monkey;
cheek-pouches. (From a fresh dissection.)]

In many mammals reservoirs of similar import are added
directly to the stomach and form an integral part of
the organ. Examples are furnished by the compound stomachs of many
rodents, ruminants, cetaceans and herbivorous edentates. The peculiar
appearance of these stomachs is explained if the additional reservoirs
are in imagination removed and the digestive stomach proper restored so
to speak to the type-form. The proximal or cardiac portion of the
stomach in many rodents is devoid of gastric glands and must be
interpreted as a storage chamber for food (Fig. 81). The same
significance attaches to the corresponding portion of the manatee's
stomach (Fig. 68).

[Illustration: FIG. 81.--Stomach of _Cricetus vulgaris_, hamster.
(Nuhn.)]

Similar contrivances are found in the ruminant stomach. The first
and second divisions (rumen and reticulum) are nothing but sac-like
gastric reservoirs or pouches, in which the food is collected, to be
subsequently returned to the mouth for mastication. When swallowed for
the second time the bolus is carried, by the closure of the so-called
oesophageal gutter, past the first and second stomach into the digestive
apparatus proper (the abomasum) (Figs. 82 and 83). Many ruminants
(_e. g._, _Moschus_) only have these three compartments. Most, however,
have four, the leaf stomach or psalterium being intercalated between
the retinaculum and the abomasum. The psalterium contains no digestive
glands. It may possibly serve for the absorption of the liquid portions
of the foods.

[Illustration: FIG. 82.--Stomach of _Ovis aries_, sheep. (Columbia
University Museum, No. 1807.)]

[Illustration: FIG. 83.--Scheme of ruminant compound stomach. (Nuhn.)]

The rumen or first stomach of the camels and llamas is provided with
so-called "water-cells," for the storage of water. These cells are
diverticula lined by a continuation of the gastric mucous membrane. The
entrance into these compartments can be closed by a sphincter muscle
after they are filled with water (Fig. 84).

[Illustration: FIG. 84.--Mucous membrane of stomach of _Camelus
dromedarius_, dromedary, showing water-cells. (Columbia University
Museum, No. 1123.)]

[Illustration: FIG. 85.--Stomach of _Phocaena_, porpoise. (Nuhn.)]

The three stomachs of the cetaceans are similar to those of the
ruminants (Fig. 85). The first is a crop-like reservoir for the
reception of the food when swallowed. The mucous membrane is entirely
devoid of digestive glands. In the dolphins the mucous membrane is
provided with a hard horny covering, which serves to break up the food
mechanically by trituration. The second stomach and the gut-like pyloric
prolongation constituting the third stomach contain gastric glands and
are hence digestive in function.

(_b_) Stomach forms, in which a portion of the organ is converted into
an apparatus for mastication, are seen especially in birds, in which
animals, on account of the absence of teeth, mastication cannot be
performed in the mouth.

The stomach of the bird is usually composed of two segments, one placed
vertically above the other.

The first appears like an elongated dilatation of the oesophagus, forming
the _Proventriculus_ or glandular stomach.

The second is larger, round in shape, with very strong and thick
muscular walls (Figs. 86 and 87).

[Illustration: FIG. 86.--Stomach of _Urinator imber_, red-throated loon.
(Columbia University Museum, No. 1808.)]

[Illustration: FIG. 87.--Scheme of stomach of granivorous bird. (Nuhn.)]

The proventriculus furnishes the gastric juice exclusively.

The second or muscular stomach, devoid of gastric glands, functions
merely as a masticating apparatus for the mechanical division of the
food. The thick muscular walls of this compartment may measure several
inches in diameter and carry on the opposed mucous surfaces lining the
cavity a hard horny plate with corrugated and roughened surface (Fig.
88). These hard plates are designed to crush the food between them, as
between two mill stones. The muscle stomach is best developed in
herbivorous birds, while both the muscular wall and the horny plate are
much weaker and thinner in carnivore wading and swimming birds (Fig.
89).

[Illustration: FIG. 88.--OEsophagus and stomach of _Gallus bankiva_, hen.
(Columbia University Museum, No. 1809.)]

[Illustration: FIG. 89.--Stomach of _Botaurus lentiginosus_, bittern.
(Columbia University Museum, No. 23/1810.)]

In birds of prey, especially in the owls, the stomach walls are scarcely
more massive than in other animals, and the mucous membrane is soft and
devoid of a horny covering. The glandular and masticatory stomachs are
less sharply divided from each other in these forms, and the entire
organ conforms more to the general vertebrate type (Fig. 90).

[Illustration: FIG. 90.--Stomach of owl sp. (Nuhn.)]

In some birds (herons, storks, etc.) a small rounded third stomach, the
so-called pyloric stomach, is placed between the muscle stomach and the
pylorus (Fig. 91). It contains no gastric glands, and possibly may
function as an additional absorbing chamber.

[Illustration: FIG. 91.--Stomach of _Ardea cinerea_, heron. (Nuhn.)]

[Illustration: FIG. 92.--Stomach of crocodile. (Nuhn.)]

Among reptiles the stomach of the crocodile resembles the organ in birds
(Fig. 92). It is flat and rounded in shape, the muscle wall carries a
tendinous plate, and there is a pyloric stomach. There is, however, no
glandular stomach or proventriculus, as in birds, and the mucous
membrane is not covered by a horny plate, but is soft and contains the
peptic glands. Figs. 93 and 94 show the stomach of _Alligator
mississippiensis_, in the ventral view and in section.

[Illustration: FIG. 93.--Stomach of _Alligator mississippiensis_.
(Columbia University Museum, No. 1811.)]

[Illustration: FIG. 94.--Same in section. Thin-walled cardiac segment
continues into cavity of pyloric ventriculus.]

[Illustration: FIG. 95.--Stomach of _Bradypustridactylus_, three-toed
sloth. I. First stomach, devoid of gastric glands, corresponding to
rumen of ruminants.

II. Second stomach, the homologue of the ruminant reticulum.

III. Digestive stomach proper, provided with gastric glands connected by
a gutter with the oesophagus.

IV. Muscular stomach, the walls formed by a thick muscular plate and
provided on the mucous surface with a dense corneous covering for
purposes of trituration.]

(_c_) The combination of the two accessory functions just described in
the same stomach is found in the three-toed sloth (Fig. 95).

There are here two large reservoirs, which correspond to the rumen and
retinaculum of the ruminants, and a digestive compartment containing
gastric glands, which corresponds to the ruminant abomasum, and is
connected by an oesophageal gutter directly with the oesophagus. At the
pyloric extremity the muscle wall is greatly increased and the mucous
membrane of this portion carries a thick horny covering, forming a
masticatory stomach greatly resembling the corresponding structure in
the bird. Its function is evidently to complete the mechanical division
of the food which has only been partly masticated in the mouth.

The same significance is probably to be attached to the thickened
muscular walls which the pyloric segment of the stomach in _Tamandua
bivittata_, another edentate, presents (Fig. 96), in strong contrast
with the thinner walled cardiac segment and fundus.

[Illustration: FIG. 96.--Stomach of _Tamandua bivittata_, collared
ant-eater, (Columbia University Museum, No. 68/1485.)]


INTESTINE.

Continuing our consideration of the development of the alimentary canal
we find that changes from the simple primitive straight tube below the
stomach depend upon two factors:

1. The increase in the length of the intestinal tube, which exceeds
relatively the increase in the length of the body cavity in which it is
contained.

2. The differentiation into small and large intestine, the development
of the caecum and ileo-caecal junction, and the development of the
accessory digestive glands, liver and pancreas, by budding from the
proximal portion of the primitive entodermal intestinal tube.

1. In embryos up to 5 mm. cervico-coccygeal measure (Fig. 97) the
intestinal tube follows the body curve without deviation. Subsequently
the elongation of the intestine causes a small bend, with the convexity
directed ventrad, to appear in the umbilical region. This bend gradually
increases until the gut forms a single long loop, beginning a short
distance below the pylorus and directed ventro-caudad. The apex of the
loop, to which the vitello-intestinal duct is attached (Fig. 98) (cf. p.
34) projects beyond the abdominal cavity into the hollow of the
umbilical cord, constituting the so-called "umbilical or embryonal
intestinal hernia." This entrance of the apex of the intestinal
umbilical loop into the umbilical cord begins in embryos of about 10 mm.
During the succeeding weeks--up to the tenth--the segment of the
intestine thus lodged within the hollow of the umbilical cord increases.
After this period the intestinal coils are gradually withdrawn within
the abdomen. The explanation of this temporary extrusion of the
intestine into the umbilical cord is probably to be found in the strain
produced by the yolk-sac which is attached by the vitello-intestinal
duct to the apex of the umbilical loop. As we have seen (p. 35) the site
of the original apex of the loop may still be indicated in the adult by
the persistence of a portion of the vitello-intestinal duct as a
"Meckel's diverticulum."

[Illustration: FIG. 97.--Alimentary canal of human embryo of 5 mm. x 15.
(Reconstruction after His.)]

[Illustration: FIG. 98.--Schema of human embryonic intestinal canal,
with intestinal umbilical loop, but before differentiation of the large
and small intestine.]

In its simplest primitive condition the loop presents a proximal,
descending or efferent limb, an apex, and an ascending, returning or
afferent limb (Fig. 98). In the human embryo these segments of the loop
furnish the jejuno-ileum and portions of the large intestine, in a
manner to be subsequently detailed.

This stage in the development of the higher vertebrate intestine is well
illustrated by the alimentary tract of the mud-puppy, _Necturus
maculatus_, shown in Fig. 99, which represents the entire situs viscerum
of an adult female animal.

[Illustration: FIG. 99.--Viscera of _Necturus maculatus_, mud-puppy, _in
situ_. (Columbia University Museum, No. 1175.)]

The stomach is tubular, not distinctly differentiated from the
oesophagus, placed vertically in the long axis of the body. The pyloric
end is marked by a constriction separating stomach from midgut and
immediately beyond this point the pancreas is applied to the intestine.
The rest of the intestinal canal forms a simple loop, the descending
limb presenting one or two primitive convolutions. There is no marked
differentiation between large and small intestine, the canal possessing
a nearly uniform caliber from pylorus to cloaca.

2. The differentiation of the small from the large intestine, marked by
the appearance of the caecal bud or protrusion (Fig. 100), takes place in
the ascending segment of the umbilical loop a short distance from the
apex. In the human embryo the caecal bud appears in the 6th week as a
plainly marked protuberance, which grows very slowly in length and
circumference. It shows very early an unequal rate of development; the
terminal piece, not keeping pace in growth with the proximal portion, is
converted into the vermiform appendix, while the proximal segment
develops into the caecum proper. The increase in the length of the loop,
which begins to be marked in the 7th week, is not uniform. The apex is
the first portion to present the evidences of this growth. Subsequently
the descending limb grows in length very rapidly and is early thrown
into numerous coils of the future mobile portion of the small intestine
(jejuno-ileum). Even before the withdrawal of the apex of the loop
within the abdominal cavity a prominent coil of these convolutions is
found protruding in the umbilical region (Fig. 544). The ascending limb
of the loop from which a portion of the large intestine is developed,
grows comparatively slowly at this time.

[Illustration: FIG. 100.--Schema of human embryonic intestinal canal
after differentiation of the large and small intestine.]

The future portions of the human adult alimentary tract below the
stomach may be referred, in reference to their derivation, to this
primitive condition of the tube as follows:

1. The segment of small intestine situated between the pylorus and the
beginning or point of departure of the proximal or descending limb of
the umbilical loop, develops into the _duodenum_. This portion of the
small intestine is indicated early in embryos of 2.15 mm. (Fig. 101),
by the origin of the hepatic duct from the intestinal tube. Somewhat
later, in embryos of 4.10-5 mm. length, (Fig. 102) it becomes
additionally marked by the origin of the pancreatic diverticulum. The
duodenum, at first straight, now begins to curve, forming a short
_duodenal loop_ or _bend_. In embryos of 6 weeks the duodenum forms a
simple loop placed transversely below the pyloric extremity of the
stomach (Figs. 103 and 104).

[Illustration: FIG. 101.--Human embryo of 2.15 mm., twelve days old.
Seessel's sac is the cephalic blind termination of the embryonic
foregut before the communication with the ectodermal invagination of
the stomadaeum has been formed. (Reconstruction after His.)]

[Illustration: FIG. 102.--Representation of alimentary canal and
appendages of human embryo of 4.1 mm.; isolated. x 15. (Kollmann, after
His.)]

[Illustration: FIG. 103.--Alimentary canal and appendages of human
embryo of 12.5 mm. x 12. (Kollmann, after His.)]

[Illustration: FIG. 104.--A. Schematic representation of alimentary
canal, with umbilical loop and mesenteric attachments in human embryo of
about six weeks. B and C, stages in the intestinal rotation.]

2. The descending limb, the apex and a small part of the ascending limb
of the umbilical loop form the jejuno-ileum.

3. The remainder of the ascending limb forms the caecum and appendix, the
ascending and transverse colon.

4. The distal straight portion of the primitive tube forms the terminal
portion of the transverse colon (the splenic flexure), the descending
colon, sigmoid flexure and rectum.

The primitive condition of the embryonal mammalian alimentary tract,
after differentiation of the large intestine is well illustrated by some
of the lower vertebrates in which development never proceeds beyond this
stage. Fig. 112 shows the entire alimentary canal of a teleost fish, the
conger eel (_Echelus conger_) isolated.

[Illustration: FIG. 112.--Alimentary canal, isolated and in section, of
_Echelus conger_, the conger eel. (Columbia University Museum, No.
1812.)]

The preparation forms a good illustration of the embryonal stage of the
higher vertebrates in which development has not proceeded beyond the
formation of the simple umbilical loop, about corresponding to the
schematic Fig. 98. The stomach is differentiated both by its caliber and
by the formation of a pyloric ring valve.

The midgut forms a simple loop with a descending and ascending limb
closely bound together by mesenteric attachment. Different from the
course of development followed in the human embryo is the situation of
the ileo-colic junction. The same appears in the terminal straight
segment of the canal--corresponding to the human descending colon--while
in the human embryo the differentiation of small and large intestine
takes place in the course of the ascending limb of the loop. This
condition depends upon the relatively much shorter extent of the
teleost endgut compared with the human large intestine. Other examples
are afforded by the alimentary tract of some of the Amphibia and
Reptilia. Fig. 105 shows the alimentary canal of _Rana catesbiana_, the
common bull frog. The stomach, fairly well differentiated, is succeeded
by the small intestine of considerable length and uniform caliber. The
proximal portion of the small intestine is characterized as duodenum by
its connection with liver and pancreas. In the remaining portion of the
intestinal canal it is not difficult to recognize the elements of the
umbilical loop of the higher mammalian embryo. The larger mass of the
jejuno-ileal coils is developed from the descending limb of the loop; a
smaller number of convolutions belong to the returning or ascending
limb, which also includes the ileo-colic junction. The very short large
intestine of the frog passes straight down to enter the cloaca. Another
example, in which the early embryonal stages of the higher mammalia are
illustrated by the permanent structure of one of the lower vertebrates,
is given in Fig. 106, which shows the alimentary tract of a chelonian,
_Pseudemys elegans_, the pond turtle. The bilobed liver fits over the
well-differentiated stomach in the manner of a saddle. The stomach
itself, as in chelonians generally, has a markedly transverse position
and passes under cover of the right lobe of the liver into the duodenum.
The coils of small intestine form a prominent mass, which, however, when
unravelled as shown in the figure, permits us to recognize its identity
with the mammalian embryonic umbilical loop. The well-marked ileo-colic
junction is situated at the termination of the returning limb of the
loop, close to the beginning of the descending limb. This close
approximation of the duodenum and colon (duodeno-colic isthmus) forms
one of the most important factors in the further development of the
mammalian intestinal canal and will again be referred to below.

[Illustration: FIG. 105.--_Rana catesbiana_, bull-frog. Alimentary canal
and appendages. (Columbia University Museum, No. 1454.)]

[Illustration: FIG. 106.--_Pseudemys elegans_, pond turtle. Alimentary
canal. (Columbia University Museum, No. 1437.)]

From the ileo-colic junction the large intestine of the turtle continues
caudad to the cloaca in a nearly straight line. The same primitive
condition of the intestinal canal may be observed in some members of
man's own class, the mammalia--as in certain edentates. Figs. 107 and
108 show the entire abdominal portion of the alimentary tract in
_Tamandua bivittata_, the little ant-eater of Brazil. The stomach is
turned cephalad and the great omentum elevated. The intestines are
turned over to the right side.

[Illustration: FIG. 107.--Abdominal viscera of _Tamandua bivittata_, the
little ant-eater, seen from the left, with the intestines turned to the
right. (From a fresh dissection.)]

[Illustration: FIG. 108.--The same view, from another specimen. Figures
107 and 108 should be studied and compared together, as each supplements
the other.]

It will be observed that in spite of the numerous coils of the small
intestine the general arrangement of the alimentary canal corresponds to
the primitive scheme shown in Fig. 98. The entire intestinal canal is
attached by a continuous vertical mesentery to the dorsal median line of
the abdominal cavity ventrad of the vertebral column and aorta. The
growth in length of the small intestine has necessitated a corresponding
lengthening of the attached border of the mesentery--consequently the
membrane presents a pleated or crenated appearance. The caecum is well
developed, the ileo-caecal junction being situated within the returning
limb of the loop, a little distance from the apex.

In Figs. 109 and 110, taken from the same specimens, the entire mass of
the small intestines has been turned to the left so as to exhibit the
right leaf of the common dorsal mesentery and the mesoduodenum, the
latter containing the head of the pancreas. It will be noted that the
mesentery, expanding beyond the duodeno-colic isthmus, is common to the
small and to the proximal portion of the large intestine, _i. e._, to
those segments of the alimentary canal which are developed from the two
limbs of the umbilical loop. Figs. 107-110 should be studied and
compared together, as each supplements the others.

[Illustration: FIG. 109.--Abdominal viscera of _Tamandua bivittata_, the
little ant-eater, seen from the right, with the intestines turned to the
left. (From a fresh dissection.)]

[Illustration: FIG. 110.--The same view, from another specimen.]

It will be observed, in reference to the change from the primitive loop
to the subsequent increase in the length of the tube and the resulting
arrangement of the mesentery, that three successive stages are to be
considered, represented schematically in Fig. 111. In the earliest stage
(Fig. 111, I.) the two segments of the loop are of equal length,
parallel to one another, the distance between the beginning and
termination of the loop (1-2) being maintained throughout its
extent. Hence the mesentery is of equal width in all its parts within
the loop, only drawn out, _i. e._, away from the vertebral column, in
accordance with the length of the loop. In the next stage (Fig. 111,
II.) the increase in the length of the intestine is accompanied by a
corresponding widening of the mesentery. The points 1 and 2 are still
approximately the same distance apart as in the earlier stage, but the
increase in the length of the tube between these points forces the two
limbs of the loop to abandon their early parallel course, and to form
curved lines with the concavity turned toward the mesenteric attachment.
In this condition the mesentery consequently forms a widely expanded
membrane framed by the intestine and narrowing between the points 1 and
2 to a neck or isthmus which effects the transition between the expanded
segment surrounded by the intestine and the rest of the dorsal primitive
mesentery. Finally in the stage represented in Fig. 111, III., the
increase in the length of the small intestine has reached a point where
a single curve is no longer sufficient for the accommodation of the
growth. Consequently the tube now appears coiled and convoluted, and the
mesentery, as it is attached to the gut, of necessity follows all the
twists and appears fluted or pleated in its distal attached portion.

[Illustration: FIG. 111.--Schematic representation of the development of
the mesentery of the umbilical loop.]

If we now carefully examine the conditions presented by the intestine
and mesentery in a form like _Tamandua_ (Figs. 107 and 108) we will find
that they correspond to the developmental facts thus far considered. The
termination of the duodenum (1) and the bend in the colon (2) mark the
two points at which in the primitive schema (Fig. 111, I.) the umbilical
loop begins and terminates. The proximal of these two points (1)
corresponds to the termination of the duodenum, which segment extends
from here cephalad to the pyloric extremity of the stomach. The distal
point (2) is placed on the colon where the returning limb of the loop
resumes the original median vertical course of the large intestine.
These two points mark the neck of the loop, which we can describe as the
_duodeno-colic neck_ or _isthmus_.

The same condition is well shown in the intestinal canal of the snapping
turtle (Fig. 113). The duodenum and colon approach each other very
closely at the isthmus and between these points the convolutions of the
intestine extend in a wide circle. We will find this approximation of
duodenum and colon a feature which persists throughout all the later
developmental stages of the higher vertebrates and has an important
bearing on the final arrangement of the intestinal canal in the human
adult.

[Illustration: FIG. 113.--_Chelydra serpentina_, snapping turtle;
intestinal canal, pancreas, and spleen, isolated. (Columbia University
Museum, No. 1369)]

=Further Changes in the Development of the Human Alimentary Canal.
Rotation of the Intestine. Formation of the Segments of the Colon. Final
Permanent Relations of the Segments of the Intestinal Tube.=--The next
important stage leading up to the final adult disposition of the
intestine in man and the higher mammals is the _rotation_ of the
portions developed from the two limbs of the primitive loop around an
oblique axis drawn from the duodeno-colic isthmus to the apex of the
loop. The portion of the large intestine, developed from the ascending
limb of the loop, moves in the third month to the middle line, coming
into contact with the ventral abdominal wall. From here the large
intestine passes, ventrad of the jejuno-ileal coils, toward the cephalic
end of the abdominal cavity and lies transversely along the greater
curvature of the stomach. The growing coils of the small intestine crowd
the colon more and more cephalad. In the fourth month the caecum turns to
the right, coming into contact with the caudal surface of the liver,
ventrad of the duodenum, and subsequently reaches the ventral surface of
the right kidney. As the result of this rotation the ileo-colic
junction, caecum and succeeding portion of the colon are carried from the
original position in the distal and left part of the abdomen cephalad
and to the right across the proximal (duodenal) portion of the small
intestine, while the coils of the jejuno-ileum, developed from the
descending limb and apex of the loop, are turned in the opposite
direction, caudad and to the left underneath the preceding (Figs. 114
and 115). This change in the relative position of the parts of the
intestinal tract and the resulting altered bearing of the colon to the
duodenum will be best appreciated by considering in the first place the
effect of the change on the arrangement of the primitive mesentery and
the intestinal vessels, and secondly by repeating actually the rotation
in the intestinal tract of a mammal (cat) in which the adult arrangement
of the intestine and peritoneum permits us to perform the manipulations
and note the result.

[Illustration: FIG. 114_A_.--Intestinal canal in stage of umbilical
loop--before rotation.]

[Illustration: FIG. 114_B_.--First stage in rotation, colon crossing
duodenum.]

[Illustration: FIG. 115_A_.--Second stage in rotation--rotation of small
intestine.]

[Illustration: FIG. 115_B_.--Schema of intestinal canal after complete
rotation and descent of caecum.]

=I. Effect of Rotation on the Disposition of the Primitive Mesentery and
on the Relative Position of Duodenum and Colon, and Consequent
Arrangement of the Intestinal Blood Vessels.=--It will be appreciated
that in Fig. 111, representing a profile view of the original
arrangement, or in Figs. 107 and 108, showing the intestinal canal of
_Tamandua_, the left layer of the primitive mesentery is turned toward
the observer. The membrane is seen to pass from the ventral aspect of
the vertebral column and aorta, through the narrow neck of the
duodeno-colic isthmus, to expand in the manner already indicated toward
its intestinal attachment. In the rotation of the intestine the twist
takes place at the duodeno-colic neck, carrying, as already stated, the
large intestine cephalad and to the right, while the jejuno-ileum is
turned in the opposite direction caudad and to the left. During this
rotation the duodeno-jejunal angle (Figs. 114, _B_ and 115, _A_) passes
to the left underneath the proximal segment of the colon, which now lies
ventrad and to the right of the duodenal portion of the small intestine.
The mesenteric peritoneum, occupying the bight of the umbilical loop,
will, after the rotation, in the left profile view shown in Fig. 104,
_A_ and _B_, turn its original right leaf toward the beholder, _i. e._,
toward the left, while the original left leaf is turned toward the
right.

Observation of the difference in the position of the ileo-colic junction
will still further accentuate the change in the relative position of the
parts which has been effected by the rotation. In the primitive
condition shown in Fig. 104, _A_, the ileum enters the large intestine
from right to left, and the concavity of the caecal bud turns its
crescentic margin ventrad and to the right.

After rotation is accomplished (Fig. 104, _B_ and _C_, and Fig. 115) the
ileo-colic entrance takes place in the opposite direction, from left to
right and the caecum turns its concave margin caudad and to the left.

Figs. 116 and 117 show the intestinal tract of _Tamandua bivittata_
arranged so as to correspond to the human embryonic condition after
rotation. The caecum has been brought up and to the right across the
proximal duodenal portion of the small intestine, while the jejuno-ileal
coils have been turned down and to the left. The rotation has been
accomplished by a twist at the duodeno-colic isthmus, and the original
right leaf of the mesentery has become the left and _vice versa_.
Comparison with Figs. 107 and 108, representing the condition before
rotation in the same animal, will indicate the changes which have been
accomplished by imitating the course of development followed in the
higher mammals.

[Illustration: FIG. 116.--Abdominal viscera of _Tamandua bivittata_,
with the intestine rotated to correspond to the development in the human
subject. (From a fresh dissection.)]

[Illustration: FIG. 117.--The same view as Fig. 116, from another
specimen.]

Failure of rotation and arrest of development at the primitive stage,
with consequent persistent embryonic condition of the mesentery, occurs
occasionally in man. Such cases have been reported by W. J. Walsham, in
St. Barthol. Hosp. Rep., London, Vol. 16. The following four instances
of this condition, taken from the Columbia University museum, will
illustrate the disposition of the abdominal contents.

Fig. 118 shows the arrangement of the abdominal viscera in an adult
female body. Beginning at the pyloric extremity of the stomach the
entire course of the duodenum can be overlooked and its continuation
into the jejuno-ileal division traced. The small intestines occupy the
ventral and right part of the cavity. The ileo-colic junction is placed
in the lower left-hand corner of the abdomen and the small intestine
enters the large from right to left, the ascending colon is situated to
the left of the median line and at its point of transition into the
segment representing the transverse colon is connected by several
adhesions with the ventral surface of the duodenum. The transverse
colon, folded into several coils bound together by adhesion, occupies
the upper left portion of the abdomen.

[Illustration: FIG. 118.--Abdominal viscera of adult human female, in a
case of arrested rotation of the intestines. (Columbia University
Museum, Study Collection.)]

[Illustration: FIG. 119.--The same preparation with the intestinal coils
displaced upward and to the left.]

Fig. 119, taken from the same specimen, shows the entire mass of
intestines lifted up and turned to the left, exposing the background of
the abdominal cavity lined by parietal peritoneum. The duodenum is still
entirely free and non-adherent to the parietal peritoneum. The
continuity of the mesoduodenum with the jejuno-ileal mesentery is well
shown. The primitive right leaf of the mesentery is turned to the
observer. This layer after completed rotation would form the left layer
of the adult mesentery of the jejuno-ileum.

Fig. 120 illustrates another instance of the same condition in the
adult. In this case the duodenum was coiled twice upon itself and
adherent to the prerenal parietal peritoneum.

[Illustration: FIG. 120.--Abdominal viscera of adult human male;
non-rotation of intestine. (Columbia University Museum, Study
Collection.)]

Fig. 121, presenting the same adhesion of the duodenum, illustrates very
perfectly the persistence of the narrow duodeno-colic isthmus in cases
of non-rotation, as well as the development of the different segments of
the adult tract from the limbs of the embryonal umbilical intestinal
loop.

[Illustration: FIG. 121.--Abdominal viscera of adult human male;
non-rotation of intestine. (Columbia University Museum, Study
Collection.)]

It will be observed that beyond the duodeno-colic isthmus the coils of
the jejuno-ileum have resulted from the increase in length of the
descending limb, the apex and the proximal part of the ascending or
recurrent limb, carrying the ileo-colic junction and caecum. The
remainder of the ascending limb, terminating in the embryonic condition
at the splenic flexure by passing into the descending colon, has in the
course of further development in this individual produced a straight
segment--the misplaced ascending colon--and a convoluted and bent
representative of the normal transverse colon.

The same disposition of the large intestine may be noted in the other
preparations.

Fig. 122 shows an instance of non-rotation observed in the human infant
at two years of age.

[Illustration: FIG. 122.--Abdominal viscera of child, two years old;
non-rotation of intestine. (Columbia University Museum, Study
Collection.)]

[Illustration: FIG. 123.--Human foetus at term; abdominal viscera,
hardened _in situ_; non-rotation of caecum. (Columbia University Museum,
No. 1813.)]

Fig. 123, taken from a foetus at term, shows the result of failure to
completely rotate in the region of the caecum and ileo-colic junction.
The rest of the large intestine has rotated as usual and assumed the
normal position. The terminal ileum, however, passes behind the caecum
and enters the large intestine on its right side; the caecum is turned
upwards and to the right and the appendix lies ventrad of the beginning
of the ascending colon. In order to produce the normal arrangement,
shown in Fig. 124, taken from another foetus at term, it would be
necessary to turn the caecum and ileo-colic junction in Fig. 123 through
half a circle. The caecum would then turn upwards and to the left, the
ileum entering the large intestine from left to right, and the appendix
would be placed behind the caecum and ileo-colic junction. Figs. 125 and
126 show the normal and abnormal arrangement presented by these two
preparations diagrammatically. The instances in which in the adult the
ileo-colic entrance is placed on the right side of the large intestine
and in which the appendix is situated laterad of the ascending colon
unquestionably find their explanation in the failure of the intestine to
completely rotate at the ileo-colic junction.

[Illustration: FIG. 124.--Human foetus at term; abdominal viscera,
hardened _in situ_; normal position of completely rotated caecum and
appendix. (Columbia University Museum, No. 1814.)]

[Illustration: FIG. 125.--Just before final rotation of caecum and
terminal ileum. Concavity of caecum directed cephalad and to right.
Terminal ileum enters colon from right to left.]

[Illustration: FIG. 126.--Rotation completed. Concavity of caecum turns
caudad and to left. Terminal ileum enters colon from left to right.]

[Illustration: FIGS. 125, 126.--Schematic representation of final stages
in rotation of caecum and large intestine.]

The resulting conditions are shown in Figs. 127 and 128, taken from
adult human subjects in which the final stage of rotation of the large
intestine has not taken place.

[Illustration: FIG. 127.--Adult human subject with non-rotated caecum.
The terminal ileum turns caudad from right to left to enter right side
of colon.]

[Illustration: FIG. 128.--Adult human subject with non-rotated caecum,
the ileum entering large intestine from the right and behind, and the
appendix placed to the right of the ascending colon. (From a fresh
dissection.)]

In Fig. 127 the terminal ileum is sharply bent on itself and adherent to
the prerenal parietal peritoneum. It passes from right to left and
downwards to enter the right posterior circumference of the large
intestine. The caecum is turned cephalad and the appendix is in contact
with the right lobe of the liver. The caecum passes with a sharp bend
into the obliquely directed ascending colon.

In Fig. 128 the ileum enters the colon from the right and below. The
apex of the caecum is turned cephalad and to the right and the appendix
extends beneath peritoneal adhesions along the lateral border of the
proximal segment of the colon.

In the next place it is desirable to clearly understand the vascular
supply of the intestine before and after rotation and the final relation
of the superior mesenteric artery to the transverse portion of the
duodenum.


Development of Aortal Arterial System.

The thoracic and abdominal aortae are at first double, the first aortic
arches continuing as so-called "primitive aortae" ventrad of the
vertebral column to the caudal end of the body.

The cephalic portions of the two vessels unite in the chick on the third
day and from this point fusion into a single vessel proceeds slowly
caudad.

In the rabbit the fusion of the primitive aortae begins on the ninth day
in the region of the lung-buds and progresses from here caudad until by
the sixteenth day a single aorta is formed (Fig. 129).

[Illustration: FIG. 129.--Diagrams illustrating the arrangement of the
primitive heart and aortic arches. (After Heisler, modified from Allen
Thompson.)]

That the entire descending aorta in man results from the fusion of two
vessels is shown by the rare cases in which the aorta is divided
throughout its entire length by a septum.

The arteries of the allantois are originally the terminations of the
primitive aortae. After fusion of the primitive aortae to form the
abdominal aorta the allantoic arteries, now passing as the umbilical
arteries to the placenta, appear as the branches of bifurcation of the
abdominal aorta, in the same way as the common iliacs do in the adult.

They furnish branches, which at first are very small, to the budding
posterior extremities and the pelvic viscera. In time these rudiments of
the future external and internal iliac arteries become larger, but as
the umbilical arteries continue to develop throughout the entire
intra-uterine period they appear even in the foetus at term as end
branches of the aorta, a condition which is only changed after birth by
the obliteration of the umbilical arteries and their conversion into the
lateral ligaments of the bladder, while the iliac vessels now appear as
the terminal aortic branches. The statement that the umbilical arteries
appear as the terminal branches of the embryonal aorta requires to be
modified in the following respect:

When the allantois develops its arteries are in fact end-branches of the
two primitive aortae. After their fusion and after the formation of the
single aorta this vessel is continued beyond the umbilical arteries as
a small trunk, the caudal artery or rudiment of the adult sacralis
media. Consequently the umbilical arteries are really lateral branches
of a median vessel, viz., aorta abdominalis and arteria sacralis media.
But as the umbilical vessels are very large and the caudal aorta very
small, the former, even under these conditions, appear as the real
terminal branches of the abdominal aorta.

The arteries supplying the yolk-sac and subsequently the intestinal
canal are the vitelline or omphalo-mesenteric. At first they are
branches derived from the two primitive aortae, and after the fusion of
these vessels they arise from the resulting single abdominal aorta. The
omphalo-mesenteric arteries are at first multiple and later are reduced
to two. When the primitive intestine loses its original close contact
with the vertebral column and the common dorsal mesentery develops, the
two omphalo-mesenteric arteries unite to form a single vessel, running
between the layers of the mesentery. After a short course this artery
divides again into two branches, passing one on each side, around the
intestinal tube, which has in the meanwhile become closed. Ventrad of
the intestine these branches reunite so that the gut is surrounded by a
vascular circle. The left half of this loop becomes obliterated and the
trunk of the omphalo-mesenteric artery now passes on the right side of
the intestine to the umbilicus. The peripheral segment of the
omphalo-mesenteric artery disappears with the cessation of the vitelline
circulation. The proximal portion, situated between the layers of the
mesentery, gives numerous anastomosing branches to the intestine and is
converted into the main trunk of the superior mesenteric artery.

The derivation of the superior mesenteric as the fully developed
proximal segment of the embryonic omphalo-mesenteric artery passing to
the yolk-sac is responsible for the rare anomaly in the adult of a
branch of the superior mesenteric artery continuing beyond the intestine
to the umbilicus. I have encountered one instance of this persistence of
the intra-abdominal portion of the omphalo-mesenteric artery in a male
subject 54 years of age. A connective strand, containing a
small artery derived from the superior mesenteric vessels, extended
between the right layer of the mesentery, some distance from its
attached border, and the ventral abdominal wall at the umbilicus. The
vessel which was pervious throughout, was the size of one of the digital
arteries.

Hyrtl has observed the same variation. An example of partial persistence
of the omphalo-mesenteric artery in the adult is well seen in the case
of Meckel's diverticulum shown in Fig. 37, where the arterial vessel
continued upon the diverticulum represents the embryonic
omphalo-mesenteric artery.

The remaining intestinal arteries are at first more numerous and paired.
In man and most mammals they are early reduced in number, passing from
the abdominal aorta to the dorsal or attached border of the intestine,
between the two peritoneal layers of the primitive dorsal mesentery
(Fig. 104). The arterial blood supply of the intestinal canal then
presents three general divisions:

1. Vessels pass from the proximal part of the abdominal aorta to the
stomach and pyloric portion of the duodenum. This set of vessels forms
the rudiment of the future coeliac axis. With the development of the
liver and pancreas by budding from the duodenum, and with the appearance
of the spleen in the mesoderm of the dorsal mesentery, branches
corresponding to these organs (hepatic and splenic arteries) are added
to the gastric and duodenal vessels and the adult arrangement of the
coeliac axis is thus obtained (Figs. 130, 131, 132 and 133).

[Illustration: FIG. 130.--Diagrammatic representation of the arteries
proceeding to the alimentary canal and appendages prior to rotation of
intestine (stage of simple umbilical loop).]

[Illustration: FIG. 131.--Diagrammatic representation of the arteries of
the alimentary canal in the first stage of intestinal rotation, showing
relation of superior mesenteric artery to the transverse portion of the
duodenum.]

[Illustration: FIG. 132.--Arteries of alimentary canal in the later
stages of intestinal rotation.]

[Illustration: FIG. 133.--Final arrangement of arteries of alimentary
canal after completed rotation of the intestines.]

These vessels have an important bearing on the formation of the adult
peritoneal cavity in the retro-gastric space, and will be considered in
detail below with that portion of the subject.

2. The next vessel in order derived from the aorta and supplying the
duodenum, pancreas, the small and a part of the large intestine is the
above-mentioned superior mesenteric artery, which arises from the aorta
a short distance caudad of the coeliac axis (Figs. 130, 131, 132 and
133).

At the time when the intestine still presents the primitive arrangement
of the umbilical loop (Figs. 104 and 130) this vessel passes between
the layers of the dorsal mesentery through the narrow duodeno-colic neck
to reach the two limbs and the apex of the intestinal loop. In its
course it gives off successively branches to the gut from each side.
Those from the right side of the main vessel pass to the duodenum,
pancreas, jejunum and ileum. Those from the left side of the main vessel
accede in succession to the colic angle of the isthmus, the proximal
portion of the colon, the caecum and the ileo-colic junction. The
terminal portion of the superior mesenteric artery supplies the ileum
near the ileo-colic entrance. After rotation it will be found that the
turn has occurred at the point _X_ (Fig. 130), _i. e._, in that part of
the vessel which occupies the duodeno-colic isthmus. Hence it will be
found that the first branches derived from the right side of the
primitive superior mesenteric artery, supplying the duodenum and
pancreas (Art. pancreatico-duodenalis inferior) still arise after
rotation from the right side. They are succeeded, beyond the point _X_,
by the original highest _left_ branches passing to colon, caecum and
ileo-colic junction, while all the original right-sided vessels, except
the inferior pancreatico-duodenal, appear now as branches from the left
side of the main artery, supplying the coils of the jejuno-ileum. Hence
in the adult (Fig. 133) the succession of branches derived from the
right or concave side of the superior mesenteric artery is as follows:

    1. Arteria pancreatico-duodenalis inferior.
    2. Arteria colica media.
    3. Arteria colica dextra.
    4. Arteria ileo-colica.

On the other hand, the first branches from what has now become the left
or convex side of the vessel are the original lower right-hand vessels
to the small intestine developed from the descending limb of the loop.
Hence in the adult the left side of the superior mesenteric vessel gives
rise to the vasa intestini tenuis.

3. The caudal intestinal arterial branch derived from the aorta is the
inferior mesenteric artery supplying parts of the transverse colon, the
descending colon, sigmoid flexure and rectum (Figs. 130, 131, 132, and
133).

On the other hand in the cases of non-rotation of the intestine as above
described in Figs. 118-122, the embryonic type of the intestinal
arterial supply persists, as indicated schematically in Fig. 134. Not
only the pancreatico-duodenalis inferior, but all the remaining branches
to the small intestine are derived from the right side of the superior
mesenteric artery. The terminal branches of the main artery supply the
ileo-colic junction, while the arterial supply of the large intestine,
A. colica dextra and media, are given off from the left side of the
parent vessel.

[Illustration: FIG. 134.--Schematic representation of intestinal
arterial supply from superior mesenteric artery in cases of arrested
rotation of the intestine.]

II. =Demonstration of Intestinal Rotation in the Cat.=--The changes in
the relative position of the different intestinal segments and the final
disposition of the mesenteries and blood vessels can best be understood
by the direct examination of the abdominal contents in an animal whose
permanent adult arrangement corresponds to one of the early embryonal
human stages, and in which the necessary manipulations can readily be
carried out and their results noted.

It is doubtful if the above detailed developmental stages in man can
ever be clearly comprehended unless the student will for himself examine
the conditions and perform the manipulations in one of the lower
mammals.

The necessity of keeping the three dimensions of space in mind and the
fact that certain structures during and after rotation cover and obscure
each other, make diagrams and drawings unsatisfactory unless the actual
examination of the object itself is combined with their study.
Fortunately, among the common domestic animals of convenient size easily
obtained the cat answers every purpose of this study admirably. The
student is earnestly urged to pursue his study of the development and
adult arrangement of the human abdominal viscera and peritoneum in the
light which the anatomy of this animal can shed on the complicated and
obscure conditions encountered in the human subject. The plan of having
the opened abdominal cavity of the cat directly side by side with the
human subject, while the arrangement of the abdominal viscera and
peritoneum is considered, cannot be recommended too highly.

=Directions.=--After killing the animal with chloroform the abdominal
cavity is to be freely opened by a cruciform incision and the skin flaps
turned well back and secured in this position. It is well to select a
male animal or an unimpregnated female, as the size of the pregnant
uterus in the later stages renders the examination of the abdominal
viscera and peritoneum more difficult.

For purposes of careful study and comparison of the vascular relations
of the abdomen, it is highly desirable to inject the animal with
differently  gelatine, starch or plaster of Paris mass. The
arterial injection can be made through the carotid artery, the systemic
venous injection through the femoral vein, and the portal circulation
can be filled after opening the abdomen, by injection through the
superior mesenteric or splenic veins. Animals prepared in this manner
are especially useful for the study of the upper portion of the
abdominal cavity and of the peritoneal relations of liver, stomach,
spleen, pancreas and duodenum. They may be kept for permanent reference
in a 5 per cent. solution of formaline or 50 per cent. alcohol.

After opening the abdominal cavity turn the great omentum up over the
ventral surface of the thorax and secure it in this position, thus
exposing the underlying intestines completely (Fig. 135). Trace in the
first place the entire course of the intestinal tube from the pyloric
extremity of the stomach down. It will be noticed that the first portion
of the small intestine (duodenum) is freely movable, completely invested
by peritoneum and attached to the dorsal midline by a mesoduodenum
between the layers of which a portion of the pancreas is seen.

[Illustration: FIG. 135.--Abdominal viscera of cat; great omentum
raised; intestines turned down and to left. (From a fresh dissection.)]

Following the duodenum caudad it will be observed that the gut can be
traced directly continuous with the remaining coils of the small
intestine. The ileo-colic junction and the beginning of the large
intestine are marked by a short pointed caecum. The large intestine is
short, as it is in all carnivore mammals, and passes from the caecum
almost directly down into the pelvis.

Take the caecum and the first portion of the large intestine and turn
them caudad and over to the left side as far as the peritoneal
connections will permit.

Spread out the coils of the small intestine in the opposite direction,
_i. e._, over to the right side.

The arrangement of the intestinal tract after these manipulations should
appear as shown in Figs. 136 and 137.

[Illustration: FIG. 136.--Abdominal viscera of cat, hardened; omentum
removed to display derivation of intestines from umbilical loop and the
relation of the superior mesenteric artery and common dorsal mesentery
to the small and large intestines. (Columbia University Museum, No
728.)]

[Illustration: FIG. 137.--Abdominal cavity of cat. (From a fresh
dissection.)]

It will be seen that all the essential features described for the
corresponding stage in the human embryo (Fig. 104, _A_) exist here. The
proximal portion of the small intestine (duodenum) retains its freedom
and mobility, being attached to the ventral surface of the vertebral
column by the portion of the primitive mesentery which now constitutes
the mesoduodenum. The gut itself forms a bend with the convexity turned
to the right.

Observe in the next place that the point (Fig. 136, _X_), where small
intestine and colon approach each other closely, marks the situation of
the foetal duodeno-colic isthmus. The small intestine at this point
corresponds to the future duodeno-jejunal angle as will be seen after
rotation has been accomplished.

Recalling the development of the jejuno-ileum it will not be difficult
to recognize in the numerous coils of small intestine which succeed to
the duodeno-colic isthmus the results of the increase in length of the
descending or efferent limb of the human embryonal umbilical loop.
Tracing these coils it will be found that the terminal portions of the
ileum correspond to the apex and to the proximal part of the ascending
or recurrent limb of the primitive loop, while the remainder of this
limb furnishes the caecum and the next succeeding segment of the large
intestine. Following the tube up to this point the colic boundary of the
duodeno-colic isthmus will be reached; from here the short large
intestine of the carnivore descends straight into the pelvis, attached
to the ventral surface of the vertebral column by a mesocolon which
corresponds to the distal part of the original primitive dorsal
mesentery.

Now with the parts still in this position examine carefully the
arrangement of the mesentery and of the intestinal blood vessels.
Starting with the duodenum it will be seen that the primitive sagittal
mesentery of this portion of the intestine has followed the gut in its
turn to the right, so that the original right layer of the sagittal
membrane is now directed dorsad and lies in contact with the parietal
peritoneum which invests the background of the abdominal cavity in the
right lumbar region below the liver and covers the ventral surface of
the right kidney. Beneath this parietal peritoneum the inferior vena
cava is seen, receiving the right renal vein and ascending to enter the
dorso-caudal aspect of the right lobe of the liver. If now we assume
that in the cat the opposed serous surfaces of the original right leaf
of the mesoduodenum, now directed dorsad, and of the parietal peritoneum
adhere to each other, and that the visceral peritoneum covering the
dorsal surface of the descending duodenum likewise becomes obliterated
by adhesion to the subjacent parietal peritoneum, we will obtain the
arrangement found in the adult human subject, in which the descending
duodenum is fixed by adhesion below the right lobe of the liver and
ventrad of the medial portion of right kidney, right renal vein and
inferior vena cava. During this process of anchoring the head of the
pancreas, which is found between the two layers of the free mesoduodenum
of the cat, would also become fixed to the abdominal background by
adhesion of the original right leaf of the mesoduodenum, investing what
has now become the dorsal surface of the pancreas, to the parietal
peritoneum. The original left layer of the primitive mesoduodenum would
then appear as _secondary_ parietal peritoneum covering what has now
become the ventral surface of the transversely disposed head of the
gland. The stages may be represented schematically in Figs. 138-140.

[Illustration: FIGS. 138-140.--Diagrammatic representation of three
stages in the development of the mesoduodenum, duodenum, and pancreas
leading to the secondary "retroperitoneal" position of these viscera.]

[Illustration: FIG. 138.--Free mesoduodenum in sagittal plane, including
head of pancreas between right and left layers.]

[Illustration: FIG. 139.--Mesoduodenum folded to right; left leaf has
become ventral; right dorsal, directed toward primitive prerenal
parietal peritoneum.]

[Illustration: FIG. 140.--Fixation of head of pancreas and duodenum
under cover of secondary parietal peritoneum by adhesion of apposed
surfaces of mesoduodenum and primitive parietal peritoneum.]

Figs. 138 and 139 shows the arrangement in the cat where a free
duodenum and mesoduodenum exists, with the pancreas included between its
layers.[2]

[2] The student should not be confused by the fact that a considerable
portion of the pancreatic gland in the cat will be found included
between the layers of the great omentum, extending over to the left side
of the abdomen. This circumstance will be found of importance in
studying the development of the dorsal mesogastrium and of the
structures connected with it. For the present attention should only be
given to the right extremity or head of the pancreas, situated close to
the duodenum and included between the layers of the mesoduodenum.

It will be noticed that the duodenum in the cat can be carried over to
the median line (Fig. 138) exposing the entire ventral aspect of the
right kidney and the inferior vena cava beneath the primary lumbar
parietal peritoneum. This manipulation will also expose the dorsal
surface of the head of the pancreas, covered by what originally was the
right leaf of the mesoduodenum.

Fig. 140 indicates the results of adhesion of the duodenum, pancreas and
mesoduodenum to the parietal peritoneum as it normally occurs in the
human subject. It will be seen that the primary parietal peritoneum can
be traced mesad over the ventral surface of the right kidney as far as
the point _X_, and that from here on to the median line the peritoneum
is _secondary_ parietal peritoneum, consisting of the visceral
peritoneal investment of the ventral surface of the duodenum and of the
original left leaf of the mesoduodenum, beneath which the ventral
surface of the pancreas is seen. Pancreas and duodenum occupy in the
adult secondarily a "retro-peritoneal" position, _i. e._, the peritoneum
now covering the ventral surface of these viscera appears as a
continuation of the parietal peritoneum, the transition between primary
and secondary parietal peritoneum occurring along the line marked _X_ in
Fig. 140. The opposed peritoneal surfaces indicated by the dotted lines
have become adherent and converted into loose connective tissue in which
the pancreas and duodenum lie imbedded. In the human embryo this process
of adhesion begins in the eighth week, starting at the duodeno-jejunal
flexure and ascending gradually toward the pylorus. At the end of the
fourth month the union is complete.

Proceeding caudad it will next be observed that the peritoneum of the
mesentery occupies the narrow neck of the duodeno-colic isthmus, and
that large vessels (the superior mesenteric) pass between its two layers
at this point to supply the segments of the intestine forming the loop.
In conformity with the greatly increased length of the intestine it will
be found that the mesentery expands from the narrow pedicle at the neck
in a fan-shaped manner in order to develop a sufficiently long margin
for attachment to the intestine. The following points should be
carefully borne in mind in studying the mesentery with the intestines in
this position:

1. The mesentery presents two free surfaces, right and left. With the
coils of the small intestine turned over to the right, the left leaf of
the mesentery is turned toward the observer.

2. Inasmuch as the descending limb of the embryonic loop has developed
the greater part of the small intestine, while a portion of the large
intestine (caecum and colon up to the isthmus) is the result of
differentiation within the ascending or returning limb of the loop, it
will be at once apparent that the double peritoneal layer which extends
between the duodeno-colic isthmus and the attached border of the gut is
partly mesentery of the small intestine, partly mesocolon passing to the
large intestine (caecum and proximal colon). This condition may be
indicated schematically in Fig. 141.

[Illustration: FIG. 141.--Schematic representation of mesentery of
umbilical loop, common to small intestine and proximal portion of large
intestine.]

The curved line _A_ may be taken as an arbitrary division between the
portion of the membrane which on the right of the figure passes to the
small intestine, and the portion which proceeds to the left to be
attached to the large intestine. In other words the line will
schematically separate the true mesenteric from the mesocolic segment of
the primitive membrane.

With the parts in their present position this line might be assumed to
indicate a strip along which the opposed serous surfaces of the parietal
peritoneum and the right leaf of the primitive mesentery became
adherent. In that case an actual division into a mesenteric and
mesocolic segment would have been effected.

Ventrad and to the right of this line of adhesion we would trace that
portion of the primitive membrane which now passes to the coils of the
small intestine as the true mesentery, having an apparent origin in the
background of the abdomen to the dotted line of adhesion. In the same
manner the peritoneal layers passing to the left to reach the caecum and
beginning of the colon would appear as a free mesocolon with the same
line of apparent origin from the background of the abdomen. (cf. p. 80.)

These considerations should be followed out in the dissection of the cat
in order to become familiar with the principle of _secondary lines of
origin_ for peritoneal layers. As we will see later this factor is of
importance in correctly estimating the value of the human adult
conditions.

3. A brief consideration of the mechanical conditions and comparison
with the earlier stages will show why the peritoneal layers which occupy
the bight of the fully developed umbilical loop are especially prone to
develop secondary lines and areas of adhesion to other serous surfaces.
If we compare the dorsal mesentery in its primitive condition, before
the straight intestinal tube has become differentiated into the
subsequent segments, and before the umbilical loop has been formed (Fig.
142), with the later stages represented by the intestines of the cat as
now arranged (Figs. 143 and 144), it will be seen that the vertical line
of attachment to the ventral surface of the vertebral column, between
the points _a_ and _b_ corresponds in the advanced stages to the
interval _ab_ separating the two points of the duodeno-colic isthmus;
also that the entire mesenteric peritoneal surface beyond the isthmus is
the result of drawing out and lengthening the intestinal tract.
Consequently folding or overlapping of this extensive membrane affords
opportunities for adhesions between its own serous surfaces or between
it and the remaining visceral and parietal peritoneum of the abdomen.

[Illustration: FIGS. 142-144.--Schematic representation of three stages
in the development of the mesentery of the umbilical intestinal loop.]

[Illustration: FIG. 142.--Early stage before differentiation of
intestinal canal.]

[Illustration: FIG. 143.--Stage of umbilical loop. Differentiation of
common dorsal mesentery of earlier stage into dorsal mesogastrium,
mesoduodenum, primitive mesentery of umbilical loop, and descending
mesocolon.]

[Illustration: FIG. 144.--Final stage. With complete differentiation of
large and small intestine, the primitive mesentery of the umbilical loop
contains not only the mesentery of the future jejuno-ileum, but also the
mesocola and the ascending and transverse colon, developed from the
ascending or afferent limb of the umbilical loop.]

Moreover, it will be appreciated that the entire extensive coil of
intestines extending between the two boundaries of the duodeno-colic
isthmus (_a_, _b_) is suspended from the back part of the abdomen by a
narrow pedicle and that consequently rotation will readily occur around
the axis drawn through the neck of the isthmus.

Now proceed to illustrate on the cat the result of the rotation as it
occurs normally during the development of the primate intestinal tract.
Take the caecum and commencement of the colon and draw the same over to
the right across the duodeno-colic isthmus and the duodenum. Twist or
rotate the entire mass of small intestines around the isthmic pedicle,
so that the original left leaf of the mesentery will look to the right
and _vice versa_ (Fig. 145). The conditions thus established will be
found to correspond to the schemata shown in Figs. 114 and 115. The main
features of the intestinal tract in the rearranged position will be as
follows:

1. The two points, _a_ and _b_, of the duodeno-colic isthmus (Fig. 145)
are still close together, but reversed in position, _b_ is in front and
to the right, _a_ behind and to the left, whereas before the rotation
_b_ was situated below and to the left, _a_ above and to the right (Fig.
135).

[Illustration: FIG. 145.--Abdominal viscera of cat, with intestines
rotated to correspond to the stage in the development of the human canal
in which the caecum has reached the subhepatic position, but before the
establishment of the ascending colon. (From a fresh dissection.)]

2. The direction of the ileo-colic entrance is reversed, the ileum now
entering the large intestine from below and the left upwards and to the
right, instead of from right to left.

3. The descending duodenum is now situated dorsad to the colon.

4. The original left leaf of the mesentery has become the right, and
_vice versa_.

5. The superior mesenteric artery crosses over the transverse portion of
the duodenum, and with the exception of the inferior
pancreatico-duodenal artery the original right-sided branches now arise
from the left side of the vessel and _vice versa_.

It is now time to compare the conditions established in the cat by the
manipulations just detailed with the arrangement of the adult human
intestinal tract and peritoneum below the level of the transverse colon
and mesocolon.

I. The shortness of the large intestine in the cat will require careful
manipulation in order to produce a disposition in conformity with the
arrangement of this portion of the human intestinal tract. By
stretching the gut somewhat and pulling it well out of the pelvis
sufficient length will be obtained to establish an ascending, transverse
and descending colon. Move the caecum from the subhepatic position which
it occupies immediately after rotation (Fig. 145) down to the lower and
right-hand corner of the abdomen. Pull the distal portion of the large
intestine well out of the pelvis and obtain thus sufficient length to
establish an ascending, transverse and descending division each provided
with a free mesocolon (Fig. 146). In the formation of the three definite
main segments of the human large intestine, ascending, transverse and
descending colon, the following stages may be recognized:

[Illustration: FIG. 146.--Abdominal viscera of cat, with the intestines
rotated to correspond to the adult human disposition, with ascending,
transverse, and descending segments of the colon. (From a fresh
dissection.)]

1. Immediately after rotation the large intestine lies transversely
along the greater curvature of the stomach, with the caecum on the right
side in front of the duodenum and closely applied to the caudal surface
of the right lobe of the liver (Fig. 147).

[Illustration: FIG. 147.--Human foetus, 6.6 cm., vertex-coccygeal
measure; liver removed. (Columbia University Museum, Study Collection.)
x 4.]

PERSISTENCE OF SUBHEPATIC POSITION OF CAECUM IN ADULT.--The period at
which the caecum descends into the iliac fossa is liable to a
considerable range of variation.

Treves found in two foetus, measuring respectively 41/2" and 51/2", the
caecum on a level with the caudal end of the right kidney, while in
several individuals at full term the caput coli was still placed
immediately below the liver, with no large intestine in the place of the
ascending colon. This condition is well illustrated in the foetus shown
in Fig. 124.

The caecum may remain undescended throughout life. Treves, in an
examination of 100 bodies, found this condition in two subjects, both
females, one 41, the other 74 years of age. Both cases presented an
identical disposition. There was no large intestine in the place of the
ascending colon. The caecum was placed on the right side, immediately
underneath the liver, just to the right of the gall-bladder; it was
quite horizontal in position, continuing the long axis of the transverse
colon and included between the layers of the transverse mesocolon. From
the extremity of the caecum a horizontal fold was continued to the
abdominal parietes and upon it the edge of the liver rested. In one of
these instances the colon from the tip of the caecum to the splenic
flexure measured 38". The great omentum was attached only to the left
half of this portion. The descending colon was very long, measuring 15".

In the other case the distance from the tip of the caecum to the splenic
flexure was 27", the great omentum commencing 5" from the former point.
The descending colon was of normal length.

In both bodies the remaining viscera were normal.

2. The caecum next descends ventrad of right kidney to the iliac fossa.
The future ascending colon is at this time placed very obliquely on
account of the large size of the foetal liver, and passes without a
marked angle into the transverse segment. Thus in Fig. 148, from a foetus
5" in length, the descending colon is vertical and the splenic flexure
well marked, forming the highest point of the colic arch. There is no
hepatic flexure, and no ascending and transverse colon, but instead of
these an oblique segment passing upwards and to the left between caecum
and splenic flexure.

[Illustration: FIG. 148.--Abdominal viscera of human foetus of 12.5 cm.,
vertex-coccygeal measure, hardened _in situ_; transverse and ascending
colon not yet differentiated. (Columbia University Museum, No. 1815.)
Natural size.]

This disposition, due to the large size of the liver, is still marked at
times in the foetus at term, and occasionally even in children up to 2 or
3 years of age.

3. The ascending colon is subsequently differentiated from the
transverse segment and the hepatic flexure formed consequent upon the
diminution of the relative size of the liver, which permits the foetal
oblique segment of the colon extending in the earlier stages between the
right iliac fossa and the spleen to become divided by a right-angled
(hepatic) bend or flexure into an ascending and a transverse segment
(Fig. 149).

[Illustration: FIG. 149.--Abdominal viscera of human foetus at term,
hardened _in situ_; hepatic flexure formed and ascending and transverse
colon differentiated. (Columbia University Museum, No. 1816.)]

4. The splenic flexure develops early and is well marked. It indicates
the point of transition of the original ascending limb of the umbilical
loop into the remaining vertical median segment of the large intestine,
from which the descending colon is formed.

In the adult the ascending and descending portions of the colon are
vertical. The transverse colon is not quite horizontal since the
splenic flexure is higher and placed more dorsally than the hepatic
flexure. In the embryo the rapidly-growing coils of the small intestine
push the descending colon to the left and dorsad into close contact with
the dorsal abdominal wall.

A small bend which appears about the middle of the third month in the
left iliac fossa indicates the rudiment of the future sigmoid flexure or
omega loop.

The rest of the endgut follows the body wall in a well-marked curve,
whose termination lies within the concavity of the caudal portion of the
embryo (Fig. 150). From this terminal part the rectum develops after the
division of the cloaca and the union of the proctodaeum with the
entodermal intestinal pouch has taken place as detailed above.

[Illustration: FIG. 150.--Caudal portion of human embryo of 5 mm., with
the end- and caudal gut at the highest stage of its development. x 25.
(Reconstruction after His.)]

The early position of the colon produced by the large size of the foetal
liver, and before the descent of the caecum has occurred, is shown in
Fig. 124. In Fig. 123, where the liver has regained its normal
proportions with reference to the abdominal cavity and viscera, and the
caecum has descended into the right iliac fossa, the hepatic flexure is
well marked and the first segment of the colon has acquired the vertical
position on the right side, the single obliquely transverse segment of
Fig. 124, having become divided into an ascending and a transverse
colon.

[Fig. 124. Early stage. Liver relatively large. Proximal portion of the
colon extends obliquely between the right lumbar region and the spleen.
The caecum has not yet descended.

Fig. 123. Later stage. The caecum occupies the right iliac fossa.
Relative reduction in the size of the liver allows the colic segment to
be divided by the hepatic flexure into an ascending colon and a
transverse colon.]

At times the transverse colon, whose normal average length in the adult
is 20", greatly exceeds this measurement and forms an arch which hangs
down or makes a well-marked V-shaped bend with the apex directed toward
the pubes. This is the normal arrangement of this portion of the large
intestine in many of the lower primates. Fig. 151 shows the abdominal
viscera of _Macacus rhesus_, hardened _in situ_, seen from the front
and the right side, with the omentum turned up over the stomach. The
transverse colon forms an extensive V-shaped bend, whose apex reaches to
the pubes, from which point the large intestine turns again cephalad and
dorsad to form the splenic flexure and then descends to the pelvis.

[Illustration: FIG. 151.--Abdominal viscera of _Macacus rhesus_, rhesus
monkey, hardened in situ. (Columbia University Museum, No. 1817.)]

The average length of the ascending colon in the adult, measured from
the tip of the caecum to the hepatic flexure, was found by Treves in his
series of 100 bodies to be 8", while the descending colon, from the
splenic flexure to the beginning of the sigmoid loop, measured 81/2".

The descending colon may at times be much longer, up to 15", and become
convoluted.

II. In the next place, in order to understand the arrangement of the
peritoneum in this lower larger compartment of the abdomen, disregard
for the present the peritoneal connections of the stomach, liver,
pancreas and spleen, and the folds of the great omentum entirely. This
latter membrane is adherent in the adult human subject by its dorsal
surface to the upper margin of the transverse colon, so that in turning
the omentum up over the ventral chest wall the transverse colon will be
carried with the omentum and the lower layer of the transverse mesocolon
will be put upon the stretch. This membrane forms in adult man by its
transverse attachment to the abdominal background the cephalic limit of
the larger lower compartment of the abdomen, which is framed laterally
by ascending and descending colon, continuous below with the pelvic
cavity and occupied chiefly by the freely movable coils of the
jejuno-ileum.

Remember that the duodenum starting from the pyloric extremity of the
stomach first turns cephalad and dorsad in contact with the caudal
surface of the right lobe of the liver, forming the first portion or
hepatic angle of the duodenum; that in the next place the second or
descending portion of the duodenum passes down in front of the medial
part of the ventral surface of the right kidney and the inferior vena
cava, but _behind_ the right extremity (hepatic flexure) of what after
rotation and formation of the ascending colon appears as the transverse
colon; that consequently the descending duodenum is divided by its
intersection with the transverse colon into a cephalic supra-colic and a
caudal infra-colic segment.

Also remember that the second angle of the duodenum (transition between
the descending and transverse portions) is consequently situated to the
right of the vertebral column below the level of the transverse colon
and the secondary attachment presently to be considered of the
transverse mesocolon to the background of the abdominal cavity.

The third portion of the duodenum extends from this point more or less
transversely--depending upon the type--to the left, across the vertebral
column and aorta. This transverse portion, after the rotation of the
primitive loop at the duodeno-colic angle, is crossed in the direction
caudad and ventrad by the superior mesenteric vessels, which hence
divide this portion of the intestine into a right and left segment.

The latter turns cephalad and ventrad on the left side of the vertebral
column (4th or ascending portion) to become continuous at the
duodeno-jejunal angle with the free or floating small intestine
(jejunum).

If we imagine in the cat the duodenum anchored or fixed by adhesion of
the dorsal (originally right) leaf of the mesoduodenum and of its own
dorsal visceral peritoneum to the abdominal parietal peritoneum in the
manner above indicated (p. 70) as far as the duodeno-jejunal angle we
will have conditions established which correspond to those found in the
human adult abdominal cavity.

III. It is next necessary to study carefully the disposition of the
primitive dorsal mesentery connected after rotation with the different
segments of the intestinal tube, ascending, transverse and descending
colon and free small intestine.

In order to obtain in the cat a cephalic limit to the region now under
consideration which will correspond to the arrangement of the adult
human peritoneum, we will begin with the peritoneal membrane attached to
the portion of the colon which in the rearranged intestinal tract
represents the human transverse colon. This transverse segment of the
large intestine is now made to extend directly across the abdomen from
the liver to the spleen. The two layers composing the transverse
mesocolon are an upper or cephalic and a lower or caudal layer.

Now it will be seen in the cat that the upper or cephalic layer of the
transverse mesocolon thus established is continuous on each side with
the dorsal (originally right) leaf of the ascending and with the dorsal
(originally left) leaf of the descending mesocolon, which peritoneal
layers are in direct opposition to the parietal lumbar and prerenal
peritoneum. On the other hand, the inferior or ventral layer of the
transverse mesocolon is continuous on each side of the median line with
the ventral (originally respectively left and right) leaves of the same
mesocola, while at the site of the duodeno-colic isthmus the two layers
of the transverse mesocolon are continuous as originally with the two
layers of the mesentery of the jejuno-ileum (Fig. 146).

Now fix the transverse mesocolon firmly against the background of the
abdomen and place the ascending and descending colon as far as possible
over to the right and left side respectively. We will assume a line of
secondary adhesion between the transverse mesocolon and the parietal
peritoneum investing the dorsal abdominal wall. Along this line the
upper or cephalic surface of the transverse mesocolon would become
continuous with the dorsal parietal peritoneum, while the lower or
caudal layer would still be continuous with the left leaf of the
ascending and the right leaf of the descending mesocolon. We have
already seen that the duodenum and mesoduodenum become anchored in the
subhepatic region and that the visceral ventral peritoneum of the gut
and the original left leaf of the mesoduodenum appear then as secondary
parietal peritoneum. Hence a sagittal section through the right lumbar
region, right kidney and descending duodenum would, immediately after
rotation and establishment of the transverse mesocolon, show
the peritoneal arrangement indicated in Fig. 153. After adhesion of the
transverse mesocolon continuity would be established between its upper
or cephalic layer and the secondary parietal peritoneum investing the
supra-colic portion of the descending duodenum (Fig. 154) while its
caudal layer becomes continuous with the secondary parietal peritoneum
covering the infra-colic segment of the duodenum and the lower portion
of the ventral surface of the right kidney.

[Illustration: FIGS. 152-154.--Schematic representation of peritoneum in
fixation of descending duodenum and formation of transverse colon and
mesocolon.]

[Illustration: FIG. 152.--Sagittal section through right kidney and
descending duodenum before adhesion of latter to parietal peritoneum.]

[Illustration: FIG. 153.--Adhesion of descending duodenum to primitive
parietal peritoneum. Colon and mesocolon after rotation of the
intestine, but before adhesion.]

[Illustration: FIG. 154.--Adhesion of mesocolon to duodenum and
primitive parietal peritoneum, resulting in formation of root of
transverse mesocolon.]

Reference to the schematic Figs. 152, 153 and 154, will show that the
adult duodenum becomes fixed to the posterior parietes of the abdomen by
adhesion of its visceral serous covering and of the dorsal layer of the
mesoduodenum to the primitive parietal peritoneum. The supra-colic
segment of the adult descending duodenum lies under cover of a single
peritoneal layer, derived from its own visceral investment and appearing
as secondary parietal peritoneum by continuity laterad along the line of
adhesion with the primitive parietal peritoneum covering the upper part
of ventral surface of right kidney, while mesad, the layer covering this
segment of the duodenum, is continued into the secondary parietal
peritoneum derived from the left or ventral leaf of the mesoduodenum and
covering the ventral surface of the pancreas (cf. Figs. 138-140).

On the other hand, the infra-colic segment of the descending duodenum,
as well as the lower and mesal angle of the ventral surface of right
kidney, between ascending and transverse colon, is covered by a layer of
secondary parietal peritoneum derived from the ventral layer of the
ascending mesocolon and continuous with the caudal layer of the
transverse mesocolon. Beneath this secondary parietal peritoneum are two
obliterated layers, on the one hand the dorsal layer of the mesocolon,
on the other the visceral infra-colic duodenal serosa and the primitive
prerenal parietal peritoneum.

In the further development of the adult human arrangement the changes
below the level of the transverse colon and mesocolon result in the
fixation of the ascending and descending colon to the background of the
right and left lumbar regions. The opposed serous surfaces of the
ascending and descending mesocola and of the dorsal parietal peritoneum
adhere and the process also usually involves the dorsal visceral
peritoneum of the ascending and descending colon, so that these portions
of the gut obtain a fixed position.

Adhesion of the mesocolon to the dorsal body wall (parietal peritoneum)
does not occur at all points at the same time. Usually adhesion proceeds
from the midline laterad. The fixation of the ascending colon in the
human embryo begins about the fourth month.

In the descending segment by the same time adhesion has usually
proceeded nearly up to the descending colon, but the intestine itself is
as yet free. In the fifth month the descending colon has usually become
fixed between the splenic flexure and the beginning of the sigmoidea. In
the latter region a free mesocolon usually persists throughout life.

Differences in the rate of growth between the length of the body wall
and the length of the mesocolon may play an important part in the
production of peritoneal _fossae_, small pouches which in some regions of
the abdomen may assume considerable proportions. Such fossae are found
around the duodeno-jejuneal angle, the caecum and appendix, and the
sigmoid flexure. They will be considered more in detail with these
respective regions, especially in reference to their relation to
retro-peritoneal hernia.

In a certain proportion of cases adhesion between the parietal
peritoneum and the ascending and descending mesocolon is incomplete or
entirely wanting, resulting in the formation of a more or less
completely free ascending and descending mesocolon. Treves, in an
examination of 100 bodies, obtained the following figures:

In 52 subjects there was neither an ascending nor a descending
mesocolon, the intestine being fixed in the manner which is regarded as
normal.

In 22 there was a descending, but no trace of an ascending mesocolon.

In 14 a mesocolon was found in both the ascending and descending
segments of the large intestine.

In 12 there was an ascending mesocolon, but no corresponding fold on the
left side. Hence from this series a mesocolon may be expected on the
left side in 36 per cent., on the right side in 26 per cent.

Both development and comparative anatomy would lead us to expect that
the descending mesocolon would be found more frequently than the
ascending.

In the lower animals the descending mesocolon is always an extensive and
conspicuous membrane. It is well developed in all monkeys and the
anthropoidea, as the remains of the primary vertical fold of the dorsal
mesentery, while the ascending mesocolon is a secondary production,
acquired during the development of the bowel by rotation.

In most of the lower monkeys the ascending mesocolon is also largely or
entirely free. The descending mesocolon can always in these animals be
reflected to the median line (cf. Fig. 155).

[Illustration: FIG. 155.--Abdominal viscera of _Macacus cynomolgus_, Kra
monkey, hardened _in situ_. (Columbia University Museum, No. 1801.)]

The line of attachment in man of the descending mesocolon is usually
along the lateral border of the left kidney and vertical, while the line
of attachment of the ascending mesocolon is usually less vertical,
crossing the caudal end of the right kidney obliquely from right to left
and with an upward direction (Fig. 156).

[Illustration: FIG. 156.--Schema of visceral and peritoneal relations of
ventral surface of right kidney.]

In like manner when both the ascending and descending mesocola are
absent as free membranes the left or descending colon is adherent along
the lateral border of the kidney to the abdominal parietes, while the
ascending colon is fixed at the hepatic flexure a little obliquely
across the ventral surface of the caudal end of the corresponding gland
ascending toward the medial margin.

Treves found in the cases of persistent ascending mesocolon in the adult
that the membrane varied in breadth from 1" to 2" while the persistent
fold on the left side varied between 2" and 3" in breadth.

In the foetus, up to 5"-6" in length, the descending mesocolon is usually
an extensive fold. Its attachment is vertical, but nearer to the median
line than in the adult, usually along the medial border of the left
kidney. It is at times found attached along this line in the adult.

An ascending mesocolon is rare even in the foetus. The caecum and
beginning of the ascending colon are completely invested by peritoneum,
but above the parts so invested the colon is usually adherent along an
oblique line to the ventral and medial aspect of the right kidney.

In the foetus at full term, if the caecum is still undescended and in
contact with the liver, it is not uncommon to find the cephalic portion
of the descending colon provided with a mesocolon, while the caudal part
of the descending colon is fixed by adhesion to the ventral surface and
lateral border of the left kidney. This free membrane is then really a
part of the transverse mesocolon. Where the caecum descends to the iliac
fossa the portion of the foetal descending colon so invested is drawn
over to the right and incorporated in the transverse colon.

Treves in two out of 100 bodies found the caecum in the right iliac
region, but both it and the whole of the ascending colon were entirely
free from any peritoneal connections with the dorsal parietes of the
abdomen.

The gut from the tip of the caecum to the hepatic flexure was entirely
invested by peritoneum continuous with the mesentery. The ascending
colon was covered in the same manner and by the same fold as the small
intestine. The segment of large intestine thus free measured 8" in both
instances.

The mesentery lacked the usual attachment to the dorsal abdominal wall
and its root was represented by the interval between the duodenum and
the transverse colon. The membrane had no other than its original
primary attachment, and small intestine and ascending colon formed
together a loop that practically represented the condition of the great
primary intestinal loop. (Compare p. 73.)

The arrangement presented in these two subjects corresponds to that met
in many animals, such as the cat.

A cross-section of the cat's abdomen arranged as above would show the
following disposition of the peritoneum, corresponding to the stage in
the human development preceding the fixation of the two vertical colic
segments (Fig. 157). It will be seen that the right and left mesocola
can be reflected to the median line where they become continuous ventrad
of the vertebral column and aorta with the mesentery of the small
intestine. The ventral surfaces of both kidneys are seen to be covered
by the primitive parietal peritoneum of the abdominal cavity.

[Illustration: FIGS. 157, 158.--Schema showing peritoneal arrangement in
transection of infra-colic compartment of abdomen before and after
fixation of ascending and descending colon.]

Fig. 158 shows the adult human arrangement of the same parts, after
fixation of the vertical colic segments by adhesion of the opposed
surfaces of their mesocola and the primitive parietal peritoneum. The
background of the abdomen is now seen to be covered by a layer of
secondary parietal peritoneum, _viz._, the original left leaf of the
ascending and right leaf of the descending mesocolon, continuous above
with the lower or caudal layer of the transverse mesocolon.

This adhesion is so complete that the original condition is disregarded
in adult descriptive anatomy. The layer which has adhered to the
parietal peritoneum can no longer be recognized and the other has
assumed the role of parietal peritoneum.

The connection of the transverse mesocolon with the dorsal lamella of
the great omentum will be considered below.

The course of the vessels in the ascending and descending mesocola is
not altered by the secondary adhesions. These vessels are in the adult
situated behind the secondary parietal peritoneum derived from the
mesocola.

The origin of the transverse mesocolon obtains by the fixation of the
hepatic and splenic flexures high up in the abdomen a transverse course,
and the transverse growth of the abdomen holds the membrane in this
position cephalad of the duodeno-jejunal flexure, so that on elevating
the transverse colon the mesocolon appears as separating the upper from
the lower abdominal compartment. This posterior line of attachment or
so-called "root of the transverse mesocolon," is nothing more than the
upper limit of the area of adhesion between the primitive parietal
peritoneum and the opposed surfaces of the ascending and descending
mesocola. Reference to the abdominal cavity of the cat after complete
rotation (Fig. 146) will show the original continuity of the three
mesocola very clearly. A secondary connection is established along the
lateral border of ascending and descending colon (Fig. 158), between the
primitive parietal peritoneum and the ventral visceral peritoneal
investment of the large intestine. Both of the vertical segments of the
colon now appear fixed. Their dorsal surfaces are uncovered by
peritoneum and can be reached in the lumbar region, laterad of the
kidney, without opening the peritoneal cavity (lumbar colotomy).

The caudal portions of both kidneys are covered, beneath the secondary
parietal peritoneum, by a layer of loose connective tissue representing
the result of obliteration by adhesion of the first and second of the
original three layers of prerenal peritoneum, _viz._, the primitive
parietal (1) and the two layers of the mesocola (2 and 3).

LINE OF ATTACHMENT OF THE MESENTERY OF THE JEJUNO-ILEUM.--Examination of
the caudal surface of the transverse mesocolon in the cat, with the
parts in the above outlined position, will show how and why in the adult
human abdomen the duodeno-jejunal angle appears to dip out from beneath
the transverse mesocolon, becoming gradually more and more free until
complete transition to the mobile jejunum is obtained. From this point,
situated to the left of the second lumbar vertebra, the dorsal
attachment of the adult human mesentery of the jejuno-ileum extends
somewhat obliquely caudad and to the right to terminate in the right
iliac fossa at the ileo-colic junction.

Returning to the conditions presented by the cat's intestines to obtain
an explanation of this line of fixation we must recall the fact that in
the peritoneum included within the limits of the umbilical loop, after
differentiation of small and large intestine, but before rotation, we
have both the elements of the mesentery of the small intestines and of
the ascending and transverse mesocolon combined (Fig. 141). For it will
be seen that this membrane carries at this time vessels both to the
jejuno-ileum and to the segments of the large intestine (caecum,
ascending and transverse colon). This fact will be at once recognized if
the cat's intestines are arranged to correspond to the primitive
condition (Fig. 136) and the mesentery examined.

After rotation and differentiation of the colic segments and after the
adhesion of the ascending and descending colon in man, the course of the
main trunk of the superior mesenteric artery passes, after crossing the
third portion of the duodenum, down and to the right to terminate near
the ileo-colic junction by anastomosis with its ileo-colic branch. The
adhesion of the right and left mesocola to the dorsal parietal
peritoneum proceeds mesad as far as this line, leaving free the
mesentery of the small intestines, which contains the vasa intestini
tenuis derived from the left side of the main vessel. The secondary line
of attachment of the mesentery to the abdominal background is therefore
along this line. To obtain a clear idea of these processes of
development in man assume that in the cat, after rotation and
establishment of the three divisions of the colon, the two vertical
(ascending and descending) mesocola become adherent to the dorsal
parietal peritoneum, leaving the mesentery of the small intestine free.

Fig. 159 illustrates schematically the area of mesocolic adhesion in the
human subject after complete rotation, and the line of the mesentery of
jejuno-ileum.

[Illustration: FIG. 159.--Schematic figure to show lines of mesocolic
adhesion, formation of root of transverse mesocolon and root of
mesentery of jejuno-ileum in human subject.]

Fixation of the ascending and descending cola and of their mesocola
proceeds cephalad as far as the line _AB_, which thereby constitutes the
root of the free transverse mesocolon.

The secondary parietal peritoneum derived from the ventral layer of the
ascending mesocolon covers the lower and inner portion of the ventral
surface of the right kidney, the infra-colic division of the descending
and the dextro-mesenteric segment of the transverse duodenum, while
along the root of the jejuno-ileal mesentery it becomes continuous with
the right layer of that membrane. The secondary parietal peritoneum
derived from the ventral layer of the descending colon covers the lower
part of the ventral surface of the left kidney and the
sinistro-mesenteric segment of the transverse duodenum and becomes
continuous along the mesenteric radix with the left layer of the
jejuno-ileal mesentery.

Caudad the adhesion of the descending colon and mesocolon to the
parietal peritoneum proceeds only to the point _C_, following the dotted
line mesad and resulting in the formation of the free mesocolon of the
sigmoid flexure.

=Resume of the Adult Arrangement of the Human Peritoneum in the Lower
Compartment of the Abdomen, Below the Level of the Transverse Colon and
Mesocolon.=--We should now consider the arrangement of the human
peritoneum in the adult below the dorsal attachment of the transverse
mesocolon in the light of the embryological and comparative anatomical
facts just stated. In doing this it will be advisable to study both the
actual conditions encountered and their significance in the sense of
determining the derivation of the peritoneal layers from the primitive
dorsal mesentery. Open the abdominal cavity in the usual manner by a
cruciform incision.

Turn the great omentum up on the chest wall, exposing the underlying
intestines. This manipulation, as already stated, will cause the omentum
to carry the transverse colon with it, on account of the adhesion, in
the adult, of the gut to the dorsal layer of the omentum. Hence the
cephalic or upper layer of the transverse mesocolon will not be seen at
this stage because the omental adhesion just referred to prevents us
from passing between the greater curvature of the stomach and the
transverse colon without tearing peritoneal layers. It will, however, be
possible to trace on the right side the duodenum from the pylorus down
ventrad of the right kidney until the descending portion disappears
behind the hepatic flexure of the colon. With the omentum and transverse
mesocolon turned up, as stated, and the transverse mesocolon put upon
the stretch, it will be seen that the abdominal space now overlooked is
bounded cephalad by the lower layer of the transverse mesocolon and its
attachment to the dorsal abdominal wall. The lateral limits of the space
are given by the ascending and descending colon respectively. The
attachment of the mesentery of the small intestine to the oblique line
extending from the left of the vertebral column at about the level of
the second lumbar vertebra to the right iliac fossa subdivides the
entire space into a secondary right and left compartment.

Begin by following the caudal layer of the transverse mesocolon dorsad
on the right side. In the angle between ascending and transverse colon
(hepatic flexure) pressure will locate the caudal portion of the ventral
surface of the right kidney. Remember that the peritoneum touched in
these procedures appears in the adult as parietal prerenal peritoneum,
but that in reality it is the left leaf of the originally free ascending
mesocolon, whereas the original right leaf of this membrane and the
primitive parietal peritoneum have, by adhesion of their serous
surfaces, been converted into the loose subserous connective tissue
covering the ventral aspect of the kidney beneath what now appears as
parietal peritoneum.

Mesad of the resistance offered to the finger by the right kidney the
caudal (infra-colic) portion of the descending duodenum and the angle of
transition between it and the third or transverse portion will be found,
invested in the same way by secondary (mesocolic) parietal peritoneum.
It will be seen, especially if the duodenum is injected or inflated,
that the hepatic flexure of the colon lies ventrad of the vertical
descending second portion of the duodenum, so that one part of this
intestine is situated cephalad the other caudad of the colon. (Supra-
and infra-colic segments of descending duodenum.)

Individual differences are observed in the area of colic attachment to
the duodenum. Usually the two intestines are in contact with each other
and adherent over a considerable surface. Exceptionally the transverse
mesocolon extends across to the right so as to include the hepatic
flexure. In this latter case the uncovered non-peritoneal surface of the
descending duodenum is small, represented by the interval between the
layers of the transverse mesocolon, and the hepatic flexure is then not
directly adherent to the gut.

If we now trace the transverse duodenum from right to left we will
encounter the right layer of the root of the jejuno-ileal mesentery. The
caudal layer of the transverse mesocolon, the right leaf of the
mesentery and the secondary parietal peritoneum investing the ventral
surface of the transverse duodenum all meet at this point. Surround the
mesentery of the free small intestine with the fingers of one hand so
that the entire mass of intestinal coils can be swung alternately from
side to side.

Turning them over to the left, as already stated, the proximal portion
of the transverse duodenum can be traced from right to left as far as
the root of the mesentery. Here the peritoneum investing the ventral
surface of the duodenum becomes continuous with the right leaf of the
mesentery. Now swing the whole mass of small intestines over to the
right, exposing the parietal peritoneum in the space to the left of the
vertebral column, between the attachment of the mesentery to the median
side, the root of transverse mesocolon cephalad and the descending colon
to the left. Remember that the same significance attaches to this
secondary parietal peritoneum as on the right side. It appears in the
adult as parietal peritoneum, but is in its derivation the original
right leaf of the descending mesocolon. Close to the root of the
mesentery the continuation from the right side of the transverse
duodenum will be seen, crossing the median line from right to left
ventrad of aorta and vertebral column and usually turning cephalad on
the left side of the lumbar vertebrae, as the fourth or ascending
duodenum, to reach the caudal surface of the transverse mesocolon near
its attachment, where the gut turns ventrad to form the duodeno-jejunal
angle and become continuous with the free small intestine.

From the fact that the transverse duodenum is thus seen on each side of
the root of the mesentery it will be recalled that after rotation of the
primitive intestine the superior mesenteric artery crosses the
transverse portion of the duodenum to reach its distribution between
the leaves of the mesentery. Hence this portion of the small intestine
consists of a dextro- and sinistro-mesenteric segment. This intersection
of mesentery and duodenum marks the site of the primitive duodeno-colic
isthmus through which the superior mesenteric artery passed to reach its
distribution to the gut composing the embryonic umbilical loop.

To the left of the ascending duodenum a portion of the caudal surface of
the pancreas will be seen, covered by the continuation of the caudal
leaf of the transverse mesocolon into the parietal peritoneum. The
consideration of this relation of peritoneum and pancreas will
profitably be deferred until we have studied the developmental changes
in the region of the dorsal mesogastrium and great omentum.

In the angle between termination of the transverse colon and proximal
part of descending colon (splenic flexure) the caudal part of the
ventral surface of the left kidney will be felt. The disposition of the
peritoneum and its significance is the same as on the right side.
Inasmuch as we have already seen that the secondary parietal peritoneum
covering the dorsal abdominal wall on each side of the small intestine's
mesenteric attachment is derived from the primitive ascending and
descending mesocolon, it will be readily understood why the blood
vessels supplying the ascending and descending colon (arteria
ileo-colica, a. colica dextra, a. colica sinistra) are placed _behind_
the parietal peritoneum, while the colica media, supplying the
transverse colon, runs between the layers of the transverse mesocolon.
Originally the same condition obtained for the two vertical colic
segments, but with the anchoring of these portions of the large
intestine and the adhesion of their mesocola to the parietal peritoneum
the blood vessels which formerly ran between the two layers of the
membrane, as long as it remained free, now appear as retroperitoneal
vessels placed beneath the parietal peritoneum derived secondarily from
the mesocola.

This fact must be borne in mind in studying the arrangement of certain
folds and fossae of the parietal peritoneum which are now to be
considered.

=Duodenal Fossae. Fossa of Treitz and Retro-peritoneal Hernia.=--The
peritoneal cavity of the cat can be used to great advantage in order to
obtain a clear idea of the formation of these folds and fossae, whose
relation to the so-called "retro-peritoneal hernia" has led to an
exaggerated elaboration of minute detail and a somewhat puzzling
terminology in human descriptive anatomy.

=Directions for Examining the Folds and the Formation of the
Duodeno-jejunal Fossa in the Cat.=--Turn the omentum and the coils of
the small intestine cephalad out of the abdomen until they rest upon the
ventral thoracic wall. Press the large intestine over to the left side,
putting the mesocolon on the stretch until the parts are arranged as
shown in Fig. 160. The loop of the duodenum with the head portion of the
pancreas will be seen caudad of the liver and ventrad of the right
kidney. A well-marked peritoneal fold, somewhat sickle-shaped, with the
concavity of the free edge directed caudad and to the right, will be
seen extending from the convex border of the duodenum, directly opposite
the mesenteric or attached margin, to the right leaf of the mesocolon.
This fold indicates the beginning adhesion of the duodenum to the
mesocolic peritoneum, the first step toward the subsequent complete
fixation of the gut as it is found in man.

[Illustration: FIG. 160.--Abdominal cavity of cat, with intestines
everted and elevated to show duodenal fold. (From a fresh dissection.)]

[Illustration: FIG. 161.--Abdominal viscera of _Nasua rufa_, brown
coaiti. (From a fresh dissection.)]

Fig. 161 shows the abdominal cavity of Nasua rufa, the brown
Coati-mundi, a South American arctoid carnivore, with the intestines
everted and turned to the left side. In this animal the large intestine
is very short, there is no caecum, the ileo-colic junction is only marked
on the surface by a pyloric-like constriction of the tube and in the
interior by the projection of a ring-valve (Fig. 408).

The duodenal fold is very well developed, passing between the convex
surface of the duodenal loop and the adjacent right leaf of the short
mesocolon.

In Primates, in which complete rotation of the intestine, on the plan of
the human development, takes place, still further and more extensive
agglutination of the serous surface of the duodenum to the peritoneum of
the mesocolon occurs. Fig. 162 shows the condition in _Hapale
vulgaris_, one of the marmosets. The ascending and descending mesocola
and the mesoduodenum of this animal are still free, but the surface of
the duodenum has become fastened to the opposed mesocolon. With fixation
of the hepatic flexure and adhesion of the ascending colon, such as
occurs in man, the duodenum is carried dorsad against the ventral
surface of the right kidney, and now anchoring of the duodenum, by
obliteration of the mesoduodenum and adhesion to the prerenal parietal
peritoneum, takes place as already detailed above. To return now to the
formation of the duodeno-jejunal fossa by means of this fold, as
illustrated in the cat. Perform the manipulations already described in
rotation of the intestine. The appearance of the parts then will be as
shown in Fig. 163. The large intestine is drawn over so as to represent
the human ascending and transverse colon in one segment, the descending
colon in the other, and the mesocolon appears correspondingly as
transverse and descending. In other words the cat's intestines as
arranged in the figure would represent the stage in the human
development in which caecum and beginning of large intestine are still
subhepatic in position ventrad of the right kidney, before
differentiation of ascending and transverse colon by descent of caecum
into right iliac fossa.

[Illustration: FIG. 162.--Abdominal viscera of _Hapale vulgaris_, the
marmoset. (Columbia University Museum, No. 1818.)]

[Illustration: FIG. 163.--Abdominal viscera of cat; intestines rotated
and turned to the right to show duodenal fold. (From a fresh
dissection.)]

In the human subject, as we have seen, the transverse mesocolon obtains
a secondary attachment to the background of the abdominal cavity, its
caudal surface remaining free.

The descending mesocolon turns its original right leaf ventrad, its left
leaf dorsad, and the latter adheres to the primitive parietal peritoneum
covering the left lumbar region and ventral surface of left kidney. This
area of adhesion extends up to and usually involves the dorsal surface
of the descending colon, anchoring the same in the left lumbar region,
down to the point where the sigmoid flexure begins and where the
original mesocolon again appears free.

In the cat, therefore, with the intestines arranged to correspond to the
course of the human large intestine after rotation has been
accomplished, the lines representing the peritoneal human adhesions
should be fixed, as shown in the schema, Fig. 159: AB, line of secondary
attachment after rotation resulting in the formation of the "root" of a
free transverse mesocolon. BC, line of limit of secondary adhesion to
the original parietal peritoneum involving the entire left (now dorsal)
layer of the descending mesocolon and the dorsal surface of the
descending colon, resulting in the fixation of the latter part of the
large intestine.

This establishes, as already stated, a secondary parietal peritoneal
surface in the left lumbar region derived from the original right leaf
of the descending mesocolon. Inasmuch as the inferior mesenteric vessels
originally passed to the descending colon between the layers of the
mesocolon they will now apparently be placed beneath the (secondary)
parietal peritoneum of the left lumbar region.

If now the duodenal fold in the cat be examined after rotation of the
intestine it will be found presenting the original relations (Figs. 160
and 163), viz., passing from the convex margin of that portion of the
duodenal loop which would correspond to the human fourth or ascending
portion, to the original right layer of the mesocolon, which in man
becomes secondarily converted into the parietal peritoneum of the left
lumbar region. Hence the connections of the fold are as follows:

_On the right_: ventral surface of the ascending duodenum.

_On the left_: right layer of mesocolon (secondary lumbar parietal
peritoneum in the adult human subject).

_Cephalad_ it abuts against the caudal layer of the transverse mesocolon
along the line which would correspond to the root of the mesocolon in
the adult human subject.

The concave _caudal_ edge is free and bounds the entrance into a fossa,
the "superior duodenal fossa" of anthropotomy. This fossa opens caudad
and extends cephalad to the root of the transverse mesocolon. The
ventral and left wall of the fossa is formed by the fold in question,
its background by the mesocolon (right leaf); to the right the left
circumference of the ascending duodenum enters into the formation of
the fossa, and its fundus is formed by the confluence of the fold and of
the caudal layer of the transverse mesocolon. The inferior mesenteric
vessels are found near the left margin of the entrance into the fossa.

Fig. 164 shows the appearance of the fold in _Nasua rufa_ after rotation
of the intestine. The short course of the large intestine in this
animal, and the consequent reduction of the mesocolon, brings the fold
much below the level which it occupies in the cat.

[Illustration: FIG. 164.--Abdominal viscera of _Nasua rufa_, the brown
coaiti, showing the position of the duodenal fold after rotation of the
intestine. (From a fresh dissection.)]

If we now look for the corresponding structures in man we will find
certain modifications depending chiefly upon still closer adhesion
between duodenum and the mesocolon which is destined to become the left
parietal peritoneum after anchoring of the descending colon. We have
already encountered an example of such closer connection in the marmoset
shown in Fig. 162.

In all cases the "superior duodenal" fold, corresponding to the fold
just encountered in the cat, is the original condition, and the duodenal
fossa consequently opens caudad. In many instances this will be the only
fold and fossa encountered in the adult human subject. In other
instances more extensive duodeno-mesocolic adhesions result in the
addition of an "inferior fold," bounding a fossa the entrance into which
is directed cephalad toward the transverse mesocolon. Such a condition
is seen in Fig. 165 taken from a foetus at term. The duodenal fossa in
this case is bounded by an "upper" and "lower" duodenal fold continuous
with each other on the left side, but separated on the right at their
attachment to the duodenum. It will be seen that the inferior mesenteric
vein runs in the left margin of the fold, following along the left
border of the entrance into the fossa. A segment of the colica sinistra
artery may occupy the same position. This position of the vein, or
artery, or of both vessels, is not the cause leading to the formation of
the duodenal fossa, but is more or less accidental and variable. In many
cases the vessels run at some distance from the folds bounding the
fossa.

[Illustration: FIG. 165.--Abdominal viscera of human foetus at term,
arranged to show duodenal folds and fossa. The jejuno-ileum, ascending
and transverse colon have been removed. (Columbia University Museum, No.
1819.)]

In some subjects the "inferior" fold is the only one found, and the only
duodenal fossa then encountered looks cephalad. This condition, when
associated with the course of the inferior mesenteric vessels in the
free edge of the fold, constituted the classical "fossa
duodeno-jejunalis" of Treitz, and is described as "Treitz's fossa."

Fig. 166 shows the condition in which only a small inferior fold
attaches itself to the termination of the transverse duodenum. There is
practically an entire absence of duodenal or duodeno-jejunal folds and
fossae. The inferior mesenteric vessels course under cover of the
mesocolic secondary parietal peritoneum, but do not produce a fold.

[Illustration: FIG. 166.--Abdominal viscera of human foetus at term.
(Columbia University Museum, No. 1820.)]

[Illustration: FIG. 167.--Abdominal viscera of adult human subject,
showing duodenal folds and fossa. (From a fresh dissection.)]

Fig. 167, from an adult human subject, illustrates the further
development of the fossa from the foetal conditions shown in Fig. 165.
The well-marked duodenal fossa is bounded by a superior and inferior
duodenal fold, uniting laterally in a crescentic margin containing a
segment of the inferior mesenteric vein and colica sinistra artery. The
lower division of the peritoneal recess thus produced corresponds to the
typical (vascular) "fossa of Treitz." Mesally the projection of the
fourth portion of the duodenum bounds the fossa.

In Fig. 168, also taken from an adult human subject, an extensive
duodenal recess is bounded in the same way by a superior and inferior
duodenal fold. In the interior of the fossa a third duodenal
reduplication of the peritoneum ("intermediate duodenal fold") is seen,
as is also the trunk of the inferior mesenteric vein, while the main
trunk of the colica sinistra artery courses laterally behind the
secondary mesocolic parietal peritoneum near the margin of the
descending colon.

[Illustration: FIG. 168.--Abdominal viscera of adult human subject,
showing duodenal folds and fossa. (From a fresh dissection.)]

It will be seen that the freedom of the ascending or fourth portion of
the duodenum depends largely upon the disposition and extent of these
folds. Inasmuch as they are the product of varying degrees of adhesion
of this segment of the intestine they are subject to great individual
variations and have given rise to an unnecessary and complicated
classification of the duodenal folds and fossae. The close relation
maintained between the duodeno-jejunal angle and the caudal layer of the
transverse mesocolon near its root at times leads to the production of
a peritoneal fold connecting this membrane with the duodeno-jejunal
knuckle of intestine (duodeno-jejunal or mesocolic fold) and may result
in the formation of a duodeno-jejunal or mesocolic fossa of the
peritoneum. An instance of this fold is seen in Fig. 168.

The importance of the duodenal fossae, and of similar peritoneal recesses
in other parts of the abdominal cavity, is founded on the fact that by
gradual enlargement they may lodge the greater part of the movable small
intestine in their interior, leading to the formation of intra- or
retro-peritoneal herniae.[3]

[3] For full details of the anatomical and pathological conditions
involved consult B. G. A. Moynihan "On Retro-peritoneal Hernia"--London,
1899.

=Fossa Intersigmoidea.=--A second peritoneal pocket or fossa is
encountered in the region of the sigmoid flexure and its mesocolon. The
formation of this fossa is closely associated with the adult disposition
of the sigmoid mesocolon as part of the original primitive vertical
dorsal mesentery. In the typical arrangement of the parts the sigmoid or
omega loop of the large intestine has a free mesocolon. The adhesion of
the descending mesocolon to the parietal peritoneum usually ceases along
a line drawn horizontally from the lateral margin of the left psoas at a
level with the crest of the ilium to the medial side of the iliac
vessels. This line, along which the mesocolon ceases to be adherent to
the parietal peritoneum, joins the attachment of the distal portion of
the sigmoid mesocolon, which partially retains its primitive vertical
origin to the dorsal midline, at a right angle. This angle is the site
of the _intersigmoid fossa_, the entrance into which is seen usually as
a round opening of variable size on elevating the sigmoid flexure and
putting its mesocolon on the stretch. Fig. 159 shows the area of
adhesion between the primitive descending mesocolon and the parietal
peritoneum (from C mesad) which results in the formation of a free
mesocolon for the sigmoid flexure. Frequently in the angle formed by the
horizontal and vertical line of attachment of the sigmoid mesocolon a
non-adherent strip of the primitive mesocolon roofs in a more or less
extensive intersigmoid fossa, whose fundus is directed upwards and
inwards.

=Caecum, Appendix and Ileo-colic Junction.=--Several peritoneal fossae and
folds are found in the ileo-colic region in connection with the caecum,
appendix and termination of the ileum. The practical importance of this
portion of the intestinal tract and the great morphological interest
which attaches to the same make it worth while to consider its anatomy
in a separate chapter.




PART II.

ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT OF THE ABDOMEN.


We have already seen that the transverse colon and mesocolon effect a
general division of the adult human abdominal cavity into a cephalic
supra-colic compartment, situated between the diaphragm and the level of
the transverse colon and mesocolon, comprising in general the
hypochondriac and epigastric regions, and a larger caudal infra-colic
space which includes the entire rest of the abdominal cavity and is
continued caudad into the pelvic cavity. The arrangement of the
peritoneum and viscera in this latter space has just been considered.
The fact will be recalled that the second or descending portion of the
duodenum, passing dorsad of the hepatic colic flexure, forms so to speak
the visceral connection between the portions of the alimentary tube
situated in the supra-colic compartment and those situated in the
infra-colic space. The fixation of this segment of the duodenum and its
consequent secondary retroperitoneal position in the adult human subject
masks this continuity of the alimentary canal to a certain extent so
that it requires more than a superficial examination in order to trace
correctly the course of the duodenum from the pylorus to the
duodeno-jejunal angle, dorsad of the colon, root of transverse mesocolon
and mesentery, and under cover of the secondary parietal peritoneum.

We have now to turn our attention to the viscera contained in the
cephalic or supra-colic compartment of the abdomen and to consider the
disposition of the serous membrane investing them and connecting them
with each other and with the abdominal parietes.

The visceral contents of the supra-colic compartment comprise the
liver, pancreas, spleen, stomach and the proximal portion of the
duodenum, including the hepatic angle and the supra-colic part of the
descending duodenum. Less directly the cephalic portions of the right
and left kidney and the corresponding suprarenal capsules belong to this
visceral group.

In this region of the abdomen we meet with the most extensive
modifications of the primitive dorsal peritoneal membrane, producing
conditions which, considered without reference to development and
comparative anatomy, are complex and difficult of comprehension. These
changes lead to the formation of the so-called "lesser sac," a term
which in some respects is unfortunate as it implies a more complete
degree of separation from the general peritoneal cavity or "greater sac"
than actually exists.

In order to clearly understand the adult arrangement of the peritoneum
in this region it is advisable to consider the subject in two distinct
subdivisions, dealing successively with the two cardinal facts which
contribute to effect the change from the simple primitive to the
complicated adult condition.

These two main elements are:

1. Developmental changes in the position of the stomach, alterations in
the disposition of the proximal part of the primitive dorsal mesentery
attached to the stomach, and the development of pancreas and spleen in
connection with this membrane.

2. The development of the liver and the successive stages in the
production of the final adult vascular and serous relations of this
organ.

=1. Stomach and Dorsal Mesogastrium.=--We have already considered the
early stages in the differentiation of the stomach from the primitive
intestinal tube of uniform caliber (p. 40). It will be recalled that the
stomach at a certain period, while it already presents the main
structural features familiar in the adult organ, occupies a vertical
position in the abdominal cavity, turning its concave margin (lesser
curvature) ventrad, while the convex dorsal border (greater curvature)
is directed toward the vertebral column, being attached to the same by
the layers of the proximal part of the primitive dorsal mesentery. At
this time the stomach presents right and left surfaces, and the
oesophageal entrance is at the highest or cephalic point of the organ,
while the pyloric transition to the small intestine occupies the distal
caudal extremity.

The primitive dorsal mesentery, as already stated, passes as a thin
double-layered membrane between the ventral surface of the vertebral
column and the dorsal border of the stomach, which, as we will presently
see, becomes during the later stages of development the caudal (lower)
margin or greater curvature.

It will be seen that the embryonic differentiation of the intestinal
tract into successive segments justifies the application of a
terminology based on this differentiation to the corresponding portions
of the primitive common dorsal mesentery.

Thus the proximal portion extending between the vertebral column and the
dorsal border or greater curvature of the stomach becomes the
_mesogastrium_; we differentiate this portion still further as the
"_dorsal mesogastrium_" to distinguish it from a "_ventral
mesogastrium_" which we will presently encounter in considering the
development of the liver and the connected peritoneum.

In the same way the section of the primitive common dorsal mesentery
attached to the duodenal loop becomes the _mesoduodenum_, that connected
with the mobile part of the small intestine (jejuno-ileum) the
_mesentery_ proper, while the portion passing to the colon forms the
_mesocolon_, to be subsequently still further subdivided, after the
different segments of the large intestine have become mapped out, as the
_ascending_, _transverse_ and _descending mesocolon_, the
_mesosigmoidea_ and the _mesorectum_.

In tracing the development of the adult human peritoneum it is well to
consider certain stages, which we will find illustrated by the permanent
conditions presented by some of the lower vertebrates:

These stages comprise:

(_a_) Changes in the position of the stomach.

(_b_) Changes in the direction and extent of the dorsal mesogastrium.

(_c_) Development of the pancreas and spleen in connection with the
mesogastrium.


A. Changes in the Position of the Stomach.

The primitive position of the organ above outlined (p. 41) is changed
during the course of further development by a twofold rotation.

1. The primitive vertical position, in which the oesophageal entrance
occupies the highest cephalic extremity, while the pyloric opening is at
the opposite caudal end, is exchanged for one directed more
transversely, approximating the two gastric orifices to the same
horizontal level. In human embryos of 13.9 mm. the fundus has already
descended, the pylorus moving cephalad and to the right, while the
cardia becomes shifted more to the left. At the same time the greater
growth and prominence of the convex border or greater curvature becomes
marked in comparison with the relatively short extent of the opposite
margin or lesser curvature.

2. Coincident with this change in position is a rotation around the
vertical axis, by means of which the original left side of the stomach
is turned ventrad, becoming the ventral or "anterior" surface, while the
original right surface of the organ now looks dorsad toward the
vertebral column, becoming the dorsal or "posterior" surface of human
anatomy. The oesophageal or cephalic end is placed to the left of the
median line, while the caudal or pyloric end is situated on the right
side (Figs. 169 and 170).

[Illustration: FIGS. 169, 170.--Two front views of the entodermal canal.
(Minot, after His.)]

[Illustration: FIG. 169.--Embryo Sch. 1 of His.]

[Illustration: FIG. 170.--Embryo Sch. 2 of His.]

The original ventral border, now the "lesser curvature" or "upper
border," looks cephalad and to the right, toward the caudal surface of
the liver, while the original dorsal border, as the "greater curvature"
or "lower border" is directed in the main caudad and to the left.

The prominence of this border is still further increased by the greater
development of the stomach to the left of the oesophageal entrance
resulting in the formation of the "fundus" or "great cul-de-sac."

This rotation of the stomach explains the asymmetrical position of the
vagus nerve in the adult, the left side of the embryonic stomach,
innervated by the left vagus, becoming the "anterior" surface of adult
descriptive anatomy and _vice versa_.

It will be readily appreciated that a comparatively flat organ like the
stomach, will, as long as it occupies a sagittal position, with right
and left surfaces, help to divide the upper part of the abdominal cavity
to a certain extent into a right and left half, even if the peritoneal
connections of the organ are left out of consideration. As soon,
however, as the above-described changes in position take place and the
surfaces of the stomach are directed ventrad and dorsad, the relative
arrangement and extent of this right and left abdominal space becomes
altered by the different disposition of the septum, _i. e._, the
stomach. The original right side of the organ is now directed dorsad,
and the rotation of the organ has created a space between this dorsal or
"posterior" surface of the stomach and the background of the abdominal
cavity, which is the inception of the "lesser peritoneal cavity" or
retrogastric space. We will find that this space becomes well defined
and circumscribed by the peritoneal connections of the stomach, but we
will realize, even at this stage, that the _dorsal_ surface of the
stomach will form a part of the general _ventral_ wall of the lesser
peritoneal space.

On the other hand, the partial division of the abdomen into a right and
left half, effected by the stomach in its primitive sagittal position,
disappears after rotation of the organ. We now pass uninterruptedly from
left to right across the ventral (original _left_) surface of the
stomach.


B. Changes in the Direction and Extent of the Dorsal Mesogastrium.

The effects of the altered position of the stomach on the disposition of
the abdominal space have just been considered in relation to the organ
itself, without reference to its natural connections with the parietes
and with adjacent viscera. Their true significance and their influence
on the adult anatomical arrangement of the abdomen is, however, only
appreciated when the changes in the arrangement of the peritoneal
membrane which they involve, are taken into account.

The dorsal mesogastrium changes more than any other portion of the
peritoneum in the course of development. It not only becomes displaced
and altered in direction by the rotation of the stomach, but in addition
it grows so extensively that it finally hangs down like an apron over
the entire mass of small intestines, forming the great omentum.

If we begin with the primitive disposition of the sagittal stomach and
dorsal mesogastrium shown in Fig. 171 it will be observed that both
structures together actually divide the dorsal portion of the abdominal
cavity into symmetrical right and left halves (Fig. 172).

[Illustration: FIG. 171.--Schematic representation of dorsal
mesogastrium before rotation of stomach.]

[Illustration: FIG. 172.--Semi-diagrammatic representation of
mesogastrium in human embryo of the sixth week. (Kollmann.)]

After rotation of the stomach (Fig. 173) the mesogastrium loses its
original sagittal direction. It follows the altered position of the
original dorsal border of the stomach, which has now become the caudal
margin or "greater curvature," by turning caudad and to the left, being
at the same time considerably elongated. This occurs during the second
month. Hence the dorsal mesogastrium, after leaving the vertebral
column, turns ventrad and to the left to reach its gastric attachment
along the greater curvature. This is the first indication of the
formation of the great omental or epiploic bursa.

[Illustration: FIGS. 173-175.--Schema of dorsal mesogastrium after
rotation of stomach.]

[Illustration: FIG. 173.--Early stage.]

[Illustration: FIG. 174.--Later stage, extension of mesogastrium beyond
stomach to left, with fundus of blind retrogastric pouch thus created at
X.]

[Illustration: FIG. 175.--After adhesion over area of dotted line
between dorsal mesogastrium and primitive parietal peritoneum. Secondary
line of transition from dorsal mesogastrium to parietal peritoneum at
X.]

The stomach is here considered as developing in situ and as influencing
by its growth and change of position the arrangement and direction of
the peritoneal layers with which it is connected. As a matter of fact it
is well to note that the stomach at first lies above the primitive
diaphragm or septum transversum, migrating, however, at an early period
into the subhepatic abdominal position. This migration produces a
corresponding increase in the length of the oesophagus (Fig. 34) and the
stomach, in consequence of this change in position, acquires its ventral
and dorsal mesogastrium. For the purpose of explaining the adult
peritoneal relations of the organ it is, however, more
convenient to regard the stomach as an abdominal organ from the
beginning and to deal with the subsequent changes in position from this
standpoint. The inaccuracy is slight and renders the comprehension of
the succeeding stages easier.

It will be noticed (Fig. 173) that the rudimentary retro-gastric space
or "lesser peritoneal sac" is bounded ventrally by the dorsal (the
primitive _right_) surface of the stomach, while its dorsal boundary is
furnished by the ventral (originally _right_) layer of the dorsal
mesogastrium.

In the primitive condition, therefore, dorsal mesogastrium and stomach
form together a straight line sagittal in direction and placed in the
median plane of the body. As the result of the developmental changes
above outlined this straight line becomes bent at the point where the
mesogastrium reaches the stomach (Fig. 173, x). The two component
elements of the line (stomach and mesogastrium) hinge on each other
here, and the angle which they form opens to the right.

The changes which are to be observed in the later stages depend
principally upon a peculiar feature characteristic of the development of
the dorsal mesogastrium. This feature consists in the extreme redundancy
of the membrane which grows out of proportion to the requirements of its
visceral connections, and to a certain extent becomes independent of the
direct mechanical purpose of carrying blood vessels to the viscera.
Hence in a transverse section at this period (Figs. 174 and 175) the
mesogastrium no longer passes in a direct line between its points of
attachment, viz. the greater curvature of the stomach and the vertebral
column, but extends beyond the stomach to the left. We will appreciate
the significance of this extensive growth of the mesogastrium especially
in considering the development of the spleen and pancreas. For the
present it will suffice to note (Figs. 174 and 175) that the growth has
carried the mesogastrium well to the left of the stomach, consequently
the retrogastric space is now bounded toward the left by the bend which
the original right leaf of the primitive sagittal mesogastrium takes in
order to reach its gastric attachment. The retrogastric space therefore
terminates toward the left in a blind pocket formed by this
reduplication of the mesogastrium.

One more factor is to be taken into consideration, namely the tendency,
already noted, of peritoneal surfaces to become adherent to each other.
Such adhesion involves the apposed surfaces of the mesogastrium and of
the primitive parietal peritoneum to the left of the vertebral column.
The dorsal (original _left_) layer of the mesogastrium adheres to the
parietal peritoneum covering the left side of the abdominal background
and the cephalic portion of the ventral surface of the left kidney up to
the end of the blind pouch which forms the extreme left limit of the
retrogastric space. Hence, after this process of adhesion is completed,
the dorsal wall of the retrogastric space is lined by secondary parietal
peritoneum covering the left kidney (original right leaf of primitive
mesogastrium) (Fig. 175). We obtain (Fig. 175 at x) an apparent
continuity of the parietal peritoneum with that portion of the
mesogastrium which, derived from the original left layer of the
membrane, appears now to extend, as the ventral one of two layers,
between the stomach and the abdominal parietes near the lateral border
of the left kidney. (Primitive gastro-splenic omentum.)

It should be remembered that the disposition of the peritoneum just
indicated is modified by the development of the pancreas and spleen,
both of which organs are intimately associated with the mesogastrium.
The foregoing statements and diagrams are therefore merely given for the
purpose of affording a general view of the extent, growth and changes of
the dorsal mesogastrium before proceeding to consider the development of
the pancreas and spleen in and from the membrane itself.

[Illustration: FIG. 176.--Schematic ventral view of stomach, duodenum,
and dorsal mesogastrium, after rotation of stomach and extension of
omental bursa caudad beyond greater curvature of stomach. The ventral
mesogastrium is detached along the lesser curvature.]

[Illustration: FIG. 177.--Semi-diagrammatic representation of peritoneal
membrane in human embryo. (After Kollmann.)]

In the view directly from in front the redundancy of the peritoneum
forming the mesogastrium is shown in Figs. 176 and 177. Just as the
membrane extends further to the left than required by its visceral
connection with the stomach, so the downward growth exceeds the demand
made by the rotation of the attached border (greater curvature) caudad
and to the left. The mesogastrium, forming, as it now does, the great
omentum, enlarges in descending toward the transverse colon (Fig. 177).
The bag thus formed can be distended with air in a foetus of from 8 to 9
cm. vertex-coccygeal measure, as shown in the figure. Consequently in
sagittal section the membrane is seen to extend caudad beyond the level
of the greater curvature, and must turn on itself and pass again
cephalad in order to reach the stomach (Fig. 178). By reason of this
excessive growth the limits of the primitive retrogastric space are
enlarged, not only toward the left, but more especially in the caudal
direction. The bend made by the mesogastrium in returning to the stomach
forms the blind extremity of a pouch which continues the retrogastric
space caudad beyond the stomach, and whose dorsal and ventral walls are
formed by the reduplicated mesogastrium. This pocket or pouch
constitutes the _omental_ or _epiploic bursa_ of the lesser peritoneal
cavity, for the great omentum is the direct product of this redundant
growth of the mesogastrium caudad. It will be observed that the great
omentum is made up of four peritoneal layers, the folding of the
double-layered mesogastrium naturally producing this result. The first
or ventral and the fourth or dorsal layer are derived from the original
left layer of the primitive sagittal mesogastrium; the intermediate
second and third layers, separated from each other at this stage by the
cavity of the omental bursa, are products of the primitive right leaf of
the mesogastrium. Since the entire retrogastric space with its
extensions becomes the "lesser cavity" of the human adult peritoneum, it
will be seen that its serous membrane is derived from the original right
leaf of the mesogastrium (second and third omental layers). After the
above-described adhesion of the mesogastrium to the parietal peritoneum
overlying the ventral surface of the left kidney, the membrane would be
traced in sagittal section (Fig. 179) from the dorsal surface of the
stomach caudad, lining the interior of the omental bursa (second layer)
to the turn or blind end of the pouch; thence cephalad as the third
omental layer, forming the dorsal wall of the epiploic bursa, to invest,
as secondary parietal peritoneum, the cephalic segment of the ventral
surface of the left kidney.

[Illustration: FIG. 178.--Schematic sagittal section through stomach and
dorsal mesogastrium, after rotation and formation of omental bursa.]

[Illustration: FIG. 179.--Schematic sagittal section through stomach and
dorsal mesogastrium after adhesion to prerenal parietal peritoneum.]


C. Development of Spleen and Pancreas in the Dorsal Mesogastrium and
Changes in the Disposition of the Great Omentum.

In order to obtain a correct conception of the adult human conditions it
is finally necessary to consider the development of the spleen and
pancreas in their connection with the dorsal mesogastrium and to note
the changes which are produced by adhesion of portions of the great
omentum to adjacent serous surfaces. It will be advisable to discuss
these subjects at first separately, and to subsequently combine all the
facts in an attempt to gain a correct impression of their share in
determining the disposition of the adult human peritoneum.

=1. Development of Spleen.=--The spleen develops from the mesoderm
between the layers of the dorsal mesogastrium, near its point of
accession to the greater curvature, in the region of the subsequent
fundus. It has therefore, like the stomach, originally free peritoneal
surfaces. After rotation of the stomach the organ lies between the two
layers of the membrane at the extreme left end of the retrogastric space
(Fig. 180).

[Illustration: FIG. 180.--Schematic transverse section of the abdomen,
showing early stage of development of spleen from extreme left end of
dorsal mesogastric pouch.]

=Vascular Connections.=--The splenic artery accedes to the mesal surface
of the spleen from the vessel which originally passed directly to the
dorsal border (subsequent greater curvature) of the stomach, between the
layers of the mesogastrium.

With the further growth of the spleen the segment of this vessel
situated between its origin from the coeliac axis and the hilum of the
spleen becomes relatively larger, forming the adult splenic artery,
while the continuation of the original vessel to the greater curvature
of the stomach appears now as a branch of the splenic artery, viz., the
arteria gastro-epiploica sinistra.

Through the development of the spleen the dorsal mesogastrium has been
subdivided into a proximal longer vertebro-splenic, and a distal shorter
gastro-splenic segment. The former, as we have seen, loses its identity
as a free membrane in the human adult, by fusing with the parietal
peritoneum investing the ventral surface of the left kidney. Hence,
after this adhesion has taken place, the splenic artery courses from the
coeliac axis to the spleen behind peritoneum which functions as part of
the general parietal membrane, but which is derived from the original
right leaf of the proximal vertebro-splenic segment of the primitive
mesogastrium (Fig. 181). On the other hand the distal segment of this
membrane, beyond the spleen, remains free, carrying, as the
gastro-splenic omentum, the left gastro-epiploic artery between its
layers from the splenic artery to the greater curvature of the stomach.

[Illustration: FIG. 181.--Schematic transverse section of the abdomen,
showing later stage of development of spleen and arrangement of
peritoneum after adhesion of dorsal layer of mesogastrium and primitive
prerenal parietal peritoneum.]

The lateral limit of the area of adhesion between mesogastrium and
parietal peritoneum is situated along the lateral border of the left
kidney. Hence, in the final condition of the parts, the main splenic
vessels at the hilum are situated between two peritoneal layers of which
the ventral (Fig. 181) appears as the parietal peritoneum forming the
dorsal wall of the retro-gastric space, while the dorsal layer (Fig.
181) forms a reflection from the mesal surface of the spleen, along the
dorsal margin of the hilum, to the adjacent lateral border of the left
kidney (lieno-renal ligament) and to the diaphragm. At this point of
adhesion subsequently firmer strands of connective tissue develop in the
serous reduplication forming the _ligamentum phrenico-lienale_ of
systematic anatomy. This process of adhesion takes place during the
second half of intra-uterine life. A connection with the colon, produced
by adhesion of the mesogastrium to the splenic flexure of the large
intestine, forms the adult _lig. colico-lienale_, while a similar
adhesion between great omentum, transverse mesocolon and phrenic
parietal peritoneum just caudad of the spleen, gives rise to the
_colico-phrenic_ or _costo-colic "supporting" ligament_ of the spleen.

On the other hand, the ventral one of the two layers constituting the
gastro-splenic omentum and including between them the left
gastro-epiploic artery, is formed by the distal part of the primitive
left layer of the mesogastrium, while the dorsal layer of the same fold
is the portion of the primitive right layer beyond the spleen, which
has not been converted into secondary parietal peritoneum, but forms now
part of the ventral wall of the lesser peritoneal sac between the spleen
and the stomach (Fig. 181) (lig. gastro-lienale). Since, therefore, the
gastro-splenic omentum is a specialized part of the fully-developed
dorsal mesogastrium, and since we have seen that the great omentum is
formed directly by the excessive growth of this membrane caudad, it is
not difficult to understand why in the adult human subject the ventral
layer of the gastro-splenic omentum is directly continuous with the
ventral layer of the great omentum along the greater curvature of the
stomach to which both are attached. The dorsal layer of the
gastro-splenic omentum would, in the same way, be continuous with the
second layer of the great omentum, lining the ventral wall of the
omental bursa, if it were not for the fact that in the adult adhesions
usually obliterate the cavity of the bursa.

Fig. 182 shows the stomach, left kidney, spleen and splenic flexure of
the colon hardened in situ and removed from the body of a two-year-old
child. The great omentum has been divided along the line of adherence to
the transverse colon.

[Illustration: FIG. 182.--Part of the abdominal viscera of child, two
years old, hardened _in situ_ and removed from body. The great omentum
has been detached along the line of the transverse colon. (Columbia
University, Study Collection.)]

[Illustration: FIG. 183.--The same preparation with the spleen removed,
showing lines of peritoneal reflection on mesial surface of the organ.]

In Fig. 183 the spleen has been removed from the preparation by division
of its peritoneal and vascular connections, and is shown in its mesal
aspect (gastric and renal surfaces, intermediate margin and hilum). It
will be seen that the peritoneal reflections are arranged in the form of
two concentric elliptical lines. The two ventral lines form the
gastro-splenic omentum and correspond to the reflection of the
peritoneum from spleen to left end of stomach carrying the gastric
branches derived from the splenic artery. The third line from before
backwards results from the division of the secondary parietal peritoneum
of the lesser sac, covering splenic artery, and ventral surface of
pancreas and derived from the dorsal mesogastrium; while the most dorsal
fourth line represents the divided reflection of the peritoneum from the
renal surface of spleen to lateral border of left kidney and diaphragm
(lig. lieno-renale).

Between the second and third lines of peritoneal reflection appears the
portion of the mesal surface of the spleen in contact with and invested
by the extreme left end of the lesser peritoneal sac.

Fig. 184, taken from an adult human subject with the viscera hardened in
situ, shows the left or splenic extension of the lesser peritoneal
cavity.

[Illustration: FIG. 184.--Upper abdominal viscera of adult human
subject, hardened _in situ_, with liver and colon removed and stomach
turned up. (Columbia University, Study Collection.)]

=2. Development of the Pancreas.=--The pancreatic gland is derived from
the hypoblast of the enteric tube. The secreting epithelium and that
lining the ducts of the adult gland is formed by budding and
proliferation of the intestinal epithelium. The gland develops primarily
from two outgrowths which are at first separate and distinct from each
other.

1. The proximal and dorsal bud grows directly from the hypoblast lining
the duodenum immediately beyond the pyloric junction.

In embryos of 8 mm. (four weeks) (Fig. 185) it appears as a small
spherical outgrowth connected by a slightly narrower stalk with the
epithelial intestinal tube.

[Illustration: FIG. 185.--Pancreatic and hepatic buds of human embryo of
four weeks. (Kollmann.)]

2. The distal and ventral outgrowth is separated from the preceding and
is from the beginning closely connected with the similar embryonic
outgrowth from the enteric tube which is to form the liver. This portion
of the pancreas is, strictly speaking, derived primarily from the
epithelium of the primitive hepatic duct and not directly from the
duodenum. This primary arrangement of the gland, being formed of two
main collections of budding hypoblastic cells, corresponds to the adult
system of the pancreatic excretory ducts. The proximal or dorsal
outgrowth furnishes that portion of the head of the gland whose
excretory system terminates in the _secondary pancreatic duct_ or _duct
of Santorini_, while the distal (ventral) outgrowth includes within its
area the termination of the principal pancreatic duct or _canal of
Wirsung_, which is closely connected with the end of the common
bile-duct at the intestinal opening common to both (Figs. 186-187). The
method of union of the two pancreatic outgrowths and their respective
share in building up the adult gland explains the usual adult
arrangement of the excretory system and its variations.

[Illustration: FIG. 186.--Pancreatic buds of human embryo of five weeks.
(Kollmann, after Hamburger.)]

[Illustration: FIG. 187.--Pancreatic buds of human embryo of six weeks.
(Kollmann, after Hamburger.)]

In the embryo of five weeks (Fig. 186) the two portions have grown in
length. The dorsal or proximal outgrowth, developing between the layers
of the mesoduodenum, is at this time the larger of the two, composed of
a number of glandular vesicles clustered around the stalk represented by
the parent duct.

The distal or ventral pancreatic growth, connected with the liver duct,
is as yet small and presents only a few vesicular appendages. The duct
of this portion empties in common with the hepatic duct into the
duodenum.

In embryos of the sixth to seventh week (Fig. 187), the two glandular
outgrowths have become connected with each other at a point which
corresponds exactly to the divergence of the duct of Santorini from the
main pancreatic duct (canal of Wirsung) in the adult gland (Fig. 188).

[Illustration: FIG. 188.--Human adult. Corrosion of pancreatic and
common bile-ducts: ventral view. (Columbia University Museum, No.
1712.)]

The secondary pancreatic duct (of Santorini) of the adult corresponds to
that section of the proximal or larger embryonic outgrowth situated
between the intestine and the point where the two glandular diverticula
fuse with each other. Hence the canal of Wirsung in the adult is a
compound product. It includes the duct system developed, in connection
with the bile duct, in the head of the gland, forming the intestinal
termination of the main duct. Its distal body portion on the other hand
is derived from the duct system of the originally larger proximal
outgrowth, including the entire peripheral portion which has become
secondarily added to the duct of the ventral outgrowth to form together
with it the canal of Wirsung. On the other hand the proximal portion of
the duct system of this originally larger part becomes secondarily
differentiated as the duct of Santorini.

Fig. 188 shows the normal adult arrangement of the pancreatic and
biliary ducts in a corrosion preparation of the canal.

The duct of Santorini in this case opened by a separate orifice into the
duodenum above the common opening of the biliary and pancreatic ducts
(cf. p. 113).

=Explanation of Adult Arrangement of Human Pancreatic Ducts and Their
Variations Dependent Upon the Embryonic Development.=--The smaller
distal embryonic outgrowth is, as we have seen, from its inception in
close connection with the duodenal end of the common bile-duct (Fig.
185).

The proximal outgrowth, situated nearer to pylorus and derived directly
from the duodenal epithelium, is the larger and forms the greater part
of the bulk of the adult pancreas (Figs. 186, 187).

If, notwithstanding this primitive arrangement, the distal duct (canal
of Wirsung) appears as the main pancreatic duct in the adult, while the
proximal (duct of Santorini) is secondary, this depends upon a union of
the products of the two outgrowths in such a manner that the greater
part of the duct system of the proximal and larger portion is
transferred to the distal duct to form the adult canal of Wirsung, while
the smaller segment of the proximal duct, between its opening into the
duodenum and the point of fusion of the two outgrowths, forms the adult
secondary duct of Santorini. This duct opens usually into the duodenum
upon a small papilla situated about 2.5 cm. above the common duodenal
termination of the bile-duct and canal of Wirsung (papilla Vateri) (Fig.
193). The duct of Santorini usually tapers toward the duodenal opening
from its point of departure from the main duct, its caliber gradually
diminishing in the direction indicated, so that it is smaller at the
duodenal opening than at the point of confluence with the main duct
(Fig. 189). Hence the secretion from the proximal head portion of the
pancreas, conveyed by this duct and its tributaries, passes usually into
the main pancreatic duct and not directly into the intestine through the
duodenal opening of the duct of Santorini. The latter is, however, thus
enabled to vicariously take upon itself the conduct of the pancreatic
secretion in cases of obstruction or obliteration of the main duct
(calculi, ulcers, cicatrices, etc.). In these cases of obstruction of
the main duct the duct of Santorini enlarges and performs its functions.

[Illustration: FIGS. 189-192.--Series of schemata showing normal and
variant adult types of biliary and pancreatic ducts.]

[Illustration: FIG. 189.--Usual human adult type.]

[Illustration: FIG. 190.--Persistence of early embryonal type.]

[Illustration: FIG. 191.--Duct of Santorini has no duodenal orifice.]

[Illustration: FIG. 192.--Duct of Santorini forms the only pancreatic
duct. Separate duodenal openings of biliary and pancreatic ducts,
resulting from failure of development of distal embryonal pancreatic
bud.]

[Illustration: FIG. 193.--Mucous surface of human duodenum, showing
entrance of biliary and pancreatic ducts and diverticulum Vateri.
(Columbia University Museum, No. 1842.)]

Occasionally, without obstruction of the main duct, the duodenal opening
of the duct of Santorini is large, and the flow of secretion evidently
the reverse of the usual, _i. e._, directly into the intestine.

In other cases, also without pathological conditions, the proximal duct
is the larger of the two and serves as the principal channel of
pancreatic secretion, the canal of Wirsung being small. This is
evidently a persistence and further development of the early embryonic
relative condition of the two outgrowths above described (Fig. 190). On
the other hand the duct of Santorini may not open at all into the
duodenum, terminating in small branches which drain the proximal part of
the head of the gland (Fig. 191).

Schirmer has examined the arrangement of the pancreatic ducts in 105
specimens. In 56 of these the duct of Santorini passed from the main
duct into the duodenum, opening upon a papilla situated 2.5 cm. above
the common opening of the bile duct and canal of Wirsung.

In 19 the duct of Santorini was well developed but did not open into the
duodenum.

In but 4 cases the duct of Santorini formed the only pancreatic duct,
the lower opening being occupied by the bile duct alone (Fig. 192). We
may assume in these cases failure of development of the distal outgrowth
connected with the primitive hepatic bud, leaving only the proximal
duodenal outgrowth to form the entire adult gland.

Figs. 188 and 189 show the normal arrangement of the duodenal openings
of the biliary and pancreatic ducts.

Figs. 190 to 192 show schematically the variations in the relative
development and the adult arrangement of the pancreatic ducts.

=Diverticulum and Papilla Vateri.=--From what has been said regarding
the embryonic union of the distal pancreatic outgrowth with the hepatic
bud it will be easy to recognize the corresponding features in the
arrangement of the adult duodenal termination of the common bile-duct
and canal of Wirsung. The dilated interior of the duodenal papilla
(diverticulum Vateri) corresponds to the embryonic segment between the
intestinal opening of the primitive liver duct and the point when this
duct gives off the distal larger pancreatic outbud (Figs. 186, 187, 188,
193 and 194).

[Illustration: FIG. 194.--Adult human subject. Mucous membrane of
pyloro-duodenal junction and of duodenum. (Columbia University Museum,
No. 1840.)]

The union of the pancreatic and biliary ducts to form the recess of the
diverticulum Vateri, which then opens by a single common orifice into
the duodenum, is better marked in some of the lower vertebrates than in
man.

[Illustration: FIG. 195.--Duodenum, with entrance of pancreatic and
biliary ducts and well-developed diverticulum Vateri in the cassowary,
_Casuarius casuarius_. (Columbia University Museum, No. 1821.)]

Fig. 195 shows the proximal portion of the duodenum of the cassowary
(_Casuarius casuarius_) with the biliary and pancreatic ducts and the
diverticulum at their confluence in section.

The development of these two main digestive glands as diverticula from
the intestinal canal also explains the direct continuity of the mucous
membrane of their ducts with that lining the duodenum, a fact which is
of considerable importance in the pathological extension of mucous
inflammations from the intestine to the duct system of the glands.

=Development of the Pancreas in Lower Vertebrates.=--In the embryo of
the _sheep_ two pancreatic buds are found, but the duct of the dorsal
(proximal) outgrowth (duct of Santorini) subsequently fuses entirely
with the main duct.

In the _cat_ there are likewise two pancreatic outgrowths.

In the _chick_ three pancreatic buds are visible about the fourth day.

_Amphibia_ likewise present three embryonic pancreas buds.

The ventral (distal) outgrowth is double, the two portions proceeding
symmetrically from each side of the hepatic duct. The single dorsal
outgrowth is derived directly from the duodenal epithelium. Later on all
these outgrowths fuse to form the single adult gland.

_Fish_ also possess several (up to four) embryonic pancreatic
outgrowths.

Recently in human embryos of 4.9 mm. cervico-coccygeal measure three
pancreatic outgrowths have been observed, all entirely distinct from
each other, one dorsal, budding from the epithelium of the primitive
duodenum and two ventral, proceeding from the grooved gutter which
represents the primitive ductus choledochus at this period. In embryos
of from 6 to 10 mm. the two ventral outgrowths have already fused, hence
only two buds, a single ventral and a dorsal, are now encountered.[4]

[4] Iankelowitz, Arch. f. Mikr. Anat., Bd. 46, 1895.

These observations place the development of the human pancreas in line
with the triple pancreatic outgrowths, two ventral and one dorsal
characteristic of the majority of the lower vertebrates, which have been
hitherto carefully examined. The ventral or distal bud is probably
double in the majority of vertebrates. The two segments fuse, however,
so early that the derivation of the pancreas from a double outgrowth, as
described above for the human embryo, practically obtains. In forms in
which the adult gland presents a number of separate openings into the
duodenum (cf. p. 118), the development would probably show multiple
embryonic outgrowths from the intestinal hypoblast.

In any case the dorsal pancreatic bud appears to have developed in the
vertebrate series before the ventral outgrowth and to be hence
phylogenetically the older structure.


COMPARATIVE ANATOMY OF THE PANCREAS.

With the exception of _Amphioxus_ and probably also of the
_Cyclostomata_, the gland appears to be present in all vertebrates,
varying, however, much in size, shape and relation to the intestinal
tube. Usually it appears as an elongated, flattened, more or less
distinctly lobulated organ, in close apposition to the duodenum between
the layers of the mesoduodenum. In all forms in which the gland is found
it is connected with the post-gastric intestine and marks the beginning
of the midgut. In structure the gland is usually acinous, resembling the
salivary glands. It is well developed in the selachians, forming a
triangular body connected with the beginning of the midgut (Fig. 202).
In some instances the gland elements do not extend beyond the intestine
itself, but remain imbedded in the wall of the midgut, as in
_Protopterus_. In certain adult teleosts the pancreas is surrounded by
the liver (Fig. 196), in others it does not appear as a compact gland
but is distributed in the form of finely scattered lobules throughout
the mesentery between the two layers of this membrane. On account of
this concealed position of the gland it was formerly believed that the
adult teleosts did not possess a pancreas. The pyloric caeca (cf. p. 119)
found in these forms were consequently considered to be homologous with
the pancreas of the higher vertebrates.

[Illustration: FIG. 196.--A portion of alimentary canal of _Pleuronectes
maculatus_, the flounder, with pancreas attached to biliary duct and
concealed in the substance of the liver, which has been removed.
(Columbia University Museum, No. 1491.)]

In _Myxinoids_ a peculiar lobulated glandular organ is found imbedded in
the peritoneal coat of the intestine near the entrance of the bile-duct,
into which its lobules open separately. This organ possibly corresponds
to the higher vertebrate pancreas.

An organ which may represent a dorsal pancreas is also developed in
_Ammocoetes_ (larva of _Petromyzon_), but its exact homology is still
doubtful. It is possible that a true pancreas has not yet developed in
the cyclostomata. In _Amphioxus_ no trace of a pancreas is found. In all
other vertebrates the gland is present. In certain amphibians, as the
frog, the single pancreatic duct opens into the common bile duct (Fig.
197).

[Illustration: FIG. 197.--Pancreas and biliary ducts of _Rana
esculenta_, frog. (Wiedersheim, after Parker; both from Ecker.)]

In lacertilians and in some chelonians a lateral offshoot of the
pancreas is directed transversely and is adherent to the spleen. Fig.
113 shows the gland in _Chelydra serpentaria_. While the gland usually
has a single duct, yet two ducts are found in a number of animals (many
mammals, birds, chelonians and crocodiles). At times three ducts are
encountered, as in the chicken and pigeon.

The arrangement of the pancreatic duct system among mammalia presents
the following variations:

1. Mammals with _one_ pancreatic duct, either connected with the
bile-duct or entering the intestine independently:

Monkeys, most rodents (except the beaver), marsupials, carnivora (except
dog and hyena), many ungulates (pig, peccary, hyrax, etc.), most
ruminating artiodactyla.

(_a_) The pancreatic duct joins the common bile-duct before entering the
duodenum in the monkeys, marsupials, carnivora, in the sheep, goat and
camel.

The point of entrance of the combined duct into the intestine varies. In
some forms it is near the pylorus, in others at some distance from the
same. The common opening is situated 11/2" to 2" beyond the pylorus in
carnivora, and one foot behind the same point in the goat and sheep.

(_b_) The pancreatic duct does not join the bile-duct, but empties
separately into the intestine, in most rodents and in the calf and pig.

In the calf the pancreatic duct opens into the duodenum 15' beyond the
bile-duct and 3' beyond the pylorus.

In the pig the pancreatic opening is 5"-7" beyond that of the bile-duct
and 6"-8" behind the pylorus.

2. Mammals with _two_ pancreatic ducts, of which one usually joins the
bile-duct: perissodactyla (except the ass according to Meckel),
elephant, beaver, several carnivora, dog, hyena, and according to
Bernard the cat. In the perissodactyla the proximal of the two
pancreatic ducts empties, either combined with the bile-duct, or
separate from it, but very close to it, 3"-4" behind the pylorus. The
second distal duct is smaller and opens several inches further down.

[Illustration: FIG. 198.--_Necturus maculatus_, mud puppy. Dissection of
intestinal canal, liver, pancreas, and spleen, with blood-vessels
injected. (Columbia University Museum, No. 1863.)]

[Illustration: FIG. 199.--Pancreas and pancreatic ducts of rabbit.
(Nuhn.)]

In most rodents the pancreatic entrance is placed at some distance from
the pylorus. Fig. 199 shows the arrangement of the parts in the rabbit,
in which animal the main distal pancreatic duct empties at a distance of
13"-14" from the pylorus into the end of the duodenum, which intestine
forms a very long loop, while the biliary duct, receiving the smaller
proximal pancreatic duct, opens near the pylorus.

In the _beaver_ the smaller proximal duct joins the bile-duct or even
enters the duodenum anterior to the bile-duct, nearer the pylorus,
while the distal larger pancreatic duct opens into the intestine
16"-18" behind the biliary duct. Of the two ducts found in the dog
(Fig. 200) the smaller proximal either joins the bile-duct or opens
into the intestine close to it, 1" to 11/2" beyond the pylorus. The
larger distal duct opens into the duodenum 1" to 11/2" behind the biliary
duct. Fig. 201 shows the dog's stomach and proximal portion of the
duodenum in section. The proximal smaller pancreatic duct here joins
the biliary duct, and opens with it by a single orifice into the
duodenum. The distal larger pancreatic duct opens independently into
the intestine further caudad.

[Illustration: FIG. 200.--Abdominal viscera of dog, showing arrangement
of pancreatic ducts. (Nuhn.)]

[Illustration: FIG. 201.--Section of dog's stomach, and proximal portion
of duodenum, with entrance of biliary and pancreatic ducts. (Columbia
University Museum, No. 1822.)]

The parts in _Hyaena_ present a similar arrangement.

Bernard always found _two_ pancreatic ducts in the _cat_, one large
principal duct and a second smaller accessory duct. Of these, the one
situated nearest to the pylorus always united with the bile-duct. The
pancreatic duct thus joining the bile-duct was sometimes the main duct,
sometimes the accessory smaller duct.

Since the main function of the pancreatic juice is the conversion of
starch into sugar, the gland appears better developed in general in
herbivora than in carnivora, without, however, disappearing in the
latter. In fact it is of considerable size in the carnivora, because the
secretion also acts on the albuminous food substances and, though to a
lesser degree, on the fats.


PYLORIC CAECA OR APPENDICES.

In the _Cyclostomata_ and _Selachians_ the intestinal canal is in the
main free from caecal appendages, while a large portion of the tube is
provided with a special fold of the mucous membrane which projects into
the lumen of the gut (spiral valve). Fig. 43 shows the straight
intestinal tract with the spiral valve of the longer distal segment in a
cyclostome, _Petromyzon marinus_ or lamprey. In Figs. 202 and 203 the
selachian (shark) intestine is represented in two examples, while the
similar spiral valve in a Dipnoean or lung fish, _Ceratodus_, is seen in
Fig. 204.

[Illustration: FIG. 202.--Alimentary tract with spleen and pancreas of
_Squalus acanthias_, the dog-fish. (Columbia University Museum, No.
1405.)]

[Illustration: FIG. 203.--Alimentary canal of _Galeus canis_, dog-shark,
in section, showing spiral intestinal valve. (Columbia University
Museum, No. 1429.)]

[Illustration: FIG. 204.--Alimentary canal with spiral valve of
_Ceratodus forsteri_, the Australian lung-fish (Barramunda). (Columbia
University Museum, No. 1645.)]

On the other hand in the Ganoids and in many Teleosts longer or shorter
finger-shaped diverticula of the midgut are found immediately beyond the
pylorus in the region of the bile-duct.

These pouches or diverticula of the intestine form the so-called pyloric
caeca or appendices of these fish. They vary very much in length,
diameter and number in different forms.

Thus but a single diverticulum appears in _Polypterus_ and _Ammodytes_
(Fig. 205). _Rhombus maximus_ and _Echelus conger_ (Figs. 112 and 206)
have two, and the same number appear in _Lophius piscatorius_ (Fig.
207). Perca has three and the _Pleuronectidae_ have three to five.

[Illustration: FIG. 205.--Alimentary canal of _Polypterus bichir_.
(Columbia University Museum, No. 1823.)]

[Illustration: FIG. 206.--Alimentary tract of _Echelus conger_, Conger
eel. Stomach, mid- and end-gut, liver, and spleen. (Columbia University
Museum, No. 1430.)]

[Illustration: FIG. 207.--Stomach, duodenum, and pyloric caeca of
_Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)]

Fig. 208 shows the stomach and the beginning of the midgut with four
pyloric caeca in _Pleuronectes maculatus_, and Fig. 209 the same parts of
this animal in section.

[Illustration: FIG. 208.--_Pleuronectes maculatus_, window-pane. Stomach
and mid-gut with pyloric caeca and hepatic duct. (Columbia University
Museum, No. 1432.)]

[Illustration: FIG. 209.--_Pleuronectes maculatus_, window-pane. Stomach
and mid-gut with pyloric caeca, in section. (Columbia University Museum,
No. 1433.)]

[Illustration: FIG. 210.--_Paralichthys dentatus_, summer flounder.
Stomach and mid-gut with pyloric caeca and liver. (Columbia University
Museum, No. 1431.)]

Fig. 210 shows the stomach and midgut of _Paralichthys dentatus_, the
summer flounder, with three well-developed conical pyloric caeca. On the
other hand in some forms the number of pyloric appendices is enormously
increased, while their caliber diminishes. Thus 191 caecal appendages are
found surrounding the beginning of the midgut in _Scomber scomber_. A
well-marked example of prolific development of the pyloric appendages is
furnished by the common cod, _Gadus callarias_ (Fig. 211). The
appendices are in the natural condition bound together by connective
tissue and blood vessels, so as to form a compact organ, resembling a
gland (Fig. 211, A), and a similar arrangement is found in _Thynnus
vulgaris_ and _alalonga_, _Pelamys_ and _Accipenser_ (Fig. 212).

[Illustration: FIG. 211.--Pyloric caeca of _Gadus callarias_, codfish.
(Columbia University Museum, No. 1825.) _A._ Bound together by
connective tissue and blood-vessels.

_B._ Dissected to show confluence of caeca to form a smaller number of
terminal tubes of larger calibre entering the intestine.]

[Illustration: FIG. 212.--Alimentary canal of _Accipenser sturio_,
sturgeon. Numerous pyloric caeca are bound together to form a gland-like
organ. (Columbia University Museum, Nos. 1826, 1827, and 1828.)

In the smaller upper figure on the left the stomach, mid-gut, and
pyloric caeca are seen in section, showing the lumen of the latter and
their openings into the mid-gut.

The lower left-hand figure shows the mid- and end-gut in section, the
latter provided with a spiral mucous valve.]

In some Teleosts (Siluroidea, Labroidea, Cyprinodontia, Plectognathi and
Leptobranchiates) the appendices are entirely wanting. If there are not
more than 8-10 appendices they usually surround the gut and empty into
the same in a circle. In other cases they are arranged in a single line,
or in a double row, opposite to each other (Fig. 213). Each appendix may
open into the intestine independently, this especially where the number
is limited and the individual pouches large (cf. Figs. 206-210), or
several may unite to form a common duct.

[Illustration: FIG. 213.--_Melanogrammus aeglifinus_, haddock. Stomach,
mid-gut, and pyloric caeca; spleen. (Columbia University Museum, No.
1598.)]

Fig. 211, _B_, shows the appendices in _Gadus callarias_, the cod, freed
by dissection from the investing connective and vascular tissue. It will
be noticed that a considerable number of the tubes unite to form ducts
of larger caliber which open into the intestine, as seen in the section
shown in Fig. 214.

[Illustration: FIG. 214.--Stomach and mid-gut of _Gadus callarias_,
codfish, in section, showing intestinal openings of pyloric caeca.
(Columbia University Museum, No. 1830.)]

The pyloric appendices apparently have the same _significance_ as the
spiral intestinal fold of the Selachians, Cyclostomes and
Dipnoeans, _i. e._, the production of an increase in the area of
the digestive and absorbing surfaces of the intestinal mucous membrane.
Hence, as stated, the appendices and the spiral fold are found to vary
in inverse ratio to each other. Thus, for example, _Polypterus_ (Fig.
205) still has a fairly well developed spiral fold and only a single
pyloric appendix, while _Lepidosteus_, with but slightly developed
spiral fold, has numerous appendices. It was formerly held that the
pyloric caeca and the pancreas were mutually incompatible structures, and
that where one is found the other will be wanting.

Hence the appendices were regarded as homologous with the pancreas of
the higher forms. Recent observations have shown that this view is not
strictly and entirely correct, while at the same time it merits
consideration in several respects.

It is true that the pancreas in certain teleosts is now known to be
present although concealed from observation in the liver or scattered in
the form of small lobules between the layers of the mesentery (cf. p.
117), and that in a number of fish, such as _Salmo salar_, _Clupea
harengus_, _Accipenser sturio_, both the appendices and the pancreas are
encountered. Consequently these structures are not identical or even
completely homologous, since they occur side by side in the same form.

On the other hand Krukenberg has demonstrated that the appendices
pyloricae may function physiologically as a pancreas by yielding a
secretion which corresponds to the pancreatic juice in its digestive
action. In the majority of forms, however, they apparently merely
increase the intestinal absorbing surface, secreting only mucus.

These structures are nevertheless very interesting and instructive since
they furnish a perfect gross morphological illustration of the embryonal
stages just considered in connection with the development of the
mammalian pancreas. In the adult ganoid or teleost these blind
diverticula or pouches, varying greatly in shape, number and size,
protrude from the intestine immediately beyond the pylorus, usually in
close connection with the duodenal entrance of the bile-duct. Two or
more of these pouches may unite to form a common duct or canal opening
into the intestine.

These forms, therefore, offer direct and valuable morphological
illustration of the manner in which the pancreas of the higher
vertebrates develops, _i. e._, as a set of hollow outgrowths or
diverticula from the hypoblast of the primitive enteric tube. We can
establish a consecutive series, beginning with forms in which only one
or two diverticula are found, and extending to types in which the number
of the little cylindrical pouches reaches nearly two hundred and in
which they are bound together by connective tissue and blood vessels so
as to closely resemble the structure of a glandular pancreas. This is
one of the most striking instances in which the minute embryological
stages of the higher types are directly illustrated by the permanent
adult conditions found in the lower vertebrates. [The same statement, as
we will see, holds good in reference to the development of the _liver_.]


RELATION OF THE PANCREAS TO THE PERITONEUM.

The gland becomes very intimately connected with the serous layers of
the primitive dorsal mesentery. In order to clearly comprehend the adult
serous relations it is necessary to make a distinction between two
divisions or portions of the gland, based upon the altered relations of
the primitive dorsal mesentery which result from the differentiation of
the primitive simple intestinal tube into stomach and duodenum.

1. The primary outgrowth of the pancreatic tubules from the duodenum,
_i. e._, the part which is to form the "head" of the adult gland, is
situated between the two layers of that division of the primitive dorsal
mesentery which forms, after differentiation of stomach and small
intestine, the _mesoduodenum_. Coincident with the rotation of the
stomach, as we have seen, the duodenum and mesoduodenum exchange their
original sagittal position in the median plane of the body for one to
the right of the median line, balancing, so to speak, the extension of
the stomach to the left (Fig. 218).

The original right layer of the mesoduodenum and the right surface of
the duodenum now look dorsad and rest in contact with the parietal
peritoneum investing the right abdominal background and the ventral
surface of the right kidney and inferior vena cava. We have already seen
that the descending portion of the duodenum in man becomes anchored in
this position by adhesion of these apposed peritoneal surfaces. This
fixation includes, of course, the structures situated between the layers
of the mesoduodenum, _i. e._, the head of the pancreas. Consequently,
after rotation and adhesion, this portion of the gland turns one surface
ventrad, invested by secondary parietal peritoneum, originally the left
leaf of the free mesoduodenum, while the original right surface of the
gland has become the dorsal and has lost its mesoduodenal investment by
adhesion to the primary parietal peritoneum.

2. In order to understand the way in which the body and tail of the
pancreas obtain their final peritoneal relations it is necessary to
consider the development of the dorsal mesogastrium to form the omental
bag. If we regard the primitive dorsal mesentery in the profile view
from the left side (Fig. 215) it will be seen that, as already stated,
the mesoduodenum is the first part of the membrane to be invaded by the
pancreatic outgrowth from the intestine. Cephalad of the mesoduodenum
the primitive dorsal mesogastrium (Fig. 215) is seen to protrude to the
left and caudad to form, as already explained, the cavity of the omental
bursa of the retrogastric space ("lesser peritoneal sac"). The further
growth of the pancreas carries the developing gland from the district of
the mesoduodenum into that portion of the dorsal mesogastrium which now
forms the dorsal wall of the omental bursa (Fig. 216).

[Illustration: FIG. 215.--Cephalic segment of primitive mesentery in
schematic profile view.]

[Illustration: FIG. 216.--Schematic profile view of primitive
mesenteries with formation of omental bursa and developing spleen and
pancreas.]

[Illustration: FIG. 217.--_Sos scrofa foet._, foetal pig. Portions of
thoracic and abdominal viscera hardened in situ. (Columbia University
Museum, No. 1449.)]

This double relation of the pancreas to the mesoduodenum and to the
mesogastrium forming the omental bursa is well seen in foetal pigs
between two and three inches in length (Fig. 217).

The head portion of the pancreas is seen developing between the layers
of the mesoduodenum, while the body and tail of the gland, extending to
the left, grows between the two dorsal layers of the omentum bursa
towards the spleen, which organ is found connected with the left and
dorsal extremity of the omental sac derived from the dorsal
mesogastrium.

Before the growth of the great omentum is pronounced the continuity of
the mesoduodenum and dorsal mesogastrium can be readily appreciated
(Fig. 218). But after the redundant growth of the membrane has carried
the great omentum further caudad, the stomach and the two omental layers
attached to the greater curvature lie in front of the structures
included between the two dorsal layers and conceal them from view (Fig.
177).

[Illustration: FIG. 218.--Schematic view of primitive mesentery after
intestinal rotation and incipient formation of omental bursa from dorsal
mesogastrium.]

In sagittal sections to the left of the median line (Figs. 221 and 222)
the pancreas now appears included between the layers of the great
omentum near their point of departure from the vertebral column. (This
point is of course identical with the prevertebral attachment of the
primitive dorsal mesogastrium from which the omentum is developed.)

[Illustration: FIGS. 221, 222.--Schematic sagittal sections through
stomach, pancreas, great omentum, and left kidney.]

[Illustration: FIG. 221.--Before adhesion between dorsal and
mesogastrium and parietal peritoneum.]

[Illustration: FIG. 222.--After adhesion.]

The foregoing considerations will, therefore, lead to the conclusion
that the pancreas presents, in regard to its peritoneal relations, two
distinct segments:

1. The portion adjacent to duodenum (head and neck of the gland) is
developed between the layers of the mesoduodenum.

2. The distal portion of the gland, comprising the body and tail,
develops between the layers of the great omentum (dorsal segment),
derived from the primitive dorsal mesogastrium.

The transections of the dorsal mesogastrium shown in Figs. 180 and 181
will now have to be amplified by the introduction of the body of the
pancreas between the two layers of the vertebro-splenic segment, in
addition to the splenic artery (Figs. 219 and 220).

[Illustration: FIGS. 219, 220.--Schematic transection of dorsal
mesogastrium, pancreas, spleen, and stomach.]

[Illustration: FIG. 219.--Before adhesion to primitive parietal
peritoneum (arrow indicates the direction in which the adhesion takes
place).]

[Illustration: FIG. 220.--After adhesion and formation of secondary line
of transition between mesogastrium and parietal peritoneum (lieno-renal
ligament).]

Hence the following facts will be understood:

1. In the adult the splenic artery supplies a series of small branches
to the pancreas as it courses along the cephalic border of the gland on
its way to the spleen.

2. After the above-described adhesion of the original left leaf of the
dorsal mesogastrium (vertebro-splenic segment) to the parietal
peritoneum (Fig. 220), the dorsal surface of the body of the pancreas
loses its peritoneal investment and becomes attached by connective
tissue to the ventral surface of the left kidney.

3. The ventral surface of the body of the pancreas is in the adult lined
by peritoneum of the "lesser sac"; in other words the organ has
practically assumed a "retro-peritoneal" position, its ventral
peritoneal covering appearing now as the dorsal parietal peritoneum of
the retro-gastric space.

4. When completely developed the extreme end (tail) of the pancreas
extends to the left, following the splenic artery, until it touches the
mesal aspect of the spleen at the hilus.

5. If we, therefore, leave out of consideration for the moment the
transverse colon and duodenum, which will be taken up presently, and
confine ourselves to the arrangement of the stomach, pancreas and great
omentum, a sagittal section to the left of the median line would result
as shown in Fig. 222, after the adult condition of adhesion has been
established.

The same process of fixation, which resulted in the anchoring of
duodenum and head of pancreas, extends to the body of the gland and the
investing omentum. The peritoneum lining the original left, now the
dorsal surface of the gland, fuses with the primitive parietal
peritoneum covering the diaphragm and the left kidney. The main body of
the pancreas in the adult appears prismatic, giving a triangular
sagittal section. The dorsal surface is adherent to the ventral surface
of the left kidney; the ventral surface is covered by the secondary
parietal peritoneum (original right layer of mesogastrium) which lines
the dorsal wall of the retrogastric space and omental bursa (lesser
peritoneal sac). The great omentum now appears to take its dorsal point
of departure along the sharp margin which separates this ventral surface
of the pancreas from a third narrower surface directed caudad. This
surface, under the conditions which we are at present examining, would
be lined by the peritoneum continued onto it from the dorsal layer of
the great omentum. This peritoneum merges along the dorsal margin of
this caudal surface of the pancreas with the general parietal peritoneum
covering the left lumbar region and the caudal part of ventral surface
of the left kidney. We have, therefore, along this line a secondary
transition from visceral to parietal peritoneum, obtained by the
obliteration of the original visceral peritoneum investing the dorsal
surface of the pancreas before adhesion to the parietal peritoneum.

The pancreas assumes, therefore, in the adult a secondary
retro-peritoneal position, covered on its ventral surface by peritoneum
of the "lesser sac," while the caudal surface is lined by part of the
general peritoneal membrane of the "greater sac." The dorsal surface,
denuded of serous covering by obliteration, is adherent to the crura of
the diaphragm, the aorta and the ventral surface of the left kidney.

It is now proper to compare the conclusions just derived from the study
of the development of the human dorsal mesogastrium and connected
structures (spleen and pancreas) with the conditions presented by the
corresponding parts in one of the lower mammalia, which illustrate some
of the human embryonal stages. Here again the abdominal cavity of the
cat forms an instructive object of study.

The purpose of the following comparison should be twofold:

I. The mesogastrium, spleen and pancreas in the cat will clearly
illustrate the process of human development above outlined.

II. The abdominal viscera of the cat, if properly arranged, will enable
us to complete the consideration of this region by including the very
important relations which the transverse colon and third portion of the
duodenum bear in man to the great omentum and pancreas.


I. SPLEEN, PANCREAS AND GREAT OMENTUM OF CAT.

After opening the abdominal cavity it will be seen that the great
omentum can be lifted up, exposing the subjacent coils of the small and
large intestine, to which it adheres at no point. In other words the
entire dorsal surface of that part of the original mesogastrium which
forms the great omentum is free. It will be remembered that this is not
the case in the adult human subject, because here the dorsal surface of
the great omentum adheres to the transverse colon. Consequently in man
only that portion of the dorsal surface of the omentum can be seen which
extends between the transverse colon and the caudal free edge of the
membrane.

It will be noted that on the left side the spleen is connected by its
mesal surface to the omentum and through it with the stomach
(gastro-splenic omentum). In other words the cat illustrates the human
embryonal stage in which the spleen has appeared between the layers of
the dorsal mesogastrium at the extreme left or blind end of the
retrogastric pouch formed by the rotation of the stomach and elongation
of the mesogastric membrane, but _before_ the adhesion has taken place
between the original _left_ (now _dorsal_) layer of the vertebro-splenic
segment of the mesogastrium and the primitive parietal peritoneum
apposed to it (Fig. 219). Consequently the dorsal wall of the "lesser"
sac in the cat is still composed of the two layers of the free
vertebro-splenic segment of the mesogastrium, the primitive right (now
ventral) layer not having been converted, as is the case in man, into
secondary parietal peritoneum by adhesion of the original left (now
dorsal) layer to the primitive prerenal parietal peritoneum.

If we now examine the relation of the pancreas to the peritoneum we can
establish the following facts:

1. The portion of the gland adjacent to the duodenum, corresponding to
the "head" of the human organ, is included between the two layers of
the mesoduodenum. This membrane is free, so that the dorsal surface of
this portion of the pancreas is seen to be invested by the dorsal layer
of the mesoduodenum (Fig. 223). The duodenum and the mesoduodenum, the
latter containing the head of the pancreas between its layers, can be
turned toward the median line, so as to expose the entire ventral
surface of the post-cava and right kidney. To illustrate the arrangement
which is found in the adult human subject the descending duodenum and
pancreas should be allowed to fall over to the right so as to cover the
vena cava and the mesal part of the ventral surface of right kidney. The
adult human condition will now be produced if we assume that the
structures are fixed in this position by the obliteration of the apposed
serous surfaces, viz., the parietal peritoneum over kidney and vena cava
on the one hand and the right layer of the mesoduodenum and the dorsal
visceral peritoneum of the duodenum on the other.

[Illustration: FIG. 223.--Abdominal viscera of cat, hardened and removed
from body, showing relation of pancreas to mesoduodenum and dorsal
mesogastrium, respectively. (Columbia University Museum, No. 728.)]

2. In following out the pancreas of the cat in its entire extent,
proceeding to the left of the pylorus, it will be seen that the body of
the gland has extended between the two dorsal layers of the great
omentum (primitive dorsal mesogastrium) over to the spleen (Fig. 223).
Consequently the arrangement in the cat corresponds to the stage in the
human development shown in Fig. 219 and Fig. 221 in which adhesion of
the dorsal surface of the pancreas to the parietal peritoneum has not
yet taken place.

It will be quite easy to reconstruct from the facts as demonstrated by
the arrangement of the parts in the cat, the stage in the development of
the lesser peritoneal sac in which the dorsal wall of the space is still
formed by the proximal portion of the free dorsal mesogastrium (great
omentum) and the structures included between its two layers.

It must then become apparent that the entire serous surface which in the
adult human subject we regard as "parietal peritoneum of the lesser sac"
lining the dorsal wall of the retrogastric space is derived from what
originally was the right layer of the primitive sagittal dorsal
mesogastrium.


II. RELATION OF GREAT OMENTUM TO TRANSVERSE COLON, TRANSVERSE MESOCOLON
AND THIRD PART OF DUODENUM.

The second purpose to be accomplished by the study of the cat's
abdominal cavity at this stage is the correct appreciation of the adult
human conditions which are produced by areas of adhesion between the
transverse colon, transverse mesocolon and third part of the duodenum on
the one hand, and the dorsal mesogastrium, as great omentum, with the
structures contained between its layers, on the other.

Perform the manipulations of the large and small intestine in the cat
(see p. 67) which are required in order that the tract may be arranged
so that it will correspond in general to the topographical conditions
presented by the adult human subject. Locate the transverse colon and
mesocolon and the third portion of the duodenum produced by these
manipulations in imitation of the corresponding human structures. Then
proceed to plot the different parts out successively as they would
appear in a sagittal section (Fig. 224).

[Illustration: FIG. 224.--Schematic sagittal section of abdominal
viscera of cat, after the intestines have been rotated to correspond to
the adult human disposition, to show lines of peritoneal reflection
before adhesion.]

The following facts are to be noted and indicated on the plan of the
section:

1. The great omentum is free, hanging down from the greater curvature of
the stomach over the coils of intestine. Turning the omentum up it will
be observed that the body of the pancreas is included between the two
dorsal layers of the membrane.

2. The omentum, containing the pancreas, can be lifted up, exposing the
next succeeding structure, viz., the transverse colon and mesocolon. In
the cat the large intestine has been brought over, by the manipulations
above indicated, into a transverse position so as to represent the human
transverse colon and its mesocolon. It is therefore necessary to
remember that in this mammal the fixation of the transverse mesocolon in
the position indicated, by adhesion of ascending and descending mesocola
to the parietal peritoneum of the abdominal background, has not yet
occurred. Consequently the membrane must be held in the transverse
position in order to represent the human arrangement.

It will of course be observed that both surfaces of the transverse
mesocolon established in this way are free, not adherent to either
omentum or pancreas on the one hand, nor to the transverse duodenum on
the other.

3. The third or transverse portion of the duodenum is seen to be
attached by the distal part of the mesoduodenum, both of the serous
surfaces of the membrane being free. The duodenum having been brought
from right to left transversely across vertebral column and aorta,
underneath the superior mesenteric artery, the mesoduodenum, in the
segment corresponding to the transverse duodenum, exchanges its original
sagittal position for one in a horizontal plane, with cephalic
(primitive left) and caudal (primitive right) surfaces.

Now compare the above arrangement of the intestines and peritoneum in
the cat at once with the conditions presented in the adult human
subject, reserving certain intermediate stages, as exhibited by some of
the lower monkeys, for subsequent study.

[Illustration: FIG. 225.--The same figure indicating the areas of
adhesion and peritoneal obliteration (shaded) which produce the
arrangement of the adult human peritoneum.

1. Area of adhesion between opposed surfaces of great omentum and
transverse mesocolon and colon.

2. Area of adhesion between parietal peritoneum, duodenum, and caudal
layer of transverse mesocolon.

3. Adhesion of opposed walls of omental bursa leading to obliteration of
distal portion of pouch and producing "gastro-colic" ligament of adult
human subject.]

The examination of a similar sagittal section representing schematically
the adult human arrangement of the parts (Fig. 225) will reveal the
following points of difference as compared with the cat:

1. The peritoneum covering the dorsal surface of the pancreas, derived
from the primitive dorsal mesogastrium, has become adherent to the
parietal peritoneum, as previously described.

2. The cephalic surfaces of the transverse colon and mesocolon fuse with
the corresponding area of the dorsal (4th) layer of the great omentum
(dorsal mesogastrium).

In the human foetus in the 4th month the connection is still so slight
that the omentum can readily be separated from the transverse colon and
mesocolon.

Further dorsad the cephalic layer of the transverse mesocolon adheres to
the serous investment of the caudal surface of the pancreas, derived, as
we have seen, from the same dorsal layer of the great omentum.

3. The duodenum and mesoduodenum are fixed by adhesion on the one hand
to the parietal peritoneum, on the other to the caudal layer of the
transverse mesocolon near the root of that membrane.

4. The cavity of the omental bursa is usually obliterated in the adult
caudad of the level of the transverse colon, by adhesion of the apposed
surfaces of the two intermediate omental layers.

We have therefore three general areas of secondary peritoneal adhesion
to deal with (Fig. 225), viz.:

  1. Dorsal layer of primitive   }    { Parietal peritoneum, cephalic
    mesogastrium (great          }    { layer of transverse
    omentum) including the       }    { mesocolon and cephalic surface
    serous investment of the     } to { of transverse colon.
    dorsal and caudal surfaces   }    {
    of the pancreas (Fig. 225,   }    {
    1).                          }    {

  2. Transverse duodenum         }    { Parietal peritoneum and
    and mesoduodenum (Fig.       } to { caudal layer of transverse
    225, 2).                     }    { mesocolon.

  3. Between the apposed serous surfaces of the intermediate omental
  layers (Fig. 225, 3).

[Illustration: FIG. 226.--Schematic sagittal section of adult human
peritoneum].

These areas of adhesion result naturally in the production of secondary
lines of peritoneal transition as follows:

1. Figs. 225, 1; 226, 1, from the omentum, dorsal layer, to the caudal
surface of transverse colon, caudal layer of transverse mesocolon and
caudal surface of the pancreas.

2. Figs. 225, 2; 226, 2, from the caudal layer of the transverse
mesocolon across the transverse portion of the duodenum to the parietal
peritoneum and mesentery of the jejuno-ileum.

3. Figs. 225, 3; 226, 3, between the intermediate omental layers,
forming the secondary caudal limit of the lesser sac.

These changes consequently result in the rearrangement of the adult
human peritoneum in accordance with the following schema (Fig. 226):

We trace the peritoneum as the ventral or superficial layer of the great
omentum from the greater curvature of the stomach caudad around the
distal free edge of the omentum and cephalad, as the dorsal layer, to
the ventral border of the transverse colon. Here apparently this layer
is continued across the caudal surface of the large intestine and beyond
as the caudal layer of the transverse mesocolon. While this condition
obtains practically in the adult it is to be remembered that the
adhesion (at 1 in Fig. 225) prevents us from lifting the omentum away
from the colon, and that consequently the apparent continuity of the
dorsal layer of the great omentum with the caudal layer of the
transverse mesocolon is the result of this peritoneal fusion.

Near the dorsal attachment or "root" of the transverse mesocolon the
caudal layer of the membrane becomes continuous with the parietal
peritoneum investing the transverse portion of the duodenum on its
ventral aspect, which peritoneum in turn passes into the free mesentery
of the jejuno-ileum (Fig. 225, 2). Comparison with the previous figures
will show that we are dealing here with another area of secondary
peritoneal fusion.

If we now open the "lesser peritoneal cavity" by dividing the two layers
of the omentum attached to the greater curvature of the stomach (Figs.
225 and 226 in direction of arrow) we will apparently reach the upper or
cephalic surface of the transverse mesocolon. This layer can be followed
dorsad to the sharp border which separates the ventral and caudal
surfaces of the pancreatic body and the membrane can be traced thence
over the ventral surface of the gland to the diaphragm. (The connections
with the liver and stomach shown schematically in the diagram (Fig. 225)
are to be considered in detail subsequently.)

In the adult the peritoneal surface just described appears as the
cephalic layer of the transverse mesocolon and its continuation dorsad.
From the facts previously considered it will be at once apparent that we
are really dealing here with a part of the third layer of the primitive
omentum. We do not see the original cephalic layer of the transverse
mesocolon. This membrane has become fused with the fourth omental layer,
and its free serous surface obliterated in the stretch between the
vertebral column and the transverse colon. Hence the human adult
transverse mesocolon is apparently composed of _two_ layers; the
cephalic of these layers appears as peritoneum of the "lesser sac," in
conformity with its derivation from the original third omental layer
lining the interior of the omental bursa. The caudal layer, on the other
hand, is a part of the general or "greater" peritoneal membrane. The
entire adult transverse mesocolon, hence, comprises _four_ peritoneal
layers, of which only two remain as permanently free serous surfaces.
These differ in their derivation, the cephalic layer being a part of the
primitive dorsal mesogastrium (third omental layer), while the caudal
layer is part of the primitive mesocolon. Between these two layers of
the adult transverse mesocolon are included the two obliterated
embryonic membranes, _viz._, the fourth omental layer and the original
dorsal layer of the transverse mesocolon.

Caudad the two layers of the adult transverse mesocolon surround the
transverse colon and are continuous along the ventral margin of the
intestine with the layers of the great omentum. Toward the vertebral
column these layers again diverge. The cephalic layer, lining the
"lesser peritoneal cavity" invests the ventral surface of the pancreas.
The caudal layer continues over the caudal surface of the body of the
gland and transverse portion of the duodenum into the parietal
peritoneum and the free mesentery of the jejuno-ileum. Consequently the
returning layers of the great omentum are said to surround the
transverse colon and unite along the dorsal border of the intestine to
form the transverse mesocolon, which membrane is continued dorsad toward
the vertebral column as two layers. At the "root" of the transverse
mesocolon these layers are then described as diverging, the cephalic
passing up to line the ventral surface of the pancreas, while the caudal
continues over the caudal surface of the pancreas and third portion of
the duodenum into the parietal peritoneum and mesentery.

Wherever in this discussion of the transverse mesocolon the transition
between the caudal layer of the membrane and the "parietal" peritoneum
is referred to it is necessary to remember that this "parietal"
peritoneum is the _secondary_ investment of the abdominal background,
formed by the surface of the ascending and descending mesocolon which
remains free after the opposite surface and the vertical segments of the
large intestine have been anchored by adhesion to the _primary_ parietal
peritoneum (cf. p. 81, Fig. 158).

A summary at this point of the course of the dorsal mesogastrium, in
forming the great omentum and its subsequent connections, would show us
that the membrane first enlarges and descends towards the transverse
colon (Fig. 177). The omental bag is formed by the descending or
superficial segment (starting from the greater curvature of the
stomach), turned toward the observer in the figure, and by the ascending
or deep layer which is attached above to the dorsal abdominal wall, in
front of the vertebral column and aorta along the original line of
origin of the dorsal mesogastrium. Gradually growing and descending
further, the deep segment becomes attached to the transverse colon. It
also becomes connected, especially on the left side, with the
diaphragmatic peritoneum (phrenicocolic lig.), so that its original
starting point is no longer distinct. Finally the development of the
spleen and pancreas between the layers of the dorsal segment and their
subsequent connections obscure the original conditions.

Fig. 297 shows the primitive condition at a time when the connection
with the transverse colon and mesocolon has not yet taken place.

The omental bag or bursa epiploica develops in the region of the dorsal
mesogastrium and the viscera included between its layers, by changes in
the position and extent of the membrane which finally result in placing
a part of the right half of the primitive coelom cavity behind the
stomach. Up to the sixth week the line of origin of the dorsal
mesogastrium is from the mid-dorsal line of the abdomen. It deviates
from this origin to the left because the great curvature of the stomach
to which it is attached turns in this direction. On this account, and
because of the rapid growth of this portion of the mesogastrium, a bag
or space is formed behind the stomach. The entrance into this space is
situated to the right of the lesser curvature, behind the peritoneal
layers connecting the same with the liver (lesser or gastro-hepatic
omentum and hepato-duodenal ligament). The ventral wall of this space is
formed by the dorsal surface of the stomach itself, the dorsal wall by
the mesogastrium, turning to the left and presenting its original right
surface, now directed ventrad. The caudal limit of the retro-gastric
space is given by the turn of the mesogastrium to reach its attachment
along the greater curvature of the stomach (rudiment of great omentum).

The stomach, in contributing to produce these changes, passes from the
vertical to the oblique and finally into the transverse position. The
pylorus, formerly directed caudad, passes up and to the right. The
fundus develops and the original left side of the stomach becomes the
ventral, the right side the dorsal. The original dorsal border, now the
greater curvature, moving caudad, carries the attached dorsal
mesogastrium with it into its new position. The mesogastrium now pouches
to form the great omentum and rapidly enlarges. At first hardly
projecting beyond the greater curvature, it increases in length until it
forms a four-layered apron which hangs down as a loose sac over the
transverse colon and the coils of the small intestine (Fig. 177). In the
foetus of six months the cavity of the omental bag extends caudad as far
as the lower edge of the omentum. Later adhesions between the peritoneal
surfaces lining the interior of the bursa limit this extension.

The omental bursa is therefore formed by a ventral lamella, consisting
of two peritoneal layers, which hangs down from the greater curvature of
the stomach and passes around the caudal free edge of the omentum into
the double-layered dorsal lamella, which ascends, over the transverse
colon, to the original starting point of the dorsal mesogastrium along
the front of the vertebral column and aorta. Hence the "great omentum"
is originally composed of four layers of peritoneum.

The dorsal double lamella becomes adherent over a considerable area to
the parietal peritoneum of the dorsal abdominal wall. In this way the
organs developed between the two layers of the lamella obtain their
final fixed position. The pancreas becomes anchored and appears in the
adult as a "retro-peritoneal" structure, while the spleen is attached by
the "phrenico-lienal ligament" to the diaphragm.

In addition the dorsal omental lamella adheres in the fourth month to
the cephalic layer of the transverse mesocolon and to the transverse
colon.

Important illustrations of some of the intermediate stages in the human
development of this portion of the peritoneal tract are afforded by the
permanent adult conditions found in the abdominal cavity of some of the
lower primates, notably certain of the cynomorphous monkeys.

[Illustration: FIG. 227.--Abdominal cavity of _Macacus rhesus_, Rhesus
monkey, with the small intestine removed. (Columbia University Museum,
No. 63/1831.)]

Fig. 227 shows the abdominal cavity and disposition of the peritoneum in
a macaque monkey (_Macacus rhesus_, male) in the ventral view, with the
coils of small intestines removed and the omentum lifted up and
reflected upon the ventral body wall. The following important points of
difference from the arrangement in the _cat_ on the one hand, and in
_man_ on the other, are to be noted:

1. The large intestine presents the typical primate course, with an
ascending, transverse and descending colon. The ileo-caecal junction is
situated in the right iliac fossa.

2. The ascending and descending mesocola are still _free_, not having
become adherent to the parietal peritoneum along the dorsal abdominal
wall. Hence the caudal portions of the ventral surfaces of the two
kidneys are still covered by the _primitive parietal peritoneum_.

3. The great omentum is not yet adherent to the transverse colon and
mesocolon except for a short distance on the extreme right. At this
point the dorsal layer of the omentum has begun to contract adhesions to
the hepatic flexure of the colon and ascending colon, but the rest of
the transverse colon is free. Differing from the human arrangement is a
line of adhesion, uniformly present in these monkeys, between the dorsal
surface of the omentum along its right edge and the ventral
surface and right border of the _caecum_ and _ascending colon_, parts
which normally are not adherent to the omentum in man.

4. Hence in tracing the omentum to the left of the limited adhesion to
the hepatic flexure and ascending colon, _i. e._, nearly throughout the
entire extent of the transverse colon, we find the membrane passing
freely without adhesion over the cephalic surface of the transverse
mesocolon, which preserves its original free condition, independent of
the omentum. This arrangement is shown in the schematic sagittal section
in Fig. 230.

5. Tracing the omentum dorsad beyond the transverse colon and mesocolon
the pancreas is reached. Here we encounter the first extensive area of
omental or mesogastric adhesion. The omental peritoneum continues over
the ventral and caudal surfaces of the gland, investing the same, but
the dorsal surface has lost its serous covering and is anchored to the
ventral surface of the left kidney. Hence a sagittal section would show
the arrangement of the monkey's omentum as indicated in the schematic
Figs. 229 and 230. Making now a general comparison of the peritoneal
membrane of this animal with that of man, and of both with the preceding
common embryonal condition, we can draw the following conclusions,
indicated schematically in the five figures 228-232.

[Illustration: FIGS. 228-232.--Schematic sagittal sections of dorsal
mesogastrium and omental bursa, in man, monkey, and cat.]

[Illustration: FIG. 228.--Common embryonal condition, as illustrated by
cat, after rotation and formation of omental bursa.]

[Illustration: FIG. 229.--Area of adhesion between dorsal mesogastrium
and primitive parietal peritoneum in _Macacus_, producing condition
shown in Fig. 230.]

[Illustration: FIG. 230.--Arrangement of great omentum as found in
_Macacus rhesus_, shown without reference to areas of peritoneal
obliteration.]

[Illustration: FIG. 231.--Corresponding section of human adult
peritoneum showing, along dotted lines, area of peritoneal adhesion.]

[Illustration: FIG. 232.--Section showing human adult peritoneum without
reference to area of adhesion.]

1. The dorsal layer of the monkey's omentum in its proximal segment
behaves in the same way as in man, _i. e._, it becomes adherent to the
primitive parietal peritoneum down as far as the caudal margin of the
dorsal surface of the pancreas included between the primitive
mesogastric layers forming by their further growth the omental apron.

Therefore we find, as in the human subject,

(_a_) The pancreas adherent to the ventral surface of the left kidney.

(_b_) A portion of the ventral surface of the kidney, cephalad of the
pancreas, and the dorsal wall of the retrogastric (lesser peritoneal)
space lined by secondary parietal peritoneum derived from the third
layer of the omentum (original right layer of dorsal mesogastrium).

2. The monkey differs from adult man in the behavior of the dorsal
omental layer in relation to the cephalic surface of the transverse
mesocolon. The adhesion, which in the human subject fuses this layer
with the transverse colon and mesocolon, does not occur in the monkey.

Hence we have in this animal the following conditions:

(_a_) The omentum is non-adherent to the transverse colon and transverse
mesocolon.

(_b_) The caudal surface of the pancreas is lined by its original
mesogastric peritoneum.

(_c_) The transverse mesocolon is formed by the original two layers of
the primitive dorsal mesentery; hence its cephalic layer is not
"peritoneum of the lesser sac" as is the case in man.

(_d_) The caudal part of the ventral surface of the left kidney below
the pancreas, is covered by the original parietal peritoneum.

(_e_) Only one point or line of _secondary peritoneal transition_
exists, where the dorsal layer of the omentum in the adult becomes
continuous with the parietal peritoneum covering the caudal surface of
the pancreas and the ventral surface of the left kidney.

_Note_: In the schematic sections shown in Figs. 228 to 232 the
transverse colon is represented as far removed from the ventral surface
of the left kidney, in order to make the peritoneal lines of the
mesocolon more clear. Actually a sagittal section which would divide the
kidney would cut the transverse colon at its extreme left end, where it
turns close to the ventral surface of the left kidney and then follows
its lateral border to form the splenic flexure (Fig. 235). The caudal
part of the ventral surface of the left kidney in the adult human
subject is covered by the peritoneum which, as secondary parietal
peritoneum, is derived from the upper part of the right leaf (later
ventral leaf) of the descending mesocolon. Hence it should be remembered
that these diagrams present _combinations_ of sections. A section which
will show the full development of the transverse mesocolon is mesad of
the kidney; while a section through the kidney would be too far laterad
to show the transverse mesocolon.

[Illustration: FIGS. 233-235.--Series of schematic sagittal sections
through left kidney and adrenal, pancreas, and transverse colon, to show
development of adult peritoneal relations.]

[Illustration: FIG. 233.--Embryonic condition, as illustrated by cat,
after rotation of intestine. Pancreas free between dorsal layers of
great omentum. Transverse colon and mesocolon free. Kidney behind
primitive parietal peritoneum.]

[Illustration: FIG. 234.--Area of adhesion between: 1. Primitive
parietal peritoneum. 2. Mesogastrium forming great omentum. 3. Colon and
mesocolon.]

[Illustration: FIG. 235.--Adult human arrangement, shown without
reference to obliterated areas.]

Figs. 233, 234 and 235 show sagittal sections through the left kidney
with the adult arrangement of the peritoneum and colon and the embryonic
and adhesion stages leading to the same.

It will be observed that in all the schematic sections of the early
embryonic stages the two layers of the transverse mesocolon are shown
without dorsal attachment, as turning with the formation of a fold (Fig.
228 at x) into two layers descending ventrad of the parietal peritoneum.
This is because the dorsal attachment of the mesocolon is at this stage
still in the median line and would hence not be encountered by a
sagittal section through the kidney, and because the two layers of the
transverse mesocolon, immediately after rotation of the large intestine,
are still directly continuous with the two layers of the descending
mesocolon. That is to say, the cephalic layer of the transverse
mesocolon is continuous with the dorsal (originally the left) layer of
the descending mesocolon, and the caudal layer of the transverse
mesocolon with the ventral (originally the right) layer of the
descending mesocolon, which is, in the human subject, to assume
subsequently the character of parietal peritoneum after the dorsal layer
and the primitive parietal peritoneum have become obliterated by
adhesion (Fig. 235).

Fig. 236 shows this continuity of the descending and transverse
mesocolon as a permanent adult condition in the macaque. The fold of
transition between the two is seen at x in Fig. 228. It will be noticed
that the ventral surface of the left kidney, caudad of the adherent
pancreas, is covered by the primitive parietal peritoneum, corresponding
to section in Fig. 230.


RELATIONS OF SPLEEN AND OMENTUM IN _MACACUS RHESUS_.

The spleen in this animal has not contracted any extensive adhesions to
the parietal peritoneum (the phrenico-lienal lig. of anthropotomy is not
developed). It can be turned mesad so as to expose the lateral border
and an adjacent segment of the ventral surface of the left kidney, as
well as the dorsal surface of the tail of the pancreas at its tip, still
covered by mesogastric peritoneum. Hence in the monkey the adhesion of
the original vertebro-splenic segment of the mesogastrium, including the
pancreas, to the primitive parietal peritoneum is less complete than in
man.

[Illustration: FIG. 236.--Abdominal viscera of _Macacus cynomolgus_, Kra
monkey. (Columbia University Museum, No. 1801.)]


MEDIAN ATTACHMENT OF DESCENDING MESOCOLON AND ITS RELATION TO THE
MESOCOLON OF THE SIGMOID FLEXURE IN THE _MACAQUE_.

Fig. 236 shows the abdominal viscera, hardened in situ, of _Macacus
cynomolgus_, the Kra monkey, in the ventral view and from the left side.

The great omentum is lifted up, the pancreas is adherent to the ventral
surface of the left kidney, the caudal portion of which is covered by
the primary parietal peritoneum, which can be exposed by turning the
still free descending mesocolon mesad. The mesocolon retains its
primitive attachment to the median line ventrad of the large
prevertebral blood vessels. It is readily seen that adhesion between the
left leaf of this free descending mesocolon and the parietal peritoneum
down to the level of the iliac crest would produce the conditions found
in the human adult, with an attached descending colon and a free sigmoid
flexure; also that limited adhesion of the mesocolon of the sigmoid
flexure to the parietal peritoneum would produce, as previously
explained (cf. p. 97), the intersigmoid peritoneal fossa.

=2. Ventral Mesogastrium and Liver.=--The peritoneal reflections from
the stomach to the liver, and the arrangement of the membrane in
connection with the latter organ, remain for consideration.

Certain complicated adult conditions, encountered in this part of the
abdominal cavity, make it desirable to arrange the subject for purposes
of study under the following subdivisions:

I. The development of the liver and of its vascular system, and the
significance of the adult circulation of the liver and of the foetal
remnants connected with the organ.

II. The anatomy of the ventral mesogastrium and the changes produced in
the arrangement of the membrane by the development of the liver.

=I. A. Development of the Liver.=--The liver, like the pancreas, is
developed from the duodenum as an outgrowth from the hypoblast lining
the enteric tube. As we have previously noted, the first outgrowth of
the hepatic diverticulum is closely associated with the distal
pancreatic outbud; in fact the latter arises as a derivative from the
hepatic duct rather than as a distinct outbud from the intestinal tube.
(This close association of the hepatic duct with the pancreas is well
seen in the arrangement of the concealed pancreas of some teleosts (cf.
p. 117, Fig. 196).)

In point of time the liver is the first accessory structure to develop
by budding from the primitive alimentary canal, the pancreas and lung
following.

[Illustration: FIG. 237.--Longitudinal section of an embryo of
_Petromyzon planeri_, four days old. (Minot, after Kupffer.)]

In the primitive type of development, as seen in _Petromyzon_ and in the
Amphibia, the liver appears very early, as a diverticulum of the
embryonic intestinal tube, near its cephalic extremity, projecting on
the ventral aspect down into the mass of yolk-cells (Fig. 237). The
short stretch of the primitive alimentary canal cephalad of the hepatic
diverticulum corresponds to the foregut. With the development of the
heart the primitive foregut becomes divided into pharynx and
post-pharyngeal segment (oesophagus and stomach). The hepatic
diverticulum then lies immediately dorsad of the caudal or venous
extremity of the heart. Hence it is probable that the liver is an older
organ in the ancestral history of the vertebrates than the pharynx or
even the heart. The liver diverticulum lies in close connection with the
omphalo-mesenteric veins which return the blood from the yolk-sac to the
heart. In the course of further development, as will be seen below, the
liver comes into very intimate relations with the venous circulation.

In human embryos of 3.2 mm. the primitive hepatic duct appears as a wide
hollow pouch composed of hypoblast cells, growing between the two layers
of the ventral mesogastrium, which membrane, extending between the
ventral border of the primitive stomach and the ventral abdominal wall,
will be subsequently considered in detail. The liver, in developing
between the layers of the ventral mesogastrium, approaches very early
the _septum transversum_ or rudimentary diaphragm and becomes connected
with the same. A mass of mesodermal cells, derived from the mesogastrium
and from the primitive mesodermal intestinal wall surrounding the
hypoblastic lining of the tube, covers the caecal termination of the
primitive hepatic duct, forming the so-called embryonic _hepatic ridge_.
This mesodermal tissue accompanies the duct in its further growth and
branching, forming the connective tissue envelope, known in the adult as
the capsule of Glison. The primitive hepatic duct is directed cephalad
in the mesogastrium between the vitelline duct and the stomach (Fig.
101).

In embryos measuring 4.25 mm. the duct is 0.24 mm. long. Later (in
embryos of 8 mm.) the primitive single duct divides into two secondary
branches, indicating, even at an early stage, the adult arrangement of
the duct, as formed by the union of the right and left hepatic ducts
(Fig. 185).

The gall-bladder in embryos of this size (8 mm.) is a well-defined caecal
diverticulum, branching caudad from the main hepatic duct.

The vesicular mucous surface is thus derived from the enteric hypoblast
in the same way as the epithelial lining of the bile-ducts and
capillaries. The external muscular and fibrous coats of the gall-bladder
are developed from the mesoderm of the mesogastrium.

It is to be noted that at an early stage the gall-bladder is derived
from the main duct close to the intestine, the latter duct being very
short. Later on the common duct grows in length, making the liver more
and more a gross anatomical organ distinct from the intestine. The
cystic duct develops as the result of a similar increase in length of
the cystic diverticulum. The two principal secondary branches of the
hepatic duct give origin to sprouts or buds. These are derivatives of
the hypoblastic cells of the larger ducts and may from the beginning be
hollow, possessing a lumen continuous with that of the parent duct
(Selachians, Amphibians). In warm-blooded animals these sprouts are at
first solid, forming the s. c. _hepatic cylinders_, and only
subsequently become hollowed out with the further development of the
biliary duct system of the liver. The rapid growth of the organ leads to
a great increase in the number of the hepatic cylinders. They spread out
on all sides, finally coalescing with adjacent buds so as to form an
interlacing network whose meshes are filled by blood vessels. After the
hepatic cylinders have become canalized they preserve the same
arrangement, hence the resulting biliary capillaries of the adult form
an anastomosing network. Amphioxus and the amphibians have a single
hepatic outgrowth (Fig. 49).

In the Selachians the liver arises as a ventral outgrowth at the hinder
end of the foregut immediately in front of the vitelline duct, thus
bringing the liver from the beginning into close proximity with the
vitelline veins entering the heart. Almost as soon as formed the
outgrowth develops two lateral diverticula, opening into a median canal.
The two diverticula are the rudimentary lobes of the liver and the
median canal uniting them is the rudiment of the common bile-duct and
gall-bladder.

In the Teleosts the liver arises quite late (in the trout about the 25th
day) as a solid outgrowth from the intestinal canal close to the heart.
In the Amniota the liver arises in the same position as in the Anamnia,
but, at least in birds and mammals, shows its bifurcation almost, if not
quite, from the start. The two forks embrace between them the
omphalo-mesenteric or vitelline veins just before they empty into the
sinus venosus of the heart.

In the chick the liver appears between the 56th and 60th hour, the right
fork being always of greater length but less diameter than the left. The
hepatic outbud in the rabbit appears during the 10th day, and during the
11th day begins to send out branches.

In man, as above stated, the bud appears well marked in embryos of 3
mm.

[Certain adult variations make it appear possible that there are two
human embryonic hepatic buds, a cranial and a caudal, as is the case in
birds.]

=I. B. Comparative Anatomy of the Liver.=--The liver, phylogenetically a
very old organ, occurs in all vertebrates, for the caecal diverticulum of
the intestine of amphioxus (Fig. 49) has probably the significance of a
hepatic outbud.

The primitive form of the liver is symmetrically bilobed, a type which
is seen well in the chelonian organ (Fig. 238).

[Illustration: FIG. 238.--_Pseudemys elegans_, pond turtle. Alimentary
canal. (Columbia University Museum, No. 1437.)]

In size the liver is subject to great variations. It is usually larger
in animals whose food contains much fat. Hence carnivora in general have
a larger liver than herbivorous animals.

Its shape also varies considerably, depending on the form of the body
cavity and on the amount and disposition of the available space. Hence
in the snakes the organ appears long drawn out, flattened, almost
ribbon-like (Fig. 239), while the relatively very large coronal diameter
of the body cavity in the turtles permits the liver to expand
transversely (Fig. 238).

[Illustration: FIG. 239.--Stomach, mid-gut, pancreas, and liver of _Boa
constrictor_, boa. (Columbia University Museum, No. 1832.)]

[Illustration: FIG. 240.--Liver of _Macacus cynomolgus_, Kra monkey.
(Columbia University Museum, No. 28/1833.)]

[Illustration: FIG. 241.--Liver of _Pleuronectes maculatus_, flounder.
(Columbia University Museum, No. 1679.)]

In general, when the liver is large and the available space for its
reception limited, it is usually split into several (two to seven)
lobes, which permit, by mutual displacement, the accommodation of the
organ to varying space-conditions of the body cavity (Fig. 240). Under
the opposite circumstances, on the other hand, even the primitive
bilobed character may disappear and the liver is then unlobed (Fig.
241).

The presence or absence of a gall-bladder depends apparently largely on
the character of the food and on the habitual type of digestion. In many
vertebrates digestion is carried on nearly continuously, without marked
interruption, especially in many ungulates, ruminants and rodents. In
such animals the gall-bladder is absent. It is also absent in several
birds (most Parrots, Doves, Ostrich, Rhea americana, the Cuculidae,
Rhamphastos, etc.). This variability emphasizes the morphological fact
that the biliary bladder is only a modified portion of the hepatic duct
system, as shown by the development above outlined.

A great variety is observed in the arrangement of the biliary ducts,
through which, at the period of intestinal digestion, bile passes from
the liver and gall-bladder into the intestine, while in the intervals of
digestion the secretion is only carried from the liver to the bladder.
The following main types of the biliary duct system may be recognized:

1. The hepatic duct joins the cystic to form the common bile-duct,
entering the duodenum by passing obliquely through the intestinal wall
(Fig. 242). This form is encountered in man and in most mammals. It is
also found in some birds (_Buceros_), many amphibians, and in some fish
(_Lophius_). Instead of one hepatic duct two may join the cystic duct
separately to form the common bile duct (_Phoca litorea_), or the number
of hepatic ducts may be further increased. The separate hepatic ducts
then unite successively with the cystic duct. This occurs in many
mammals (as _Tarsius_, _Galeopithecus_, monotremes) and in some fishes
(_Xiphias_, _Trigla_, _Accipenser_) (Fig. 243).

[Illustration: FIG. 242.--Schema of hepatic and cystic ducts. (Nuhn.)]

[Illustration: FIG. 243.--Schema of hepatic and cystic ducts. (Nuhn.)]

2. Of two hepatic ducts only one helps to form with the cystic duct the
common duct, while the other leads from the liver transversely into the
bladder, especially into the neck, forming the hepatico-cystic duct
(Fig. 244). This arrangement is found in several mammals (calf, sheep,
dog).

[Illustration: FIG. 244.--Schema of hepatic and cystic ducts. (Nuhn.)]

[Illustration: FIG. 245.--Schema of hepatic and cystic ducts. (Nuhn.)]

3. No common bile-duct is formed. The hepatic and cystic ducts each
empty separately into the intestine (hepato-enteric and cysto-enteric
ducts), while a hepato-cystic duct carries the bile directly from the
liver to the gall-bladder (Fig. 245).

_Lutra vulgaris_ among mammalia, the majority of the birds and several
reptilia present this type.

When the gall-bladder is absent a single large hepato-enteric duct is
found, or instead a number of smaller ducts which enter the intestine
successively.

=I. C. Development of Vascular System of Liver.=--In order to comprehend
the peritoneal relations of the adult liver it is absolutely necessary
to have a clear understanding of the development of the vascular system
in connection with the gland.

For our purpose, in the first place, a serial consideration of the
successive stages, illustrated by schematic diagrams, will prove most
practicable. These diagrams represent the structures in the dorsal view,
_i. e._, in the position which they would occupy in the adult liver with
the gland resting on its upper or convex surface and with the ventral
sharp margin turned toward the beholder (see Fig. 259).

The development of the venous system, especially in connection with the
liver, presents a somewhat complicated series of successive conditions.
After having become familiar with the principal typical embryonal
stages, as shown in the following diagrams, the student is strongly
recommended to cement this knowledge by the comparative examination of
the venous system. The permanent veins of the lower vertebrates, while
in many cases not strictly homologous to those of the higher forms, yet
are excellent objects for study, since they serve to illustrate
temporary stages in the development of the mammalian venous system, and
to that extent are of aid in comprehending one of the most difficult and
important chapters in human anatomy. At the conclusion of the
diagrammatic consideration of the mammalian development a number of
comparative facts will be put together for this purpose.

=1. Early Stage.=--In the earlier developmental stages in mammalian
embryos the primitive dorsal aorta extends caudad along the ventral
aspect of the vertebral axis, giving off paired vitelline or
omphalo-mesenteric arteries to the yolk-sac and allantoic arteries to
the embryonic urinary bladder or allantois (Figs. 246 and 247).

[Illustration: FIG. 246.--Diagram of embryonic vascular system, without
the portal circulation. (Parker, after Wiedersheim.) The dorsal aorta is
formed by the junction of the right and left aortic roots arising from
the confluence of the branchial arterial arches.]

The blood is returned from the vascular area of the yolk-sac by two
vitelline or omphalo-mesenteric veins, which unite near the heart to
form a common trunk, continued as the _sinus venosus_ into the caudal or
auricular extremity (venous end) of the primitive tubular heart (Figs.
246, 247 and 248).

[Illustration: FIG. 247.--Diagram of the circulation of the yolk-sac at
the end of the third day of incubation in the chick. (After Balfour.)
The median portion of the first aortic arch has disappeared; but its
proximal end forms the external, its distal the internal carotid
arteries. The whole blastoderm has been removed from the egg and is
viewed from below. Hence the left appears on the right, and _vice
versa_.

Arteries in black.

Veins in outline.]

[Illustration: FIG. 248.--Schema of vitelline veins.]

=2. Development of Allantois. Stage of Placental Circulation.=--The
placental circulation, replacing the temporary vitelline circulation of
the earliest stages, is inaugurated by the appearance of two umbilical
veins, which pass cephalad, imbedded in the tissue of the ventral
mesogastrium, to empty into the sinus venosus near the vitelline veins
(Fig. 249). The umbilical veins return the oxygenated blood from the
placenta to the embryo. At first the right umbilical vein is the larger
of the two.

[Illustration: FIG. 249.--Schema of umbilical veins, early stage.]

The sinus venosus at this time also receives two large veins,
transversely directed, called the ducts of Cuvier, which are formed near
the heart by the union of the anterior cardinal (primitive jugular) and
posterior cardinal veins, draining respectively the head end of the
embryo, and the body walls and Wolffian bodies.

The vitelline veins are placed on each side of the primitive small
intestine, and become connected with each other by a broad anastomotic
branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the
vitelline veins send out branches which break up into a wide-meshed
capillary network in the mesodermic tissue enveloping the hepatic
cylinders. Hence at this period the circulation in the vitelline veins
is made up of three districts:

(_a_) Distal segment of veins, coursing along duodenum, and joined by a
transverse anastomosis, before reaching the liver bud (subintestinal
veins).

(_b_) Middle segment, from which capillary vessels are derived,
ramifying upon and between the developing hepatic cylinders.

(_c_) Proximal segment, formed by the continuation of the proximal part
of the vitelline veins into the sinus venosus of the heart.

[Illustration: FIG. 250.--Schema of primitive portal circulation.]

=3. Formation of Portal Circulation. A.=--With the further development
of the liver the direct connection of the distal segment of the
vitelline veins with the sinus venosus becomes lost, the intermediate
segment being entirely broken up into an intrahepatic network (Fig.
250). Hence all the blood brought to the liver by the vitelline veins
(venae hepaticae advehentes) passes through the hepatic capillary
circulation, before it is carried by the proximal segment of the
vitelline veins (venae hepaticae revehentes) into the sinus venosus. The
amount of this blood increases with new connections which the vitelline
veins make with the venous radicles developing in the intestinal tract
and its appendages. In proportion as, with the development of the
placenta and reduction of the yolk-sac, the original significance of the
vitelline veins as nutritive and respiratory vessels disappears, this
secondary connection of the vitelline veins with the veins of the
alimentary tract becomes more and more important, until finally the
original vitelline veins, now properly called omphalo-mesenteric veins,
return the blood from the intestinal tube, pancreas and spleen to the
liver.

The venae hepaticae advehentes, becoming connected in this way with the
developing intestine, pancreas and spleen, form the rudiments of the
future portal system, while the venae hepaticae revehentes are prototypes
of the hepatic veins of the adult circulation.

=B. Development of the Portal Vein.=--The distal subintestinal segments
of the vitelline veins are early united by a transverse anastomotic
branch. The section of the veins above this anastomosis is seen already
in Fig. 250 to have assumed an annular shape, while the veins below the
primary anastomosis are approaching each other to form a second
ring-like junction.

In Fig. 251 the subintestinal segments of the two vitelline veins are
seen to have communicated with each other by transverse anastomotic
branches around the duodenum, two of these branches being situated
ventrad and one dorsad of the intestinal tube. These branches, and the
portions of the primitive vitelline veins between their points of
derivation, form two vascular loops or rings, encircling the primitive
duodenum (Fig. 251).

[Illustration: FIG. 251.--Schema of further development of portal
circulation and connection of same with umbilical veins in early
stages.]

The distal portions of the vitelline veins, before reaching the caudal
annular duodenal anastomosis, next fuse into a single longitudinal
vessel which also receives the veins from the stomach, intestine,
spleen, and pancreas, and forms the beginning of the portal vein.

By atrophy of the right half of the lower, and of the left half of the
upper duodenal venous ring (Figs. 252 and 253), the proximal portion of
the portal vein is formed as a single vessel, taking a spiral course
around the duodenum (Fig. 256). Hence in the adult the portal vein and
its principal branch (the superior mesenteric vein) crosses over the
ventral surface of the duodenum (third portion), turns along the mesal
side of the second portion, and then continues to the liver along the
dorsal aspect of the first portion (Fig. 254). _Note_--In comparing Fig.
254 with the schematic figures it should be noted that the same presents
the parts in the _ventral_ view, while the schemata offer the _dorsal_
aspect.

[Illustration: FIG. 252.--Second stage in development of circulation
through portal and umbilical veins. The proximal segment of the main
portal vein is formed by the persistence of the left half of the distal
and right half of the proximal periduodenal vascular ring of the
omphalo-mesenteric veins. The distal segment of the main portal vein is
the product of the fusion of the omphalo-mesenteric veins, and becomes
connected with the veins of the intestinal canal, pancreas, and spleen.
The proximal terminal segment of both umbilical veins becomes included
in the system of the venae hepaticae revehentes.]

[Illustration: FIG. 253.--Third stage in development of portal and
umbilical veins during the placental period.]

[Illustration: FIG. 254.--Corrosion preparation showing course of portal
vein and tributaries in relation to duodenum. (Columbia University
Museum, No. 1857.)]

[Illustration: FIG. 255.--Human embryo of 10 mm. cervico-coccygeal
measure. Heart and ventral body-wall removed to show sinus venosus and
entering veins. (Kollmann, after His.)]

[Illustration: FIG. 256.--Final stage of development of portal and
umbilical veins in the placental period.]

=4. Changes Leading to the Final Arrangement of the Umbilical Veins.=--A
very important rearrangement of the umbilical veins takes place. These
veins originally course in the lateral abdominal wall, close to the fold
of the amnion (Fig. 255), and then turn cephalad of the developing liver
along the septum transversum to empty into the sinus venosus at each end
(Figs. 249 and 250). The right umbilical vein is at first the larger.

This symmetrical arrangement, and the direct connection of the umbilical
veins with the sinus venosus, now becomes lost by the occurrence of the
following changes:

1. At first (Fig. 249) all the blood carried to the liver by the
omphalo-mesenteric veins passes through the hepatic capillary network
before being conducted by the venae revehentes to the sinus venosus. Very
early, however, a new intrahepatic channel develops, the ductus venosus
(Figs. 250-253), which passes obliquely between the entrance of the left
omphalo-mesenteric vein into the capillary system (l. v. advehens) and
the termination of the right omphalo-mesenteric vein (r. vena revehens)
in the sinus venosus.

In human embryos of 4 mm. the ductus venosus can already be
distinguished, and in embryos of 5 mm. the vessel has assumed
considerable proportions.

2. A communication is next established on both sides between the
capillary hepatic network in the portion of the liver nearest to the
abdominal wall and the umbilical veins as they ascend imbedded in the
abdominal wall (Fig. 251).

This connection is usually from the start larger on the left side and
connects with the left omphalo-mesenteric vein just at the point where
the same is about to be continued into the ductus venosus. This
connection becomes rapidly larger, so that the ductus venosus, which at
first appeared merely as an anastomotic channel between the left
omphalo-mesenteric vein and the terminal portion of the right
omphalo-mesenteric vein, now forms the main continuation of the left
umbilical vein. This vessel grows very rapidly up to its connection with
the ductus venosus and soon exceeds the right umbilical vein in size
(Fig. 252). Beyond the ductus venosus on the other hand the proximal
segment of the left umbilical vein diminishes in size, and loses its
independent character by incorporation in the hepatic circulation. Only
its terminal portion, emptying into the sinus venosus, is preserved.
This is surrounded by the growing masses of hepatic cylinders and is
converted into a vena revehens.

The connection of the right umbilical vein with the liver vessels is at
first symmetrical to that on the left side, but less strongly developed.
The effect of this connection is to reduce in the same way the proximal
segment of the right umbilical vein and to convert its termination into
a vena revehens. With the great development of the left vein, however,
the vein on the right side gradually diminishes and finally loses its
connection with the intrahepatic circulation altogether. The right
umbilical vein is now reduced to a vessel of the ventral abdominal wall,
which carries blood in the reverse of the original direction, _i. e._,
from the abdominal wall caudad _into_ the left umbilical vein (Figs. 253
and 255).

The connection thus established between the umbilical vein and the
portal circulation results in the formation of a single large (the
original left) umbilical vein which, throughout the remainder of foetal
life, returns all of the placental blood (Fig. 253).

The newly developed hepatic portion of the left umbilical vein becomes,
however, not only connected with the ductus venosus, but also with the
right part of the upper venous ring, derived from the right
omphalo-mesenteric vein (Fig. 253). This connection forms the left
portal vein of the adult, and enlarges rapidly.

The terminations of the ductus venosus and of the venae hepaticae
revehentes undergo a number of secondary changes in relative position.
The left hepatic vein loses its direct connection with the sinus
venosus, and now opens into the termination of the ductus venosus, into
which the right hepatic vein also empties. This common vessel (v.
hepatica communis) subsequently forms the proximal segment of the
postcava when this vessel develops (Fig. 256).

The blood, therefore, returned to the liver by the left umbilical vein
divides at the transverse fissure into three streams. Two of these pass
through the connection with the portal vein and through branches
developed from the hepatic part of the umbilical vein into the capillary
system of the right and left lobe. The third continues through the
ductus venosus to the common hepatic vein and sinus venosus (Fig. 256).
The ductus venosus thus becomes the chief vessel returning arterialized
placental blood to the heart. When the postcava develops fully the
hepatic segment of this vessel also joins the terminal part of the
ductus venosus (Fig. 256) and gradually replaces the same as the main
returning venous channel, the proximal part of the ductus venosus being
incorporated in the vena cava (Fig. 257). The postcava then receives the
right hepatic veins separately, while the left hepatic veins and ductus
venosus open together into the main vein. This condition obtains up to
the time of birth and the consequent interruption of the placental
circulation.

[Illustration: FIG. 257.--Schema of relation of postcava to hepatic
veins and ductus venosus.]

While at first the ductus venosus communicates throughout its entire
length with the meshwork of the hepatic capillary system, a separation
into two segments, _i. e._, ductus venosus proper and intrahepatic
segment of umbilical vein, is established after the free communication
with the left umbilical vein takes place. This condition is exhibited in
Fig. 258, which represents the corroded venous system of the foetal
liver, and in Fig. 259, showing an injected liver in the foetus at term.

[Illustration: FIG. 258.--Corrosion preparation of venous system of
human liver in foetus at term. (Columbia University Museum, No. 1834.)]

[Illustration: FIG. 259.--Injected and hardened human liver from foetus
at term. (Columbia University Museum, No. 1853.)]

It will be observed that the umbilical vein on entering the liver gives
off a large branch to the left lobe, and a smaller branch on the right
side to the quadrate lobe, which act as the main venae advehentes of
these portions of the liver. Arrived at the transverse fissure the
umbilical vein divides into three branches, at right angles to each
other. The left branch enters the left lobe, the right branch becomes
directly continuous with the left main division of the portal vein,
while the central branch, continuing the direction of the umbilical
vein, passes dorsad, as the ductus venosus proper, to join the left
hepatic vein close to its entrance into the postcava.

=5. Changes Consequent upon the Establishment of Pulmonary
Respiration.=--After birth the umbilical vein and its continuation, the
ductus venosus, become obliterated, the former constituting the round
ligament of the liver, the latter the ligament of the ductus venosus,
both structures imbedded in corresponding portions of the sagittal
fissure on the caudal and dorsal surfaces of the adult liver (Figs. 284
and 286). The lateral branches of the umbilical vein, however, in its
course from the ventral margin of the liver to the transverse fissure
(Fig. 258), remain pervious and are transferred to the portal
circulation.

[Illustration: FIG. 260.--Diagram of intrahepatic foetal venous
circulation.]

[Illustration: FIG. 261.--Diagram illustrating the changes in the
intrahepatic venous circulation resulting from the cessation of the
placental circulation at birth.]

It will be noticed, in reference to the _direction_ of the blood
current, that at birth a sudden reversal takes place in the right
terminal branch of the umbilical vein at the transverse fissure (Figs.
260 and 261). Before birth the blood current of the umbilical vein
divides into three streams, right, left and central. The latter enters
the ductus venosus. The left enters the liver directly, the right
traverses, from left to right, the segment between the termination of
the umbilical and the bifurcation of the portal vein. This segment in
the adult carries blood from right to left, as left branch of the portal
vein. In the foetus, however, the blood traverses this segment from left
to right, in passing from the umbilical to the right branch of the
portal vein. The blood entering the liver through the portal vein passes
chiefly into the right division of that vessel (Fig. 260).

After birth all the venous blood entering the liver passes
through the portal vein. In the right division the direction of the
current is the same as in the foetus.

On the left side, however, the current is now from right to left, from
the bifurcation of the portal into the channels of the left lobe
formerly connected with the umbilical vein (Fig. 261).

Hence the direction of the current in this segment is reversed at birth.


SUMMARY OF HEPATIC CIRCULATION.

The foregoing consideration of the development shows us that the hepatic
circulation presents successively three main stages:

=1. Omphalo-mesenteric or Vitelline Stage=, which results in the laying
down of the primary capillary circulation of the liver and in the
establishment of its connection with the developing veins of the
alimentary tract (primitive portal channels).

=2. Umbilical or Placental Stage=, in which the greater part of the
blood circulating through the liver is oxygenated blood returned from
the placenta by the umbilical vein, accounting for the rapid growth and
relatively large size of the organ during foetal life.

The placental blood uses the preformed capillary channels of the
vitelline or primitive portal system in the liver, and the same rapidly
extend and enlarge with the accelerated growth of the gland. During this
stage venous blood is also returned from the alimentary tract to the
liver by the portal vein, produced by fusion of the distal segments of
the primitive vitelline veins and their secondary connection with the
mesenteric, splenic and pancreatic veins (omphalo-mesenteric development
of primitive vitelline veins).

=3. Adult or Portal Stage.=--With the interruption of the placental
circulation the portal vein assumes again its original position as the
only vein carrying blood to the liver. With the establishment of
intestinal digestion and absorption this vessel grows rapidly in size.


COMPARATIVE ANATOMY OF THE HEPATIC VENOUS CIRCULATION.

For the purpose of fixing the main facts in connection with the
development of the higher mammalian hepatic circulation, and in order to
obtain a demonstration of the cycle through which the different veins
pass, the student is recommended to examine, preferably by personal
dissection, a limited series of lower vertebrates which can be readily
procured and easily injected. The following series has been selected,
but it will be understood that other forms can be substituted, according
to the local conditions which govern the supply of the material.

1. _Fish._ _A Selachian_, the common skate (_Raja
  ocellata_) or dog-fish (_Acanthias vulgaris_).

2. _Amphibian._
  (_a_) Urodele. _Necturus maculatus._
  (_b_) Anura. The common _frog_.

3. _Reptile._

Preferably, on account of the ease of injection, one of the larger
lizards, as _Iguana tuberculata_.

The turtles, although somewhat more difficult objects to prepare, can be
substituted.

4. _Bird._ The common fowl.

5. Human foetus at term.

=1. Fish.=--The venous system can be injected by tying a canula in the
lateral vein, and injecting both cephalad and caudad, or by injecting
cephalad through the caudal vein. The injection of the systemic veins
can also be made caudad through one of the ducts of Cuvier, combined
with an injection cephalad of the caudal vein.

[Illustration: FIG. 262.--Diagram of the veins of a selachian.
(Wiedersheim, after Parker.)

The lateral vein arises from a venous network surrounding the cloaca,
receiving one or more cutaneous veins of the tail, veins of the
body-wall, and veins of the pelvic fins.

The caudal vein divides at the posterior end of the kidney into the two
renal-portal veins, from which the advehent veins of the renal-portal
system are derived. The revehent renal-portal veins join to form the
posterior cardinal veins, which, after dilating enormously to form the
cardinal sinuses, join with the anterior jugular, subclavian, and
lateral veins to form the ducts of Cuvier. The latter receive the
inferior jugular veins, from the deep parts of the head and neck and the
terminations of the hepatic portal system (hepatic sinus).

The hepatic portal vein is formed by the veins of the oesophagus,
stomach, and intestines. After traversing the capillary vessels of the
liver, the revehent hepatic veins unite to form an extensive hepatic
sinus before entering the heart.]

The following main facts are to be noted in the venous system of the
Selachian (Fig. 262):

=1. There are Two Portal Systems.= (_a_) _Renal Portal System._--The
caudal vein divides near the vent into two branches which course along
the lateral border of the kidneys, sending _afferent_ or _advehent_
veins into the organ. The blood traverses the renal capillaries and is
gathered together by the _efferent_ or _revehent_ veins, which empty
into median paired vessels, the posterior cardinals.

(_b_) _Hepatic Portal System._--The veins of the digestive tract and
appendages unite to form a hepatic portal vein. The blood after
traversing the capillary system of the liver is collected by hepatic
veins, which form a dilated hepatic sinus emptying into the sinus
venosus of the heart.

2. The middle segment of the intestine, presenting a spiral valve in the
interior, gives rise to a vein emptying into the portal vein which
corresponds to the subintestinal vitelline vein of the mammalian embryo
(Fig. 202).

3. The posterior cardinal veins, also greatly dilated and forming the
posterior cardinal sinus, join, near the heart, the veins returning
blood from the head, the anterior cardinal or jugular, to form a
transversely directed trunk, the duct of Cuvier, which empties into the
sinus venosus at the auricular extremity of the heart. Into the duct of
Cuvier empties on each side a _lateral vein_ returning the blood from
the body walls. This vein can be considered, for our present purpose, as
representing in general the abdominal vein of amphibians and reptiles,
and the umbilical vein of the mammalian embryo.

The adult selachian venous system is therefore to be considered as
illustrating the following conditions above encountered in our study of
the embryology of the mammalian venous system.

1. The heart illustrates excellently the stage in the mammalian
development, in which auricular and ventricular segments have
differentiated, but before the division of the cavities into a pulmonary
and systemic portion by the development of the auricular and ventricular
septa and the division of the arterial trunk into pulmonary artery and
aorta.

The sinus venosus still exists, as an ante-chamber to the auricular
cavity proper, receiving on each side the ducts of Cuvier, which
represent the fusion product of the systemic veins, anterior and
posterior cardinal.

2. The hepatic portal circulation corresponds to the mammalian stage in
which the vitelline veins have become omphalo-mesenteric by joining the
intestinal veins.

The spiral vein remains as a portion of the original vitelline vein
corresponding to the subintestinal segment of the mammalian embryo (cf.
Figs. 248 and 249).

The selachian portal vein represents the united vitelline veins, into
which the veins of the digestive tract open.

In the liver we find a simple system of venae advehentes, derived from
the branching of the portal vein, a hepatic capillary network, and venae
revehentes, the proximal remnants of the original vitelline veins which
carry the liver blood to the sinus venosus. The condition of the hepatic
circulation corresponds therefore to the stage shown in Fig. 250 of the
mammalian development. There is as yet no association of the hepatic
venous system with the representative of the umbilical vein (the lateral
vein of the selachian).

3. The lateral veins, which we can, as stated, regard for purposes of
illustration, without prejudging their genetic significance, as
representing the mammalian embryonic umbilical veins, still present the
condition corresponding to the early mammalian embryonal stage shown in
Fig. 250. They are veins of the body walls, emptying cephalad of the
liver, directly into the ducts of Cuvier, and through them into the
sinus venosus of the heart.

Fig. 262 shows the arrangement of the venous system in a typical
selachian diagrammatically.

=2. Amphibian.= (_a_) =Urodele.=--The following points are to be noted
in comparison with the preceding form:

1. The two ducts of Cuvier entering into the sinus venosus are formed by
the anterior cardinal and subclavian veins, which latter, having
appeared with the full development of an anterior extremity, receives
the posterior cardinal veins, representing the mammalian azygos system.

2. The renal portal circulation persists. The caudal vein is, however,
no longer the only afferent vein of this system. With the full
development of a posterior extremity an iliac vein returns the blood
from the same and gives a large branch (afferent to the portal renal
system), while the trunk continues cephalad as an anterior abdominal
vein, corresponding to the lateral selachian vein, emptying in the
hepatic portal vein.

3. The efferent veins of the renal portal system no longer unite to form
the posterior cardinal, as in the Selachian, but empty into a new median
vessel, the inferior vena cava, or postcava, which has replaced the
distal segments of the posterior cardinal veins.

The postcava now carries the blood from the kidneys directly to the
heart. The original posterior cardinal veins still persist in their
proximal segments, as smaller trunks connecting the distal part of the
postcava with the ducts of Cuvier through the subclavian veins. The
ducts of Cuvier represent the precavae (venae cavae superiores) of mammalia
and the postcardinals the mammalian azygos veins.

4. The hepatic portal system differs in two respects from the Selachian
type.

(_a_) The blood returned to the liver from the digestive tract by the
portal vein becomes mixed before entering the gland with the blood
returned from the posterior extremities and abdominal walls by the
abdominal vein.

This vein, paired below and continuous with the lateral of the two
branches into which the iliac vein divides, becomes united into a single
trunk above and empties into the portal vein.

The abdominal vein represents the lateral vein of the Selachian and
corresponds to the umbilical vein of the higher vertebrates.

(_b_) The venae hepaticae revehentes do not empty directly into the sinus
venosus, but into the proximal portion of the postcava.

Hence the adult urodele venous system illustrates, in reference to the
mammalian development, these stages:

1. The umbilical (abdominal) vein has lost its direct connection with
the sinus venosus. The proximal segment, cephalad of the liver, has
disappeared, and its blood now passes directly into the hepatic
circulation by its union with the portal vein.

(Cf. stage schema Figs. 251 and 252.)

2. The postcaval vein has made its appearance, largely replacing the
posterior cardinal veins, whose proximal segments became converted into
secondary vessels (azygos) uniting the system of the postcava with that
of the duct of Cuvier (mammalian praecava), while their distal segments
are transformed into the distal portion of the postcava.

The postcava, therefore, is made up of two districts:

(_a_) The proximal portion is a new vessel, developed in connection with
the hepatic venous system.

(_b_) The distal portion is derived from the distal segments of the
original posterior cardinal veins.

The termination of the hepatic veins in the postcava corresponds to the
stage shown in schema Fig. 256.

Fig. 263 gives a schematic representation of the arrangement of the
venous system in a typical urodele amphibian (_Salamandra maculosa_).

[Illustration: FIG. 263.--Diagram of the veins of urodele amphibian
(_Salamandra maculosa_). (Wiedersheim.)

The caudal vein bifurcates at the posterior extremity of the kidneys to
form the afferent trunks of the renal-portal system along the lateral
border of the kidneys, from which the advehent veins of the renal-portal
system are derived. The iliac or femoral vein divides into an anterior
and a posterior branch, the latter opening into the afferent
renal-portal vein, while the former, uniting with the one of the
opposite side, forms the abdominal vein, and receives vessels from the
bladder, cloaca, and end-gut. The revehent veins of the renal-portal
system, emerging upon the ventral surface of the kidneys, empty into a
single median vessel, the distal or renal section of the postcava or
vena cava inferior. Proceeding cephalad, the proximal or hepatic section
of this vessel, after traversing the liver and receiving the revehent
hepatic veins of the hepatic portal system, empties into the sinus
venosus of the heart. Previous to entering the liver the postcava gives
off the two posterior cardinal or azygos veins, which continue cephalad,
receiving tributary segmental veins from the body-walls and reach the
sinus venosus by joining the subclavian veins. These latter uniting with
the anterior cardinal (jugular) veins form the ducts of Cuvier (precaval
veins).

The abdominal vein continues cephalad in the ventral mesogastrium to the
liver, giving off a number of smaller branches, which enter the hepatic
portal circulation by penetrating the ventral surface of the liver
between the layers of the ventral mesogastrium, while the main
continuation of the vessel joins the hepatic portal vein at its point of
entrance into the liver.

The hepatic portal vein is formed by tributaries returning the blood
from the digestive tract (intestinal canal, spleen, pancreas). The
blood, after traversing the hepatic portal circulation, is conducted by
the hepatic revehent veins to the proximal section of the postcava. A
number of secondary or accessory portal veins pass from the anterior
portion of the intestinal canal (oesophagus, stomach) directly to the
liver.]

In Fig. 264 the dissected venous system of _Necturus maculatus_, the mud
puppy, is shown in an injected preparation.

[Illustration: FIG. 264.--Dissection of veins of _Necturus maculatus_,
mud-puppy. (Columbia University Museum, No. 1835.)

The postcava has been divided at the cephalic end of the liver just
before entering the sinus venosus, and the postcardinals have been cut
prior to their junction with the subclavian veins.

The stomach has been turned caudad. The abdominal vein has been divided
after the common trunk has been formed by branches from the iliac veins.
The latter are seen entering the afferent renal-portal vein, derived
from the bifurcation of the caudal vein, along the lateral border of the
kidneys.

The junction of the main trunk of the abdominal vein with the hepatic
portal vein takes place close to the liver under cover of the pancreas.
A series of accessory portal veins continuous with the abdominal vein
enter the ventral surface of the liver between the layers of the ventral
mesogastrium. The inter-renal segment of the postcava receives the
revehent renal-portal veins. The iliac vein enters the advehent
renal-portal veins derived from the caudal vein.]

[Illustration: FIG. 265.--Venous system of _Rana esculenta_, frog.
(Ecker.)]

(_b_) =Anure.=--The venous system of _Rana esculenta_ is shown in Fig.
265. Comparison with venous system of _urodele_:

1. The abdominal vein, corresponding to the mammalian umbilical vein,
has assumed a greater importance in reference to the hepatic
circulation. It is a large trunk, continuous below with the pelvic vein,
terminating above in two branches, which enter the liver as afferent
veins, being joined just prior to the division by the hepatic portal
vein.

2. A small cardiac vein, coming from the heart, empties into the angle
of bifurcation of the abdominal vein.

3. The postcava is well developed, formed by large efferent renal veins.
It entirely replaces the posterior cardinal veins which are absent in
the adult animal.

4. A right and left praecaval vein is formed by the union of two jugular
trunks with the vein of the anterior extremity and a large
musculo-cutaneous vein.

Comparison with the mammalian development: the venous system of this
amphibian can be used to illustrate the mammalian embryonal stage shown
in schema Fig. 252, in which the abdominal or umbilical vein has become
the most important vessel in the afferent hepatic venous system.

The communication existing by means of the cardiac vein between the
heart and the hepatic afferent system may suggest, but _purely for
illustrative purposes_, the direct connection of the umbilical vein with
the heart by the ductus venosus in the mammalian embryo (cf. schema
Figs. 250-256).

=3. Reptile.=--In _Iguana_ the renal portal system is well developed.
The caudal vein, returning the blood from the tail and the cavernous
tissue of the genital organs, continues for a short distance upon the
fused caudal end of the two kidneys (Fig. 269) and then divides into two
afferent renal veins which ascend on the ventral surface of the glands,
giving branches to the renal capillary system. About the middle of the
kidney each afferent vein is joined by a large transverse branch from
the abdominal vein (Fig. 266).

[Illustration: FIG. 266.--Systemic veins of _Iguana tuberculata_. The
alimentary canal and appendages, together with the hepatic portal vein
and the intrahepatic segment of the postcava, have been removed. The
liver occupies the space between the divided ends of the postcava. The
vertebral vein represents the rudimentary proximal segment of the
postcardinal vein corresponding to the mammalian azygos vein. (Columbia
University Museum, No. 1320.)]

[Illustration: FIG. 267.--Veins of _Iguana tuberculata_. Connection of
systemic veins with sinus venosus of heart. The rudimentary system of
the vertebral (azygos) veins and their proximal connection with the
subclavian vein are shown. (Columbia University Museum, No. 1859.)]

[Illustration: FIG. 268.--Corrosion preparation of venous system of
liver in _Iguana tuberculata_. The hepatic portal system and its
connection with the abdominal vein, as well as the relation to the
postcava, are shown. The preparation supplements Fig. 266, showing the
parts which have been removed in the latter. (Columbia University
Museum, No. 1860.)]

[Illustration: FIG. 269.--_Iguana tuberculata_, male. Genito-urinary
tract, dorsal view, with renal-portal, postcardinal, and postcaval
veins. (Columbia University Museum, No. 1862.)]

The renal efferent system begins by a number of inter-renal anastomoses
which unite along the mesal border of the right kidney into a large
ascending trunk, while the corresponding vessel of the left side,
starting from the same anastomosis, is considerably smaller (Figs. 266
and 269). Each of these vessels also receives blood from the testis,
epididymis, vas deferens and adrenal body in the male, and from the
ovary and oviduct in the female. They represent, in fact, the distal
functional part of the right and left embryonic postcardinal vein. Just
caudad of the left testis the vein of the left side crosses obliquely
ventrad of the aorta and joins the right vessel to form the trunk of the
postcava, which enters, immediately beyond the cephalic pole of the
right testis, the prolonged caval lobe of the liver (Figs. 266 and 269).
Ascending in the substance of this gland and receiving the afferent
hepatic veins (Fig. 268), the vena cava emerges from the cephalic
surface of the liver greatly enlarged and proceeds to the right
auricle.

The abdominal vein divides below into two branches which pass caudad on
each side of the bladder, receiving tributaries from the same, to the
lateral border of the kidneys (Figs. 266 and 269). Here the vessel is
connected by the transverse branch above described with the afferent
renal portal system derived from the caudal vein. At the same point it
receives the sciatic vein, the principal venous vessel of the posterior
extremity. Above, the main abdominal vein, resulting from the union of
the two branches referred to, ascends on the dorsal surface of the
ventral abdominal wall, receiving a few twigs from the ventral
mesogastrium within whose free caudal edge the vessel runs. Just before
reaching the liver the abdominal vein turns dorsad on the caudal surface
of the gland and joins the hepatic portal vein (Figs. 268 and 275).
Several accessory veins, two or three in number, belonging to the system
of the abdominal vein, pass above this point from the ventral body wall
between the layers of the ventral mesogastrium, to enter the liver
separately on its convex ventral surface, above the fusion of the main
abdominal vein with the portal vein. These additional branches on
entering the liver join the portal system, forming a set of ventral
accessory portal veins.

The hepatic portal vein derives its principal tributaries from the
splenic, gastric, pancreatic and intestinal veins. One or two additional
branches (accessory vertebral portal veins), as above stated, connect
the system of the segmental and vertebral veins with the portal
circulation, entering the liver separately. In like manner one or two
gastric veins (accessory gastric portal veins) enter the dorsal aspect
of the liver separately, passing from the stomach to the gland between
the layers of the gastro-hepatic omentum (Fig. 275).

Compared with the development of the mammalian type, the venous system
of Iguana serves to illustrate the stage in the history of the umbilical
vein (represented by the abdominal vein of the reptile) in which the
connection of the vessel with the portal vein has been formed and
transmits the greater part of the blood returned by the
umbilical vein to the liver, while the proximal segment above this
point, originally continued into the sinus venosus, has begun to
disappear, being, however, still represented by the vessels which, as
accessory ventral portal veins, pass in the ventral mesogastrium, from
the body wall to the liver.

It will be noted that all the hepatic portal blood, whether conducted by
the main portal and abdominal vein, or by the accessory portal branches,
traverses the capillary circulation of the liver before entering the
postcava.

The vertebral and segmental venous system, representing the azygos veins
of the mammalia, is very rudimentary (Figs. 266 and 267). The distal
portions of the postcardinal veins form the efferent renal branches and
the ascending trunks of the postcava.

The next segment of the vertebral veins appears as a trunk on the right
side which enters the portal circulation. A second vein higher up is
connected with both the gastric portal system and with the longitudinal
chain of the vertebral veins. Finally a proximal venous branch on each
side of the vertebral column, representing the upper portion of the
postcardinal veins, receives the proximal segmental veins and empties
into the subclavian vein (Fig. 267).

[Illustration: FIG. 270.--Veins of pigeon, _Columba livia_. (Modified
from Parker and Haswell.) The renal-portal vein of the right side is
supposed to be dissected to show its passage through the right kidney.]

=4. Bird.=--The characteristic change in the venous system of the bird,
as compared with that of the amphibian and reptile, is found in the
nearly complete abolition of the renal portal system. The caudal vein
bifurcates, sending on each side a large trunk, which receives the
pelvic (int. iliac) veins, to the kidney (renal afferent portal vein),
but only a few small branches enter the substance of the gland (Fig.
270, afferent renal V). The main vessel continues cephalad through the
kidney and, after receiving the vein from the posterior extremity
(femoral), unites as common iliac vein with the vessel of the opposite
side to form the postcava. This vessel traverses the liver, receiving
the hepatic afferent veins of the portal system. The portal vein is
formed by tributaries from the intestinal canal, pancreas and spleen,
and is also joined by a large coccygeo-mesenteric vein, which is given
off at the point of bifurcation of the caudal vein and receives
tributaries from the lower part of the alimentary canal. The abdominal
vein of amphibians and reptiles is represented probably by the
epigastric vein, which returns the blood from the omental mass of fat to
the hepatic veins.

Compared with the mammal on the one hand, and with the lower types on
the other, the venous circulation of the bird illustrates the following
points:

1. Extensive reduction of the renal portal system and direct formation
of postcava by the iliac veins, foreshadowing the condition found in the
mammal.

2. Complete separation of the portal and systemic venous circulation in
the adult. Disappearance of the ventral abdominal vein as a vessel of
the body wall.

=5. Human Foetus at Term.=--The student is recommended to examine, by
dissection and injection, the venous system of a foetus at term, noting
the following facts:

1. Course of umbilical vein in ventral abdominal wall and along free
edge of falciform ligament to liver (Fig. 241), corresponding to the
position of the amphibian and reptilian abdominal vein (Figs. 264 and
275).

2. Connection of umbilical vein in liver:
  (_a_) With portal system (Figs. 258 and 271).
    ({~GREEK SMALL LETTER ALPHA~}) With portal vein.
    ({~GREEK SMALL LETTER BETA~}) With portal system of left and quadrate lobes by
      branches derived directly from umbilical vein while
      situated in the umbilical fissure (Fig. 258).
  (_b_) With hepatic veins and postcava by the ductus venosus
    (Figs. 258 and 271).

[Illustration: FIG. 271.--Human foetus at term. Corrosion preparation of
heart and vascular system. (Columbia University Museum, No. 1858.)]

3. Connection of the postcaval and precaval systems by the azygos veins
representing the proximal segments of the embryonic postcardinal veins
(Fig. 272).

[Illustration: FIG. 272.--Human foetus at term. Postcava and azygos
veins. (Columbia University Museum, No. 1861.)]

If possible the dissection of an injected foetus should be combined with
the examination of corrosion preparation of the foetal circulation and
especially of the venous system of the foetal liver (Figs. 258 and 271).

3. The remnants of foetal structures in the adult liver (round ligament
and ligament of the ductus venosus) should be compared with the
structures from which they are derived in the foetus at term (umbilical
vein and ductus venosus).


II. THE VENTRAL MESOGASTRIUM.

This membrane has been heretofore mentioned on several occasions. It now
remains for us to carefully consider its arrangement in detail, both as
regards the peritoneal relations of the liver and in reference to its
influence on the abdominal space as a whole. We can best accomplish this
purpose by considering the membrane in the first place in a purely
schematic manner. In contradistinction to the primitive common dorsal
mesentery, which extends the entire length of the alimentary tube, the
ventral mesentery, or properly the ventral mesogastrium, is confined to
the stomach and proximal portion of the duodenum. We can represent the
membrane as extending between the ventral abdominal wall and the ventral
border (later the lesser curvature) of the stomach and of the hepatic
angle of the duodenum. Cephalad it is connected with the embryonic
septum transversum (future diaphragm). Caudad its two layers pass into
each other in a free concave edge, including between them the umbilical
vein (free edge of falciform ligament of adult). Consequently a
schematic profile or lateral view of the membrane and its attachments in
the earlier stages would appear as represented in Fig. 273, while the
arrangement in transection would be as shown in Fig. 274. It will be
observed that the separation of the cephalic portion of the abdominal
cavity into symmetrical right and left halves, previously indicated in
discussing the primitive stomach and the dorsal mesogastrium, is
actually completed by the ventral mesogastrium. This complete separation
of the lateral halves of the coelom cavity ceases at the point where the
ventral mesogastrium terminates in the free concave edge carrying the
umbilical vein. Hence caudad of this falciform edge the two halves of
the cavity communicate freely with each other ventrad of the intestine
and dorsal mesentery.

[Illustration: FIG. 273.--Schematic profile view of ventral mesogastrium
with developing liver.]

[Illustration: FIG. 274.--Schematic transection of abdomen in region of
ventral mesogastrium.]

This difference in the extent of the mesogastria is perhaps best
understood by reference to their relation to the first portion of the
duodenum. We have seen that the duodenum in the early stages is attached
dorsally by a portion of the common dorsal mesentery, which, after
differentiation of the intestinal tract, immediately follows the dorsal
mesogastrium proper, forming the mesoduodenum (Fig. 172). The proximal
portion of the duodenum (hepatic angle) is still included within the
fold of the ventral mesogastrium which membrane terminates immediately
beyond this point in the free edge surrounding the umbilical vein
(subsequent round ligament) (Fig. 172). The remainder of the duodenum is
devoid of any ventral attachment, being only connected to the dorsal
body wall by the mesoduodenum (Fig. 197).

Subsequently, after the fourth month, while the right surface of the
mesoduodenum and descending duodenum adhere to the parietal peritoneum,
the peritoneal investment of the first portion or hepatic angle remains
free. This peritoneal covering of the proximal duodenal segment is
situated at the point where the caudal end of the ventral mesogastrium,
after surrounding the first portion of the duodenum, becomes continuous
with the dorsal mesentery forming the mesoduodenum. Obliteration of the
latter membrane by adhesion to the parietal peritoneum leaves the first
portion of the duodenum invested on both surfaces by the _lesser
omentum_, derived from the ventral mesogastrium. The ventral surface of
the gut is covered by the ventral layer, the dorsal surface by the
dorsal layer of the lesser omentum. These two layers become continuous
around the right free edge of the lesser omentum (hepato-duodenal
ligament) forming the ventral boundary of the foramen of Winslow (cf.
infra, p. 177).

Returning to the schematic consideration of the ventral mesogastrium
above outlined (Figs. 273 and 274) we have to note the first important
change in the arrangement depending upon the development of the liver.
This organ, growing, as we have seen, from the duodenum, extends between
the two layers of the ventral mesogastrium, receiving a serous
investment from the same. At an early period the liver, developing thus
between the mesogastric layers, reaches the septum transversum and
becomes closely connected with it, laying the foundation for the
subsequent extensive attachment of the gland to the diaphragm.

Extending caudad the liver grows beyond the caudal free edge of the
ventral mesogastrium on each side, carrying the serosa with it.
Consequently the ventral margin of the liver becomes indented at this
point; the umbilical vein and subsequently its fibrous remnant, the
round ligament, are imbedded in a notch and fissure (umbilical notch and
fissure) continued from the ventral margin dorsad along the caudal
surface of the liver (Fig. 259).

This growth of the liver has now effected a division of the primitive
ventral mesogastrium into two segments:

1. Ventral portion, between diaphragm and liver, forms the broad
falciform or suspensory ligament of the liver.

2. The dorsal portion, between liver and stomach, forms the lesser or
gastro-hepatic omentum.

The caudal free edge of the ventral mesogastrium extends between the
umbilicus and the caudal surface of the liver, carrying the umbilical
vein between its layers. The growth of the liver serves to bury this
free edge and the contained vein in a fissure on the caudal surface of
the liver. The same obtains in the case of the ductus venosus continued
from the umbilical vein (umbilical fissure and fissure of ductus venosus
of adult liver). Consequently the original continuity of the broad
ligament and lesser omentum, as parts of the primitive ventral
mesogastrium, is not readily seen in the adult.

The broad ligament extends across the convex cephalic surface of the
liver uniting it to the ventral abdominal wall and diaphragm, while its
free falciform edge apparently stops at the umbilical notch in the
ventral border of the organ. Actually, however, the obliterated vein is
surrounded in the bottom of the fissure, by a peritoneal fold which
effects the junction between broad ligament and lesser omentum.

We will see later in what way the permanent adult arrangement of the
lesser omentum is brought about. For the present we can state, on the
hand of the schematic Fig. 273, that the free caudal edge of the
falciform ligament containing the umbilical vein, and the free edge of
the gastro-hepatic omentum form together originally the caudal free edge
of the ventral mesogastrium, which membrane becomes separated, by the
growth of the liver, into suspensory or broad ligament and lesser or
gastro-hepatic omentum.

[Illustration: FIG. 275.--Abdominal viscera of _Iguana tuberculata_.
(Columbia University Museum, No. 1313.)]

This primitive disposition of the ventral mesogastrium and the viscera
connected with the same, is well shown in some of the lower vertebrates
in whom the development never proceeds beyond the early mammalian
stages. Fig. 275 shows in profile view from the right side the situs
viscerum and peritoneum in _Iguana tuberculata_.[5] The two dorsal
aortic roots are seen to unite to form the main aorta, which descends
between the layers of the dorsal mesentery, sending branches to the
dorsal margin of oesophagus and stomach. From the opposite border of the
stomach the ventral mesogastrium is derived. Its dorsal segment
(gastro-hepatic omentum) connects liver and stomach, carrying between
its layers the portal vessels, hepatic artery and biliary duct. The
ventral segment of the membrane, forming the suspensory or broad
ligament, extends between abdominal wall and ventral surface of the
liver. Caudad, the lesser omentum and the suspensory ligament are seen
to have a common concave falciform edge.

[5] _Iguana tuberculata_, one of the large lizards native of South
America. This animal forms an excellent object for the comparative study
of the visceral and vascular anatomy of the abdomen. It possesses a
well-differentiated intestinal tract, several coils of small intestine,
a well-marked caecum and large intestine. The examination of this or a
similar reptilian form is to be highly recommended. Iguana is easily
obtained in any of our large cities, as a considerable number of these
animals are annually imported from Mexico and the South American states.

The ventral abdominal vein ascends between the layers of the suspensory
ligament and near the liver becomes connected by a large branch with
the portal vein. A few smaller branches are seen passing from the
abdominal wall beyond this point. In this reptile, therefore, the
permanent vascular arrangement corresponds to an early human embryonic
stage.

The reptilian ventral abdominal vein is the homologue of the umbilical
vein of the placentalia. The large branch passing to the portal vein
represents the connection established in the human embryo between the
umbilical and portal veins. The small branches, continuing cephalad
between the mesogastric layers, represent the temporary proximal
remnants which in the human embryo the umbilical veins form in
connection with abdominal walls. The permanent adult arrangement of this
part of the vascular system in this animal corresponds therefore to one
of the stages of development in the human embryo, as previously
indicated (cf. p. 149; Figs. 251 and 252).


PERITONEAL RELATIONS OF LIVER.

It is well to begin the study of the peritoneal connections of the liver
with the consideration of the embryonic stage shown in Fig. 273
schematically.

If we imagine this embryonic liver detached from its connections in such
a manner as to leave the divided peritoneal layers of the ventral
mesogastrium as long as possible, and if we regard the preparation from
behind, the appearance of the parts could be represented in Fig. 276.[6]

[Illustration: FIG. 276.--Schematic view of embryonic liver detached
from its connections, seen from behind, with lines of peritoneal
reflection.]

[6] I am indebted to Dr. J. A. Blake, former Assistant Demonstrator of
Anatomy at Columbia University, for the valuable suggestion which led to
the preparation of Figs. 276, 277 and 278 together with the correlated
text.

It will of course be seen that the area of direct adhesion to the
diaphragm, extending transversely, would separate the lesser omentum
from the suspensory ligament.

As is seen in the transection (Fig. 274), the right and left layers of
the suspensory ligament, at its attachment to the liver, turn into the
visceral peritoneum investing the organ on its ventral and cephalic
surfaces. Continuing around the borders of the liver this visceral
peritoneum then invests in like manner the dorsal or caudal surface
directed toward the stomach, until, at the region of the future portal
or transverse fissure, this visceral peritoneum becomes in turn
continuous with the two layers of the lesser or gastro-hepatic omentum.
Consequently in the embryonic detached liver the lines of peritoneal
reflection would be nearly cruciform, the vertical limb of the cross
being formed on the cephalic surface by the two layers of the suspensory
ligament, while on the caudal surface it is formed by the layers of the
lesser omentum. The horizontal arm of the cross is formed by the upper
and lower limits of the area of diaphragmatic attachment, along which
the parietal diaphragmatic peritoneum turns into the visceral hepatic
investment (forming the two layers of the primitive coronary ligament).
In the liver shown thus schematically from behind we would overlook the
dorsal and adjoining portions of the cephalic and caudal surfaces of the
adult human liver.

The primitive biliary duct, portal vein and hepatic artery reach the
liver between the layers of the lesser omentum. The venae revehentes
(hepatic veins) reach the sinus venosus at the attachment of the liver
to the septum transversum (primitive diaphragm).

The first important change, resulting in a rearrangement of these
peritoneal layers, is produced by the connection of the umbilical with
the rudimentary portal vein.

This junction occupies a relatively wide area on the caudal surface of
the liver, and the layers of the lesser omentum are separated somewhat
at this point to accommodate the enlarging vascular structures between
them. More especially is this the case with the right leaf of the
primitive gastro-hepatic omentum. A species of lateral diverticulum is
formed by this leaf so as to include the umbilical vein at its junction
with the portal. The membrane in the region of this diverticulum turns
its surfaces dorsad and ventrad, and its free edge toward the right
(Fig. 277). With the gradual increase in the size of the vessels, and
with the transverse position which the rotation of the stomach imparts
to the opposite border of the lesser omentum attached to the lesser
curvature, this transversely disposed portion gradually exceeds in
length and size the part of the original omentum enclosing the umbilical
vein. This vessel and the investing peritoneum become lodged in a
sagittal depression on the caudal surface of the liver (rudimentary
umbilical fissure), while the transverse portion, developed as
indicated, surrounds the structures connected with the liver at the
future transverse or portal fissure.

[Illustration: FIG. 277.--Schematic view of embryonic liver, showing
influence of vascular connections on the arrangement of the lines of
peritoneal reflection.]

[Illustration: FIG. 278.--Later stages, showing development of
transverse fissure, Spigelian and caudate lobes.]

Schematically this rearrangement of the hepatic peritoneal lines of
reflection can be shown in Fig. 278.

It will be observed that in this way a small part of the caudal surface
of the right lobe has become partially marked off from the remainder as
a rudimentary Spigelian lobe, bounded ventrally by the transverse
fissure and lesser omentum attached to the same; to the left by the two
layers of the lesser omentum containing the ductus venosus; while the
limit cephalad is afforded by the reflection of peritoneum from liver to
diaphragm, forming part of caudal layer of right coronary ligament. To
the right this rudimentary Spigelian surface is directly continuous with
the rest of the dorsal and caudal surface of the right lobe (Fig. 277).
Finally a definite right limit is given to the Spigelian lobe by the
increasing size of the postcava and its closer connection with the
liver. This vessel now assumes the position of the main venous trunk
entering the heart from below.

This inclusion of the vena cava in the fissure or fossa of that name on
the dorsal surface of the liver affords, so to speak, the vertical
measure of the non-peritoneal area of the liver attached directly to the
diaphragm. As the vein develops the interval between the two layers of
the right coronary ligament increases, producing the well-known large
non-peritoneal area on the dorsal surface of the adult liver, which is
directly attached to the diaphragm.

Immediately to the left of the vena cava, however, the original
condition persists. The area of direct diaphragmatic attachment is
narrow and consequently the two layers of the coronary ligament are
close together at this point.[7]

[7] It should be remembered that in the final adult arrangement of the
abdominal viscera the liver shifts relatively backwards, so that the
diaphragmatic attachment, originally directed cephalad, now looks dorsad
and forms part of the dorsal or "posterior" surface of the adult organ.
The original ventral surface looks cephalad, as well as ventrad, forming
the convex surface which in the adult rests in contact with the
abdominal wall and diaphragmatic vault, while the surface originally
directed dorsad toward the stomach finally in large part has an
inclination caudad forming the "inferior" surface of human anatomy.

In this way a species of recess (Spigelian recess or hepatic antrum of
lesser sac) is formed. A portion of the dorsal liver surface lying just
to the left of the vena cava, between it and the ductus venosus, remains
invested by peritoneum which is reflected from the boundaries of this
space to the diaphragm. This forms the Spigelian lobe (Fig. 278).

The lobe is bounded to the right by the postcava, to the left by the
reflection of the lesser omentum to the stomach along the fissure for
the ductus venosus; cephalad the boundary is formed by the reflection of
the caudal layer of the coronary ligament to the diaphragm.

The caudal boundary is afforded by the transverse position which the
lesser omentum has assumed in the region of the transverse or portal
fissure.

It will be seen that the original continuity of the Spigelian lobe with
the caudal surface of the right lobe is maintained by the narrow bridge
of liver tissue connecting the caudal right angle of the rectangular
Spigelian lobe with the right lobe. This narrow isthmus, situated
between vena cava dorsad and the free right edge of lesser omentum
ventrad, forms the so-called _caudate lobe_.

[Illustration: FIG. 279.--Liver of human foetus at eighth month. View of
caudal and dorsal surfaces. (Columbia University Museum, No. 1854.)]

Fig. 279 shows a human foetal liver at the end of the eighth month in the
view from below and behind. The original continuity of the layers of the
lesser omentum, attached along the fissure for the ductus venosus, with
the fold of the falciform ligament occupying the umbilical fissure can
still be made out for a short distance beyond the left extremity of the
transverse fissure. The section of the lesser omentum which occupies the
transverse fissure and, including the portal vein, hepatic artery and
duct between its layers, terminates in the free right margin, is
evidently derived by a lateral extension from the right layer of the
primitive sagittal lesser omentum, whose original direction is preserved
along the fissure of the ductus venosus.

In Fig. 280 the lines of peritoneal reflection on the cephalic, dorsal
and caudal surfaces of a human foetal liver at term are shown.

[Illustration: FIG. 280.--Human foetal liver at term, showing lines of
peritoneal reflection on cephalic, dorsal, and caudal surfaces.
(Columbia University Museum, No. 1855.)]

We can now proceed to trace the reflection of the peritoneum from the
liver to adjacent structures.

Begin with the caudal layer of the coronary ligament on the extreme
right, where fusion with the corresponding cephalic layer produces the
right triangular ligament. The caudal layer of the coronary ligament
proceeds from right to left along the caudal margin of the
non-peritoneal dorsal diaphragmatic surface of right lobe, being
reflected along this line from the liver to the adjacent portions of the
diaphragm and ventral surface of right kidney and suprarenal capsule
(hepato-renal ligament). A small cephalic part of ventral surface of
right suprarenal capsule lies above this line of reflection, is hence
non-peritoneal and firmly connected with the liver just to the left of
entrance of vena cava into the caval fissure. Continuing, the caudal
layer of the coronary ligament crosses the ventral surface of the vena
cava and turns, immediately to the left of the vein, at a right angle,
ascending to form the left boundary of the Spigelian recess, being
reflected along this line from the left margin of the caval fissure to
the pillars of the diaphragm. Arrived at the opening of the central
tendon permitting passage of vena cava into pericardium, and at the
level of the entrance of the left hepatic vein into the cava, the
peritoneum turns again at a right angle and runs from right to left,
forming the cephalic limit of the Spigelian recess. Turning caudad along
the fissure for the ductus venosus, as right leaf of that portion of the
lesser omentum which is attached to this fissure and has preserved its
sagittal position, the peritoneal line of reflection reaches the left
extremity of the portal or transverse fissure. It now turns to the
right following the fissure as the dorsal layer of the transverse
segment of the lesser omentum, and becomes continuous, with the
formation of a free right edge, with the ventral layer of the same
membrane, passing from right to left, the two layers including between
them the structures entering and leaving the liver at the transverse
fissure (portal vein, hepatic artery, duct). Arriving at the left
extremity of the transverse fissure the ventral layer of the transverse
segment of the lesser omentum--as we practically trace it in the adult
as a free membrane--turns directly into the left leaf of the sagittal
segment attached along the fissure for the ductus venosus, and becomes
continuous along the dorsal border of the left lobe with the caudal
layer of the left coronary ligament. This direct continuity, as just
stated, exists practically in the adult. From the development of the
membrane, however, it will be seen that the ventral layer of the
transverse lesser omentum, at the left extremity of the portal fissure,
becomes continuous with the right layer of the primitive mesogastrium
enclosing the umbilical vein. After surrounding this vein it is
continued into the left leaf of the same membrane, which in turn passes
into the left layer of the portion attached along the fissure for the
ductus venosus.

This original connection can at times be traced very clearly in young
specimens (Fig. 279), and occasionally is also still evident in the
adult liver.

Usually, however, the round ligament of the adult and its investing
peritoneum is buried so deeply in the umbilical fissure, or even bridged
over in part by liver tissue, that the connection is not evident. The
ventral layer of the transverse omentum then appears directly continuous
with the left layer of the sagittal omentum attached along the fissure
for the ductus venosus.

We can sum up the facts just considered as follows:

1. The rotation of the stomach from the sagittal into the transverse
position, and the development of the umbilical and portal veins,
rearrange the original sagittal plane of the lesser omentum, dividing it
into two districts:

(_a_) Cephalic portion, remaining in the original sagittal plane,
follows the fissure for the ductus venosus. With the incorporation of
the Spigelian lobe in the adult dorsal or "posterior" surface of the
liver, this segment of the omentum assumes a vertical direction, forming
the left boundary of the Spigelian recess, being reflected from the
fissure for the ductus venosus to the abdominal portion of the oesophagus
and the part of the lesser curvature of stomach adjacent to the cardia.

(_b_) Distal caudal portion of the lesser omentum is twisted laterally
and turned to the right by the change in the position of the stomach and
the development of the structures connected with the liver at the
transverse fissure. It is reflected from this fissure to the distal part
of the lesser curvature and to the first portion of the duodenum. This
transverse segment of the lesser omentum is a secondary derivative from
the right leaf of the primitive membrane, produced by the enlarged area
for entrance of umbilical and portal veins at the transverse fissure. It
lies ventrad of caudal border of Spigelian lobe.

2. The distal segment of the original omentum containing the umbilical
vein (round ligament), continues imbedded in the umbilical fissure, to
the ventral margin of the liver, where it joins the layers of the
suspensory ligament passing over the cephalic surface.

3. The adult lesser omentum at the transverse fissure may be regarded as
a diverticulum of the right leaf of the primitive embryonal sagittal
omentum.

With the reduction of the umbilical vein after birth to form the round
ligament this structure becomes deeply buried in the umbilical fissure.
The ventral and dorsal layers of the lesser omentum at the transverse
fissure thus become continuous with respectively the left and right
layers of the second segment of the omentum which ascends vertically
along the fissure for the ductus venosus.

4. The cephalic layer of the coronary ligament (Fig. 280) remains
practically in the embryonic condition. The adult convex cephalic
surface of the liver is traversed in the sagittal direction by the
suspensory ligament which connects it with the abdominal surface of the
diaphragm, and thus effects the division into right and left lobes on
the convex surface. Arrived at the dorsal border of this surface
(junction of "superior" and "posterior" surfaces) the right and left
leaves of the falciform ligament turn at right angles into the cephalic
layer of the right and left coronary ligament, which at each extremity
meet the right and left caudal layers to form the triangular ligaments.
It will thus be seen that the apparent irregularity in the relative
arrangement of the s. c. "upper" and "lower" layers of the coronary
ligaments, produced by the Spigelian recess, is only a difference in the
interval between the two layers, caused by the vertical extent of the
non-peritoneal direct diaphragmatic attachment of the right lobe to the
right of the vena cava.

=Comparative Anatomy of Spigelian Lobe and Vena Cava in the Cat.=--The
lines of peritoneal reflection in the _cat's_ liver and the arrangement
of the Spigelian lobe and recess are seen in Fig. 281, taken from a
preparation hardened in situ.

[Illustration: FIG. 281.--Liver of cat, hardened _in situ_. (Columbia
University Museum, No. 1836.)]

Compared with the human liver it will be noted that the area of
diaphragmatic adhesion is much less developed. The dorsal surface of the
right lobe to the right of the postcava is peritoneal, there being no
extension laterad of the right coronary and triangular ligaments. The
postcava enters the liver in a special prolongation of the liver
substance (caval lobe).

The boundaries of the Spigelian recess and the lines of attachment of
the gastro-hepatic omentum correspond to the human arrangement.


RELATION OF THE HEPATIC PERITONEUM TO THE "LESSER SAC."

_Foramen of Winslow._--We have previously seen that the rotation of the
stomach and the further growth of the dorsal mesogastrium lead, in the
first instance, to the formation of the "lesser peritoneal cavity." This
cavity is in fact primarily the retrogastric space created by the
transverse position of the stomach, augmented by the cavity of the
omental bursa developed from the dorsal mesogastrium.

We have now to consider the additional boundaries of this space
contributed by the peritoneal connection of the lesser curvature with
the liver.

The lesser omentum follows, of course, along its gastric attachment to
the lesser curvature the general direction of the stomach, passing from
the cardia transversely downwards and to the right. We distinguish the
two layers of the adult membrane as ventral and dorsal, which meet in
the free right edge and include between them the main structures
entering and leaving the liver at the transverse fissure, viz.: the
portal vein, hepatic artery and bile-duct.

The lesser omentum therefore prolongs the plane of the stomach cephalad
towards the liver and thus forms the continuation of the ventral
boundary of the lesser peritoneal sac. We can now consider the line of
its hepatic attachment in the light of the facts previously adduced, and
combine the same with the line of gastric attachment to the lesser
curvature. Fig. 282 shows the foetal liver and stomach in their relative
position in the dorsal view, and Fig. 283 gives the lines of the
peritoneal reflections. The vertical segment of the omentum, occupying
the fissure for the ductus venosus, passes to the cardiac part of the
lesser curvature, its ventral layer covering the ventral and left side
of the oesophagus, while its dorsal layer passes to the dorsal and right
side of the oesophagus at its entrance into the stomach. The transverse
segment of the omentum, attached on the liver to the portal or
transverse fissure, accedes to the pyloric part of the lesser curvature.
Of course the ventral and dorsal layers of the omentum are continuous
with the serous visceral investment of the ventral and dorsal surfaces
of the stomach.

[Illustration: FIG. 282.--Dorsal view of human liver and stomach in
foetus at term, showing lines of hepatic and gastric attachment of lesser
omentum.]

[Illustration: FIG. 283.--Schema of lines of reflection of peritoneum on
dorsal surface of liver and in the formation of the gastro-hepatic
omentum. _A B_, transverse section of lesser omentum attached to
transverse fissure of liver and to pyloric section of lesser curvature
(_A' B'_); _B C_, vertical section of lesser omentum passing between
fissure of ductus venosus and cardiac section of lesser curvature of
stomach (_B' C'_); _C D_, line of reflection of peritoneum from cephalic
border of Spigelian lobe to diaphragm; _D E_, line of reflection of
peritoneum from right border of Spigelian lobe to left margin of
postcava and diaphragm.]

[Illustration: FIG. 284.--Portion of abdominal viscera of adult human
subject, hardened _in situ_. (Columbia University, Study Collection.)
The segment of stomach between cardiac and pyloric orifices has been
removed, dividing the lesser omentum to this extent, but leaving the
right extremity of the membrane (lig. hepato-duodenale) intact. Behind
this portion the arrow passes through the foramen of Winslow.]

[Illustration: FIG. 285.--Liver and stomach of _Macacus pileatus_.
(Columbia University, Study Collection.)]

Fig. 284 shows this right-angled course of the lesser omentum at the
hepatic line of attachment in a preparation of the abdominal viscera
hardened in situ, with the segment of the stomach between the cardiac
and pyloric orifices removed. The arrow is passed behind the right free
edge of the lesser omentum. This portion of the membrane is still
intact, not having been disturbed by the removal of the body of the
stomach, and includes between its layers the structures connected with
the liver at the transverse fissure (duct, hepatic artery and portal
vein). The lesser omentum is seen to be attached to the liver along the
transverse fissure (Fig. 284, _A_) and along the fissure for the ductus
venosus (Fig. 284, _B_), constituting the transverse and vertical
segments above referred to, which pass into each other at the angle of
junction between the transverse fissure (left end) and the fissure for
the ductus venosus (Fig. 284, _C_). The caudal and left border of the
Spigelian lobe is exposed by the division of the omentum, and the extent
of the Spigelian or hepatic recess of the lesser peritoneal sac is
shown. Fig. 285 shows the liver, stomach and lesser omentum of a Macaque
monkey hardened in situ, and demonstrates still more conclusively that
the uniform curve of the omentum along the lesser curvature of the
stomach becomes a broken line at the hepatic attachment, the angle being
placed at the left end of the transverse fissure at the point where the
same encounters the fissure for the ductus venosus.

[Illustration: FIG. 286.--Abdominal viscera of adult human subject,
hardened _in situ_; with liver lifted up after incision of the
gastro-hepatic omentum. (Columbia University Museum, No. 1845.)]

In Fig. 286 finally the hardened abdominal viscera of an adult human
subject are shown in the ventral view with the lesser omentum incised.
The cut through the lesser omentum exposes the hepatic recess of the
lesser peritoneal cavity immediately to the left of the foramen of
Winslow. Toward the right free margin of the omentum the divided portal
vein, hepatic artery and duct are seen between the layers of the omentum
imbedded in the pancreas and coursing behind the first portion of the
duodenum on their way to the transverse fissure.

To the left of these structures the omental tuberosity of the pancreas
projects above the level of the lesser curvature under cover of the
secondary parietal peritoneum forming the dorsal wall of the lesser sac,
while the lower edge of the Spigelian lobe appears in the upper angle of
the incision.

If we remember that the liver is itself welded to the diaphragm between
the layers of the coronary ligament (Fig. 280), it will become
apparent that the serous surface of the Spigelian lobe forms part of the
ventral wall of a peritoneal recess situated behind the lesser omentum,
between this membrane and the diaphragm. Access to this recess, without
the division of peritoneal layers, can only be obtained by passing from
right to left, along the caudate lobe, between the vena cava behind,
covered by parietal peritoneum, and the free right edge of the lesser
omentum in front. (In the reverse direction of the arrow shown in Fig.
284.) This hepatic or Spigelian recess of the lesser peritoneal cavity
has categorically the following boundaries (Figs. 282 and 283):

Dorsal: Parietal peritoneum, reflected along the line CD, from the
caudal layer of the coronary ligament to the diaphragm.

Ventral: Visceral peritoneum investing the Spigelian lobe and the
gastro-hepatic omentum.

Right: Reflection of peritoneum along the line DE (caval fissure) to
become the parietal peritoneum covering the diaphragm.

Left: Right layer of lesser omentum, reflected along the fissure for the
ductus venosus (CB) to the cardiac portion of the lesser curvature,
continuous with the dorsal layer of the lesser omentum reflected from
the transverse fissure to the pyloric segment of the lesser curvature
(AB).

We will presently see that certain relations of the vessels connected
with the liver at the transverse fissure and of the duodenum prevent the
finger, when passed from right to left behind the free right edge of the
lesser omentum and along the caudate lobe of the liver, from proceeding
downward at this point. A narrow channel of communication is thus formed
between the Spigelian recess and rest of the lesser sac on the one hand,
and the general greater peritoneal cavity on the other. This channel is
the so-called foramen of Winslow.

Having once passed this narrow space the finger will be in the Spigelian
recess and can palpate its boundaries. Further progress cephalad and to
the right is barred by the diaphragmatic adhesions of the liver just
detailed. But in the direction downward behind the lesser omentum and
along the dorsal surface of the stomach, as well as to the left toward
the spleen the excursion is limited only by the length of the examining
finger.

After opening the abdominal cavity of the human adult, elevating the
liver and depressing the stomach, the hepatic attachment of the lesser
omentum can be traced as already described. It will then be observed
that the gastric attachment of the membrane lies in one plane following
the lesser curvature while the hepatic attachment forms a broken line,
with the angle situated at the left extremity of the transverse fissure.
The vertical segment of the hepatic attachment, occupying the fissure
for the ductus venosus, turns at this angle into the transverse segment
which follows the transverse fissure to its right extremity where the
two layers pass into each other around the right free omental margin
(hepato-duodenal ligament). Consequently we overlook, in an abdominal
cavity thus exposed, the entire caudal surface of the liver, including
the caudal surfaces of right, left, and quadrate lobes. The junction of
right and caudate lobes can be seen between vena cava and right edge of
the omentum, or rather, it can be felt at this point. But the Spigelian
lobe, turning its surface dorsad against the parietal peritoneum
covering the diaphragm, forms part of the "posterior" liver surface and
is not visible, although--as just stated, it can be palpated by passing
the finger through the foramen of Winslow. The Spigelian lobe cannot be
overlooked in its entire extent until the liver is removed from the body
and regarded from behind. The caudal edge (continuation of its right
angle into the caudate lobe and papillary tubercle) can be seen by
tearing through the layers of the lesser omentum and lifting the liver
up forcibly (Fig. 286).

=Caudal Boundary of Foramen of Winslow.=--We have above referred to the
fact that the finger introduced through the foramen of Winslow meets in
this canal with resistance if an attempt is made to pass downwards.
After passing this constricting point the free excursion into the
Spigelian recess and behind the omentum and stomach and toward the
spleen can be performed.

In considering the elements which produce this narrowing of the
communication between the two peritoneal sacs at the foramen of Winslow
we have to deal with two factors, one primary and constant, the other
secondary and inconstant.

1. The first of these is afforded by the arrangement of the arterial
vessel supplying the liver. The hepatic artery is a branch of the coeliac
axis, furnishing arterial blood to the liver tissues and supplying, in
addition, branches to the stomach, duodenum and pancreas.

This vessel is, of course, placed primarily, like all other arterial
branches supplying the alimentary tract, between the layers of the
primitive dorsal mesentery. Originally the vessel supplies the distal
(pyloric) portion of the stomach along its dorsal attached border
(subsequently the greater curvature) corresponding to the adult
gastro-epiploica dextra of the hepatic (gastro-duodenalis).

It likewise gives branches to the adjacent pyloric portion of the
duodenum and the pancreas, as that gland develops from the intestine,
corresponding to the adult superior pancreatico-duodenal branch, and to
the ventral border (lesser curvature) of stomach, corresponding to the
adult pyloric branch of the hepatic.

With the development of the liver from the duodenum arterial branches
derived from this primitive gastro-duodenal vessel pass to the sprouting
hepatic cylinders by continuing around the duodenum, beneath its serous
investment, to reach the interval between the two layers of the ventral
mesogastrium, in which the liver develops, near the free margin of this
membrane.

After the rotation, which turns the right side of the stomach, duodenum
and mesoduodenum dorsad, the branch which passes over the dorsal surface
of the duodenum to reach the liver becomes more favorably situated and
develops into the main hepatic artery which reaches the liver at the
transverse fissure between the folds of the lesser omentum. The original
right side of the duodenum, now turned dorsad, adheres to the parietal
peritoneum. The hepatic artery which reached the liver by passing over
this surface of the duodenum, beneath its visceral serous covering,
becomes imbedded in connective tissue by the adhesion of the visceral
duodenal and the primitive parietal peritoneum. Hence in the adult the
hepatic artery courses imbedded in the connective tissue which binds the
duodenum to the abdominal background to reach the interval between the
two omental layers which carry it to the transverse fissure.

The hepatic artery, therefore, derived from one of the primitive
intestinal branches (gastro-duodenal) is, notwithstanding its hidden
position in the adult, originally situated between the layers of the
free primitive dorsal mesogastrium.

It now becomes necessary to regard the development of the great omentum
from the primitive dorsal mesogastrium in relation to this course of the
hepatic artery. We have seen that the great omentum and the cavity of
the omental bursa is produced by the extension of the dorsal
mesogastrium to the left and caudad, subsequent to the rotation of the
stomach. The splenic artery and the left gastro-epiploic branch pass
from the coeliac axis to the left between the layers of the mesogastrium,
as previously seen (Figs. 291 and 292).

The hepatic artery, however, is so to speak placed on the border line
between the portion of the primitive mesentery which, as dorsal
mesogastrium, is to turn to the left and caudad to form the great
omentum, and the portion which, as mesoduodenum, turns to the right and
passes to the duodenal loop (Fig. 287).

[Illustration: FIG. 287.--Primitive dorsal and ventral mesogastrium with
course of hepatic artery.]

[Illustration: FIG. 288.--The liver divides the ventral mesogastrium
into a dorsal segment, the gastro-hepatic or lesser omentum, and a
ventral segment, the suspensory or falciform ligament of the liver.]

[Illustration: FIG. 289.--Stages in the development of the dorsal
mesogastrium (omental bursa) and mesoduodenum to show relation of
hepatic artery to these two segments of the primitive common dorsal
mesentery.]

In the further course of development the dorsal mesogastrium grows more
and more, forming the omental bag, while the mesoduodenum on the other
hand becomes anchored early and obliterated as a free membrane by
adhesion of its original right layer to the primitive parietal
peritoneum. The hepatic artery runs on the line dividing these two
different mesenteric segments. We can imagine, so to speak, that the
redundant growth of the omentum to the left and caudad, takes place over
the hepatic artery as a resistant support (Figs. 288 and 289). Cephalad
of the hepatic artery is the developing omentum, caudad of the vessel
the mesoduodenum. The artery follows the cephalic limit of the
mesoduodenum and becomes, as stated, adherent to the abdominal
background in the segment between its origin from the coeliac axis and
the point where, after having crossed the dorsal surface of the
duodenum, it enters the right edge of the lesser omentum on its way to
the liver.

=Pancreatico-gastric Folds.=--If we open the lesser peritoneal cavity by
dividing the gastro-hepatic omentum and look into the background of the
retro-omental space, we will see a fold of the secondary lining parietal
peritoneum (derived from the mesogastrium), which can be traced from the
cephalic border of the pancreas to the pyloric extremity of the stomach.
This fold carries the hepatic artery to the lesser omentum behind the
first portion of the duodenum, and is called the right or main
pancreatico-gastric fold. A similar fold, further to the left, carries
in a like manner the coronary artery of the stomach to the cardiac end
of the lesser curvature. This fold forms the left or secondary
pancreatico-gastric fold. Between the two folds the caudal margin of the
Spigelian lobe projects into the lesser cavity.

The appearance of the two pancreatico-gastric folds in the adult human
subject is well seen in Fig. 284.

[Illustration: FIG. 290.--Abdominal viscera of _Nasua rufa_, brown
coaiti, with stomach turned up and great omentum divided. (From a fresh
dissection.)]

Fig. 290 shows the abdominal cavity of _Nasua rufa_, with great omentum
divided to bring into view the vessels passing from coeliac axis to liver
and stomach and elevating the retrogastric parietal peritoneum to
produce the pancreatico-gastric folds.

(The course of the hepatic artery from coeliac axis to liver in the
dorsal view in the cat is seen in Fig. 223.)

[Illustration: FIG. 291.--Schematic transection through foramen of
Winslow before adhesion of dorsal mesogastrium and mesoduodenum to
parietal peritoneum.]

[Illustration: FIG. 292.--The same section after the adult conditions
have been established by adhesion.]

Figs. 291 and 292 represent schematically cross-sections directly
through the foramen of Winslow, showing the method by means of which the
hepatic artery reaches the upper border of the duodenum and the effect
of the adhesion of duodenum and mesoduodenum upon the disposition of the
vessel.

The coronary artery, like the splenic, is at first situated between the
layers of the dorsal mesogastrium (vertebro-splenic segment). Like the
splenic the coronary artery becomes anchored to the abdominal background
and placed secondarily behind the parietal peritoneum of the lesser sac
by the adhesion of this mesogastric segment to the primitive parietal
peritoneum. To reach the lesser curvature at the cardia and to run
thence from left to right along the lesser curvature between the layers
of the gastro-hepatic omentum, the vessel raises the investing parietal
peritoneum (originally the right leaf of the dorsal mesogastrium) into a
crescentic fold, extending between its origin from the coeliac axis at
cephalic margin of pancreas and the beginning of the lesser curvature of
the stomach. Hence this fold is called the left pancreatico-gastric
fold. (Seen well in Fig. 284.)

In the next place it must be borne in mind that the relation of the
primitive hepatic artery to the vascular supply of the stomach, pancreas
and duodenum produces a permanent shortening of the primitive mesentery
at this point. This result is indicated in the schematic figures 287,
288 and 289.

In the original condition the dorsal mesentery, passing to a practically
straight intestinal tube, is of uniform sagittal measure (Fig. 287).

As development proceeds, and as the liver grows from the duodenum, the
hepatic artery develops from the primitive pyloric vessel as above
indicated. This vessel, assuming greater importance with the rapid
growth of the liver, is not lengthened out as happens with the remaining
purely intestinal branches which follow the increase in the length of
the intestinal canal. The hepatic artery, therefore, will mark the point
where the original short sagittal extent of the primitive mesentery will
tend to be preserved. Cephalad of this point the dorsal mesogastrium
grows out into the great omentum (Figs. 288 and 289); caudad of the same
point the membrane, in following the development of the intestine,
becomes drawn out into the permanent mesentery and mesocolon.

The hepatic artery, in addition, marks the cephalic limit of the
adhesion which anchors the duodenum and mesoduodenum to the parietal
peritoneum. Consequently in the adult the vessel courses in as direct a
manner as possible, taking the shortest course from the coeliac axis to
the liver, passing dorsad of the duodenum and giving what now appear as
secondary branches to supply the intestine, the stomach and pancreas
(pyloric and gastro-duodenal arteries (pancreatico-duod. superior and
gastro-epiploica dextra)).

Even if no fixation of the duodenum and mesoduodenum takes place this
course of the hepatic artery will produce a constricted passage between
the liver (caudate lobe) cephalad, abdominal parietes and aorta dorsad,
lesser omentum and pyloric duodenum ventrad, and hepatic artery caudad.
This passage leading from the general peritoneal cavity into the
retrogastric space is the _primitive foramen of Winslow_. This condition
is well represented in the abdominal cavity of some of the lower
mammalia, in which duodenum and mesoduodenum remain permanently free.

Fig. 293 shows a view of the abdominal cavity from the right side in a
specimen of the ant-eater, _Tamandua bivittata_.

[Illustration: FIG. 293.--Abdominal viscera of _Tamandua bivittata_, the
little ant-eater, with the intestines turned downward and to the left.
(From a fresh dissection.)]

The right kidney is seen in the background, covered by the parietal
peritoneum. The duodenum and mesoduodenum are free and can be turned
toward the median line. The opening of the foramen of Winslow leading
into the retrogastric space is seen between the liver cephalad, kidney
and vena cava dorsad, lesser omentum and pyloric extremity of the
stomach ventrad, and a fold of peritoneum carrying the hepatic artery
caudad. Exactly similar conditions prevail in the cat and in many other
mammals.

It will be seen in all these instances that neither portal vein nor
bile-ducts limit the foramen caudad. These structures can be lifted up
and turned toward the median line with the free duodenum and
mesoduodenum. But the hepatic artery must pass to the liver from the
_retroperitoneal coeliac axis_. In doing this the vessel traverses the
cephalic border of the pancreas, and the pyloric extremity of the
stomach and duodenum, to reach the lesser omentum which conveys it to
the liver.

Consequently there must always be a narrow peritoneal neck between the
liver cephalad, aorta dorsad, hepatic artery caudad, and pyloric
extremity of stomach and duodenum together with the lesser omentum
ventrad. It should be remembered that the vessel which extends after the
development of the liver into the lesser omentum as the _hepatic_
artery, was originally destined for the supply of these latter
structures. In the adult these primary embryonic terminal branches to
the intestine appear as secondary branches derived from the hepatic as
the main vessel. Their origin, however, serves to keep the beginning of
the small intestine in comparatively close connection with the hepatic
artery which courses over the dorsal surface of the duodenum to reach
the liver. The narrow space thus left between aorta, hepatic artery,
duodenum, lesser omentum and liver forms the framework of the foramen of
Winslow and appears always as a confined and narrow channel. This
relation is shown in the accompanying schematic Figs. 294 and 295 which
represent a sagittal section through the foramen. This primitive foramen
is thus bounded cephalad by the liver (caudate lobe, connecting
Spigelian and right lobes), ventrad by the first portion of the duodenum
and the lesser omentum, with hepatic artery behind the intestine and
between the omental layers; dorsad by the abdominal background and large
retroperitoneal vessels, and caudad by the coeliac axis and beginning of
the hepatic artery.

[Illustration: FIG. 294.--Schematic sagittal section through foramen of
Winslow before fixation of pancreas by adhesion of mesoduodenum.]

[Illustration: FIG. 295.--The same section after adhesion of
mesoduodenum and pancreas. The pancreas appears secondarily
retroperitoneal, after adhesion of apposed surfaces of mesoduodenum and
primitive parietal peritoneum over dotted area, producing fixation of
dorsal surface of pancreas.]

2. In the forms which possess in the adult an adherent duodenum and
mesoduodenum, as in man, the foramen of Winslow obtains a secondary
caudal limit by the agglutination of the descending duodenum and the
parietal prerenal peritoneum. This is the secondary and inconstant
factor referred to above in the caudal boundary of the foramen. The
result of this anchoring of duodenum and mesoduodenum is to bring the
margin of the foramen further to the right and to bury the hepatic
artery still further from view. Thus in the adult human subject the
structures bounding the foramen at the margin of the entrance into the
narrow channel would be above caudate lobe of liver, behind postcava,
below duodenum adherent to ventral surface of right kidney, in front
first portion of duodenum and lesser omentum. The hepatic artery will be
felt on introducing the finger through the foramen in its original
position, but it will be seen that the actual boundaries of the foramen
have been moved so to speak a little further to the right by the
duodenal adhesion.

[Illustration: FIG. 296.--Dissection of adult liver, pancreas, spleen,
and duodenum, with vessels, to show structures concerned in the
formation of the foramen of Winslow. (Columbia University, Study
Collection.)]

Fig. 296 shows a complete dissection of the adult human viscera and
vessels concerned in the formation of the foramen, hardened in situ.

The stomach is removed, dividing of course the coronary artery and vein
and the left gastro-epiploic artery. The portal vein, hepatic artery and
bile-duct are seen entering and leaving the liver at the transverse
fissure. Behind them and to the right the vena cava enters the liver.
The hepatic artery distributes its pancreatico-duodenal branches to the
duodenum and pancreas. The left angle of the Spigelian lobe and the
fissure for the ductus venosus appear to the left of the portal vein and
hepatic artery. The right angle of the Spigelian lobe and its
continuation into the right lobe by means of the caudate lobe is hidden
by the structures occupying the transverse fissure. We would enter the
beginning of the foramen of Winslow by passing between the vena cava
behind, the structures in the transverse fissure (portal vein, hepatic
artery and duct) in front, caudate lobe of liver above and duodenum
below, the latter in the undisturbed condition of the parts adherent to
the right kidney. Continuing to the left the finger would pass between
aorta behind, coeliac axis and hepatic artery below and in front, and
liver above. These structures bound the permanent and primary narrow
channel of communication between the retrogastric or lesser peritoneal
space and the general peritoneal cavity, which exists even if a free
duodenum and mesoduodenum allow us to lift the intestine away from vena
cava and right kidney.

The main facts pertaining to the structure of the lesser peritoneal sac
and its connection with the greater peritoneal cavity by means of the
foramen of Winslow may be summed up as follows:

The mesogastrium as a whole, expanding originally in the sagittal plane
in a fan-shaped manner between the vertebral column and the ventral
abdominal wall, from the level of the umbilicus to the septum
transversum (diaphragm), divides the cephalic part of the abdominal
cavity into a symmetrical right and left half.

Figs. 172 and 273 represent the membrane as seen in a profile view from
the left side. We distinguish the segment dorsad of the stomach as the
dorsal mesogastrium, directly continuous with the remaining segments of
the common primitive dorsal mesentery, while the portion ventrad of the
stomach forms the ventral mesogastrium in which the liver develops. The
segment of the ventral mesogastrium between liver and stomach becomes
the lesser or gastro-hepatic omentum, while that between liver and
ventral abdominal wall forms the falciform or suspensory ligament.

A transection, showing the dorsal and ventral mesogastrium at the level
of the fundus of the stomach, is given in Fig. 298. The mesogastria are
here seen to be short, while in the schematic Figs. 291 and 292 the
membrane is, for the sake of distinctness, represented as being of
considerable extent.

[Illustration: FIG. 297.--Schematic sagittal section of the ventral and
dorsal mesogastria and epiploic bursa in a human embryo of eight weeks.
(Modified from Kollmann.)]

[Illustration: FIG. 298.--Transection of human embryo of 3 cm.,
vertex-coccygeal measure. (Kollmann.)]

The ventral mesogastrium surrounding the liver and stomach extends
caudad to include the first portion of the duodenum. Beyond this point
it terminates in a thickened free edge which includes the umbilical
vein. This vein extends from the umbilicus to the transverse fissure of
the liver (Fig. 297), lying within the umbilical fissure on the caudal
surface of the gland.

At the point where the vein enters the liver the thickened margin of the
ventral mesogastrium is continued, as ligamentum hepato-duodenale, to
the upper part of the duodenum and forms the ventral boundary of the
foramen of Winslow. Between the layers of the mesogastrium which meet in
this margin are situated the portal vein, biliary duct and hepatic
artery, together with the nerves and lymphatics of the liver.

The mesogastrium originally divided the abdominal cavity between
umbilicus and diaphragm into symmetrical right and left halves of equal
size and extent. This early symmetrical arrangement becomes disturbed
about the seventh week by the rotation of the stomach and the resulting
altered course of the mesogastrium, which render the two original equal
halves of the abdominal cavity unequal and asymmetrical. The original
right half becomes placed behind the stomach and is converted into a
blind sac with its opening directed to the right.

The communication of the general abdominal cavity with the retrogastric
space by means of this channel is still wide in the embryo, but
gradually becomes narrowed in the course of further development to form
the foramen of Winslow. This opening is situated between the
hepato-duodenal ligament and the parietal peritoneum covering the vena
cava. It is constricted from below by the curve of the hepatic artery as
this vessel passes from the coeliac axis to reach the liver at the
transverse fissure between the layers of the lesser omentum.

The earlier developmental stages of the higher mammalian embryos are in
general well illustrated by the permanent adult conditions found in some
of the lower vertebrates, in which development does not proceed beyond
the primitive condition.

In reptiles, birds and mammals the epiploic bursa is generally formed,
while in amphibia the dorsal mesogastrium is very short and connects the
stomach directly to the dorsal midline of the abdominal cavity without
forming the sac-like extension of the great omentum.

The dorsal mesogastrium with the stomach, and the ventral mesogastrium
including the liver between its layers, divides in these animals the
cephalic part of the body cavity into two halves, corresponding to the
earlier embryonic stages in man and in the higher mammalia.

The foramen of Winslow of the higher forms appears in the lower
vertebrates as the wide-open space leading from below into the right
half of the coelom cavity. The dorsal mesogastrium remains short, not
forming the pouch-like extension of the great omentum. The stomach
retains more or less its primitive vertical position without rotation or
elevation of the pyloric extremity, and the intestinal canal is simple,
short and comparatively straight.




PART III.

LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION AND CAECUM.


In considering the anatomy of the human caecum and vermiform appendix
many structural conditions are encountered which can only be correctly
appreciated in the light of the physiology of the digestive tract. The
alimentary canal as a whole affords one of the most striking examples of
the adaptation of structure to function. The constant renewal of the
tissues of the body by the absorption of nutritive material, the
necessary concomitant egestion of undigestible remnants, the variety in
the quantity and character of the food habitually taken, all serve to
explain why the alimentary canal responds structurally in individual
forms so completely to the physiological demands made upon it. This will
become especially evident if we extend our observations to include, in
addition to man, a review of the corresponding structures in
representative types of the lower vertebrates. Moreover the human caecum
and appendix are in part rudimentary structures, representing a portion
of the alimentary tract which, in accordance with altered conditions of
food supply and nutrition, has lost its original functional significance
to the organism and which consequently exhibits the wide range of
structural variation which characterizes the majority of rudimentary and
vestigial organs.

The vermiform process of man and the higher primates is thus one of
several indications given in the structure of the alimentary canal (the
character of the dentition is another example) which suggests that at
one phylogenetic period the forms composing the order or their immediate
ancestors were largely or entirely herbivorous, and hence possessed a
more extensively developed caecal apparatus than their omnivorous
descendants of to-day. In approaching, therefore, the study of the
human caecum and appendix we will at once meet with conditions which call
for the simultaneous physiological and morphological consideration of
the adjacent small and large intestine.

Again many of the structural peculiarities which characterize the human
caecal apparatus can only be correctly valued by comparison with the
corresponding parts in the lower vertebrates. Our inquiry will,
therefore, most profitably include the following subdivisions of the
subject:


I. General review of the functional and structural characters of the
vertebrate large and small intestine.

II. Systematic consideration of the ileo-colic junction and the
connected structures in the vertebrate series.

III. Phylogeny of the types of vertebrate ileo-colic junction and caecum,
and their probable lines of evolution.

IV. Detailed morphology of the human caecum and vermiform appendix.


I. GENERAL REVIEW OF THE MORPHOLOGY AND PHYSIOLOGY OF THE VERTEBRATE
INTESTINE.

We have seen that the intestinal tube of all vertebrates is the product
of two of the embryonal blastodermic layers, the entoderm and mesoderm.
The former furnishes the characteristic and cardinal elements of the
digestive tract, viz., the secretory and absorbing epithelium of the
mucous membrane and of the accessory digestive glands, the liver and
pancreas.

From the mesoderm, on the other hand, are derived the muscular and
connective tissue coats which surround the epithelial tube and
contribute to the thickness of the intestinal wall, as well as the blood
vessels and lymphatics. The alimentary canal separates from the yolk-sac
of the embryo by the development of cranial, caudal and lateral folds,
and at an early period communicates with the neural canal by the
primitive postanal gut (cf. p. 23). This connection subsequently becomes
lost. The oral and anal openings, by means of which the alimentary canal
communicates with the exterior, are formed _secondarily_ by entodermal
invaginations which finally break through into the lumen of the canal
(cf. p. 24).

At an early embryonic stage the alimentary canal appears therefore as a
straight cylindrical tube running cephalo-caudad in the long axis of the
body-cavity, and suspended by the primitive mesentery from the ventral
aspect of the chorda dorsalis.

In Amphioxus, the cyclostomata, certain teleosts, dipnoeans and lower
amphibians the canal remains permanently in this condition (cf. Fig.
310).

In the remaining vertebrates the uniform non-differentiated tube of the
embryo develops further and appears more or less distinctly divided into
a proximal segment, the _foregut_, a central segment, the _midgut_, and
a distal segment, the _hindgut_, or _endgut_. This differentiation of
the tube into successive segments is closely connected with the
character and quantity of the food habitually taken and with the method
and rapidity of its elaboration in the process of digestion, absorption
and excretion. In general the _foregut_ is formed by the segment which
succeeds to the oral cavity, and includes the _pharynx_, _oesophagus_ and
_stomach_. The _midgut_ is composed of a longer or shorter narrower tube
of nearly uniform caliber, the _small intestine_, which follows the
gastric dilatation. Even in forms in which the stomach is not distinctly
differentiated (cf. p. 40) the connection of the biliary duct with the
intestinal canal serves to separate the fore- and midgut. The _hindgut_
or _large intestine_ is usually separated from the preceding segment by
an external circular constriction, with a corresponding annular valve or
fold of the mucous membrane in the interior.

The beginning of the large intestine is marked in many forms by the
development of an accessory pouch or diverticulum, the _caecum_. The
hindgut extends from its junction with the midgut to the cloacal or anal
opening.


1. Midgut or Small Intestine.

The small intestine is the segment of the alimentary canal in which
digestion of the non-nitrogenous food substances takes place, and which
affords the necessary area of mucous surface for the absorption of all
digested matters. Consequently the character and habitual quantity of
the food here elaborated exerts a very marked influence on the _length_
of the small intestine, _i. e._, on the extent of the digestive and
absorbing surface represented by its mucous membrane.

The relative length of the small intestine in any individual form will
vary with both the quantity and volume of the food and with the rapidity
of the metabolic processes. Animals, in which digestion is rapid and the
usual food small in bulk and concentrated in its nutrient qualities,
have a relatively short intestine, while the canal is longer in forms
subsisting on food which is bulky and which demands considerable time
for its elaboration. Hence we find the relatively shortest intestine in
carnivora, the longest in herbivora, while the canal in omnivora
occupies an intermediate position in regard to its relative length.

The rapidity of tissue-metabolism also exerts a marked influence on the
length and development of this portion of the alimentary canal.

In the warm-blooded animals (mammals and birds) the tissue-changes are
constant and rapid and call for a large amount of nutrition within a
given period, while the metabolic processes in the cold-blooded
vertebrates (reptiles, amphibia and fishes) are slow, these animals
being able to go without food for long periods. Consequently in the
former class the small intestine is relatively much longer than in the
latter. Thus in certain birds and herbivorous mammals the small
intestine exceeds the total length of the body many times. This
influence of the quantity and quality of the food on the length of the
intestinal canal is seen, for example, very well during the course of
development in the frog.

The increase in the length of the intestine, and the consequent varying
degrees of coiling and convolution, are therefore secondary
acquired characters, depending for their development upon the habitual
kind and volume of the food. Additional provisions for increasing the
efficiency of the digestive apparatus are encountered throughout the
whole of the intestinal canal. In many forms the digestive secretory and
absorbing area is augmented by the development of folds, valves,
diverticula, villi and papillae from the mucous surface of the intestine.
Certain valves and folds, moreover, both control the direction in which
the contents of the canal move and retain the same for a longer period
in the intestinal segment in which they develop. Such folds appear
especially well developed in the intestine of certain cyclostomes,
selachians and dipnoeans (cf. Figs. 203 and 204). In these forms the
alimentary canal is usually short and straight, and the fold which has a
typical spiral course and projects far into the lumen of the gut,
evidently makes up to a very large extent for the shortness of the
intestine, serving the threefold purpose of

(_a_) Increasing the digestive and absorbing surface;

(_b_) Prolonging the period of retention of the food-substances in the
intestine, and thus increasing the time available for elaboration and
absorption.

(_c_) Regulating the direction in which the intestinal contents move.

We will see presently that a similar spiral mucous fold is also
encountered in some of the higher vertebrates, especially in the large
intestine. Examples are found in the well-developed spiral valve in the
caeca of the ostrich (Fig. 341), the similar fold in the large intestine
of many rodents (Figs. 387 and 388) and in the crescentic plicae of the
primate large intestine (Figs. 471, 472 and 473).

To the same physiological category belong the _digestive diverticula_ of
the intestinal canal, such as the pyloric appendices of the midgut found
in many teleosts and ganoids (cf. p. 119) and the varieties of caeca or
blind diverticula of the hindgut encountered throughout the vertebrate
series. They all function as reservoirs which increase the available
digestive and absorbing surface and which in addition are especially
adapted to retain substances difficult of digestion until the processes
of elaboration have been completed.

=Divisions of the Small Intestine.=--In the higher forms the segment of
the small intestine which succeeds to the pylorus is distinguished as
the _duodenum_. Into it empty the ducts of the liver and pancreas. In
some animals a pear-shaped enlargement is found, corresponding to the
_duodenal antrum_ of the human intestine, as the dilated proximal
portion of the duodenum immediately beyond the pylorus is called.
Examples of this condition are furnished by the cetaceans, several
rodents, the llama and dromedary and the koala (Phascolarctos).

In the birds and in many mammals (_e. g._, dog, Fig. 200, and many
rodents, as the rabbit) the duodenum is drawn out into a long loop
surrounding the pancreas.

=Structure of the Small Intestine.= =1. Secretory Apparatus.=--The
glands whose ducts empty into the small intestine and which furnish the
digestive secretions, may be divided as follows:

(_a_) Glands situated in the substance of the intestinal walls.

Two kinds are distinguished:

1. Brunner's glands, small acinous glands confined to the first part of
the duodenum.

2. Glands of Lieberkuehn, small caecal pits distributed not only over the
entire small intestine, but also found in the mucous membrane of the
large intestine.

These structures furnish the intestinal juice, whose chief function is
the conversion of starches into sugar, while aiding in carnivorous
animals also the digestion of proteid substances. The glands are hence
best developed in herbivora, while in carnivora they are present in
diminished numbers since they assist in the digestion of proteid
substances.

The size and number of these glands also depends on the amount of food
digested within a given period. When a considerable quantity of
digestive fluid is required, in order to obtain the nutritive value of
the food for the organism rapidly, the glandular apparatus of the
intestine will be well developed. Hence mammalia, in whom these
conditions exist, possess both the glands of Brunner and of Lieberkuehn.
In birds the latter structures are still found, but the former are
absent, while amphibia and fishes are devoid of both kinds. In these
lower vertebrates the typical intestinal glandular apparatus of the
higher forms is to a certain extent replaced by small pits and
depressions of the mucous membrane bounded by reticular folds.

(_b_) _Glands situated outside the intestinal tube, into whose lumen
their ducts empty._

The liver and pancreas fall under this head. The liver functions in the
digestion of the fatty substances of the food, while the secretion of
the pancreas converts the starches into sugars, and aids in the
digestion of albumenoid substances and to a lesser degree in that of the
fats.

=2. Absorbing Apparatus of Small Intestine.=--The mucous membrane of the
intestine is provided with villi, containing lymphatics, by whose agency
the digested matters are absorbed. These structures are developed in
individual forms in direct proportion to the ease and rapidity with
which the food is habitually absorbed.

The more rapid and complete the digestion is the greater will be the
amount of digested nutritive material at any given time in the
intestine, and the greater will be the development of the absorbing
structures. Hence the villi of the small intestine are especially large
and prominent in the carnivora, while they are small and insignificant
in herbivora and omnivora. Intestinal villi are found in nearly all
mammals and in many birds. Fig. 300 shows the villi of the intestinal
mucous membrane in a carnivore mammal (_Ursus maritimus_, polar bear)
and Fig. 301 the same structures in the cassowary (_Casuarius
casuarius_) in which bird they are very well developed. The villi are
not confined to the two highest vertebrate classes, but are encountered
also in the mucous membrane of the midgut in certain reptiles, notably
the ophidia.

[Illustration: FIG. 299.--Schematic sagittal section of abdomen to
illustrate the intestinal branches of the abdominal aorta. The gastric
and hepatic arteries are shown for the sake of convenience as arising
together from the coeliac axis (_B_), hence the left and right
gastro-pancreatic folds carrying these vessels appear fused at their
beginning, separating the hepatic recess of the lesser peritoneal sac
(_A_) from the cavity of the omental bursa (_C_).]

[Illustration: FIG. 300.--Small intestine, of polar bear, _Ursus
maritimus_. Mucous surface. (Columbia University Museum, No. 782.)]

[Illustration: FIG. 301.--Duodenum, with entrance of pancreatic and
biliary ducts and well-developed diverticulum Vateri in the cassowary,
_Casuarius casuarius_ (Columbia University Museum, No. 1821.)]

[Illustration: FIG. 302.--Mucous membrane of midgut of _Boa
constrictor_. (Columbia University Museum, No. 1837.)]

Fig. 302 shows the intestine mucous membrane of the boa constrictor with
well-developed and prominent villous projections.

Some birds, such as the snipes, herons and crows, have in place of the
intestinal villi projecting folds of the mucosa, often arranged in a
reticular manner. This type is prevalent in amphibia and fish (Fig. 112,
distal segment of midgut). Collections of lymphoid tissue in the mucous
membrane of the small intestine, either aggregated to form Peyer's
patches (Fig. 309) or as solitary follicles, are only found in the two
highest vertebrate classes, birds and mammals. In the former they appear
scattered over the surface of the mucous membrane, in the latter they
may be arranged in aggregations or regular rows. They are not secreting
structures, but their exact function in absorption is not known. This
lymphoid or adenoid tissue in certain forms is especially well developed
at the ileo-colic junction, forming the _lymphatic sac_ of some rodents,
as lepus (cf. Fig. 386). It is not confined to the small intestine, but
is found in the large intestine as well. At times it appears especially
well developed in the terminal portion of the caecal pouch (appendix), as
in _Lepus_ (Fig. 388).

The _valvulae conniventes_ or _valves of Kerkring_ of the human small
intestine serve to very greatly increase the secreting and absorbing
mucous surface. They are not found in this complete development in any
other mammals, although a very few forms present a transverse
reduplication of the intestinal mucosa and the circular layer of
muscular fibers. An example of this is found in the intestinal mucous
membrane of a species of antelope, shown in Fig. 303.

[Illustration: FIG. 303.--Mucous surface of small intestine of a species
of African antelope, _Cervicapra arundinacea_. (Columbia University
Museum, No. 1843.)]

The complete development of the valvulae conniventes in man is possibly
also associated with a mechanical function in connection with the
upright posture. In some mammalia, as in certain rodents and the
porpoise (Fig. 304), the mucous membrane of the terminal part of the
small intestine is thrown into _longitudinal folds_.

[Illustration: FIG. 304.--Mucous surface of small intestine of _Phocaena
communis_, porpoise. (Columbia University Museum, No. 1057.)]

[Illustration: FIG. 305.--Mucous membrane of mid-gut of _Lophius
piscatorius_, the angler, 18 cm. caudad of pylorus. (Columbia University
Museum, No. 1838.)]

The mucosa of the midgut in the lower vertebrates may be smooth, or
thrown into longitudinal folds, or the longitudinal folds may become
connected by oblique and transverse secondary folds, resulting finally
in a more or less complicated reticulated pattern of crypts. A very good
example of the type-form from which the more complicated conditions are
derived is seen in Fig. 305, showing the mucous membrane of the midgut
in _Lophius piscatorius_, the angler. The specimen is taken 18 cm. from
the pylorus and shows a ground plan of longitudinal plicae connected by
short oblique cross folds.

[Illustration: FIGS. 306, 307.--Intestinal mucous membrane of
logger-head turtle, _Thalassochelys caretta_. (Columbia University
Museum, No. 1839.)]

[Illustration: FIG. 306.--Mid-gut.]

[Illustration: FIG. 307.--End-gut.]

Fig. 306, showing the midgut mucosa of the loggerhead turtle
(_Thalassochelys caretta_), exhibits the same arrangement further
developed, resulting in a fine reticulated pattern, while in the endgut
of the same animal the primitive longitudinal folding is resumed (Fig.
307).

The number and size of the human valvulae conniventes vary in different
parts of the small intestine (Fig. 309). They are not usually found in
the beginning of the duodenum (Fig. 308), but commence in the second or
descending portion.

[Illustration: FIG. 308.--Adult human subject. Mucous membrane of
pyloro-duodenal junction and of duodenum. (Columbia University Museum,
No. 1840.)]

[Illustration: FIG. 309.--Adult human subject. Mucous membrane of small
intestine, showing arrangement of valvulae conniventes in successive
portions of jejunum and ileum. (Columbia University Museum, No. 1841.)]

They become very large and closely packed immediately beyond the common
entrance of the biliary and pancreatic ducts and continue to be well
developed and numerous throughout the rest of the duodenum and upper
half of the jejunum (Figs. 308 and 309). From here on they become
smaller, more irregular and less closely packed, and finally in the
terminal two feet of the ileum disappear almost entirely (Fig. 309).
This varying development of the valvulae is the chief reason why a given
segment of the ileum weighs less than a corresponding length of the
jejunum. This reduction in the fold-formation of the intestinal mucosa
toward the terminal portion of the midgut is seen even in the lower
vertebrates. Thus in Fig. 112, showing the entire intestinal tract of
the conger eel, _Echelus conger_, in section, the plicae of the mucous
membrane in the proximal segment of the midgut, at and immediately
beyond the entrance of the biliary duct, are prominent and numerous.
This redundancy continues but slightly reduced in the descending limb of
the intestinal loop, while in the ascending limb and up to the
ileo-colic junction the folds are reduced to a fine reticulated
meshwork. Beyond the ileo-colic valve plate, in the short endgut, the
mucosa again presents numerous pointed reduplications.


II. ENDGUT OR LARGE INTESTINE.

In this segment of the intestinal canal the undigested remnants of the
food are collected and evacuated from time to time.

In addition, the mucous membrane of the large intestine absorbs all
_digested_ material which is passed from the small intestine. While
digestion of food-substances will not be _inaugurated_ in the large
intestine, material already in the process of digestion and mixed with
the intestinal juices of the preceding segment, will be further
elaborated in this portion of the canal and the nutritive products
absorbed. This is especially the case in herbivora and omnivora, whose
food is bulky, containing a large amount of refuse material, and is
hence only slowly digested. On the other hand the food of the carnivora
is easily and rapidly digested and absorbed. After passing through the
small intestine hardly any substances remain which are capable of
digestion and absorption. Hence the large intestine of herbivora and
omnivora is uniformly longer in proportion to the small intestine than
it is in carnivorous animals. In the former this segment of the canal
functions as an accessory digestive apparatus and hence, as we will see,
often develops accessory structural modifications, such as a large caecum
and spiral colon, while in the latter it acts almost solely as a canal
for the evacuation of the indigestible remnants.

Again, the large intestine is better developed in the higher animals, in
mammalia and to a lesser degree in birds, in whom the functional demands
for nutrition are active and require that a relatively large amount of
food should pass through the digestive tract in a given time. On the
other hand in the lower cold-blooded vertebrates the metabolism is less
active, less food is taken and it is not necessary to secure all the
nutrient material contained in the same for the organism. The great
differences observed in the vertebrate series in regard to length, width
and structure of the large intestine depend upon these physiological
conditions. The divisions of the human large intestine into caecum,
ascending, transverse and descending colon, sigmoid flexure and rectum
are found only in the primates, and here not uniformly.

In the lower vertebrate classes the endgut is very short, corresponding
only to the pelvic segment of the Mammalia (rectum), a colon proper
being absent in these forms (cf. Fig. 112, _Echelus conger_). The human
large intestine exhibits a very characteristic structure. Throughout the
greater part of the colon the longitudinal muscular layer is mainly
disposed in the form of three bands or taenia (ligamenta coli). The canal
itself is longer than these bands, thus producing a folding of the walls
in the form of three rows of pouches (cellulae coli), in the intervals
between the bands. The pouches of each row are separated from each other
externally by constrictions, internally by projecting crescentic folds
(plicae coli) (Figs. 471, 472 and 474).

This arrangement of the large intestine is also found in the monkeys
(Fig. 473) and in certain Rodents (Fig. 474).

In other mammals the large intestine is smooth and cylindrical and the
longitudinal layer of muscular fibers uniform (Fig. 475).

In general the vertebrate large intestine is _wider_ than the small,
usually in the proportion of 5:1 or 6:1.

In some ruminant Herbivora, however, the great length of the colon leads
to a reduction of the caliber in certain segments so that the large
intestine does not exceed the width of the small, or even falls below
the same.

The _length_ of the large intestine, as in man, is usually much less
than that of the small intestine. As already stated this disproportion
is more marked in Carnivora than in Herbivora.

The ratio in length of the large to the small intestine is very low in
the Seals (1:14), and in several Edentates, as _Myrmecophaga_,
_Tamandua_ and _Bradypus_ (1:9-11).

In the carnivorous mammals it ranges 1:5-7.

In some of the ruminant Herbivora, as the cow and sheep, it is 1:4,
while in the deer, horse, certain Rodents (as _Lepus_ and _Cricetus_) it
reaches as high as 1:2 or 1:3.

The large intestine is usually relatively short in birds, reptiles,
amphibia and fish.

In the Cassowary the length of the large to the small intestine is 1:6.

In some of the birds of prey (eagle) the proportion falls as low as 1:68
or 70.

Exceptions to the general rule are furnished by some of the herbivorous
Cetaceans and by the Dugong (_Halicore_) in whom the large intestine is
twice as long as the small. Again in the Ostrich the large intestine in
one example measured 40', while the length of the small intestine was
only 22'. This unusual development of the large intestine indicates the
necessity of retaining the food, which is bulky and difficult of
digestion, until the elaboration is completed. The same significance
belongs to the enormously developed caeca of these birds (cf. p. 204).

The separation of the small and large intestine is marked externally by
the _caecum_, when present, and internally by the _valve of the colon_.
The details of the vertebrate ileo-colic junction will be considered in
the following pages.


II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED STRUCTURES IN
VERTEBRATES.

I. FISHES.

In the Cyclostomata there is no differentiation between the mid- and
hindgut. Fig. 310 shows the entire alimentary canal of _Petromyzon
marinus_, the lamprey, caudad of the pericardium.

[Illustration: FIG. 310.--_Petromyzon marinus_, lamprey. Entire
alimentary canal below pericardium. (Columbia University Museum, No.
1575.)]

In some fishes the midgut is differentiated from the hindgut by an
external circular constriction, corresponding to an annular projecting
fold of the mucosa in the interior which resembles the pyloro-duodenal
valve. There is no caecum, and the short hindgut empties into the
cephalic and ventral aspect of the cloaca. Fig. 311 shows the entire
intestinal tract of a Teleost fish, _Echelus conger_, the conger eel.
The midgut, provided at the beginning with a short globular pyloric
appendix (cf. p. 119), constitutes the longest individual segment of the
canal. The hindgut, separated from the preceding by aconstriction, is
very short and of large caliber. Fig. 312 shows the broad annular valve
with central circular opening which separates mid- and hindgut in the
interior, and Fig. 313 the ileo-colic junction in section in the same
animal.

[Illustration: FIG. 311.--_Echelus conger_, Conger eel. Alimentary
canal, stomach, mid- and hindgut, liver, and spleen. (Columbia
University Museum, No. 1430.)]

[Illustration: FIG. 312.--_Echelus conger_, Conger eel. Ileo-colic
junction, opened. (Columbia University Museum, No. 1434.)]

[Illustration: FIG. 313.--_Echelus conger_, Conger eel. Section of
mid- and end-gut, with ileo-colic junction, hardened. (Columbia
University Museum, No. 1349.)]

A similar type of ileo-colic junction is seen in other Teleosts, as in
_Gadus callarias_, the cod (Fig. 314), _Pleuronectes maculatus_, the
flounder (Fig. 315), and in some Ganoids, as _Accipenser sturio_, the
sturgeon (Fig. 212). In some Selachians an appendicular diverticulum,
the so-called "rectal" or "digitiform gland," is found connected with
the terminal segment of the gut near the entrance of the same into the
cloaca (Fig. 316).

[Illustration: FIG. 314.--_Gadus callarias_, cod-fish. Ileo-colic
junction. Intestine on each side opened, with probe passed through
constricted opening of ileo-colic valve. (Columbia University Museum,
No. 1260.)]

[Illustration: FIG. 315.--_Pleuronectes maculatus_, flounder.
Ileo-colon, opened to show ileo-colic valve. (Columbia University
Museum, No. 1493.)]

[Illustration: FIG. 316.--_Galeus canis_, dog-shark, male.
Genito-urinary tract and cloaca _in situ_. (Columbia University Museum,
No. 1694.)]

[Illustration: FIG. 317.--_Accipenser sturio_, sturgeon. Alimentary
canal. (Columbia University Museum, Nos. 1826, 1827, and 1828.)]


II. AMPHIBIA.

The alimentary canal is simple and usually comparatively short. There is
no caecal pouch. Differentiation of mid- and endgut is usually marked
externally by a constriction and by the increased caliber of the
terminal intestinal segment.

[Illustration: FIG. 318.--_Rana catesbiana_, bull-frog. Alimentary canal
and appendages. (Columbia University Museum, No. 1454.)]

[Illustration: FIG. 319.--_Necturus maculatus_, mud-puppy. Alimentary
canal and appendages. (Columbia University Museum, No. 1582.)]

[Illustration: FIG. 320.--_Cryptobranchus alleghaniensis_, hellbender.
Ileo-colic junction. (Columbia University Museum, No. 1711.)]

Fig. 318 shows the alimentary canal of the bull-frog, _Rana catesbiana_,
Fig. 319 that of a Urodele Amphibian, _Necturus maculatus_, and Fig. 320
the ileo-colic junction isolated in _Cryptobranchus alleghaniensis_, the
hellbender.


III. REPTILIA.

In reptiles a well-marked differentiation of small and large intestine
is the rule.

Four types of ileo-colic junction are encountered in this class:

1. The transition from small to large intestine is marked by the greatly
increased caliber of the latter and by an annular valve in the interior.
An example of this type is furnished by _Alligator mississippiensis_
(Fig. 321) and a similar form is encountered in some lizards, as
_Heloderma suspectum_, the Gila monster (Fig. 322).

[Illustration: FIG. 321.--_Alligator mississippiensis_, alligator.
Ileo-colon; dried preparation. (Columbia University Museum, No. 179.)]

[Illustration: FIG. 322.--_Heloderma suspectum_, Gila monster. (Columbia
University Museum, No. 69/1536.)]

2. The large intestine immediately beyond the ileo-colic junction
protrudes along the convex border to form a rudimentary lateral caecum.
This type is found in many Chelonians, _e. g._, in _Pseudemys elegans_,
the pond turtle (Figs. 323 and 324) and _Chelydra serpentaria_, the
snapping turtle (Fig. 325).

[Illustration: FIG. 323.--_Pseudemys elegans_, pond-turtle. (Columbia
University Museum, No. 1069.)]

[Illustration: FIG. 324.--_Pseudemys elegans_, pond-turtle. Ileo-colic
junction, opened. (Columbia University Museum, No. 1524.)]

[Illustration: FIG. 325.--_Chelydra serpentaria_, snapping turtle.
Intestinal canal, pancreas, and spleen. (Columbia University Museum, No.
1369.)]

3. The ileo-colic junction is provided with a well-developed sacculated
caecal pouch derived from the proximal segment of the colon and divided
in the interior by folds into several secondary compartments.

[Illustration: FIG. 326.--_Iguana tuberculata_, iguana. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
243.)]

This type is found in some of the phytophagous lizards, as _Iguana
tuberculata_ (Figs. 326 and 327). The small intestine of this animal is
of considerable length and of uniform caliber from the pylorus to the
ileo-colic junction. The caecum is a large sacculated pouch developed
chiefly along the convex border of the large intestine opposite to the
mesenteric attachment.

[Illustration: FIG. 327.--_Iguana tuberculata_, iguana. Mid-gut,
ileo-colic junction, caecum, and end-gut; dried preparation. (Columbia
University Museum, No. 178.)]

[Illustration: FIG. 328.--_Iguana tuberculata_, iguana. Ileo-colic
junction and caecum in section. (Columbia University Museum, No. 1321.)]

[Illustration: FIG. 329.--Drawing taken from same preparation (No. 1321)
to elucidate more clearly internal structure of caecal pouch.]

[Illustration: FIG. 330.--_Cyclura teres_, smooth-backed cyclura.
Ileo-colic junction and caecum in section. (Columbia University Museum,
No. 1523.)]

The examination of the interior of this pouch reveals a complicated
structure (Figs. 328 and 329). Fig. 330 shows the same structures in a
closely allied form, _Cyclura teres_. The entrance of the small
intestine is guarded by an annular sphincter valve, whose central
circular opening leads into a proximal compartment of the caecum. This
compartment is in turn separated from the remainder of the caecal pouch
by a second circular valvular fold with central opening. Beyond this
valve the interior of the pouch carries a number of crescentic mucous
folds, corresponding to the external constrictions between the caecal
sacculations. The entire pouch gradually diminishes in caliber and
finally passes with a sharp angular bend into the terminal portion of
the endgut. At this point the lumen of the canal is slightly diminished
by a sphincter-like thickening of the muscularis, producing an annular
projection of the mucous membrane. The entire caecal pouch appears as a
specialized segment of the large intestine interposed between the
termination of the midgut and the terminal portion of the endgut, which
latter is characterized by uniform caliber and increased thickness of
the muscular walls.

The highly developed and complicated structure of the caecal apparatus in
_Iguana_ and allied forms exemplifies very clearly the influence of
_vegetable_ food on the development of this segment of the alimentary
tract when compared with the simple type of ileo-colic transition found
in _carnivorous_ lizards, as _Heloderma_ (Fig. 322). _Iguana_ subsists
on leaves, fruits and other vegetable matter and the caecal pouch is
invariably found filled with the firmer and less digestible portions of
this food. These are undoubtedly retained in the pouch by the series of
valves and folds until digestion and absorption of all available
nutritive material forwarded from the small intestine is completed. On
the other hand _Heloderma_ lives almost entirely on bird eggs, a
concentrated and easily digested food. Consequently the ileo-colic
junction in this lizard is exceedingly simple and rudimentary, marked
merely by a slight external constriction, with an annular valve in the
interior, and an increase in the caliber of the short hindgut,
resembling the form found in many teleost fishes.

4. Finally in some Ophidians a typical lateral caecal pouch of
considerable dimensions is found connected with the endgut immediately
beyond the ileo-colic junction.

An example of this reptilian type, closely resembling the corresponding
structure in many Mammalia, is presented by _Eunectes marinus_, the
anaconda, shown in Figs. 331 and 332.

[Illustration: FIG. 331.--_Eunectes marinus_, anaconda. Mid- and
end-gut, with ileo-colic junction and caecum. (Columbia University
Museum, No. 72/1535.)]

[Illustration: FIG. 332.--_Eunectes marinus_, anaconda. Mid- and
end-gut, with ileo-colic junction and caecum laid open. (Columbia
University Museum, No. 1709.)]


IV. ILEO-COLIC JUNCTION IN BIRDS.

In the birds the length of the intestine is subject to great variations.
The canal is short in species subsisting on fruits and insects, long in
those feeding on seeds, plants and fish. The large intestine,
immediately beyond the ileo-colic junction, is provided typically with
two symmetrical lateral caeca which extend in some forms for a
considerable distance cephalad on each side of the small intestine to
which they are bound by peritoneal connections.

[Illustration: FIG. 333.--_Buteo harloni_, black hawk. Ileo-colic
junction and caeca. (Columbia University Museum, No. 1502.)]

[Illustration: FIG. 334.--_Phalacrocorax dilophus_, double-crested
cormorant. Ileo-colic junction and caeca. (Columbia University Museum,
No. 67/1534.)]

[Illustration: FIG. 335.--_Gallus bankiva_, hen. Ileo-colic junction and
caeca. (Columbia University Museum, No. 1486.)]

[Illustration: FIG. 336.--_Chen hyperborea_, Canada snow-goose,
Ileo-colic junction and caeca. (Columbia University Museum, No.
47/1448.)]

As a rule carnivorous birds have short and rudimentary pouches (Figs.
333 and 334), whereas they are long in herbivorous forms (Figs. 335 and
336). Some carnivorous birds, as _Corvus_, _Strix_, etc., have fairly
long caeca (Fig. 337). In the passerine birds living on seeds and
insects, the caeca are of considerable length as they are also in some of
the piscivorous divers (Figs. 338 and 339). They are long in the Ratitae,
and in the Lamellirostra, who feed chiefly on plants (Fig. 340).

[Illustration: FIG. 337.--_Bubo virginianus_, great horned owl.
Ileo-colic junction and caeca. (Columbia University Museum, No. 672.)]

[Illustration: FIG. 338.--_Urinator lumme_, red-throated loon.
Ileo-colic junction and caeca. (Columbia University Museum, No. 1001.)]

[Illustration: FIG. 339.--_Merganser serrator_, red-breasted merganser.
Ileo-colic junction and caeca. (Columbia University Museum, No. 1798.)]

[Illustration: FIG. 340.--_Casuarius casuarius_, cassowary. (Columbia
University Museum, No. 1799.)]

[Illustration: FIG. 341.--_Struthio africanus_, African ostrich.
Ileo-colic junction and caeca. (Columbia University Museum, No.
48/1573.)]

The enormously elongated caeca of the African ostrich contain a spiral
fold of the mucous membrane in the interior (Fig. 341).

In place of the usual double avian caecum a single pouch is found in a
few forms, namely in the Herons (Fig. 342).

[Illustration: FIG. 342.--_Ardea virescens_, green heron. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1132 a.)]

[Illustration: FIG. 343.--_Urinator lumme_, red-throated loon. Small
intestine with caecal pouch; the remnant of the vitello-intestinal duct.
(Columbia University Museum, No. 997.)]

[Illustration: FIG. 344.--_Urinator imber_, great northern diver. Small
intestine with caecal pouch; the remnant of the vitello-intestinal duct.
(Columbia University Museum, No. 78/1573)]

In some birds the small intestine is also provided with a caecal pouch,
the remnant of the vitello-intestinal duct corresponding in its
significance to the occasional mammalian diverticulum of Meckel (Figs.
343 and 344). (cf. p. 35.)


V. ILEO-COLIC JUNCTION, CAECUM AND VERMIFORM APPENDIX IN THE MAMMALIA.

I. Subclass: Ornithodelphia.

I. Order: Monotremata.

In many particulars the anatomical structure of these animals reveals a
close relationship to the Sauropsida. They represent the mammalian class
in its lowest stage of evolution.

The ileo-colic junction in all the existing forms is direct, without
angular bend at the entrance of the small into the large intestine. The
caecum is a long narrow pouch, slightly dilated at the extremity, derived
from the beginning of the colon and extending backward along the free
margin of the small intestine. The caecum resembles in its general shape
and structure the pouches seen in many birds, except that it is
unilateral, while the birds normally have two symmetrical caeca. The
caecum of _Ornithorhynchus anatinus_, the platypus or duck bill, is shown
in Figs. 345 and 346, and that of _Echidna hystrix_, the spiny
ant-eater, in Fig. 347. These two animals represent the two genera into
which the order is divided.

[Illustration: FIG. 345.--_Ornithorhynchus anatinus_, duck mole.
Ileo-colon and caecum. (Columbia University Museum, No. 1499.)]

[Illustration: FIG. 346.--_Ornithorhynchus anatinus_, duck mole.
Ileo-colon and caecum. (Columbia University Museum, No. 1500.)]

[Illustration: FIG. 347.--_Echidna hystrix_, spiny ant-eater. Ileo-colon
and caecum. (Columbia University Museum, No. 1501.)]


II. Subclass: Didelphia.

II. Order: Marsupialia.

The Didelphia are represented by numerous species, which are united by
certain common anatomical characters of the reproductive organs and
dentition to form the order of the Marsupialia. The individual species
included within this order differ widely in abit, food, mode of
locomotion, etc., and consequently exhibit great diversity in the
structure of the skeletal and muscular systems and of the alimentary
canal. With the exception of the Opossums inhabiting the new world, the
families composing the order are confined to the Australian continent
and the adjacent islands. In respect to the alimentary tract in general
and the ileo-colic junction in particular, we are evidently dealing with
a group of animals which, while they retain the common characters above
indicated as uniting them in the marsupial order, yet have in the
structure of their digestive canal adapted themselves to widely
divergent conditions of food supply and environment. Consequently within
the confines of this single and largely isolated order, we encounter
nearly all the types of caecum and ileo-colic junction which are found
among the remaining mammalia. The group in its individual
representatives has passed, so to speak, through the different stages of
development and evolution which, on a very much larger scale, are
exhibited by the remaining mammalian orders.

We can, independently of the systematic zoological classification,
arrange the forms composing the order under the following types:

1. _Forms with large well-developed simple caeca, of uniform caliber,
with rounded globular termination._

This type is encountered among the herbivorous Marsupials, such as the
opossums, kangaroos and wallabys. Fig. 348 shows the structures in
_Didelphis virginiana_, the common opossum, Fig. 349 in a small species
of opossum from Trinidad, and Fig. 350 the same parts in _Halmaturus
derbyanus_, the rock wallaby.

[Illustration: FIG. 348.--_Didelphis virginiana_, opossum. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1533.)]

[Illustration: FIG. 349.--_Didelphis sp._? opossum. Ileo-colic junction
and caecum. (Columbia University, Study Collection.)]

[Illustration: FIG. 350.--_Halmaturus derbyanus_, rock kangaroo.
Ileo-colic junction and caecum. (Columbia University Museum, No. 727.)]

2. _Forms with enormously developed sacculated caeca, coiled spirally,
with or without additional convolutions of the proximal colon; the
terminal portion of the caecal pouch diminishes in caliber to form a
pointed appendage._

This type of caecum characterizes the _Phalangeridae_ or Phalangers and
the _Phasolarctidae_.

[Illustration: FIG. 351.--_Trichosurus vulpinus_, vulpine phalanger.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1800.)]

[Illustration: FIG. 352.--_Phascolarctos cinereus_, koala. Ileo-colic
junction and caecum. (Drawn from preparation.) (Columbia University
Museum, No. 1528.)]

Examples are shown in Figs. 351 and 352, representing the structures in
_Trichosurus vulpinus_, the vulpine phalanger, and _Phascolarctos
cinereus_, the koala.

3. _Forms with simple caeca of moderate size._

The _Peramelidae_ or bandicoots.

Fig. 353 shows the ileo-colic junction, caecum and proximal segment of
the colon in _Perameles nasuta_, the bandicoot.

[Illustration: FIG. 353.--_Perameles nasuta_, bandicoot. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1481.)]

4. _Forms with sacculated short caeca, whose terminal portion is reduced
to constitute a typical vermiform appendix._

The caecum of the _Phascolomyidae_ or wombats, resembles, in its general
structure and in the presence of a typical vermiform appendix, very
closely the corresponding parts of the alimentary canal in man and the
anthropoid apes. Fig. 354 shows these structures in _Phascolomys
wombat_, the common wombat.

[Illustration: FIG. 354.--_Phascolomys wombat_, wombat. Ileo-caecum and
appendix. (Columbia University Museum, No. 1508.)]

5. _Forms with simple direct ileo-colic junction without caecum._

In the purely carnivorous Marsupials, comprising the family of the
_Dasyuridae_, the reduction of the caecal apparatus, foreshadowed by the
appearance of the distal rudimentary segment as a vermiform appendix in
the wombats, has been carried to the complete elimination of the pouch.
The ileo-colic junction in these animals is simple, marked externally by
a circular constriction and internally by an annular valve. The colon
forms a very short terminal segment of the alimentary canal. The parts
are shown in Fig. 355 in a typical representative of the family,
_Dasyurus viverinus_, the Tasmanian devil.

[Illustration: FIG. 355.--_Dasyurus viverinus_, dasyurus, Tasmanian
devil. Intestinal canal. Ileo-colic junction. (Columbia University
Museum, No. 1463.)]

The structural modifications encountered in the digestive tract of these
carnivorous Marsupials can properly be regarded as the result of their
habitual diet, and we will meet with analogous and identical examples of
caecal reduction in the typical Carnivores among the placental mammals
(cf. p. 212).


III. Subclass: Monodelphia.

III. Order: Edentata.

In all probability the Sloths, Ant-eaters and Armadillos composing this
order represent a highly specialized remnant of an ancient group now
largely extinct. In respect to the ileo-colic junction the Edentates may
be arranged in two groups which offer, within the limited number of
existing species, a very complete transitional series.


I. SYMMETRICAL TYPE OF ILEO-COLIC JUNCTION.

1. _Differentiation in caliber, with direct funnel-like transition of
small into large intestine. No caecum._

Beyond the ileo-colic junction the caliber of the large intestine
increases gradually. The terminal ileum is thus implanted into the apex
of a funnel formed by the proximal segment of the colon.

Examples of this type are furnished by _Myrmecophaga jubata_, the great
ant-eater (Fig. 356), and by _Choloepus didactylus_, the two-toed sloth
(Fig. 357).

[Illustration: FIG. 356.--_Myrmecophaga jubata_, great ant-eater.
Ileo-colic junction. (Columbia University Museum, No. 1519.)]

[Illustration: FIG. 357.--_Choloepus didactylus_, two-toed sloth.
Ileo-colic junction. (Columbia University Museum, No. 714.)]

2. _Abrupt demarcation of small and large intestine, with median
transition of ileum._

The caliber of the intestine enlarges rapidly immediately beyond the
ileo-colic junction. This form is derived from the preceding by the
substitution of the abrupt ileo-colic transition for the gradual
funnel-shaped development of the large intestine.

The type is illustrated by _Tatusia novemcincta_, the nine-banded
armadillo (Fig. 358), and is also found in two other armadillos,
_Tolypeutes_ and _Xenurus_.

[Illustration: FIG. 358.--_Tatusia novemcincta_, nine-banded armadillo.
Ileo-colic junction. (Columbia University Museum, No. 176.)]

3. _The colon on each side of the ileo-colic junction is prolonged
backward along the small intestine, forming two symmetrical lateral
globular colic caeca._

This type, which is to be regarded as a further development of the
preceding form, is also found in the armadillos. Fig. 359 represents the
structures in _Dasypus sexcinctus_, the six-banded armadillo, and a
similar arrangement of the parts exists in _Chlamydophorus_, another
species of armadillo.

[Illustration: FIG. 359.--_Dasypus sexcinctus_, six-banded armadillo,
Ileo-colic junction and caeca. (Columbia University Museum, No. 1478.)]

4. _The caecal pouches are more completely differentiated, communicating
with the colon by a constricted neck._

This results in an arrangement which recalls the structure of many avian
caeca (cf. Fig. 337) and is seen in the double caecal pouches of
_Cyclothurus didactylus_, the little ant-eater (Fig. 360).

[Illustration: FIG. 360.--_Cyclothurus didactylus_, little ant-eater.
Ileo-colic junction and caeca. (Columbia University Museum, No. 1512.)]


II. ASYMMETRICAL TYPE OF ILEO-COLIC JUNCTION.

The second general group of the Edentates is characterized by the
gradual development of a single lateral asymmetrical caecum, in place of
the median symmetrical ileo-colic transition found in the forms just
considered. The species composing this group thus form a link leading up
to the right-angled accession of ileum to large intestine and the
lateral caecum characteristic of most other mammalia.

[Illustration: FIG. 361.--_Manis longicauda_, long-tailed pangolin.
Ileo-colic junction; dry preparation. (Columbia University Museum, No.
95.)]

[Illustration: FIG. 362.--_Manis longicauda_, long-tailed pangolin.
Ileo-colic junction. (Columbia University Museum, No. 328.)]

1. This type may be considered as being inaugurated by the form of
ileo-colic junction found in the _Manidae_ or _Pangolins_, as illustrated
by Figs. 361 and 362, taken from the long-tailed pangolin, _Manis
longicauda_. There is no caecum and only a slight differentiation in
caliber between the small and large intestine. The gut in all the forms
examined shows a very characteristic bend at the ileo-colic junction,
being twisted into a figure of 8 and held in place by mesenteric folds.

[Illustration: FIG. 363.--_Arctopithecus marmoratus_, three-toed sloth.
Ileo-colic junction. (Columbia University Museum, No. 1479.)]

2. The second stage, illustrated by _Arctopithecus (Bradypus)
marmoratus_, the three-toed sloth (Fig. 363), reveals a distinct
increase in the caliber and convexity of the large intestine opposite
the mesenteric border immediately beyond the ileo-colic junction.

[Illustration: FIG. 364.--_Tamandua bivittata_, Tamandua ant-eater.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1590.)]

[Illustration: FIG. 365.--_Tamandua bivittata_, Tamandua ant-eater.
Ventral view of abdominal viscera from the left side. (Study Collection,
Columbia University.)]

3. This leads in the third stage, represented by _Tamandua bivittata_,
the Tamandua ant-eater (Figs. 364 and 365), to the development of a
distinct lateral caecal pouch. I have had no opportunity of examining the
structures in _Orycteropus_, but from the published descriptions[8] the
large caecum of this animal would form the final link in this series.

[8] Flower and Lyddecker, "Mammals, Living and Extinct," p. 209.


IV. Order: Sirenia.

Of the two living representatives of this remarkable mammalian order the
dugong (_Halicore_) is described as possessing a single caecum, while the
caecal pouch of _Manatus americanus_, the manatee, is symmetrically bifid
at the extremity (Fig. 366).

[Illustration: FIG. 366.--_Manatus americanus_, American manatee.
Ileo-colic junction and bifid caecum. (Columbia University Museum, No.
673.)]

V. Order: Cetacea.

In the majority of the whales the ileo-colic junction is simple without
caecum, as in _Physeter_, _Delphinus_, _Monodon_ and _Phocaena_ (Fig.
367).

[Illustration: FIG. 367.--_Phocaena communis_, porpoise. Ileo-colic
junction. (Columbia University Museum, No. 1007.)]

A few forms have a small caecal pouch.


VI. Order: Ungulata.

The intestinal canal, in conformity with the herbivorous habit of the
group, is uniformly provided with a large caecum, and in many forms the
proximal segment of the colon immediately beyond the ileo-colic junction
is more or less extensively coiled in a spiral manner. This arrangement
is, without doubt, to be regarded as being functionally accessory to the
caecal apparatus, in the sense of increasing very much the area of the
secreting and absorbing surface and of prolonging the period during
which food-substances, which are slow and difficult of elaboration, are
retained in this segment of the alimentary canal.


1. SUBORDER: ARTIODACTYLA.

A. NON-RUMINANTIA.--In the _Suidae_ the caecum is large and the spiral
colon well developed (Fig. 368).

[Illustration: FIG. 368.--_Sus scrofa foet_, foetal pig. Ileo-caecum and
spiral colon _in situ_. (Columbia University Museum, No. 1111.)]

In the peccaries (_Dicotyles_) the terminal portion of the caecal pouch
is reduced, constituting a centrally implanted appendage.

[Illustration: FIG. 369.--_Dicotyles torquatus_, collared peccary.
Ileo-colic junction, caecum, and spiral colon. (Columbia University
Museum, No. 60/1462)]

[Illustration: FIG. 370.--_Dicotyles torquatus_, collared peccary.
Ileo-colic junction and caecum, isolated. (Columbia University Museum,
No. 1464.)]

Fig. 369 shows the ileo-colic junction and spiral colon in _Dicotyles
torquatus_, the collared peccary, and Fig. 370 the caecum and appendix of
the same animal detached from the spiral colon. In the hippopotamus, on
the other hand, the caecum is said to be absent. If this is the case the
animal forms an isolated exception among the Ungulates.

B. RUMINANTIA.--The caecum is very large and the spiral coil of the colon
extensive.

Fig. 371 shows the caecum of _Capra aegagrus_, the Bezoar goat, detached
from the adjacent intestine, and illustrates the type of the ruminant
pouch, of considerable length and caliber, without terminal reduction.
The same parts in a preparation of _Boselaphus tragocamelus_, the
nilghai, are shown in Fig. 372.

[Illustration: FIG. 371.--_Capra aegagrus_, Bezoar goat. Ileo-colic
junction and caecum, isolated; dried preparation. (Columbia University
Museum, No. 194.)]

[Illustration: FIG. 372.--_Boselaphus tragocamelus_, nilghai. Ileo-colic
junction and caecum, isolated. (Columbia University, Study Collection.)]

Fig. 373 shows the caecum and ileo-colic junction, together with the
spiral coil of the colon, in _Bos indicus_, the zebu, and Fig. 374 the
same structures with a typical example of the spiral colon from _Cervus
sika_, the Japanese deer; Fig. 375 is taken from a preparation of the
parts in a foetal sheep, while Fig. 376 shows the spiral colon isolated
in _Oryx leucoryx_, the oryx.

[Illustration: FIG. 373.--_Bos indicus_, zebu. Ileo-colic junction,
caecum, and spiral colon. (Columbia University Museum, No. 676.)]

[Illustration: FIG. 374.--_Cervus sika_, Japanese deer. Ileo-colic
junction, caecum, and spiral colon. (Columbia University, Study
Collection.)]

[Illustration: FIG. 375.--_Oris aries foet_, foetal sheep. Ileo-colic
junction, caecum, and spiral colon. (Columbia University Museum, No.
1379.)]

[Illustration: FIG. 376.--_Oryx leucoryx_, oryx. Spiral colon, isolated.
(Columbia University, Study Collection.)]


2. SUBORDER: PERISSODACTYLA.

In the horse and the rhinoceros the caecum is very large and of uniform
caliber.

[Illustration: FIG. 377.--_Tapirus americanus_, American tapir.
Ileo-colic junction, caecum, and colon. (Columbia University Museum, No.
624.)]

In the American tapir (Fig. 377) the large caecum tapers at its
extremity, to form a species of rudimentary appendix, resembling
somewhat the corresponding structure in _Dicotyles_ (cf. Figs. 369 and
370). The proximal segment of the colon is bent on itself in the form of
an extensive loop with closely adherent limbs, illustrating an early
stage in the development of the ruminant spiral colon (cf. p. 233).


3. SUBORDER: HYRACOIDEA.

This suborder is formed by the single family of the _Hyracidae_. In
addition to their other isolated and puzzling structural peculiarities
the members of this small group present a most unusual arrangement of
the intestinal canal, which is unique among living mammalia. In addition
to a large sacculated caecal pouch, situated in the usual position at the
beginning of the colon, the large intestine is provided further on with
two supplementary elongated pointed conical pouches (Fig. 378).

[Illustration: FIG. 378.--_Hyrax syriacus_, hyrax or coney. Intestinal
canal, with ileo-colic junction, proximal ileo-colic caecum, and distal
paired colic caeca. (Columbia University, Study Collection.)]

This unique arrangement, which is not found in any other known
vertebrate, may possibly be led back to a type-form encountered in
certain saurians (see p. 234).


4. SUBORDER: PROBOSCIDEA.

The caecum of the elephant is a very large sacculated pouch with rounded
termination, illustrated in Fig. 379, taken from the Asiatic elephant.

[Illustration: FIG. 379.--_Elephas indicus_, Asiatic elephant.
Ileo-colic junction and caecum. (Columbia University Museum, No. 995.)]


VII. Order: Rodentia.

With the exception of a single group, the dormice (_Myoxus_) (Fig. 380),
the rodents possess a well-developed caecal apparatus.

[Illustration: FIG. 380.--_Myoxus avellanarius_, common dormouse.
Alimentary canal. (Columbia University Museum, No. 1466.)]

In some forms the terminal portion of the pouch is reduced so as to
constitute an appendix. Many of these animals, in addition to the caecum
proper, have the proximal colon elongated and coiled in a spiral, and in
some this part of the large intestine is provided in the interior with a
spiral mucous fold. This latter structure functions again to increase
the extent of the mucous absorbing surface and to prolong the retention
of substances undergoing slow digestion and absorption.

Typical examples of the capacious sacculated rodent caecum, with a
terminal pointed reduced segment, are afforded by _Castor fiber_, the
beaver (Figs. 381 and 382) and by _Erethizon dorsatus_, the Canadian
porcupine (Figs. 383 and 384). Figs. 385 and 386 show the ileo-colic
junction, caecum and appendix in _Lepus cuniculus_, the rabbit. The
interior of the caecal pouch and of the proximal segment of the colon is
provided with a complete spiral valve (Fig. 387), while the appendix is
differentiated by the histological character of its mucous membrane
which is studded with closely packed adenoid follicles (Fig. 388). A
similar aggregation of lymphoid tissue is found in this animal at the
ileo-colic junction forming the s. c. _saccus lymphaticus_ (Fig. 387).

[Illustration: FIG. 381.--_Castor fiber_, beaver. Ileo-colic junction,
caecum, and proximal colon; ventral view. (Columbia University Museum,
No. 1607.)]

[Illustration: FIG. 382.--_Castor fiber_, beaver. Ileo-colic junction,
caecum, and proximal colon; dorsal view. (Columbia University Museum, No.
1607.)]

[Illustration: FIG. 383.--_Erethizon dorsatus_, Canadian porcupine.
Ileo-colic junction, caecum, and colon; ventral view. (Columbia
University, Study Collection.)]

[Illustration: FIG. 384.--_Erethizon dorsatus_, Canadian porcupine.
Ileo-colic junction, caecum, and colon; dorsal view. (Columbia
University, Study Collection.)]

[Illustration: FIG. 385.--_Lepus cuniculus_, rabbit. Ileo-colic junction
and caecum. (Columbia University Museum, No. 1568.)]

[Illustration: FIG. 386.--_Lepus cuniculus_, rabbit. Ileo-colic junction
with saccus lymphaticus, isolated, (Columbia University, Study
Collection.)]

[Illustration: FIG. 387.--_Lepus cuniculus_, rabbit. Ileo-colic junction
with saccus lymphaticus. Caecum and proximal segment of colon opened to
show spiral mucous fold in interior. (Columbia University Museum, No.
1587.)]

[Illustration: FIG. 388.--_Lepus cuniculus_, rabbit. Caecum and appendix
inverted to show spiral fold and structure of mucosa. (Columbia
University Museum, No. 1588.)]

The coils of the proximal colon encountered in many rodents are well
seen in _Dasyprocta agouti_, the agouti (Figs. 389 and 390), which
animal also illustrates a type of caecum found in several members of the
order. The pouch here is large, sacculated, uncinate, without reduction
of the terminal portion.

[Illustration: FIG. 389.--_Dasyprocta agouti_, agouti. Ileo-colic
junction, caecum, and colon. (Columbia University Museum, No. 24/1576.)]

[Illustration: FIG. 390.--_Dasyprocta agouti_, agouti. Ileo-colic
junction, caecum, and colon. (Drawing based on preparation shown in Fig.
388.)]

[Illustration: FIG. 391.--_Lagomys pusillus._ Ileo-colic junction,
caecum, and colon. (After Pallas, from Oppel, "Lehrbuch d. Vergl.
mikrosk. Anat. d. Wirbelthiere," II., Jena, 1897, p. 577, Fig. 314.)]

[Illustration: FIG. 392.--_Arvicola pennsylvanicus_, field mouse.
Alimentary canal. (Columbia University Museum, No. 815.)]

[Illustration: FIG. 393.--_Mus decumanus_, white rat. Ileo-colic
junction, caecum, and colon. (Columbia University Museum, No. 1574.)]

The relatively enormous size of the caecum in the _Muridae_ is shown in
Fig. 392, representing the entire visceral tract of _Arvicola
pennsylvanicus_, the meadow mouse. The pouch in these animals is large,
smooth and of uniform caliber (Fig. 393).

In some the colon beyond the entrance of the small intestine is provided
with a spiral mucous valve (Fig. 394).

[Illustration: FIG. 394.--_Arvicola riparius_, meadow mouse. Ileo-colic
junction, caecum, and colon. (Columbia University, Study Collection.)]

In the single instance of _Myoxus_ among the rodents, the ileo-colic
junction is simple, without any caecal pouch (Fig. 380).


VIII. Order: Carnivora.

A. PINNIPEDIA.--In the seals and walrus the caecum is very small with a
blunt termination. Fig. 395 shows its structure in _Zalophus
gillespiei_, Gillespie's sea-lion, and Fig. 396 in _Phoca vitulina_, the
harbor seal.

[Illustration: FIG. 395.--_Zalophus gillespiei_, Gillespie's sea-lion.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 90.)]

[Illustration: FIG. 396.--_Phoca vitulina_, harbor seal. Ileo-colic
junction and caecum. (Columbia University Museum, No. 762.)]

B. FISSIPEDIA.--The _Cynoidea_, including the dogs, jackals, wolves and
foxes, form a well-marked central group with well-developed convoluted
caeca placed laterally to the ileo-colic junction (Figs. 397-399).

[Illustration: FIG. 397.--_Vulpes fulvus_, red fox. Ileo-colic junction
and caecum; dried preparation. (Columbia University Museum, No. 114.)]

[Illustration: FIG. 398.--_Canis familiaris_, dog. Ileo-colic junction
and caecum, Type I. (Columbia University Museum, No. 1550.)]

[Illustration: FIG. 399.--_Canis familiaris_, dog. Ileo-colic junction
and caecum, Type II. (Columbia University Museum, No. 1551.)]

From this type depart on the one hand the _Ailuroidea_, including the
civets, ichneumons and true cats, with the caecum uniformly present, but
short and markedly pointed, suggesting the degeneration of a formerly
better developed structure (Figs. 400-406), while on the other the
_Arctoidea_, including the bears, weasels and raccoons, constitute a
group united by many common fundamental peculiarities of structure,
among which is the entire absence of a caecal pouch (Figs. 407-415).

[Illustration: FIG. 400.--_Genetta vulgaris_, genet. Ileo-colic junction
and caecum. (Columbia University Museum, No. 1625.)]

[Illustration: FIG. 401.--_Felis concolor_, puma. Ileo-colic junction
and caecum; dried preparation. (Columbia University Museum, No. 119.)]

[Illustration: FIG. 402.--_Felis borealis, var. rufus_, red lynx.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 177.)]

[Illustration: FIG. 403.--_Paradoxurus typus_, paradoxure. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
112.)]

[Illustration: FIG. 404.--_Herpestes sp.?_, ichneumon. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
120.)]

[Illustration: FIG. 405.--_Herpestes griseus_, mongoose ichneumon.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 149.)]

[Illustration: FIG. 406.--_Proteles lalandii_, aard-wolf. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1520.)]

[Illustration: FIG. 407.--_Nasua rufa_, brown coati-mundi. Ileo-colic
junction. (Columbia University Museum, No. 1089.)]

[Illustration: FIG. 408.--_Nasua rufa_, brown coati-mundi. Ileo-colic
junction, opened, showing pyloric-like ileo-colic valve. (Columbia
University Museum, No. 1581.)]

[Illustration: FIG. 409.--_Bassaris astuta_, raccoon-fox. Ileo-colic
junction; dried preparation. (Columbia University Museum, No. 289.)]

[Illustration: FIG. 410.--_Mustela sp.?_, marten. Ileo-colic junction;
dried preparation. (Columbia University Museum, No. 199.)]

[Illustration: FIG. 411.--_Taxidea americana_, American badger.
Ileo-colic junction; dried preparation. (Columbia University Museum, No.
180.)]

[Illustration: FIG. 412.--_Procyon lotor_, raccoon. Ileo-colic junction;
dried preparation. (Columbia University Museum, No. 230.)]

[Illustration: FIG. 413.--_Cercoleptes caudivolvulus_, kinkajou.
Ileo-colic junction; dried preparation. (Columbia University Museum, No.
295.)]

[Illustration: FIG. 414.--_Ursus americanus_, black bear. Ileo-colic
junction; dried preparation. (Columbia University Museum, No. 226.)]

[Illustration: FIG. 415.--_Ursus maritimus_, polar bear. Ileo-colic
junction. (Columbia University Museum, No. 748.)]

[Illustration: FIG. 416.--_Hyaena striata_, striped hyena. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
56.)]

[Illustration: FIG. 417.--_Felis leo_, lion, Ileo-colic junction and
caecum. (Columbia University Museum, No. 1516.)]

Among the ailuroid carnivora, the hyaena and the lion occupy an isolated
position in regard to the caecum. Both of these animals possess a
well-developed long caecal pouch with blunt extremity (Figs. 416 and
417). They probably afford examples of a persistent ancestral common
type from which the remaining carnivorous forms are derived by reduction
of the caecal apparatus in conformity with the food-habits of these
animals. The caecum of both the lion and hyaena resembles very closely the
pouch of the herbivorous marsupials, such as _Halmaturus_ or _Didelphis_
(cf. Figs. 348 and 350, p. 205).


IX. Order: Cheiroptera.

In the bats the alimentary canal is uniformly simple without caecum and
scarcely any differentiation between small and large intestine (Fig.
418).

[Illustration: FIG. 418.--_Pteropus medius_, Indian fruit-bat.
Ileo-colon; dried preparation. (Columbia University Museum, No. 533.)]


X. Order: Insectivora.

In the true Insectivora the caecum is also absent and the alimentary
canal a simple non-differentiated tube.

In certain herbivorous animals included in this group on the other hand,
such as _Galeopithecus_ (Fig. 419), the caecum is present as an enormous
sacculated pouch with spiral convolutions.

[Illustration: FIG. 419.--_Galeopithecus volans_, colugo. Ileo-colic
junction, caecum, and colon. (Columbia University Museum, No. 1844.)]


XI. Order: Primates.

The caecum is uniformly present. In certain of the Lemuroidea the
terminal portion of the pouch is reduced, forming a species of appendix.
A typical vermiform appendix is regularly found in man and in the
anthropoid apes, orang, gibbon, chimpanzee and gorilla.


1. Suborder Lemuroidea.

In the typical lemurs the caecum is long, frequently terminating in a
pointed appendage. The proximal segment of the colon is looped and
coiled, resembling the spiral colon of the Ungulates and Rodents. Fig.
420 shows the caecum of _Nycticebus tardigradus_, the slow lemur, with
the typical appendage, and Fig. 421 shows the spiral arrangement of the
proximal colon immediately beyond the ileo-colic junction in the same
animal. Fig. 422, taken from another specimen of the same animal shows
the caecum, appendix and spiral colon. Figs. 423, 424, 425 illustrate the
structure of the parts in three other members of the group, _Lemur
macaco_, _Lemur mongoz_ and _Otolicnus crassicaudatus_, all showing
terminal reduction of the caecal pouch and tendency to spiral coiling of
the proximal colon. In _Tarsius spectrum_ (Fig. 426) the caecum is
relatively well-developed, but forms a simple pouch of uniform diameter,
without terminal reduction.

[Illustration: FIG. 420.--_Nycticebus tardigradus_, slow lemur.
Ileo-colic junction, caecum, appendix, and colon; dorsal view. (Columbia
University Museum, No. 20/1468.)]

[Illustration: FIG. 421.--_Nycticebus tardigradus_, slow lemur. Same
preparation as Fig. 420; ventral view, showing spiral coiling of
proximal colon. (Columbia University Museum, No. 20/1468.)]

[Illustration: FIG. 422.--_Nycticebus tardigradus_, slow lemur.
Ileo-colic junction, caecum, appendix, and spiral colon. (Columbia
University, Study Collection.)]

[Illustration: FIG. 423.--_Lemur macaco_, lemur. Ileo-colic junction and
caecum. (Drawn from preparation.) (Columbia University Museum, No.
1623.)]

[Illustration: FIG. 424.--_Lemur mongoz_, lemur. Ileo-colic junction and
caecum. (Drawn from preparation.) (Columbia University Museum, No.
1473.)]

[Illustration: FIG. 425.--_Otolicnus crassicaudatus_, galago. Ileo-colic
junction and caecum. (Drawn from preparation.) (Columbia University
Museum, No. 1626.)]

[Illustration: FIG. 426.--_Tarsius spectrum_, spectre lemur. (Drawn from
preparation.) (Columbia University Museum, No. 1521.)]


2. Suborder Anthropoidea.

A. CYNOMORPHA.

=1. Cynocephalus.=--The baboons have a well-developed capacious caecum.
The apex of the pouch is usually blunt and rounded, or only slightly
pointed. The caecum is sacculated, conforming in structure to the rest of
the large intestine. Two low vascular folds or ridges, a ventral and a
dorsal, carry the ventral and dorsal caecal branches of the ileo-colic
artery. The intermediate non-vascular fold is large, frequently fused
with the dorsal vascular fold (cf. p. 264).

Figs. 427-433 show the structures in _Cynocephalus sphinx_, _porcarius_,
_babuin_, _anubis_ and in _Cercopithecus pogonias_, _sabaeus_ and
_campbellii_.

[Illustration: FIG. 427.--_Cynocephalus sphinx_, Guinea baboon.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1082.)]

[Illustration: FIG. 428.--_Cynocephalus porcarius_, Chacma baboon.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1071.)]

[Illustration: FIG. 429.--_Cynocephalus babuin_, yellow baboon; dried
preparation. (Columbia University Museum, No. 89.)]

[Illustration: FIG. 430.--_Cynocephalus anubis_, olive baboon. (Columbia
University Museum, No. 51/1618.)]

[Illustration: FIG. 431.--_Cercopithecus pogonias_, bearded monkey.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 228.)]

[Illustration: FIG. 432.--_Cercopithecus sabaeus_, green monkey.
Ileo-colic junction and caecum. (Drawn from preparation.) (Columbia
University Museum, No. 746.)

    1. Ventral ileo-caecal vascular fold.
    2. Dorsal ileo-caecal vascular fold.
    3. Intermediate ileo-caecal non-vascular fold.
]

[Illustration: FIG. 433.--_Cercopithecus campbellii_, cercopithecus
monkey. Ileo-colic junction and caecum. (Drawn from preparation.)
(Columbia University Museum, No. 55/1542.)]

=2. Macacus.=--The caecum is of large caliber, blunt, or in some forms
slightly pointed at the apex, sacculated like the colon.

The two vascular folds are narrow and low, studded with epiploic
appendages. The intermediate non-vascular fold is large, placed nearer
to the dorsal than to the ventral vascular fold.

Figs. 434-439 show the structures in _Macacus cynomolgus_, _ochreatus_,
_rhesus_ and _pileatus_.

Fig. 439 is from a formaline hardened situs preparation of the abdominal
viscera in _Macacus cynomolgus_, the Kra monkey.

[Illustration: FIG. 434.--_Macacus cynomolgus_, Macaque monkey.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 19.)]

[Illustration: FIG. 435.--_Macacus ochreatus_, ashy-black macaque.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 11.)]

[Illustration: FIG. 436.--_Macacus rhesus_, rhesus monkey. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1126.)]

[Illustration: FIG. 437.--_Macacus pileatus_, macaque. Ileo-colic
junction and caecum. (Columbia University Museum, No. 719.)]

[Illustration: FIG. 438.--_Macacus sinicus_, bonnet macaque. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1072.)]

[Illustration: FIG. 439.--_Macacus cynomolgus_, kra monkey. Abdominal
viscera, hardened _in situ_. (Columbia University Museum, No. 1801.)]


B. ARCTOPITHECINI.

The marmosets have a long crescentic-shaped caecum, turning the concavity
of the curve upwards and to the left, terminating in a blunt point.

Typical forms are shown in Fig. 440, _Hapale jacchus_, Fig. 441, _Midas
ursulus_, and Fig. 442, _Midas geoffrei_.

[Illustration: FIG. 440.--_Hapule jacchus_, common marmoset. Ileo-colic
junction and caecum. (Columbia University Museum, No. 975.)]

[Illustration: FIG. 441.--_Midas ursulus_, <DW64> tamarin. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
235.)]

[Illustration: FIG. 442.--_Midas geoffrei_, Geoffrey's marmoset.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 197.)]


C. CEBIDAE.

=1. Ateles= and other howlers have a large caecum, crescentic in shape,
narrowed at the apex, separated from the colon by a sharp and deep
constriction, opposite the wedge-shaped ileo-colic junction.

The ileo-caecal folds are well-developed and symmetrical, two equal
vascular folds, and a free intermediate non-vascular reduplication.

Types: _Ateles ater_ (Figs. 443-445), _Chrysothrix sciureus_ (Fig. 447)
and _Nyctipithecus commersonii_ (Fig. 446). In _Mycetes_ (Figs. 448-450)
the pouch is shorter, less curved, with a slight reduction toward the
less distinctly pointed apex.

[Illustration: FIG. 443.--_Ateles ater_, black-faced coaita. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
240.)]

[Illustration: FIG. 444.--_Ateles ater_, black-faced coaita. Ileo-colic
junction and caecum, with ileo-caecal folds. (Columbia University Museum,
No. 720.)]

[Illustration: FIG. 445.--_Ateles ater_, black-faced coaita. Ileo-colic
junction and caecum, with ileo-caecal folds. (Drawn from preparation.)
(Columbia University Museum, No. 300.)

    1. Ventral vascular ileo-caecal fold.
    2. Intermediate non-vascular ileo-caecal fold.
    3. Dorsal vascular ileo-caecal fold.
]

[Illustration: FIG. 446.--_Nyctipithecus commersonii_, Vitoe monkey.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 238.)]

[Illustration: FIG. 447.--_Chrysothrix sciureus_, Viti monkey.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1624.)]

[Illustration: FIG. 448.--_Mycetes cavaya_, black howler. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1136.)]

[Illustration: FIG. 449.--_Mycetes fuscus_, brown howler. Ileo-colic
junction and caecum, with ileo-caecal folds; ventral view. (Columbia
University Museum, No. 674.)

    1. Ventral vascular ileo-caecal fold.
    3. Intermediate non-vascular ileo-caecal fold.
]

[Illustration: FIG. 450.--Drawn from the same preparation as Fig. 449;
dorsal view.

    2. Dorsal vascular ileo-caecal fold.
    3. Intermediate non-vascular ileo-caecal fold.
]

[Illustration: FIG. 451.--_Lagothrix humboldtii_, Humboldt's lagothrix.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1511.)]

[Illustration: FIG. 452.--_Pithecia satanas_, black saki monkey.
Ileo-colic junction and caecum. (Columbia University Museum, No. 641.)]

=2. Lagothrix.=--The caecum is very capacious and long, bent at a sharp
angle upwards and to the left toward the ileo-colic junction.

Type: _Lagothrix humboldtii_ (Fig. 451).

=3. Pithecia.=--The caecum resembles in general the type presented by
_Ateles_, but is less curved and less reduced and pointed at the
termination.

Type: _Pithecia satanas_ (Fig. 452).

In general the Arctopithecini and _Ateles_, _Mycetes_, _Lagothrix_ and
_Pithecia_ among the Cebidae form a group containing a series of caecal
transition types which lead up to the anthropomorphous type,
illustrating the following conditions:

(_a_) The inherent crescentic curve of the caecum, with the concavity
directed toward the left, and carrying the apex of the pouch upward
toward the lower border of the ileum and the ileo-colic junction.
(_Hapalidae_, _Ateles_, _Lagothrix_.)

(_b_) The reduction in caliber of the terminal part, foreshadowing by
the pointed and narrow extremity of the pouch the appearance of the
appendix in the anthropomorphous group. (_Hapalidae_, _Ateles_.)

(_c_) The constriction at the level of the ileo-caecal junction, with the
corresponding well-marked differentiation between caecum and colon in the
interior. (_Ateles._)

(_d_) The sharp bend in the pouch as it makes its turn upward and to the
left, repeated in certain types of adult human caeca (cf. p. 247).
(_Lagothrix._)

(_e_) _Pithecia_ forms a transitive type between the blunt sacculated
caeca of the Cynomorpha and the curved pointed pouches of the Cebidae,
partaking of the characters of both.

(_f_) The same character is seen in the caecum of _Mycetes fuscus_ the
brown howler monkey (Figs. 449 and 450).

=4. Cebinae.=--In the typical genus _Cebus_ the caecum is placed laterad
to the small intestine which is in direct linear continuity with the
colon. The pouch is slightly convoluted toward its termination,
resembling in this respect and in its position relative to the lumen of
the intestinal canal, the disposition of the parts in the cynoid
carnivora. Figs. 453 and 454 show the structures in two typical species,
_Cebus monachus_ and _C. leucophaeus_.

[Illustration: FIG. 453.--_Cebus monachus_, capuchin monkey. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
26.)]

[Illustration: FIG. 454.--_Cebus leucophaeus_, capuchin monkey.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1467.)]


D. ANTHROPOMORPHA.

The caecum is large, sacculated, provided uniformly with a vermiform
appendix.

The pouch of the four anthropoid apes agrees in curve, direction,
implantation of the appendix and the general arrangement of the vascular
and peritoneal folds with the structure in the human subject.

=1. Hylobates hoolock, Gibbon.=--Figs. 455 and 456 represent
respectively the ileo-caecum of this animal in the ventral view, and from
the left side with the ileum turned forward. The caecum is a globular
rounded pouch of nearly uniform diameter, only slightly enlarged to the
right of the root of the appendix which arises from its lowest part and
is pendent.

[Illustration: FIG. 455.--_Hylobates hoolock_, hoolock gibbon.
Ileo-colic junction and caecum; ventral view. (Drawn from Columbia
University Museum preparation No. 1530.)]

[Illustration: FIG. 456.--Drawn from same preparation as Fig. 455; view
from left side, showing formation of posterior ileo-caecal fossa.]

[Illustration: FIG. 457.--_Gorilla savagei_, gorilla. Ileo-colic
junction and caecum, with ileo-caecal folds. (Drawn from Columbia
University Museum preparation No. 1543.)]

(For arrangement of the ileo-caecal folds and fossae in this form see p.
269.)

=2. Gorilla savagei, Gorilla= (Fig. 457).--The caecum is large,
distinctly sacculated, presenting a decided curve with the concavity
directed toward the left. The appendix is implanted at the center of the
blunt apex of the pouch, the caecal sacculations on each side of the root
of the appendix being of nearly equal size (folds and fossae, cf. p.
269).

[Illustration: FIG. 458.--_Simia satyrus_, orang. Caecum and ileo-colic
junction; ventral view. (Drawn from Columbia University Museum
preparation No. 716.) 1. Appendix. 2. Ventral vascular fold.]

[Illustration: FIG. 459.--_Simia satyrus_, orang. Caecum and ileo-colic
junction; dorsal view. (Drawn from Columbia University Museum
preparation No. 716). 1. Appendix. 2. Intermediate non-vascular fold.]

=3. Simia satyrus, Orang-outang.=--Figs. 458 and 459 represent
respectively the ventral and dorsal views of the caecum and ileo-colon in
a nearly adult male specimen of orang, about 41/2 feet high.

The caecum is funnel-shaped, gradually narrowing to the origin of the
appendix from its apex, which is carried upwards to the left by the
well-marked crescentic curve of the pouch. The sweep of the funnel to
the left and upwards is characterized by the curved course of the
ventral longitudinal muscular band (Fig. 458), whose fibers spread out
over a surface 3 cm. wide. The apex is thus placed behind the terminal
ileum close to its entrance into the large intestine.

At the level of the upper margin of the ileo-colic junction the narrow
pointed termination of the caecum passes gradually into the beginning of
the appendix (Fig. 459).

The appendix measures along its free border 22.6 cm. It follows the
direction of the caecal curve for 2.7 cm., at which point it appears
somewhat constricted and takes an abrupt bend downwards for 4.3 cm.;
curving again upwards for 7.5 cm., it turns downward a second time for
5.4 cm. and terminates in a hook-like extremity 2.7 cm. long (Fig. 459).

[Illustration: FIG. 460.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum; ventral view. (Drawn from Columbia University Museum
preparation No. 675.) 1. Appendix. 2. Intermediate non-vascular
ileo-caecal fold. 3. Colon.]

[Illustration: FIG. 461.--_Troglodytes niger_, chimpanzee. Dorsal view.
(Drawn from Columbia University Museum preparation No. 675.) 1.
Appendix. 2. Intermediate non-vascular ileo-caecal fold. 3. Dorsal
vascular fold.]

=4. Chimpanzee, Troglodytes niger.=--Figs. 460 and 461 represent the
ventral and dorsal view respectively of the caecum and ileo-colon in a
young specimen.

The caecum is curved to the left and the lowest point of the pouch is
formed by the right lateral and ventral wall of the gut, but the extreme
crescentic bend which carries the origin of the appendix up and to the
left behind the ileo-colic junction is not yet developed in the young
animal; on the other hand this character of the caecum is typically
apparent in Figs. 462 and 463, taken from an adult individual of the
same species.

[Illustration: FIG. 462.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum; ventral view. (Drawn from Columbia University Museum
preparation No. 1083.) 1. Ventral vascular ileo-caecal fold.]

[Illustration: FIG. 463.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum; dorsal view. (Drawn from Columbia University Museum
preparation No. 1083.) 1. Appendix.]

[Illustration: FIG. 464.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum. (Drawn from Columbia University Museum preparation
No. 1525.)]

This extreme curve is well seen in the ventral view in Figs. 462 and
464, the latter taken from a large adult specimen. Seen from behind in
Fig. 463 the sharp bend or kink in the lumen of the caecal pouch produced
by this curve is striking and resembles the arrangement of certain types
of adult human caeca (p. 247).


II. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION AND CAECUM IN THE
VERTEBRATE SERIES.

The segments of the alimentary canal illustrate very clearly the
adaptation of structure to function. Diversity of kind and quantity of
food habitually taken and variations in the rapidity of tissue
metabolism produce marked morphological modifications in different
forms. This is more especially the case with the junction of the mid-
and hindgut, the site of development of the caecal apparatus and of
structural alterations of the large intestine possessing a similar
physiological significance. No other portion of the visceral tract,
with the possible exception of the stomach, illustrates more completely
the result of physiological demand on the development of anatomical
structure and the morphological possibilities of departure, progressive
and retrograde, from a common primitive type in accordance with varying
conditions of alimentation.

In cooerdinating, from the morphological standpoint, the structural
differences encountered in this segment of the alimentary canal, two
facts become apparent.

1. In the first place the serial study of the ileo-colic junction, as we
can briefly define the region in question by borrowing the terminology
of anthropotomy, reveals a limited number of principal structural types
from which by successive gradations the vast variety of individual forms
may be derived.

[Illustration: FIG. 465.--Schematic table of the vertebrate types of
ileo-colic junction.]

(In the schematic Fig. 465 the fundamental types and their derivatives
are indicated. In the following the individual forms illustrating these
types are referred to this schema in brackets.)

2. The observer will be impressed by the fact that representatives of
all the main types of ileo-colic junction are found within a very
limited zoological range, as within the confines of a single order.
Examples of this are furnished by the Marsupialia and, to a lesser
extent, by the Edentata. The members of these zoological groups, while
united by certain common anatomical characters, such as the reproductive
system and dentition, differ widely in habit and in the kind and
quantity of the food normally taken. These differences in the method of
nutrition have impressed their influence on the structure of the
alimentary canal and have led to the evolution of varying and divergent
types of ileo-colic junction. The study of this segment of the
intestinal tract can therefore elucidate the mutual relationship of the
vertebrate groups only to a limited degree and in special cases. On the
other hand, it renders very clear the fundamental structural ground-plan
common to all vertebrates and accentuates the specialized modifications
of this plan which develop in response to the physiological environment.
Moreover, such a review serves to reveal the significance of
rudimentary and vestigial structures, such as the human vermiform
appendix and the serous and vascular folds connected with the same.
Throughout the entire vertebrate series the alimentary canal is found to
respond with great readiness in its structure to varying grades of
functional demand. This fact becomes still more apparent if the inquiry
is not limited strictly to the region of the ileo-colic junction but
takes into account likewise the structural modifications of similar
physiological significance in other segments of the alimentary tract.

A caecal pouch or diverticulum in some form at the junction of mid- and
hindgut is a very common and widely distributed mammalian character. The
activity of the tissue-changes in warm-blooded animals, and the
consequent necessity for a rapid and complete digestive process, account
for the structural modifications of the alimentary tract so commonly
encountered among these forms. On the other hand, in the lower
cold-blooded vertebrates, notably in fishes and amphibians, the
metabolism is slow and the alimentary canal usually simple.

Specifically, the caecum appears as a pouch or diverticulum in which
food-substances, already partially digested and mixed with the
secretions of the small intestine, are retained until their elaboration
is completed and the nutritive value of the food ingested is secured for
the organism. Consequently the most complicated and highly developed
caecal apparatus is found among mammalia in the Herbivora, such as the
Ungulates and Rodents, whose food contains a comparatively small amount
of nutriment in ratio to its bulk, and hence requires considerable
elaboration before absorption. On the other hand the caecum appears as a
reduced or even rudimentary organ, or defaults entirely, in Carnivora
whose food is concentrated and easily assimilated, containing only a
small amount of non-nutritive material.

The function of the caecal apparatus may be defined as follows:

1. It provides space for the retention of partly digested substances,
and of such as are difficult of digestion, mixed with the secretions of
the preceding intestinal segment, until the digestive elaboration is
completed.

2. It increases the intestinal mucous surface for absorption, and may
develop, in certain cases, special localized areas of lymphoid tissue.

These two functional characters may be shared by other segments of the
intestinal tract, which undergo corresponding structural modifications.
It is only necessary to refer in this connection to the extreme
morphological variations encountered in the stomach. The intestinal
canal proper, however, in many instances exhibits structural
peculiarities which possess the functional significance of the caecal
apparatus. Thus the projection into the lumen of the canal of a series
of mucous folds, or the development of a continuous spiral mucous valve,
evidently serves the double purpose of prolonging the period during
which the intestinal contents are retained, and of increasing the
intestinal mucous surface for absorption.

[Illustration: FIG. 466.--_Squalus acanthius_, dog-fish. Alimentary
tract, spleen, pancreas. (Drawn from Columbia University Museum
preparation No. 1405.)]

[Illustration: FIG. 467.--_Galeus canis_, dog-shark. Alimentary tract
opened, showing spiral intestinal valve. (Drawn from Columbia University
Museum preparation No. 1429.)]

[Illustration: FIG. 468.--_Ceratodus forsteri_, Australian lung-fish.
Intestinal canal with spiral valve. (Columbia University Museum, No.
1645.)]

[Illustration: FIG. 469.--_Python molurus_, Indian python. Mid-gut,
distended and fenestrated to show spiral course of lumen. (Columbia
University Museum, No. 725.)]

This spiral mucous fold is encountered in the straight intestinal canal
of the Cyclostomata (Fig. 465, _IV_, 1, and Fig. 310), Selachians (Figs.
466 and 467) and Dipnoeans (Fig. 468). Phylogenetically it is a very old
structure, for evidences of its existence are found in the fossil
remains of some Elasmobranchs. In the Ostrich (Fig. 341) the enormously
developed caeca possess the same spiral mucous fold in the interior. The
direct combination of the caecum and spiral fold is again seen in certain
mammalia, as in _Lepus_ (Fig. 387). In some Ophidians the same
physiological purpose is served by the manner in which the convolutions
of the long intestine are bound together by a subperitoneal arachnoid
membrane. The lumen of the canal is thus made to assume a spiral course
(Figs. 331 and 469). The mucous folds of the human intestine, both the
valvulae conniventes and the crescentic folds of the large intestine,
represent the same spiral valve, perhaps modified and influenced by the
erect posture of man (Figs. 470-475).

[Illustration: FIG. 470.--Human small intestine, opened to show valvulae
conniventes. (Columbia University Museum, No. 1841.)]

[Illustration: FIG. 471.--Human large intestine, showing colic taenia and
plica. (Columbia University Museum, No. 1848.)]

[Illustration: FIG. 472.--Human large intestine, opened and in section,
showing colic plicae. (Columbia University Museum, No. 1847.)]

[Illustration: FIG. 473.--_Cynocephalus anubis_, olive baboon. Large
intestine, with cross-section showing colic taenia and plicae. (Columbia
University Museum, No. 26/1168.)]

[Illustration: FIG. 474.--Comparison of portion of human transverse
colon with distal segment of rabbit's large intestine, showing same
arrangement of longitudinal muscular bands (colic taenia) and colic
sacculations. (Columbia University Museum, No. 1589.)]

[Illustration: FIG. 475.--_Felis leo_, lion. Large intestine, with
transverse section, showing smooth carnivore lumen, without sacculations
or plicae. (Columbia University Museum, No. 1600.)]

A second modification of the intestinal canal, suggesting the same
physiological interpretation as the ileo-colic caecum, is presented by
the so-called pyloric caeca or appendices of many Teleosts and Ganoids
already referred to (p. 119). While these structures in some forms very
probably have assumed a secretory function (Figs. 476 and 477), they
evidently act in others as diverticula in which material undergoing
digestion is retained, while they increase at the same time the
intestinal mucous secretory and absorbing surface (Figs. 478 and 479).
They thus correspond physiologically to the ileo-colic caecum. In this
connection it is interesting to note that in Ganoids, which possess both
the pyloric appendices and the spiral valve, the two structures develop
in inverse ratio to each other, indicating their functional identity. In
the serial review of the structure and significance of the vertebrate
caecum and ileo-colic junction these functionally allied modifications of
other segments of the intestinal canal deserve notice.

[Illustration: FIG. 476.--Pyloric caeca of _Gadus callarias_, codfish.
(Columbia University Museum, No. 1825.)

_A._ Bound together by connective tissue and blood-vessels.

_B._ Dissected to show confluence of caeca to form a smaller number of
terminal tubes of larger calibre entering the intestine.]

[Illustration: FIG. 477.--Alimentary canal of _Accipenser sturio_,
sturgeon. Numerous pyloric caeca are bound together to form a gland-like
organ.

In the smaller upper figure on the left the stomach, mid-gut, and
pyloric caeca are seen in section, showing the lumen of the latter and
their openings into the mid-gut.

The lower left-hand figure shows the mid- and end-gut in section, the
latter provided with a spiral mucous valve. (Columbia University Museum,
Nos. 1826, 1827, and 1828.)]

[Illustration: FIG. 478.--Stomach, duodenum, and pyloric caeca of
_Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)]

[Illustration: FIG. 479.--_Pleuronectes maculatus_, window-pane. Stomach
and mid-gut with pyloric caeca and hepatic duct. (Columbia University
Museum, No. 1432.)]

The study of the vertebrate ileo-colic junction proper begins both
ontogenetically and phylogenetically with the consideration of the
primitive type in which the alimentary tube is not differentiated into
successive segments and in which consequently no distinction between
mid- and hindgut is found (Fig. 465). An example of this primitive
condition is presented by the Cyclostomata, in whom the alimentary canal
traverses the coelom cavity as a straight non-differentiated cylindrical
tube. Fig. 310 shows the alimentary canal of the Lamprey, _Petromyzon
marinus_, and it will be observed that the intestine is provided with
the spiral mucous fold above mentioned.

From this fundamental type the following main groups are to be derived:

I. Symmetrical Form of Ileo-colic Junction. Mid- and Endgut in Direct
Linear Continuity. (Fig. 465, I.)

1. _Ileo-colic junction marked externally by an annular constriction,
corresponding to a ring-valve with central circular opening in the
interior_ (Fig. 465, _I_, 1).

This form is encountered in many Teleosts. The projecting annular mucous
fold resembles the pyloro-duodenal valve.

Figs. 311-315 illustrate the structures in representative Teleosts.

Among the higher forms this type of ileo-colic junction is encountered
in the simple alimentary canal of many Amphibians (Figs. 318-320). Among
Reptiles it is found in certain lizards, as in _Heloderma suspectum_,
the gila monster (Fig. 322). This animal lives almost entirely upon
bird's eggs, and its simple and reduced ileo-colic junction contrasts
strongly with the highly developed and complicated caecal apparatus of
the phytophagous lizards, as _Iguana_ (Figs. 326-330), affording one of
the most striking illustrations of the effect which the character of the
food habitually taken has on the structure of the alimentary canal in
forms otherwise closely allied.

The same type of ileo-colic junction, as a reduction form, occurs in the
arctoid group of Carnivora among Mammalia (cf. p. 212).

2. _Differentiation in caliber of large and small intestine.
Funnel-shaped ileo-colic transition._

This type, compared with the preceding, is characterized (Fig. 465, _I_,
2) by the greatly increased caliber of the large intestine, resulting in
a funnel-shaped transition between mid- and hindgut, the small intestine
continuing into the colon at the apex of the funnel.

Examples of this type are presented by several Edentates, _Myrmecophaga
jubata_, the great ant-eater (Fig. 356), and _Choloepus didactylus_, the
two-toed sloth (Fig. 357).

3. _Abrupt demarcation of small and large intestine with caliber
differentiation_ (Fig. 465, _I_, 3).

The small intestine is still central at the ileo-colic junction,
_i. e._, the axis of its lumen is continuous with the central axis of
the colic lumen. In place of the gradual funnel-shaped transition of
the preceding type the demarcation is abrupt.

An example of this form is furnished by another Edentate, _Tatusia
peba_, the nine-banded armadillo (Fig. 358).

Among reptiles a similar well-marked ileo-colic transition is
encountered in _Alligator mississippiensis_ (Fig. 321).

4. _Colic pouch prolonged back on each side of the ileo-colic junction,
producing symmetrical colic caeca_ (Fig. 465, _I_, 4).

A growth of the colic tube cephalad, on each side of the junction with
the midgut, leads to the formation of this type, characterized by the
presence of two symmetrical globular caecal pouches. In its simplest form
this condition is illustrated by the double colic caeca of another
armadillo, _Dasypus sexcinctus_ (Fig. 359).

The bifid caecal apparatus of the American manatee (Fig. 366) belongs to
the same group.

5. _Caecal pouches of the birds_ (Fig. 465, _I_, 5).--A continuation of
the backward extension of the bilateral colic pouches leads to the
production of the typical double avian caeca in a greater or lesser
degree of development. Frequently the caeca differentiate more completely
from the colon, appearing as pouches of varying capacity joined to the
large intestine by a narrower neck.

Figs. 334-341 show the well-developed pouches as they appear in
representative avian types, while Fig. 333 illustrates the reduction of
the caecal apparatus encountered in many carnivorous birds.

6. Among mammalia _Cyclothurus didactylus_ (Fig. 360), the little
ant-eater, furnishes an example of double symmetrical globular caeca,
connected with the colon by a narrow neck (Fig. 465, _I_, 6). Reference
to the schema given in Fig. 465 will show that the types heretofore
examined all have the following common character:

They appear derived from the primitive type by a differentiation in the
caliber of the gut and by the gradual development of _symmetrical
bilateral_ caecal pouches, resulting in central median implantation of
the small intestine and its direct continuity with the colon.


II. Asymmetrical Development of a Single Caecal Pouch, Lateral to the
Ileo-colic Junction, Mid- and Endgut Preserving Their Linear Continuity.
(Fig. 465, II.)

In the second general group the symmetry of the ileo-colic junction is
disturbed. The following types are encountered, forming a series of
successive stages:

1. The increase in the caliber of the large intestine is chiefly marked
along the border opposite to the mesenteric attachment, resulting in a
greater degree of convexity in this part of the intestinal wall (Fig.
465, _II_, 1). Among Reptilia this condition is found in the ileo-colic
junction of some of the pond-turtles, as _Pseudemys elegans_ (Fig. 323),
while a mammalian example is furnished by the three-toed sloth,
_Arctopithecus marmoratus_ (Fig. 363).

2. An increase of this lateral extension of the colon leads to the
formation of a single lateral caecal pouch (Fig. 465, _II_, 2) such as is
seen in another Edentate, _Tamandua bivittata_ (Fig. 364), among
Mammalia, and in certain Ophidians among Reptiles, as in the _Anaconda_
(Figs. 331 and 332).

3. Prolongation of the pouch and reduction in caliber lead to the
formation of the slender lateral caecum found in all the Monotremes
(Figs. 345-347, Fig. 465, _I_, 3). In its general appearance the caecum
of these singular animals bears a close resemblance to the caecal pouches
of many birds.

4. Direct continuity of small and large intestine, with lateral colic
caecum, extending along the convex free border of the terminal ileum and
slightly convoluted at the extremity (Fig. 465, _II_, 4), characterizes
the entire group of the _Cebidae_ among the new-world monkeys. The caecum
in these animals is a comparatively long pouch, nearly equalling in
caliber the remainder of the intestine, occupying a distinctly _lateral_
position, with the terminal portion rounded and slightly recurved (Figs.
453 and 454).

5. The _Cynoid group_ of Carnivora, including the dogs, wolves, jackals
and foxes, presents a similar relative position of small and large
intestine and caecum (Fig. 465, _II_, 5). The caecum, compared with that
of _Cebus_, is longer and more highly convoluted (Fig. 397). Variations
encountered in certain forms indicate reversions to a more primitive
type. Thus Fig. 398 shows the usual form in the dog, while Fig. 399
exhibits an occasional type in the same animal. The caecum here is less
twisted and indicates the probable derivation of the more commonly
encountered type.

III. Rectangular Ileo-colic Junction with Direct Linear Continuity of
Caecum and Colon. (Fig. 465, III.)

The third general group, to which the large majority of Mammalia belong,
is characterized in its typical form by a right-angled entrance of ileum
into large intestine and by the direct caudal prolongation of the colon
into a caecal pouch of nearly uniform caliber with globular termination.
The axes of the small and large intestine are not in the same line as in
the two former groups, but are placed nearly at right angles to each
other. With this change in the direction of the main intestinal segments
the caecum ceases to be a lateral appendage to the canal and appears as a
caudal prolongation of the colon beyond the ileo-colic junction (Fig.
465, _III_). The type-form of this group is encountered among the
herbivorous Marsupialia, such as the kangaroos and opossums. Fig. 350
shows the ileo-colic junction and caecum in the rock wallaby, _Halmaturus
derbyanus_, and Fig. 348 the same structures in our common opossum,
_Didelphis virginiana_. The majority of the remaining mammalian forms
depend upon modifications of this type, either in the direction of
reduction of the caecal apparatus, or of increased development with
concomitant structural changes of similar physiological import in the
proximal portion of the colon.

The following subdivisions of the general group may be established.

A. 1. The caecum is long, markedly curved or uncinate, with the
crescentic medial margin turned toward the free border of the terminal
ileum. The entire pouch usually diminishes gradually in caliber to its
termination (Fig. 465, _III_, _A_, 1). This type is encountered in a
large group of new-world monkeys, including the marmosets and howlers.

Fig. 440 shows the structures in _Hapale jacchus_, one of the marmosets,
and Fig. 443 illustrates the typical caecum of this form in _Ateles
ater_, the black-handed spider monkey.

2. The caecum and appendix of man and of the anthropoid apes can be
regarded as a reduction form of this type (Fig. 465, _III_, _A_, 2).
Arrest of development of the terminal portion converts the distal
segment of the caecal pouch into an appendix whose relation to the apex
of the funnel-shaped proximal segment or caecum proper is seen in its
pure form in the human embryo (Figs. 512 and 525). With the further
development of the caecum the sharper demarcation between it and the
appendix results (Figs. 517 and 518). The displacement of the root of
the appendix cephalad and to the left, toward the lower margin of the
ileo-colic junction, as it is usually seen in adults, is due to the
relatively greater growth of the right terminal sacculation of the caecum
compared with the left (cf. types of caeca, p. 248). Throughout these
changes the initial crescentic curve of the caecum, turning its concavity
upwards and to the left, can be recognized by tracing the course of the
longitudinal colic muscular bands. The caeca and appendices of the
anthropoid apes present the same characters. The structures in the
orang, chimpanzee, gorilla and gibbon are shown in Figs. 455-464.

B. The AEluroid and Arctoid groups of the Carnivora and the Pinnipedia
constitute a very complete and instructive series illustrating the
gradual reduction of the caecum from the capacious pouch of the primitive
type and its final complete elimination from the organism (Fig. 465,
_III_, _B_).

In _Hyaena_ (Fig. 416), the large caecum with undiminished caliber of the
terminal portion persists in its full development, as seen in the
Marsupials furnishing the fundamental type (Fig. 465, _III_). The same
type of caecum is found in the lion (Fig. 417), the only true cat in
which the caecal apparatus has not undergone extensive reduction.
Phylogenetically the presence of a capacious and uniform caecal pouch in
these two animals is exceedingly important and indicates that this type
of caecum represents the ancestral form common to the aeluroid carnivore
group, which, in the remaining living representatives, has become
reduced in response to the influence which the character of the food has
on the structure of this portion of the intestinal canal. The two
instances of persistence of the primal type are all the more important
as exceptions to the rule which is otherwise universal throughout the
group.

1. The first example of this reduction (Fig. 465, _III_, _B_, 1) is
encountered in the Aard-Wolf, _Proteles lalandii_, a near relative of
hyaena (Fig. 406). The caecum in this animal is considerably shortened,
although still of fairly large and uniform caliber.

A similar type of caecal reduction is encountered in the Pinnipede
Carnivora. Fig. 396 shows the ileo-colic junction and the short blunt
caecum of the harbor seal, _Phoca vitulina_.

2. The caecum of the typical Felidae, other than the lion, is short and
the terminal portion much reduced in caliber, constituting in many forms
a species of pointed rudimentary appendix (Fig. 465, _III_, _B_, 2).
Fig. 401 represents the typical feline caecum as seen in the puma, _Felis
concolor_. Among the smaller AEluroid Carnivora related to the true cats,
as the civets and ichneumons, the terminal reduction of the short caecum
is still more marked, as seen for example in _Herpestes griseus_ (Figs.
404 and 405).

3. In the Arctoid group of Carnivora (Fig. 465, _III_, _B_, 3 and 4) the
reduction of the caecal apparatus has been carried to the complete
elimination of the pouch, restoring the primitive type of a straight
intestinal tube without diverticulum as encountered above in some of the
Edentates (Figs. 356 and 357).

In some forms allied to the true bears, such as _Procyon_, _Bassaris_,
_Cercoleptes_, _Taxidea_ and _Nasua_, the ileo-colic junction is marked
externally by a slight constriction and internally by the projection of
an annular pylorus-like valve (Figs. 407-409). The transition from the
thin-walled ileum to the thick muscular walls of the large intestine is
abrupt. The latter is very short and usually increases in caliber as it
approaches the anal orifice. The mucosa of the terminal ileum presents
very commonly one or two large oval areas of agminated follicles near
the ileo-colic junction. The mucous membrane of the large intestine is
thrown into prominent longitudinal folds. Fig. 408 shows the intestine
of the brown coati, _Nasua rufa_, opened on each side of the ileo-colic
transition.

In some of the Arctoidea, as _Procyon_ and _Nasua_, the beginning of the
colon just beyond the ileo-colic valve is bowed out opposite the
mesenteric border indicating the original site of the eliminated caecum,
and recalling the arrangement of the intestine encountered above in
_Arctopithecus_ among the Edentates (Figs. 363, 407, 412, and 465,
_III_, _B_, 3). Moreover, in the same forms rudimentary vascular and
serous folds around the ileo-colic junction, corresponding to similar
structures found in connection with a well-developed caecal apparatus in
other mammalia, point to the former existence of a caecum.

4. In the typical Ursidae even these remnants and traces of a caecal pouch
have disappeared and the intestinal canal preserves a uniform caliber,
without any differentiation of large and small intestine (Figs. 414 and
415, Fig. 465, _III_, _B_, 4).

C. The last subdivision of the third main group contains forms in which
the large uniform pouch of the primal type appears moderately reduced in
length and sacculated, terminating either in a blunt extremity or
carrying a distal constricted and rudimentary segment as an appendage.

1. The first of these types is encountered in the Old World cynomorphous
monkeys. In all of these animals the caecal pouch is wide but
comparatively short, of nearly uniform caliber and sacculated like the
rest of the colon, of which it forms the direct caudal continuation
(Fig. 465, _III_, _C_, 1). The terminal portion of the pouch is usually
blunt, globular and rounded (Figs. 428, 430 and 431), in a comparatively
small number of forms slightly pointed (Figs. 427 and 437).

2. In the second group the terminal reduced portion persists either as a
fairly distinct appendage, or in the form of a tapering pointed
extremity into which the caecal pouch proper is continued (Fig. 465,
_III_, _C_, 2). This type is encountered in certain non-ruminant
Ungulates. An example of the first condition is furnished by the caecal
apparatus of the peccary (_Dicotyles torquatus_) (Fig. 370), while the
structures in _Tapirus americanus_ (Fig. 377) illustrate the second
form.


=IV. Caecal Apparatus Combined with Structural Modifications of the
Proximal Colon of Similar Physiological Significance. (Fig. 465, IV.)=

The fourth general mammalian group comprises forms in which the caecal
pouch is large, with or without terminal appendage, while in addition
the large intestine develops structural modifications which possess the
general functional significance of the caecal apparatus. This highly
developed and complicated structure of the alimentary canal indicates
that the habitual food of these animals is bulky and difficult of
digestion. Accordingly we find the group composed in main of the
majority of the Ungulates and Rodents (with the exception of _Myoxus_),
forms in which the diet under natural conditions is purely herbivorous.
Other mammalian orders, however, also furnish representatives of this
type of caecal apparatus, the conditions as regards character and
quantity of food habitually taken corresponding to those encountered
among the Ungulates and Rodents. Thus the _Phalangers_ among Marsupials
(Fig. 352), _Galeopithecus_ (Fig. 419) as an exceptional form among the
Insectivora, and certain lemurs among Primates (Figs. 420-425) present
examples of a highly developed and specialized type of caecal apparatus.

The intestinal tract of these forms must therefore be considered from
two points of view:

I. The caecum proper.

II. The analogous structural modifications of the proximal segment of
the colon.


=I. CAECUM PROPER.=

The pouch of the Ungulates and Rodents, taking these forms as the
typical representatives of the entire group, is usually of very large
size compared with the rest of the alimentary canal. Two types are
found:

1. Large capacious smooth caecal pouch of uniform caliber (Fig. 465,
_IV_, 2). This form is met with in the Muridae among Rodents and is
illustrated in Fig. 393 showing the caecum of _Mus decumanus_, var.
_albinus_, the white rat. Fig. 392 represents the entire alimentary
canal of the meadow mouse, _Arvicola pennsylvanicus_, and indicates the
proportion which the caecal apparatus bears to the remainder of the
intestinal tract. The typical caecum of the Ungulates is shown in Fig.
371, taken from _Capra aegagrus_, the bezoar goat, and in Fig. 372, taken
from a preparation of _Boselaphus tragocamelus_, the Nilghai.

2. The caecal pouch is large, markedly crescentic in shape, sacculated,
or provided in the interior with a more or less complete spiral valve,
and reduced in caliber in the terminal segment, forming at times a
pointed appendix (Fig. 465, _IV_, 3). This form is encountered typically
among certain Rodents, as in _Castor fiber_, the beaver (Figs. 381 and
382), and _Erethizon dorsatus_, the Canadian porcupine (Figs. 383 and
384), but is not confined to this order. Thus caeca of very similar
structure are found among the Marsupials, as in _Phascolarctos_ and
_Cuscus_ (Fig. 352). In some of these forms the terminal reduction of
the caecum is very marked, resulting in a long narrow segment of the
pouch tapering to a sharp point. It is significant to note in this
connection that in one member of the marsupial order, the wombat
(_Phascolomys_), this tendency to terminal reduction of the pouch has
led to the development of a caecum and appendix identical in structure
and arrangement with the corresponding parts of man and the anthropoid
apes (Fig. 354). This is merely another illustration of the fact,
evidenced throughout the entire vertebrate series, that a primal
type-form of caecal apparatus, in responding to the conditions which
influence the development of structural modifications, will produce
identical specific types in animals otherwise widely separated in the
zoological series.

Thus again the form of caecum under discussion, found in many Rodents and
certain Marsupials, is encountered in the only Insectivore possessing a
caecum (_Galeopithecus_) (Fig. 419), and in several _Lemuroidea_ among
Primates (Figs. 420-425).


II. Structural Modifications of the Proximal Segment of the Colon
Analogous in Their Functional Significance to the Caecal Apparatus.

In these forms, in addition to the caecal apparatus proper, certain
accessory structural modifications of the adjacent large intestine are
developed which possess the physiological significance of the caecal
apparatus in general, since they serve to increase the extent of the
intestinal mucous surface and to prolong the period during which the
contents of the canal are retained for elaboration and absorption. These
modifications, which appear most fully developed in certain Rodents and
Ungulates, are of two kinds.

1. The development of the colic mucous membrane in the form of a
projecting fold or valve usually surrounding the lumen spirally (Fig.
465, _IV_, 1). The significance and phylogeny of this spiral fold has
been considered above (cf. p. 193). Functionally this reduplication must
be regarded as in general equivalent to the caecal apparatus proper, in
producing an increased surface for secretion and absorption and in
retarding the movement of intestinal contents. The caecal pouch evidently
acts as a reservoir in which partly digested substances, mixed with the
secretions of the small intestine, are retained while the slow processes
of digestion and absorption, already inaugurated in the antecedent
segment of the canal, are completed. It is reasonable to suppose that
the system of projecting mucous folds and reduplications encountered in
the colon beyond the caecum have a similar physiological import.
Moreover, in certain forms the caecum itself is provided with a similar
spiral mucous fold, as in the instances already mentioned of _Lepus_
among mammalia (Fig. 381) and of the Ostrich among birds (Fig. 341). We
have seen above (cf. p. 193) that the spiral intestinal valve is
encountered very early in the vertebrate series, in forms in which the
alimentary canal is but slightly, or not at all differentiated, short
and straight in its course. In these forms the evident purpose of the
spiral fold is to <DW44> the movement of the intestinal contents and to
increase the area of the secretory and absorbing surface. As a
structural modification possessing this character we saw the fold in
the Cyclostomata, Selachians and Dipnoeans (Figs. 310, 466, 467 and 468)
and in certain Ophidians (_Python_ and _Anaconda_, Figs. 331 and 469).
Among Mammals it is found in certain Rodentia in two forms:

(_a_) In some of the Muridae, as _Arvicola_ (Fig. 394), the mucous
membrane of the large globular caecal pouch is smooth, but the proximal
segment of the colon, immediately beyond the ileo-colic junction,
develops the spiral fold (Fig. 465, _IV_, 2).

(_b_) In other forms, as in the hares (Fig. 465, _IV_, 3), the greater
part of the caecum carries a typical spiral fold, continued up to the
root of the terminal appendage (Fig. 388), in which segment the mucous
membrane is devoid of folds, but studded thickly with lymphoid
follicles. Beyond the caecum proper the spiral fold is continued in the
opposite direction into the proximal segment of the colon, which is
large and capacious and evidently shares both the physiological and
morphological characters of the caecum proper, forming so to speak an
accessory caecal chamber. Beyond what we thus might term the caecal
division of the colon the large intestine becomes reduced in caliber,
and the previously continuous spiral fold becomes broken up into
separate semilunar haustral plicae, corresponding to the superficial
constrictions between the colic cells. In structure this distal segment
of the rabbit colon closely resembles the human large intestine (Fig.
474).

One of the most marked examples of this secondary modification of the
colon is presented by the intestinal canal of another Rodent, _Lagomys
pusillus_ (Fig. 391).

The caecum of this animal is long, curved, provided with a well-developed
spiral fold. The terminal segment of the pouch is reduced to an
appendix, with smooth mucosa containing adenoid tissue, as in the
rabbit. A second adenoid appendix, representing the globular saccus
lymphaticus of the rabbit, is derived from the caecum at the ileo-colic
junction. The first segment of the colon beyond the ileo-colic junction
is dilated and sacculated, the caecal mucous fold being prolonged into
it. This is succeeded by a narrow smooth-walled second segment. The
third division of the colon is again dilated and sacculated,
followed by a short fourth smooth-walled section. A fifth stretch is
again provided with colic cells, beyond which the terminal segment
continues of uniform caliber and with smooth walls to the vent. The
colon therefore presents three distinct sacculated portions whose
structural modifications suggest that they function in the same sense as
the caecal pouch proper. In man and in other Primates the crescentic
colic plicae are disposed in a more or less evident spiral manner around
the axis of the intestine, and it is not difficult to recognize in them
the modified remnants of the typical spiral valve of lower forms. On the
other hand, in conformity with the general reduction of the caecal
apparatus, the mucous membrane of the large intestine in Carnivora is
smooth and devoid of any trace of the spiral fold (Fig. 475).

2. The second structural modification of the large intestine, associated
in functional significance with the caecal apparatus, depends upon the
increase in the length of the proximal segment of the colon beyond the
ileo-colic junction and the twisting or coiling of this segment in a
more or less complicated definite manner, usually in the form of a
spiral, the individual turns of the coil being held in place by the
peritoneal connections. The proximal colon thus modified is admirably
adapted to <DW44> the movement of contents not yet completely digested
and to increase the absorbing surface of the intestine, and hence is
functionally allied to the caecal apparatus.

This colic modification is found in its highest degree of development in
the ruminant Ungulates, whose caecal pouch proper is also enormously
developed. In these animals the colon immediately beyond the ileo-caecal
junction is arranged in the form of a double spiral, the afferent
(caecal) and efferent (colic) tubes alternating, and continuous with each
other in the center of the coil (Fig. 465, _IV_, 5). Examples of this
type of spiral colon are shown in Fig. 373 (_Bos indicus_), Fig. 374
(_Cervus sika_), Fig. 375 (_Ovis aries_), Fig. 376 (_Oryx leucoryx_).
Ontogenetically the complicated spiral colon of the ruminants starts as
a simple loop of the proximal colon, which, with the further rapid
growth of this segment of the intestine, is bent to produce the turns of
the coil as shown in the schematic Figs. 480-482. Phylogenetically the
same gradual development can be traced in the vertebrate series. Perhaps
the earliest tendency to structurally modify the intestine in the
direction named is found in the manner in which the intestinal coils are
bound together by the subperitoneal arachnoid in many Ophidians (Fig.
331). Further in the Manidae among the Edentates there is no caecal pouch,
but the intestine at the ileo-colic junction is twisted into a figure 8
and held in this position by the peritoneal connections (Figs. 362 and
465, _IV_, 4). In certain Marsupials with well-developed caecal pouches,
such as _Phascolarctos_ and the Vulpine Phalangers (Figs. 351 and 352),
the colon immediately beyond the ileo-colic entrance is sacculated and
bent in the form of a short loop. In the tapir (Fig. 377), the proximal
segment of the colon forms a simple loop, whose afferent and efferent
limbs are closely bound together. The arrangement of the large intestine
in this animal illustrates the early embryonal stage in the development
of the complete ruminant spiral coil (cf. Fig. 480).

[Illustration: FIGS. 480-482.--Schematic representation of three stages
in the development of the ungulate spiral colon.]

The condition encountered in some Rodents presents a more advanced
stage. Thus the large intestine in the agouti (_Dasyprocta agouti_),
shows the development of the spiral coil advanced as far as the second
turn of the original loop (Figs. 389 and 390). It is readily seen that
continued growth of this segment of the intestine leads to the formation
of the complete colic spiral as found in the typical Ungulates.

The same arrangement of the large intestine obtains in certain Lemurs
among the Primates. Thus the proximal colon of the Slow Lemur
(_Nycticebus tardigradus_) is seen in Figs. 421 and 422 to present a
typical spiral coil, and similar conditions are encountered in other
members of the suborder.


=V. Caecal Apparatus and Colon in Hyrax.=

We have left for our final consideration the aberrant and unique
mammalian type found in _Hyrax_ (Fig. 378). In this remarkable little
animal the large intestine develops a typical mammalian sacculated caecum
at the ileo-colic junction, and in addition is provided further on with
two symmetrical pointed lateral colic caeca of large size. It is quite
true that this arrangement is unique among Mammalia, confined entirely
to the members of the suborder formed by the single family of _Hyrax_,
and that no strictly analogous disposition of the alimentary canal is
encountered in the entire vertebrate series. Yet these aberrant
structures are possibly capable of explanation, in regard to the method
of their development, by reference to the caecal apparatus of certain
phytophagous saurians, as _Iguana_ and _Cyclura_. In these forms (Fig.
326-330) the small intestine enters the colon somewhat asymmetrically,
the opening being guarded by a well developed annular valve.

The proximal segment of the large intestine forms an extensive
sacculated pouch. If this is opened (Figs. 328-330) it is seen that the
small intestine leads into a compartment which is separated from the
remainder of the pouch by a valvular diaphragm with central circular
opening. Beyond this primary compartment the colic pouch is incompletely
subdivided by a series of gradually diminishing crescentic folds,
corresponding to the external constrictions between the sacculations.
The entire pouch gradually diminishes in caliber until it passes with a
sharp angular bend into the terminal portion of the endgut. This
terminal segment is differentiated from the elongated colic pouch by the
greater thickness of its muscular walls and by a slight annular
projecting fold in the interior. In considering the intestinal tract of
_Hyrax_ it is conceivable that the unique condition presented by this
animal may be derived from some type conforming in general structure to
the reptilian arrangement of the parts just detailed, as indicated in
the schematic Figs. 483-485. The proximal typical caecal pouch of _Hyrax_
would then correspond to the similar colic pouch of _Iguana_. To explain
the supplementary colic caeca it is necessary to suppose that the
transition of the colic pouch into the terminal hindgut had become well
differentiated, and that on each side of this junction the colic tube
had extended backwards, resulting in the production of the supplementary
bilateral caecal pouches of _Hyrax_.

[Illustration: FIGS. 483-485.--Schematic figures illustrating possible
line of derivation of aberrant mammalian type of alimentary canal
encountered in _Hyrax_.]




PART IV.

MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX.


Not only is the anatomy of this portion of the alimentary tract of great
interest in relation to the evolution of the human structure, but in
addition the pathological and surgical importance of the region warrants
a very careful study of the caecum and appendix. This is more especially
the case since a number of variations in the arrangement of the
structures are encountered. These departures from what we consider the
normal human type have an important bearing on the development and
progress of the pathological conditions prone to involve the appendix.
We may consider the subject under the following subdivisions:


I. DEVELOPMENT OF THE CAECUM AND APPENDIX.

Much light is thrown on the adult anatomy of the parts and on the origin
of the variations observed by the study of their embryonic history. In
considering the factors which determine the variations in the position,
size, and shape of the appendix it must be remembered that the
rudimentary character of this structure is responsible for many of the
aberrant conditions encountered.

As a part of the general caecal pouch which persists in an early
developmental stage and which we can regard as destined for further
reduction and ultimate elimination in the course of evolution, the
appendix shares with other vestigial structures a wide range of
variation. Consequently the study of the development of this portion of
the alimentary tract enables us to gain a clearer view of the primary
arrangement of the structures and to trace the causes which are active
in determining the adult conditions most frequently encountered.

[Illustration: FIG. 486.--Alimentary canal and appendages of human
embryo of 12.5 mm. x 12. (Kollmann, after His.)]

[Illustration: FIG. 487.--_A_, schematic representation of alimentary
canal, with umbilical loop and mesenteric attachments in human embryo of
about six weeks. _B_ and _C_, stages in the intestinal rotation.]

At the time when the umbilical loop of the intestine has formed and has
begun to protrude into the cavity of the umbilical cord (fifth to sixth
week), the first indication of the future caecum appears as a
circumscribed thickening of the returning or ascending limb of the
intestinal loop a short distance from the apex (Figs. 486-488). This
rudiment indicates the derivation of the future definite intestinal
segments from the elements of the loop. The descending limb, apex (site
of embryonic vitelline duct, Meckel's diverticulum of adult) and a short
succeeding portion of the ascending limb furnish the ileum and jejunum.
The rest of the ascending limb develops into caecum and appendix,
ascending and transverse colon. The increase in the length of the
intestine is not uniform. The formation of convolutions begins in the
seventh week in the apex and subsequently in the descending limb. By the
eighth week a considerable number of jejuno-ileal coils have resulted
from the growth in length of these parts of the original umbilical loop,
while the growth of the segment which furnishes the colon is at this
time still inconsiderable (Fig. 489). In the meanwhile the thickening of
the tube which forms the first rudiment of the caecum has developed into
a small sac-like enlargement of the gut, budding from the left and
dorsal aspect of the ascending limb, crescentic in shape, turning its
concavity toward the parent tube. In the majority of instances examined
the small outgrowth is packed closely between the incipient ileal
convolutions, lying under cover of the more prominent bulging coils of
the umbilical protrusion, between them and a single coil of larger arc
situated dorsally and belonging to the jejunal or proximal portion of
the small intestine (Fig. 490). Fig. 497, taken from an embryo of 11 mm.
cervico-coccygeal length, represents this stage in the development of
the umbilical loop. The arrangement of the caecum which we can assume as
the typical condition at this stage and which determines in part the
subsequent final arrangement of the structures, is illustrated by this
relation of the caecal bud to the surrounding incipient convolutions of
the small intestine, with the larger part of these coils situated
ventrad of the caecum and only a single coil of larger curve placed
dorsally; the caecal pouch, derived from the ascending limb of the
umbilical loop, is situated between these two divisions, turning its
concave border to the right and embracing the parent tube. At the time
when the human caecum first appears as a distinct structure it forms a
small conical pouch with blunt extremity whose shape is well illustrated
by the caecum of some of the new-world monkeys, as _Mycetes fuscus_, the
brown howler monkey (Figs. 449 and 450). The outgrowth develops rapidly
in length and very soon assumes a distinct crescentic shape, gradually
tapering toward the extremity, a type which is found reproduced in the
caecum of _Ateles ater_, the black-handed spider monkey (Fig. 443). There
is as yet no constriction or demarcation separating the distal segment
(future appendix) from the proximal part (caecum proper) but the entire
pouch gradually narrows funnel-like to its termination.

[Illustration: FIGS. 488-496.--Series of schematic figures illustrating
stages in the rotation of the intestinal canal.]


II. CHANGES IN THE POSITION OF THE CAECUM AND APPENDIX DURING NORMAL
DEVELOPMENT, DEPENDING UPON THE ROTATION OF THE INTESTINE AND THE
SUBSEQUENT DESCENT OF THE CAECUM.

The primary cause leading to the rotation of the intestinal canal and
inaugurating the successive stages which produce the adult disposition
of the tube is to be found in the rapid increase in length of the small
intestine. Numerous convolutions of this tube succeed to the few primary
coils noted in the first stages. This condition is illustrated in Fig.
498, taken from an embryo of 4.4 cm. cervico-coccygeal measure, and the
arrangement of the intestine is indicated in schema, Fig. 491. The caecum
is found nearly in the median line imbedded among the surrounding coils
of the small intestine, which by their rapid increase have pushed the
pouch cephalad nearly into contact with the caudal surface of the liver.

[Illustration: FIG. 497.--Human embryo of 11 mm. cervico-coccygeal
measure. Enlarged view of ventral and left aspect of intestinal canal.
(Columbia University, Study Collection.)]

[Illustration: FIG. 498.--Human embryo of 4.4 cm. cervico-coccygeal
measure. Intestinal canal; Liver removed. (Columbia University, Study
Collection.)]

Three main divisions of the convolutions of the small intestine
can be made out, slightly separated from each other in the figure
to exhibit the caecum between them. The proximal (jejunal) set of
these convolutions occupy the upper and left part of the abdominal
cavity. They are the product of the single larger coil which in the
earlier stage (Fig. 497, schema Fig. 490) appeared dorsad of the
caecal diverticulum. The distal (ileal) division of small intestinal
convolutions has become greatly augmented and lies to the right of the
caecum. The concavity of the pouch is still, as in the earlier stages,
directed to the right and the entrance of ileum into colon takes
place from right to left. The caudal part of the abdominal cavity is
occupied by an intermediate set of transition convolutions which join
the proximal and distal divisions. In the two stages just described
(Figs. 497 and 498, Schema Figs. 490 and 491), the initial step in the
intestinal rotation has been taken, _i. e._, the beginning of the colon
has been displaced cephalad from its original position in the caudal
and left part of the abdominal cavity by the pressure of the rapidly
growing coils of the small intestine and now lies transversely ventrad
of the duodenum, having crossed the duodeno-colic neck or isthmus of
the primitive umbilical loop (cf. Fig. 487, _C_).

At first the distal coils of the small intestine occupy a position
_behind_ as well as to the right of the caecum, forming a dorsal
retro-caecal division connected by intermediate convolutions with the
ventral division occupying the lower and left portion of the abdominal
cavity. The apex of the caecum is frequently imbedded among these
terminal coils of the ileum. With the continued growth of the small
intestines a further displacement of the caecum cephalad and to the right
takes place, while at the same time the terminal ileal coils pass
downwards and to the left, from a retro-caecal into a subcaecal position,
thus permitting a direct apposition of the caecum to the dorsal parietal
(prerenal) peritoneum. The last steps in this process of withdrawal of
the original voluminous dorsal (retro-caecal) division of ileal
convolutions are well seen in the preparation shown in Fig. 499, taken
from an embryo of 6.7 cm. vertex-coccygeal measure, and corresponding to
the schematic stages represented in Figs. 490 and 491.
The caecum in this preparation has not yet completed its rotation and
still turns its concavity upwards and to the right, with the apex
imbedded among the terminal convolutions of the ileum.

The ileo-caecal junction takes place from right to left in a downward
direction. Nearly the entire mass of the small intestine is situated
below and to the left of caecum and colon, but a terminal ileal coil
still occupies, although evidently in the process of withdrawal, the
retro-caecal position, separating the caecum from direct contact with the
dorsal parietal peritoneum. The withdrawal of this terminal coil of the
small intestine is accompanied, or immediately followed, by a further
turn of the colon cephalad and to the right, which brings it into
contact with the caudal surface of the liver and completes the rotation,
producing a change in the relative positions of the terminal ileal coils
and the caecum. In the stages illustrated in Figs. 498 and 499 and shown
schematically in Figs. 490 and 491, the terminal coils of the ileum pass
from right to left behind the caecum to enter the colon, and the
concavity of the caecal pouch is directed upwards and to the right. After
the final rotation has occurred (schema, Fig. 492) the ileum enters the
large intestine from the left and from below, and the concave border of
the caecum is directed caudad and to the left. This change in relative
position has been accomplished by a revolution of the colon and caecum
through an arc of 180 deg. around its own long axis carrying the caecum above
and behind the small intestine and bringing it into contact with the
dorsal prerenal parietal peritoneum. At the same time the terminal coils
of the ileum turn downwards and to the left. If this final step in the
rotation of the large intestine fails to occur, with otherwise normal
development of the parts, the ileum will persist in entering the large
intestine from right to left after the caecum has obtained its final
lodgment in the right iliac fossa. We have had occasion to refer
previously to the significance of these instances of partially arrested
development (cf. p. 61, Figs. 123, 127 and 128).

[Illustration: FIG. 499.--Human embryo of 6.7 cm. vertex-coccygeal
measure. Liver removed. (Columbia University, Study Collection.)]

[Illustration: FIG. 500.--Human embryo of 4.9 cm. vertex-coccygeal
measure. Ventral view of abdominal cavity, with liver partially removed.
(Columbia University, Study Collection.)]

[Illustration: FIG. 501.--The same embryo represented in Fig. 500. The
colic coil further depressed and turned to the left; seen from the right
side.]

In Figs. 500 and 501, taken from an embryo of 4.9 cm. vertex-coccygeal
measure, the final rotation of the caecum from the position occupied in
Fig. 498 has occurred and the concavity of the pouch is directed caudad
and towards the left. At the same time the escape of the terminal ileal
coils from behind the caecum and beginning of the colon has not yet taken
place and hence the colon is still kept by these coils from direct
opposition to the dorsal prerenal parietal peritoneum. The condition
presented by this preparation can be schematically indicated by Figs.
492 and 493. The rotation has carried the beginning of the colon (Fig.
500), with the caecal bud and appendix curved on itself and turning its
concavity to the left, into the subhepatic position. The greater part of
the small intestinal coils lie now below and to the left of the caecum,
but the terminal ileal convolutions (Fig. 500) still occupy a
retro-caecal position, separating the pouch and the colon from the dorsal
parietal peritoneum. In Fig. 501 the right lateral view of the same
embryo is shown with the caecum and colon depressed and turned to the
left. The termination of the ileum reaches the ileo-colic junction by
passing behind the caecum, and the immediately adjacent ileal coils are
still retro-caecal, intervening between the pouch and the dorsal parietal
peritoneum.

In the next succeeding stage (schema, Fig. 494) these coils of the ileum
turn downward and to the left so as to lie below and mesad to the caecum
and colon, thus permitting the direct apposition of the large intestine
to the parietal prerenal peritoneum. The terminal ileum now passes from
below and to the left upwards and to the right to its junction with the
colon. This freeing of the dorsal surface of caecum and colon from
contact with the coils of the small intestines, and the consequent
direct apposition of the same to the dorsal parietal peritoneum
influences to a great extent the subsequent arrangement of the parts,
because it affords the conditions necessary to the fixation of the colon
and mesocolon by adhesion to the parietal peritoneum (cf. p. 81).

Fig. 499, taken from an embryo of 6.7 cm. vertex-coccygeal measure,
illustrates this stage, which is encountered in the majority of
instances and during which the retro-caecal coils of the terminal ileum
are withdrawn (schema, Fig. 493). The convolutions of the small
intestine have greatly increased in size and number. The retro-caecal
ileal coils, compared with Fig. 500, have shifted their position caudad
and to the left, so as to lie below and ventrad of the beginning of the
colon. Only a single coil remains behind the caecum and appendix,
intervening between these structures and the ventral surface of the
right kidney, and this coil is in the process of withdrawal from the
dorsal position as indicated by the superficial and short course of the
coil which connects it with the remaining ventral convolutions. As soon
as the withdrawal of this single remaining dorsal coil is completed the
entire mass of the small intestines will occupy a position ventrad,
caudad and to the left of the caecum and colon (Fig. 494), which will
then rest directly against the dorsal parietal peritoneum investing the
ventral surface of the right kidney.

This stage is illustrated in Fig. 502, taken from an embryo of 6.6 cm.
vertex-coccygeal measure. The caecum and appendix here occupy the
subhepatic position, well to the right of the median line and in the
background of the abdominal cavity. The terminal retro-caecal ileal coils
of the embryo shown in Figs. 500 and 501 have descended caudad and to
the left, thus freeing the dorsal surface of caecum and colon and
permitting direct contact with the prerenal parietal peritoneum.

[Illustration: FIG. 502.--Human embryo, 6.6 cm. vertex-coccygeal
measure. Liver removed. (Columbia University, Study Collection.)]

In the succeeding stages the caecum gradually descends along the
background of the right lumbar region from the subhepatic position to
the right iliac fossa, producing by this descent the ascending colon as
a distinct segment of the large intestine.

It will be observed that in the stage shown in Fig. 502 (schema, Fig.
494) the large intestine passes from the caecum to the splenic flexure
transversely from right to left across the upper part of the abdominal
cavity, caudad and ventrad of the stomach and cephalad of the coils of
the small intestine.

In the following stages the disproportionately large size of the
embryonic liver compels the colon, as the caecum descends, to assume an
oblique position. When the caecal descent is completed the colon
traverses the abdominal cavity in contact with the caudal surface of the
liver passing from the right iliac fossa obliquely cephalad and to the
left to the splenic flexure where it becomes continuous with the
descending colon, which segment has early assumed its definite position
in the background of the abdominal cavity on the left side (Fig. 495).
This oblique position of the colon is seen in Figs. 503 and 504. During
this stage the increase in the length of the colon may lead to the
arrangement seen in Fig. 505, where the future transverse segment of the
large intestine is bent caudad in form of an arch whose summit extends
nearly to the pelvis. This condition at times persists in the adult, in
cases of unusually long large intestine, and recalls the normal
arrangement found in many of the cynomorphous monkeys in whom the
transverse colon forms an extensive V-or U-shaped loop, with the apex
directed caudad toward the pubic symphysis (Fig. 506). In other
instances in the human foetus this part of the large intestine is thrown
into a number of shorter irregular coils (Fig. 507).

[Illustration: FIG. 503.--Human embryo, 7.6 cm. vertex-coccygeal
measure. Liver and small intestine from the duodeno-jejunal to the
ileo-colic junction removed. (Columbia University, Study Collection.)]

[Illustration: FIG. 504.--Human foetus, 10.6 cm. vertex-coccygeal
measure. Liver and greater part of small intestine removed. (Columbia
University, Study Collection.)]

[Illustration: FIG. 505.--Human foetus, 20.4 cm. vertex-coccygeal
measure. (Columbia University, Study Collection.)]

[Illustration: FIG. 506.--Abdominal viscera of _Macacus rhesus_, rhesus
monkey, hardened _in situ_. (Columbia University Museum, No 1817.)]

[Illustration: FIG. 507.--Abdominal viscera of human foetus at term,
hardened _in situ_; hepatic flexure formed, and ascending and transverse
colon differentiated. (Columbia University Museum, No. 1816.)]

[Illustration: FIG. 508.--Human foetus of 10.7 cm. vertex-coccygeal
measure. Liver and small intestine from the duodeno-jejunal to the
ileo-colic junction removed. The colon already presents an ascending,
transverse and descending segment. The appendix is retro-caecal, curved,
with the tip turned down, under cover of the ileo-colic junction and
mesentery. (Columbia University, Study Collection.)]

Normally, however, in the process of further development and with the
relative decrease in the size of the liver, the hepatic flexure (Fig.
505) becomes defined and passes cephalad and to the right, taking up the
slack of the bent segment and establishing the typical ascending and
transverse colon as seen in Fig. 508 (schema, Fig. 496).


III. VARIATIONS OF ADULT CAECUM AND APPENDIX.

The study of the variations of the adult caecum and appendix involves the
consideration of the following points:

(_a_) Shape of caecum and origin of appendix. (_Type of adult caecum._)

(_b_) Position, direction and peritoneal relations of the appendix.

(_c_) Arrangement of the vascular and serous ileo-caecal folds.

The peculiarities encountered in any individual case usually depend upon
the combination of all three of these factors, which together influence
and determine the arrangement of the structures in the adult. Hence the
examination of each case should be made with reference to these three
points, which we will now consider in detail.


A. SHAPE OF CAECUM AND ORIGIN OF APPENDIX. TYPES AND VARIATIONS OF ADULT
CAECUM AND APPENDIX.

The various forms of the adult caecum are all derived by modifications
from the foetal type of the pouch.

In the embryo the caecum is funnel-shaped, narrowing gradually and
symmetrically in caliber to the root of the appendix, at which point the
three colic taenia or longitudinal muscular bands of the large intestine
meet. The appendix arises from the apex of the funnel, the lateral walls
of which are equally and symmetrically developed. The entire pouch is of
a crescentic shape, the concavity of the curve turned to the left and
directed toward the caudal margin of the terminal ileum. Two
subdivisions of the foetal type are found:

I. The crescentic curve of the caecum is only slightly marked; the
appendix arises from the most pendent part of the pouch and hangs
downward (schema, Fig. 509, _I, a_).

This form, which is encountered only occasionally in the foetus and
infant, is illustrated by the preparation shown in Fig. 510, taken from
a foetus at term.

[Illustration: FIG. 509.--Schematic table of types of human caeca.]

II. In the majority of cases the inherent crescentic shape of the caecal
pouch is pronounced and carries the termination of the funnel with the
root of the appendix cephalad and to the left toward the caudal margin
of the ileo-colic junction (schema, Fig. 509, _II, a_).

At birth this typical arrangement of the caecum frequently places the
pouch in a nearly transverse position, with the apex and the root of the
appendix turned to the left, in contact with, or under cover of the
terminal piece of the ileum at its junction with the large intestine.

Figs. 511 and 512 represent the parts in the ventral view in the foetus
at term.

[Illustration: FIG. 510.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University, Study Collection.)

    1. Appendix.
    2. Reduced intermediate non-vascular fold.
    3. Ventral vascular fold.
]

[Illustration: FIG. 511.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University, Study Collection.)

    1. Appendix, coiled spirally behind terminal ileum.
    2. Non-vascular intermediate fold.
]

[Illustration: FIG. 512.--Human foetus at term (<DW64>). Caecum and
ileo-colic junction; ventral view. (Columbia University Museum, No.
692.)]

[Illustration: FIG. 513.--Human foetus at term. Caecum and ileo-colic
junction; dorsal view. (Columbia University, Study Collection.)]

[Illustration: FIG. 514.--Human foetus at term. Caecum and ileo-colic
junction; dorsal view. (Columbia University Museum, No. 1715.)]

Figs. 513 and 514, also taken from the foetus at term, show the caecum
from the dorsal aspect and illustrate well the sharp character of the
curve which carries the apex of the pouch up and to the left.

All the variations observed in the adult caecum are derived from these
two foetal types by a subsequent and usually asymmetrical enlargement and
dilatation of the pouch.

We can consider the derivatives of each form separately.

=I. Adult Caeca Derived From Type I.= (schema, Fig. 509, _I^a_, Fig.
510).

1. Further development leads to an enlargement of the caecal pouch and a
sharper demarcation between the same and the appendix. The resulting
caecum is symmetrical, with equally developed lateral sacculi, between
which the termination of the longitudinal muscular bands and the root of
the appendix is situated (schema, Fig. 509, _I^b_).

[Illustration: FIG. 515.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University Museum, No. 1510.)]

[Illustration: FIG. 516.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University Museum, No. 1548.)]

In Figs. 515 and 516 two infantile caeca are shown which illustrate this
form. The narrow and pointed apex of the foetal conical caecum is replaced
by the capacious pouch which is differentiated sharply from the
appendix. Among the anthropoid apes the same type is seen in the caecum
of the gibbon (Figs. 455 and 456), and of the young chimpanzee shown in
Fig. 460.

2. An increased development of the caecal pouch in the adult leads to the
protrusion caudad of two symmetrical sacculations on each side of the
root of the appendix which appears between them. The original apex of
the caecal pouch is still marked by the implantation of the appendix and
by the termination of the longitudinal muscular bands, but the lowest
level of the pouch is found on each side of this point at the fundus of
the secondary lateral sacculi (schema, Fig. 509, _I^c_). Treves, to whom
belongs the credit of first accurately describing and classifying the
forms of the adult caecum based on the development, found this type in
three of a series of 100 cases examined.

[Illustration: FIG. 517.--Adult human caecum and ileo-colic junction.
(Columbia University, Study Collection.)]

[Illustration: FIG. 518.--Adult human caecum and ileo-colic junction.
(Columbia University Museum, No. 234.)]

Figs. 517 and 518 illustrate this form of the pouch, which, in our
experience, is frequently associated with the retro-caecal erect
position of the appendix (cf. infra, p. 251). Fig. 472 shows this type
in the adult with pendent appendix.

=II. Adult Caeca Derived from Type II.= (schema, Figs. 509 and
511).--From this more commonly observed type of foetal caecum the
following adult forms are developed:

1. The general shape and trend of the foetal caecum is preserved. The
pouch turns sharply to the left, carrying the apex with the root of the
appendix upward toward the ileum, the appendix itself being frequently
placed under cover of the terminal coil of the small intestine (schema,
Fig. 509, _II^b_).

[Illustration: FIG. 519.--Human adult (Smith's Sound Eskimo). Ileo-colic
junction and caecum. (Columbia University Museum, No. 59/1483.)]

[Illustration: FIG. 520.--Human adult. Ileo-colic junction and caecum.
(Columbia University, Study Collection.)]

[Illustration: FIG. 521.--Human adult. Ileo-colic junction and caecum.
(Columbia University, Study Collection.)]

[Illustration: FIG. 522.--Human adult (Smith's Sound Eskimo). Ileo-colic
junction and caecum. (Columbia University Museum, No. 56/1571.)]

The apex of the caecal pouch is either conical, narrowing gradually
toward the root of the appendix (Figs. 520 and 521), or blunt and more
sharply defined against the appendix (Fig. 522). Mr. Treves encountered
this "persistent foetal type" in two per cent. of his series.

The caecum is frequently sharply bent on itself in making the turn upward
and to the left, resulting in a deep indentation of the concave border
and producing a corresponding projecting fold in the interior of the
pouch (Fig. 523). The ventral longitudinal muscular band follows the
crescentic sweep of the caecum to the root of the appendix.

[Illustration: FIG. 523.--Human adult. Ileo-colic junction and caecum;
dorsal view; dried preparation. (Columbia University Museum, No. 200.)]

[Illustration: FIG. 524.--Human foetus at term. Ileo-colic junction and
caecum; ventral view. (Columbia University Museum, No. 1717.)]

[Illustration: FIG. 525.--Same preparation as Fig. 524; dorsal view.]

Figs. 524_a_ and 525_b_, representing the caecum of a foetus at term in
the ventral and dorsal view respectively, show very clearly the
arrangement of the foetal pouch from which the adult type with sharp
angular bend is derived. This type of adult caecum is found in certain of
the anthropoid apes.

In the orang (Figs. 458 and 459) the caecum turns sharply upward and to
the left, gradually narrowing in caliber to the root of the appendix
which is coiled behind the termination of the ileum.

The same type is seen in Figs. 462 and 463, taken from a preparation of
the adult chimpanzee. Fig. 463 shows especially well the sharp bend
between the caecum and colon by means of which the apex of the pouch is
carried cephalad behind the ileo-colic junction.

Fig. 431, taken from another specimen of the same animal, shows the
characteristic crescentic curve of the caecum and the corresponding
course of the longitudinal muscular band. The apex of the pouch in this
preparation is more rounded and blunt.

The same blunt termination of the caecum of this type, with a
corresponding sharper demarcation of the appendix, is seen in the
gorilla (Fig. 457) recalling the conditions found in certain instances
in the human subject (Fig. 522).

2. In by far the larger proportion of cases (ninety per cent. in Treves'
series) the adult caecum obtains its characteristic form by an unequal
development of the walls of the intestine. The right segment between the
ventral and dorso-lateral muscular bands dilates, forming a sacculation
which projects caudad and constitutes the secondary caput coli, while
the segment between the lower border of the ileum and the original apex,
marked by the origin of the appendix, remains stationary or is further
reduced. This unequal development produces a relative displacement of
the root of the appendix upward and to the left toward the ileo-colic
junction.

In some cases the primitive crescentic curve of the caecum, as indicated
by the direction of the ventral longitudinal muscular band, is still
perceptible.

The right wall of the foetal caecum, forming the most pendent portion of
the pouch, dilates uniformly and thus constitutes the adult caput coli.
The left wall appears as a small sacculation separating the root of the
appendix from the ileo-colic junction (schema, Fig. 509, _II, c_). This
type of the adult caecum is illustrated by the preparations shown in
Figs. 526-528. In other cases part of the right wall of the caecum
between the ventral and dorso-lateral colic taenia, dilates abruptly
forming a very prominent rounded sacculation which carries the lowest
part of the pouch caudad in a sharper curve than in the preceding form
as indicated by its deviation from the direction of the longitudinal
muscular band (schema, Fig. 509, _II, d_).

[Illustration: FIG. 526.--Human adult (Smith's Sound Eskimo). Ileo-colic
junction and caecum; dorsal view. (Columbia University Museum, No.
61/1461.)]

[Illustration: FIG. 527.--Human adult. Ileo-colic junction and caecum;
ventral view. (Columbia University, Study Collection.)]

[Illustration: FIG. 528.--Human adult. Ileo-colic junction and caecum;
ventral view. (Columbia University, Study Collection.)]

Figs. 529-531 afford examples of this type, while Fig. 532, taken from
an infantile preparation, shows that the same may begin to develop at a
very early age.

[Illustration: FIG. 529.--Human juvenile. Ileo-colic junction and caecum;
dorsal view. (Columbia University, Study Collection.)]

[Illustration: FIG. 530.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University Museum, No. 115.)]

[Illustration: FIG. 531.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University, Study Collection.)]

3. Finally, in about four per cent. to five per cent., adult caeca, the
reduction of the wall to the left of the root of the appendix, between
this point and the ileo-colic junction, is complete. The entire caecal
pouch is formed by the dilated right wall between the ventral and
dorsolateral muscular bands. The ventral band terminates at the lower
border of the ileo-colic junction, from which the appendix appears to
arise, indicating the original apex of the foetal caecum (schema, Fig.
509, _II^e_).

This type is illustrated in the specimens shown in Figs. 533 and 534.

[Illustration: FIG. 532.--Human infant, Ileo-colic junction and caecum,
with secondary terminal sacculation. (Columbia University Museum, No.
1632.)]

[Illustration: FIG. 533.--Human adult. Ileo-colic junction and caecum;
dorsal view; dried preparation. (Columbia University Museum, No. 124.)]

[Illustration: FIG. 534.--Human adult. Ileo-colic junction and caecum;
ventral view; dried preparation. (Columbia University Museum, No. 14.)]

=III. Adult Caeca in Cases of Absence of the Appendix.=--A few instances
of congenital absence of the appendix have been observed.

A. v. Haller[9] describes the condition in the following words:
"Defuisse visa est in homine appendicula, ut tuberculum minimum
superesset."

[9] A. v. Haller, Elements physiologiae, Tom. 7, Liber 24, Sect. 3.

Fr. Arnold,[10] without describing any individual case, states that
"very rarely the appendix is entirely wanting."

[10] Fr. Arnold, Handbuch der Anat. d. Menschen. 1847. II. Bd., cloth,
p. 84.

E. Zuckerkandl,[11] reports having observed one case of absence of the
appendix.

[11] E. Zuckerkandl, "Ueber die Obliteration des Darmfortsatzes beim
Menschen." Anat. Hefte XI. (Bd. IV., Heft 1), 1894, p. 107.

J. D. Bryant,[12] reports a case in which he operated for appendicitis
but found "absolutely no appendix." "The point of tenderness was found
to be a glandular growth located posterior to the usual site of the
appendix."

[12] N. Y. Med. Journal, Vol. LXIX., No. 14, p. 508.

[Illustration: FIG. 535.--Human adult. Ileo-colic junction and caecum;
absence of appendix. (Columbia University Museum, No. 1077.)]

[Illustration: FIG. 536.--Human adult. Ileo-colic junction and caecum,
hardened _in situ_; absence of appendix. (Columbia University Museum,
No. 715.)]

Two instances of this variation are shown in Figs. 535 and 536, taken
from preparations in the Morphological Museum of Columbia University. In
both careful examination of the external as well as of the mucous
surface of the caecum demonstrated the entire absence of the appendix,
and the subjects from which they were obtained presented no scars or
other evidences of operative removal or of pathological processes. They
are both, therefore, authentic instances of complete congenital absence
of the appendix, not of so-called retro-peritoneal or hidden
appendix.[13]

[13] Cf. Quain.

The two examples differ from each other in some details. In the first
case (Fig. 535, schema, Fig. 509, _III^a_) the caecum is rounded and
globular. The ventral longitudinal muscular band is vertical and
continued to the lowest point of the pouch, which greatly resembles the
caecum of a typical cynomorphous monkey.

In the second case (Fig. 536, schema, Fig. 509, _III^b_) the caecum turns
upwards and to the left, terminating in a sharp point, to which several
lobes of epiploic fat are attached.

We must assume that in these cases the embryonic portion of the caecal
bud was developed just sufficiently to yield the required adult pouch
with nothing to spare, so to speak, which could remain rudimentary in
the form of an appendix.

Instances of exceedingly rudimentary and reduced appendix are also
encountered.

In the case illustrated in Fig. 537 the appendix formed a small conical
elevation without distinct lumen, measuring only 0.5 cm. in length.

[Illustration: FIG. 537.--Human adult. Ileo-colic junction and caecum,
with rudimentary appendix. (Columbia University Museum, No. 1655.)]

=B. Position and Peritoneal Relations of the Appendix.=--Statistical
records of the position of the appendix indicate a wide range of
variation. In general the results obtained by different observers show
that certain positions of the appendix are encountered in a sufficiently
large percentage of the cases to enable us to adopt a classification,
but that a very extensive series of records are required in order to
determine even approximately the preponderant relations of the appendix.
The following are the most frequently observed positions:

1. The appendix is directed upward, inward and to the left, the terminal
portion being frequently coiled under cover of the ileum and mesentery.
This position of the appendix is largely due to the normal crescentic
curve of the caecum, which carries the apex of the pouch and the root of
the appendix upward and to the left. Its production is, moreover,
favored by the tendency of the adult caecum to develop by dilatation of
the ventral and right wall at the expense of the left side of the pouch,
thus relatively shortening the interval between the origin of the
appendix and the ileo-colic junction.

Examples of this commonly encountered position of the appendix are given
in Figs. 512, 513, 514, 520, 521, 523 and 526.

2. The appendix is erected vertically behind the caecum and ascending
colon and closely attached to the dorsal wall of the large intestine. In
some instances the caecum and colon, with the adherent vertical appendix,
possess a free serous dorsal surface, not adherent to the parietal
peritoneum (Figs. 529, 538, 539 and 540). In other cases the ascending
colon is fixed and the greater part of the retro-colic appendix is
buried in the connective tissue which attaches the large intestine to
the abdominal parietes (Fig. 517). Even in these cases, however, the
dorsal surface of the caecum and the root of the appendix retain their
free serous investment.

[Illustration: FIG. 538.--Human juvenile. Caecum _in situ_ lifted up to
show vertical course of appendix, situated behind caecum and ascending
colon. The large intestine has a free peritoneal dorsal surface, and the
appendix is held in position by adhesion to the large intestine.
(Columbia University, Study Collection.)]

[Illustration: FIG. 539.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University Museum, No. 1594.)]

[Illustration: FIG. 540.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University Museum, No. 1850.)]

3. The proximal part of the appendix turns upward and to the left in
continuation of the caecal curve, but the distal portion is directed
downward and inward, hanging over the brim of the pelvis (Figs. 505, 541
and 542).

4. The appendix is directed downward, pendent from the lowest point of
the conical caecal pouch, and hangs free over the pelvic brim.

This type is encountered at times in foetal and infantile subjects (Figs.
516 and 543).

[Illustration: FIG. 541.--Human infant. Ileo-colic junction and caecum;
ventral view. (Columbia University, Study Collection.)]

[Illustration: FIG. 542.--Human infant. Ileo-colic junction and caecum;
dorsal view. The dorsal surface of caecum as far as root of the appendix
is adherent to the parietal peritoneum of the iliac fossa. (Columbia
University Museum, No. 394.)]

[Illustration: FIG. 543.--Human foetus at fifth month. Abdominal cavity
and viscera; liver and greater part of small intestine removed. (Drawn
from preparation in Columbia University Museum, No. 1814.)]

[Illustration: FIG. 544.--Human foetus at term. Abdominal cavity and
viscera; greater part of small intestine removed. (Drawn from
preparation in Columbia University Museum, No. 1813.)]

[Illustration: FIG. 545.--Human foetus at term. Ileo-colic junction and
caecum. (Columbia University Museum, No. 998.)]

5. The position of the appendix is variant and abnormal, as _e. g._
placed to the right of caecum and colon (Fig. 544) or turned up ventrad
of the ileo-colic junction (Fig. 545).

These variations in the position of the appendix and the resulting
peritoneal relations of the structure depend upon the following
factors.

1. The influence of peritoneal adhesions established during the descent
of the caecum from the subhepatic position to the iliac fossa.

2. The inherent curve of the caecal pouch.

3. The subsequent alterations in the caliber of the intestine and the
unequal development of the pouch leading to the formation of the types
of adult caeca above considered.

In determining the causes which lead to the establishment of any given
position of the appendix all three of the factors above enumerated must
be taken into account, although their influence is not exerted in every
case to an equal degree.

We have seen that normally, after completed rotation of the intestine,
the caecum with the appendix and the beginning of the colon are lodged in
the upper and right part of the abdomen, below the liver and in contact
with the prerenal parietal peritoneum (schema, Figs. 493, 502). During
the subsequent stages the caecum descends into the right iliac fossa,
thus producing the ascending colon. It is immaterial whether this change
in position is regarded as an actual descent of the pouch over the
ventral surface of the right kidney, which seems more probable, or as a
growing away from the iliac region of the remainder of the abdominal
wall, with a concomitant relative reduction in the size of the liver,
producing a relatively lower position of the caecum, or as a combination
of these processes. In either case during this period the dorsal surface
of the ascending colon and mesocolon normally becomes adherent to the
dorsal parietal peritoneum, connective tissue developing between the
opposed serous areas and leading to the usual fixation of the ascending
colon and obliteration of the free ascending mesocolon. If this process
of adhesion is inaugurated at an early stage, _i. e._, before the
descent of the caecum has been accomplished, it will act as a drag on the
dorsal surface of the colic tube during the subsequent change in
position, which carries the caecum downward into the iliac fossa. This
leads to a backward bend of the caecum and appendix which parts will in
the ventral view appear under cover of the protruding free ventral and
lateral walls of the colon. Hence in many late embryos and foetus at term
the lowest point of the large intestine in the right iliac fossa is
formed by the proximal part of the caecum or by the adjacent segment of
the colon, while the original termination of the pouch, with the root of
the appendix, is turned backward and upward, and, as we have seen, by
reason of the inherent shape of the pouch, also to the left, carrying
the beginning of the appendix frequently behind the terminal ileum and
the ileo-colic junction.

Two of the more common positions of the appendix, viz., backwards,
upwards and inwards behind the ileo-colic junction, and directly
backward, erected vertically behind caecum and colon, can therefore in
part be referred to the mechanical conditions obtaining normally during
the descent of the caecum. Of course the shape of the caecal pouch and the
later development of the adult type of caecum will modify this influence
in individual cases. We have seen that this early adhesion and the
resulting effects on the position of caecum and appendix depend on the
direct apposition of the colic tube and mesocolon to the dorsal parietal
peritoneum. Any condition which will prevent or delay this apposition
will likewise perpetuate the original embryonal condition of the tube,
completely invested by peritoneum and with a free mesocolon.

Such an element is found in the persistence of the dorsal set of ileal
convolutions in the original retro-caecal position beyond the usual
period, as indicated in the schematic Fig. 492, _IV, a_. If the turn
downward and to the left of these coils is for any reason delayed beyond
the usual time the caecal extremity of the colon will descend from the
subhepatic to the iliac position without coming directly into contact
with the dorsal parietal peritoneum, and therefore without the usual
peritoneal adhesion and obliteration of the apposed serous surfaces. The
caecum under these conditions descends without making the backward bend,
and the origin of the appendix is found at the lowest point of the
pendent funnel-shaped pouch, causing it finally to hang downward or
downward and inward over the pelvic brim. The resulting form of the
caecum and the position of the appendix is the one above described as
type _Ia_, _Ib_ and _Ic_ (Fig. 509).

Fig. 510 from a foetus at term, and Figs. 515 and 516 representing
infantile caeca, illustrate this form of the pouch, while the parts are
shown in situ in Fig. 543 taken from a preparation of a five-month
foetus.

[Illustration: FIG. 546.--Human embryo, 6.5 cm. cervico-coccygeal
measure. Abdominal cavity, with liver removed, seen from the right side.
(Columbia University, Study Collection.)]

Fig. 546 exhibits the condition obtaining during the development of this
type in the more exceptional instances of delayed apposition of the
colon to the parietal peritoneum and of increased development of the
terminal ileal coils in the original retro-caecal position. In this
embryo, measuring 6.5 cm. in vertex-coccygeal length, the development
has progressed sufficiently to establish a distinct transverse colon and
to bring the caecum and appendix into the subhepatic position. But in
place of lying in contact with the dorsal parietal peritoneum, as in the
embryo, shown in Fig. 502, over the ventral surface of the right kidney,
the increased mass of the retro-caecal ileal coils keeps the caecum,
already in the process of descent, in contact with the ventral abdominal
wall. When the final rotation of the retro-caecal small intestinal coils
downward and to the left occurs, placing the ileo-colic junction (_C_)
to the left of the large intestine (schema. Fig. 494), the ascending
colon and caecum are not yet fixed by adhesion to the dorsal parietal
peritoneum, and the appendix will present downward and to the left,
affording the necessary conditions for the establishment of the
permanent pendent position of the tube or causing the same to be
directed downward and inward over the brim of the pelvis.

[Illustration: FIG. 547.--Human embryo, 5.9 cm. vertex-coccygeal
measure. (Columbia University, Study Collection.)]

In contrast with the preceding is the condition shown in Fig. 547, taken
from an embryo of 5.9 cm. vertex-coccygeal measure. The transverse colon
in this preparation has already begun to assume an oblique position,
passing down and to the right from the splenic flexure. The caecum and
appendix are in contact with the dorsal prerenal parietal peritoneum.
The escape of the dorsal set of ileal convolutions from the retro-caecal
position, by rotation downwards and to the left, is accomplished. The
caecum and appendix are placed in the position which they would have
occupied in the embryo shown in Fig. 546 if the dorsal ileal coils had
not prevented, in the latter preparation, the apposition of the colon to
the dorsal parietal peritoneum.

In considering the effect of these variant conditions on the adult
arrangement of the structures it is necessary to bear in mind the second
of the above-mentioned factors, namely, the inherent shape of the caecal
pouch and appendix and the resulting direction of its axis.

As previously stated the normal type of the human embryonal caecum is
represented by the pouch of some of the new-world monkeys, as _Ateles_
(Fig. 443) or of certain lemurs, of which _Nycticebus_ (Fig. 420)
furnishes an excellent example. The caecum is distinctly crescentic,
turning its concave margin, after completed intestinal rotation, upwards
and to the left, toward the lower margin of the ileum. The distal
diminished segment of the pouch in _Ateles_ has already assumed the
character of a caecal appendage in _Nycticebus_ and becomes by further
reduction the typical appendix in man and the anthropoid apes, while the
proximal portion develops into the capacious sacculated caecum proper.
Consequently the initial curve of the caecum tends to carry the root of
the appendix upward and to the left toward the ileo-colic junction. This
curve of the pouch, combined with the mechanical effects produced by the
adhesion of the colon during the caecal descent, accounts for the
frequency with which the caecum in the later months of foetal life and at
birth is found curved backward, upward and to the left, placing the root
of the appendix under cover of the terminal ileal convolutions (Fig.
548). We have seen that this disposition of the structures accounts for
the preponderance of that type of adult caecum which results from the
further and unequal development and dilatation of the segment of the
pouch situated to the right of the origin of the appendix.

[Illustration: FIG. 548.--Human foetus at term. Ileo-colic junction and
caecum. Early colic and caecal adhesion with retroverted appendix.
(Columbia University Museum, Study Collection.)]

Bearing in mind the three elements just considered, viz., the effect of
adhesion during the caecal descent, the inherent shape of the pouch and
the unequal alterations in caliber in the development of the adult type,
we can at once take up the resulting variations in the peritoneal
relations of the adult caecum and appendix which have an important
influence on the progress of pathological processes in this region. It
should be remembered that in the following schematic figures the colon,
caecum and appendix are represented in the profile view in a straight
line, without indicating the characteristic turn of the crescentic caecal
pouch upwards and to the left.

Fig. 549 shows the arrangement in unimpeded caecal descent without
adhesion of colon and mesocolon to the parietal peritoneum. This
disposition of the structures, if carried into adult life, would produce
the permanently free ascending colon and mesocolon which we encountered
exceptionally in the human subject (cf. p. 82) and normally in certain
of the cynomorphous monkeys (p. 83). The ascending colon and mesocolon
can, under these conditions, be turned mesad, lifting them away from the
primary parietal peritoneum investing the ventral surface of the right
kidney. Caecum and appendix have, of course, a complete serous
investment.

[Illustration: FIG. 549.]

[Illustration: FIG. 550.]

[Illustration: FIG. 551.]

[Illustration: FIG. 552.]

[Illustration: FIG. 553.]

[Illustration: FIG. 554.]

[Illustration: FIGS. 549-554.--Schematic series illustrating the
variations in the arrangement of the caecal and colic peritoneum.]

Normally, however, in the human subject, even if the obliteration of the
apposed serous surfaces and the resulting fixation of the ascending
colon has been delayed beyond the usual period, as above indicated,
adhesion takes place subsequently, involving the dorsal surface of the
ascending colon between the ileo-colic junction and the hepatic flexure
(schema, Fig. 550). The dorsal surface of the caecum usually retains its
free serous surface in whole or in greater part. The appendix is
pendent, entirely invested by peritoneum and hangs free in the abdominal
cavity, directed toward the pelvic brim, illustrating the effect of
delayed fixation of the colon on the position of the appendix.

Examples of this condition are not frequent, and are confined almost
exclusively to foetal and juvenile subjects. Illustrations are afforded
by Figs. 515 and 516.

We have already noted (p. 246) the resulting foetal type of pendent caecum
(Fig. 510).

More commonly colic adhesion before the caecum obtains its final iliac
position results in imparting a backward turn to the pouch, leading to
the peritoneal disposition shown in schema, Fig. 551, in which the root
of the appendix is involved in the area of obliteration, while the
terminal segment remains free. An example of this condition is furnished
by the embryo shown in Fig. 508 (10.7 cm. vertex-coccygeal measure). The
colon is already segmented into an ascending, transverse and descending
portion. The caecum is retroverted and its apex with the appendix is
placed under cover of the terminal ileum which enters the large
intestine in the direction from below upward and to the right. In the
side-figure the divided end of the ileum is displaced upward to show
caecum and appendix and their relation to the ileal mesentery.

The disposition of the structures illustrated by this example probably
depends upon delayed adhesion of the colic embryonal tube to the dorsal
parietal peritoneum. The caecum and appendix appear to have descended
freely until the final position in the right iliac fossa has been nearly
attained, adhesion and fixation of the colon taking place just before
the descent is completed, and thus producing the backward turn of the
caecal end of the tube. Further development of the caecum to form the
adult caput coli in these cases leads to the unequal and exaggerated
expansion of the ventral and lateral walls of the pouch, as compared
with the fixed and adherent dorsal wall. The former are distended and
pushed downwards, producing a relative recession of the root of the
appendix upward and to the left, until it comes into relation with, or
even under cover of, the ileo-colic junction and of the terminal ileal
coil entering the colon at this point.

The resulting characteristic adult position of the appendix in these
cases is as follows:

The termination of the caecum proper, and the root of the appendix are
under cover of the terminal ileum and frequently adherent to the
parietal peritoneum of the iliac fossa (Fig. 555). The distal portion of
the appendix remains free, either hanging down and in over the brim of
the pelvis (Fig. 542), or turned upwards and to the left and coiled in
several turns (Figs. 504, 555 and 556).

[Illustration: FIG. 555.--Human foetus at term. Ileo-colic junction and
caecum; dorsal view. The area of peritoneal adhesion is seen to involve
the dorsal aspect of the caecum as far as the root of the appendix.
(Columbia University Museum, No. 1549.)]

[Illustration: FIG. 556.--Human infant. Ileo-colic junction and caecum,
with extensive adhesion to parietal peritoneum. (Columbia University
Museum, No. 301.)]

Finally the _erect vertical retro-caecal_ position of the appendix
presents several important variations in the disposition of the
peritoneal investment. In Fig. 503, taken from an embryo of 7.6 cm.
vertex-coccygeal length, the early complete recession of the retro-caecal
ileal convolutions has probably permitted an early apposition and
adhesion of the beginning of the colon to the dorsal prerenal parietal
peritoneum. The subsequent descent into the iliac fossa produces a bend
in the ventral wall of the colic tube, with a marked convexity directed
downwards and forwards, the apex of the bend situated at or near the
level of the ileo-colic junction, while the dorsal colic wall is held by
the adhesion to the parietal peritoneum, thus giving a backward
inclination to the entire caecum and appendix. During the subsequent
descent of the caecum proper this bend in the colon is gradually
diminished and the tube becomes straightened but the apex of the caecum
remains turned back and the appendix is placed in a more or less
vertical erect position behind caecum and ascending colon.

As regards the disposition of the peritoneal membrane in this type of
appendix the following conditions are to be noted:

(_a_) (Schema, Fig. 552.)--The apex of the caecum and the entire appendix
are extraperitoneal, imbedded in the loose connective tissue which
occupies the area of serous obliteration. The line of peritoneal
reflection from the dorsal wall of the secondary caput coli to the
parietal peritoneum of the right iliac fossa is placed transversely
below the true apex of the foetal caecum and the root of the appendix. The
latter tube, imbedded in connective tissue, passes vertically upwards
behind the ascending colon, its tip frequently reaching the ventral
surface of the right kidney. A well-marked example of this arrangement
in the adult is shown in Figs. 557 and 558 (ventral and dorsal view,
with peritoneal reflection and vertical retro-colic appendix).

[Illustration: FIG. 557.--Human adult. Ileo-colic junction and caecum;
ventral view. (Columbia University Museum, No. 1612.)]

[Illustration: FIG. 558.--Same preparation as Fig. 557; dorsal view. The
appendix, erected vertically between caecum and colon, is completely
imbedded in connective tissue.]

(_b_) (Schema, Fig. 553.)--In other cases, with the same position of the
appendix, the entire caecum and greater part of the ascending colon
remains free. The vertically erected appendix is closely attached to the
dorsal surface of the ascending colon, included within the serous
investment of the large intestine. The adhesion of the latter is
confined to a limited area near the hepatic flexure. Consequently caecum
and greater part of ascending colon can be turned up, away from the
parietal peritoneum of the iliac fossa, and the dorsal surface of the
appendix shows the free serous covering of the adjacent large intestine.

We may assume that this type of the peritoneal relations of the appendix
is produced in one of two ways:

1. Either the retro-colic appendix has become early attached to the
adjacent large intestine, whose dorsal surface in large part remains
free, or

2. The arrangement of the peritoneum indicated in schema, Fig. 552, may
be subsequently changed into that shown in schema, Fig. 553, by a
continued downward displacement of the caecum, producing a secondary
serous investment of the dorsal surface of appendix and part of
ascending colon.

Examples of this type are found both in infantile and adult subjects.

In Fig. 538, taken from an infant three years of age, the caecum is
lifted up to show the vertical position of the appendix behind the caecum
and ascending colon, the dorsal surface of the large intestine retaining
its free serous covering. Another illustration of this arrangement in a
juvenile subject is shown in Fig. 529. The same condition in the adult
subject is illustrated in Figs. 539 and 540.

(_c_) (Schema, Fig. 554.)--Occasionally, with the appendix erected
vertically behind the ascending colon, the apex of the caecum and the
proximal portion of the appendix are invested by peritoneum for a short
distance and the tip of the appendix likewise obtains a free serous
investment, while the intermediate greater portion of the appendix and
the corresponding segment of the dorsal surface of the ascending colon
are extraperitoneal, adherent to the abdominal parietes. Examples of
this peritoneal relation of the appendix in an infant are shown in Figs.
559 and 560, while Fig. 509 represents the same arrangement in an adult
specimen. The condition is produced from the arrangement of schema, Fig.
554, by secondary adhesion and obliteration of the serous surfaces over
the intermediate portion of the retroverted appendix and the adjacent
dorsal surface of the ascending colon.

[Illustration: FIG. 559.--Human infant. Ileo-colic junction and caecum;
dorsal view. Retro-colic appendix, adherent to the free dorsal serous
surface of the large intestine, with intermediate extraperitoneal
segment. (Columbia University Museum, No. 1638.)]

[Illustration: FIG. 560.--Human adult. Ileo-colic junction and caecum;
dorsal view. Appendix with intermediate non-peritoneal segment, while
the proximal portion and the tip are covered by serous investment.
(Columbia University Museum, No. 1615.)]


C. ILEO-CAECAL FOLDS AND FOSSAE.

Certain peritoneal folds, either mesenteric in character, _i. e._,
containing blood vessels, or non-vascular, pass between the terminal
ileum and the caecum and appendix, modifying in some instances very
markedly the position and peritoneal relations of the structures.

In considering the influence which these vascular mesenteric and
non-vascular serous folds exert in producing further changes in the
shape, position and relations of the human appendix it is necessary to
remember that in the early embryonal stages these bands and folds of the
peritoneum appear only slightly marked, but that they gain their
importance and influence on the final adult configuration of the caecal
pouch and appendix in the course of the further development of these
structures.

For this reason the comparative study of the corresponding parts in
other vertebrates, especially in certain mammalia, is of the utmost
value, if we seek to explain and understand the derivation, significance
and typical arrangement of these folds. We have seen that the caecum as
found in the large majority of mammalian forms is equivalent to the
caecum and appendix of the human subject and anthropoid apes; that in
other words the vermiform appendix represents the distal segment of a
caecal pouch, originally uniform in caliber, which has remained
undeveloped, while the proximal portion has progressed evenly with the
general development of the alimentary canal to form the caecum proper. We
have seen that this tendency to retain the distal portion of the pouch
in a rudimentary condition, _i. e._, the production of an appendage to
the caecum proper, is encountered in several of the lower forms, as
certain Marsupials, Carnivores, Ungulates and Lemurs. The morphology of
the ileo-caecal folds is hence best understood by considering these
structures as they appear in connection with the various caecal types
presented by the lower mammalia. Their arrangement and significance can
here be readily made out. On the other hand, in studying these
structures in the human appendix we are following lines which are
already becoming indistinct on account of the rudimentary character of
the organ, which we must regard as undergoing an exceedingly slow
process of reduction, with a view to its ultimate elimination from the
body. We have seen that the structural uncertainty impressed on caecum
and appendix by this evolutionary influence finds its expression in the
wide range of variation in size and arrangement which these parts
present. Necessarily, of course, this tendency to variation is shared,
and even exhibited to a more marked degree, by what we can term the
accessory structures connected with caecum and appendix, viz., the
mesenteric vascular and non-vascular serous folds passing to them from
the ileum.

We can most profitably begin our consideration of these folds in a form
in which they are preserved in their entire and original development,
and then successively trace the changes leading up to the normal
disposition in the human subject. Such a type is presented by the caecum
of _Ateles ater_, the black-handed spider monkey (Figs. 444 and 445).
The caecum of this animal presents a uniform crescentic curve, with the
concavity directed upward and to the left, and the gradual diminution in
the caliber of the pouch, from the ileo-colic junction to the apex,
denotes the tendency to retain the distal segment in a rudimentary
condition, foreshadowing the eventual formation of a vermiform appendix.

In the ventral view, with the terminal ileum lifted up, the following
arrangement of folds passing between ileum and caecum is noted (Figs. 444
and 445).

(_a_) _Vascular Mesenteric Folds._--The peritoneal vascular folds,
carrying the blood vessels to supply the caecum, are two in number, a
ventral (1) and dorsal (3). They are of nearly equal size and extent,
passing from the ventral and dorsal aspect of the ileo-colic junction
nearly to the apex of the caecum. Each contains a branch of the
ileo-colic artery, which forks in the ileo-colic mesentery, in the angle
between ileum and large intestine. The ventral branch continues in the
ventral mesenteric fold (Fig. 445) downward across the ventral surface
of the ileo-colic junction to supply the ventral part of the caecum,
while the dorsal branch descends behind the ileo-colic junction,
preserving a similar course in the dorsal mesenteric fold. The dorsal
arterial branch is somewhat larger than the ventral and its distribution
extends a little further down to the actual apex of the caecum.

(_b_) _Non-vascular Ileo-caecal Serous Reduplication._--Between the two
vascular mesenteric folds a third serous reduplication, carrying no
blood vessels, is found passing between the ileum and caecum. This fold
begins, in the preparation from which the figure is taken, on the ileum
opposite the attached mesenteric border, 2.7 cm. from the ileo-colic
junction, and passes for exactly the same distance down on the adjacent
left concave surface of the caecum. It is placed a little nearer to the
dorsal than to the ventral vascular fold, so that it passes, if the
distance between the two vascular folds on the caecum be divided into
three parts, at the junction of the dorsal third with the ventral two
thirds. The production of this intermediate non-vascular ileo-caecal
reduplication, which is of very constant occurrence in the mammalian
series, is to be led back to the development of the caecum. When the
pouch protrudes from the smooth surface of the embryonic intestine
opposite the mesenteric border, it extends backward along the future
small intestine and lifts off the serous investment of the gut in the
form of a small peritoneal plate filling the interval between itself and
the adjacent ileum. A very perfect illustration of this process can be
seen in the instance of Meckel's diverticulum shown in Fig. 561. The
proximal portion of the diverticulum is here still closely connected to
the small intestine along which it extends, both being surrounded by the
common visceral peritoneum. The distal part of the diverticulum has
separated more completely from the intestine, and in so doing has drawn
out the serous investment in the form of the triangular fold which is
seen to pass between the free margin of the intestine and the adjacent
surface of the pouch. The same process can be followed in its different
stages in certain normal mammalian caecal types.

[Illustration: FIG. 561.--Human adult ileum with Meckel's diverticulum.
Ileo-diverticular serous fold and persistent omphalo-mesenteric artery.
(Columbia University Museum, No. 1803.)]

[Illustration: FIG. 562.--Human adult. Ileum with Meckel's diverticulum,
131.5 cm. from ileo-colic junction; a distinct vascular fold is
prolonged from the ileal mesentery to the margin of the diverticulum.
(Columbia University Museum, No. 1849.)]

In this connection it may be noted that the production of the caecal
vascular folds and their relation to the mesentery is also very
perfectly illustrated in some forms of Meckel's diverticulum. Thus in
the preparation shown in Fig. 562, a broad triangular serous fold passes
from the ileal mesentery to the margin of the diverticulum, carrying the
blood vessels which supply the pouch. If the section of the intestine to
the left of the figure is regarded as representing the terminal ileum,
that to the right the colon, and the diverticulum the caecal pouch, the
formation of the fold and its relation to the mesentery, blood vessels
and intestine will correspond closely to the ileo-caecal vascular folds.

Fig. 350 shows the ileo-colic junction and caecum of _Halmaturus
derbyanus_, the rock kangaroo. The caecum here extends backwards along
the free border of the ileum to which it is closely bound by the common
investing visceral peritoneum for the greater part of its extent. In
another marsupial form, a small species of opossum from Trinidad (Fig.
349), the caecum has separated itself more completely from the adjacent
small intestine--thus drawing out the peritoneum into a narrow
connecting fold. Finally, in the Virginia opossum (Fig. 348), the ileum
has attained the usual position at right angles to caecum and colon. The
former pouch is separated from the small intestine by a considerable
interval and the angle between the two is filled out by a well-developed
triangular serous fold, connecting the free margin of the terminal ileum
and the adjacent left border of the caecum.

This is the "intermediate non-vascular" ileo-caecal fold.

Passing now from the condition presented by _Ateles_, with three fully
developed and distinct ileo-caecal folds, to the next stage leading up to
the normal human arrangement, we find the same illustrated in the caecum
of another new-world monkey, _Mycetes fuscus_, the brown howler monkey,
shown in the ventral and dorsal views in Figs. 449 and 450. The ventral
vascular fold (Fig. 449, 1) is still well developed, the contained
ventral branch of the ileo-colic artery descending over the ventral wall
of the ileo-colic junction and caecum and supplying both. The dorsal
vascular fold (Fig. 450, 2), on the other hand, is nearly completely
fused with the intermediate non-vascular reduplication (Figs. 449 and
450, 3), the approximation between these structures exhibited by
_Ateles_ having in _Mycetes_ reached the point of actual union, so that
the larger dorsal branch of the ileo-caecal artery descends to the apex
of the caecum in the following manner: The main post-caecal artery passes
over the dorsal surface of the ileo-colic junction included in a short
serous fold which corresponds to the dorsal vascular fold of _Ateles_.
Beyond the lower border of the ileo-colic junction this fold fuses with
the intermediate non-vascular fold, one arterial branch descending along
the line of attachment of this fold to the caecum, the other distributed
over the dorsal surface of the pouch.

A third type, also taken from the lower Primates, is presented by
the caecum of a cynomorphous monkey, _Cercopithecus sabaeus_, the
African green monkey, shown in Fig. 432, in the ventral and left
aspect with the terminal ileum lifted up. The caecum of this animal
is comparatively short, somewhat conical, terminating in a blunt
apex. The vascular supply is arranged on the same type as in _Ateles_
and _Mycetes_, _i. e._, a trunk of the ileo-colic artery divides
at the ileo-colic notch, one branch descending ventrad, the other
dorsad of the ileo-colic junction. The slightly larger size of the
dorsal vessel, noted in _Ateles_ and _Mycetes_, has been increased in
_Cercopithecus_ until the ventral artery (1) supplies merely the front
of the ileo-colic junction and the upper part of the adjoining ventral
wall of the caecum, while the larger dorsal vessel (2) descends behind
the ileo-colic junction, supplying the same and the entire dorsal and
apical portions of the caecum. The relation of these caecal arteries to
the peritoneum is moreover different from that encountered in _Ateles_.
In place of running in distinct mesenteric folds, as in the latter
species, the vessels pass close to the surface of the intestine, merely
covered and partly surrounded by slightly redundant visceral peritoneum
containing numerous pads of epiploic fat, which bead the course of the
vessels at regular intervals. Between the two arteries the intermediate
non-vascular fold (2) is seen, presenting much the same arrangement as
in _Ateles_ and passing between the left border of the caecum and the
adjacent margin of the ileum, nearer to the dorsal larger than to the
ventral smaller caecal artery.

We have, therefore, in the three types just considered, the following
variations in the arrangement of the vascular and non-vascular folds:

 1. (_a_) Ventral and dorsal vascular folds distinct    }
          and free. Ventral and dorsal caecal            }
          arteries of nearly equal size.                }
    (_b_) Intermediate non-vascular fold free on        }_Ateles._
          both surfaces, placed nearer to the           }
          dorsal than to the ventral vascular fold.     }
 2. (_a_) Ventral vascular fold distinct. Ventral       }
          caecal artery somewhat further reduced         }
          in size. Dorsal vascular fold distinct        }
          only over the dorsal surface of the ileo-colic}
          junction. At the lower border of              }
          the ileo-colic junction the dorsal vascular   }_Mycetes._
          fold fuses with the intermediate non-vascular }
          fold.                                         }
    (_b_) Intermediate non-vascular fold free only      }
          on ventral surface, the dorsal surface        }
          below the ileo-colic junction being fused     }
          with the dorsal vascular fold.                }
 3. (_a_) Dorsal and ventral vascular folds reduced.    }
          Dorsal artery much larger than ventral.       }_Cercopithecus._
    (_b_) Intermediate non-vascular fold well developed,}
          free on both surfaces.                        }

We may judge from this series that the following factors are capable of
materially modifying the definite arrangement of the structures:

1. The vascular folds are capable of reduction until the vessels run
close to the intestinal surface, merely covered by somewhat redundant
peritoneum containing epiploic appendages. (_Cercopithecus._)

2. The dorsal caecal artery tends to assume in all three forms the
greater share in the caecal vascular supply. This tendency is slightly
developed in _Ateles_, becomes more pronounced in _Mycetes_, and is well
marked in _Cercopithecus_, in which animal the dorsal vessel nearly
replaces the ventral branch, the latter confining itself to the ventral
surface of the ileo-colic junction and the adjacent ventral parts of the
caecal wall.

3. The intermediate non-vascular fold is placed nearer to the dorsal
larger than to the ventral smaller caecal artery. This condition, present
in both _Ateles_ and _Cercopithecus_, foreshadows the fusion of the
intermediate and dorsal vascular folds at the lower border of the
ileo-colic junction, as seen in _Mycetes_.

4. This fusion of the two folds named in _Mycetes_ results in giving
different values to the dorsal vascular fold in its proximal and distal
segments. The proximal segment descends from the ileo-colic notch behind
the ileo-colic junction to its lower border as a distinct fold. Beyond
this point its fusion with the distal (caecal) segment of the
intermediate fold rounds out a fossa, the inferior or posterior
ileo-caecal, which is consequently bounded in front by the intermediate
vascular fold, behind by the proximal segment of the dorsal vascular
fold, to the right side by the inner wall of the caecum, between the
intermediate and dorsal vascular folds, above by the lower border of
ileum and ileo-colic junction, and below by the fusion of the two
folds.

This pocket or fossa which is the most important and constant of the
peritoneal recesses in the neighborhood of the caecum, opens upward and
to the left.

5. A superior or anterior ileo-caecal fossa, formed in cases of
well-developed ventral vascular fold between the same and the ventral
wall of the ileo-colic junction, is of small size and shallow.

The cause of the greater development of the dorsal as compared with the
ventral caecal artery is probably to be sought in the adhesion of the
colon to the dorsal parietal peritoneum. In _Cercopithecus_ the dorsal
surface of the ascending colon is adherent to the parietal peritoneum
down as far as the iliac region and beginning of the caecum, whereas in
_Mycetes_ the entire caecum, as well as the ascending colon, are free and
non-adherent to the abdominal parietes. The influence of this adhesion
on the arrangement of the vascular supply of the lower portion of the
ascending colon and caecum appears to be important. Some of the
departures from the _Ateles_ type presented by _Cercopithecus_ become
still better developed in the human subject, where the adhesion of the
ascending colon and the obliteration of the apposed serous surfaces of
ascending mesocolon and parietal peritoneum is normally complete, even
if the caecum remains entirely free, or only adheres to the iliac
parietal peritoneum in the proximal part of its dorsal surface.
Comparison with forms presenting non-adherent colic and caecal tubes
indicates that the adhesion determines the relative size and arrangement
of the ileo-colic vessels.

Thus the partially adherent colon and caecum of _Cercopithecus_ presents,
compared with the free tube of _Ateles_ and _Mycetes_, a marked
reduction of the ventral and a corresponding enlargement of the dorsal
caecal artery. Further progress in the same direction is noted in the
human subject where normally the ascending colon and at times the
proximal portion of the caecum are adherent to the dorsal parietal
peritoneum.

It appears that in the adhesion of the colic tube to the parietal
peritoneum the dorsal ileo-colic vessels find an element favorable to
their more complete development and extension, replacing in part or
entirely the ventral caecal artery which becomes limited in distribution
to the region of the ileo-colic junction. The adhesion and fixation of
the dorsal wall of the intestine seems to afford an advantage to the
dorsal vessel, whereas the greater mobility and the alternating
conditions of distension and contraction, with variations of intracaecal
pressure, depending upon the contents of the pouch, appear to operate
unfavorably upon the development of the ventral vessel.

This view is borne out by the conditions observed in the exceptional
instances in which in the human subject the ventral artery assumes the
large share in the supply of caecum and appendix (cf. p. 276). In all the
cases observed the type of the caecum indicated delayed or imperfect
colic adhesion, and the ascending mesocolon remained partially free.

If we now compare the conditions above described for _Ateles_,
_Mycetes_, _Cercopithecus_ with those usually found in man and in the
anthropoid apes, we may appreciate the significance of the structures
encountered by beginning the investigation with a type in which the
derivation of the different parts is still quite evident. Such a
condition is presented by the preparation shown in Fig. 563, taken from
a child one year of age. Here the descent of the caecum has evidently
been quite rapid and uniform without dorsal adhesion. The caecum and
ascending colon remain free and can still be lifted away from the
ventral facies of the right kidney and turned toward the median line to
a point somewhat beyond the renal hilus. The caecum hangs downward
vertically and the appendix arises from the funnel-shaped apex of the
pouch.

[Illustration: FIG. 563.--Human; child one year old. Caecum and
ileo-colic junction; ventral view. (Columbia University, Study
Collection.)

1. Ventral caecal artery, surrounded by epiploic appendages.

2. Dorsal vascular fold, forming appendicular mesentery.

3. Intermediate non-vascular fold.]

The ventral caecal branch of the ileo-colic artery is slightly developed,
(1) as a small vessel descending in an epiploic fold over the ventral
surface of the ileo-colic junction as far as the root of the appendix.
The intermediate non-vascular fold (3) is well marked, measuring 2.9 cm.
in length, extending from the free border of the terminal ileum to the
caecum and appendix and crossing over the well-developed dorsal vascular
fold (2), which descends, as the appendicular mesenterolium, to the tip
of the appendix, carrying the dorsal artery. In studying the conditions
presented by this specimen, it is not difficult to trace the analogous
structures in the caeca of _Cercopithecus_, _Ateles_ and _Mycetes_. The
same vascular and non-vascular serous reduplications are found passing
between the ileum and caecum. In accordance with the type presented by
_Cercopithecus_ the ventral artery is much reduced and runs in a short
serous fold loaded with epiploic appendages. The dorsal artery, on the
other hand, is well developed and the intermediate non-vascular fold is
distinct. In their relative arrangement these folds follow the _Ateles_
type. The dorsal vascular fold forms the true mesentery of the appendix,
and, although close to and crossed by the intermediate non-vascular
reduplication, remains still quite separable and distinct from the same;
consequently the lower limit of the usual posterior ileo-caecal fossa,
produced by the fusion of the dorsal vascular and the intermediate
non-vascular fold, is absent.

A very perfect illustration of this type of the human ileo-caecal fold is
presented by the preparation of _Gorilla savagei_ shown in Fig. 457. The
ventral fold and artery appear reduced in this animal. The dorsal
vascular fold forms a broad triangular plate of serous membrane carrying
the dorsal artery in its free border and extending to the tip of the
appendix.

The intermediate non-vascular fold is narrow but distinct, continued for
a considerable distance along the ileum, opposite to the attached
border, but only for a short extent along the left border of the caecum
below the ileo-colic junction. It crosses the ventral surface of the
broad dorsal vascular fold in passing to the caecum, but remains entirely
free and is not adherent to the same.

Consequently here again the dorsal or posterior ileo-caecal fossa loses
its distal limitation. The usual arrangement of the parts, as found in
the human subject and derived from the preceding, is well illustrated by
another anthropoid ape, _Hylobates hoolock_. Fig. 455 shows the
ileo-caecum of this animal in the ventral view and the homologous parts,
as compared with _Gorilla_, are readily recognized. On turning the
terminal ileum ventrad and cephalad (Fig. 456), it is, however, seen
that the intermediate non-vascular fold does not merely cross the dorsal
vascular reduplication, as in _Gorilla_, but that it has begun to adhere
to the same at the point of intersection. Consequently a well-marked and
clearly limited posterior or dorsal ileo-caecal fossa is formed, bounded
ventrally by the intermediate fold at its accession to the caecum,
dorsally by the proximal part of the dorsal vascular fold, to the right
by the left wall of the caecum, behind by the attachment of the
intermediate fold, below by the confluence of the two folds, and above
by the lower border of ileum and ileo-colic junction.

The open mouth of the fossa looks to the left. Fig. 464, taken from an
adult specimen of the chimpanzee, _Troglodytes niger_, shows the extent
of the dorsal vascular fold and of its connection with the mesentery of
the terminal ileum.

The intermediate non-vascular fold extends from the ileum downwards
along the entire left border of the caecum to the root of the appendix,
fusing with the dorsal vascular fold and rounding out a deep posterior
ileo-caecal fossa.

The typical arrangement, as encountered in the human subject,
corresponds closely to the conditions presented by these anthropoid
apes.

[Illustration: FIG. 564.--Human adult. Caecum and ileo-colic junction.
(Drawn from preparation in Columbia University, Study Collection.)

1. Dorsal vascular fold at the beginning of the distal free portion,
forming the appendicular mesentery.

2. Proximal segment of dorsal vascular fold, fusing with

3. Intermediate non-vascular fold.

4. Rounded edge of union of dorsal vascular and intermediate folds
bounding the ileo-caecal fossa caudad.

5. Point of accession to appendix of proximal branch of appendicular
artery derived from posterior ileo-caecal artery.]

In Fig. 564, taken from an adult male human subject, the dorsal surface
of the ascending colon and of the ileo-colic junction is adherent to the
parietal peritoneum. The distention of the caecum is nearly uniform, the
right sacculation being only slightly larger than the left. The
appendix, measuring 18.4 cm. in length, arises from the dorsal surface
of the caput coli, 1.7 cm. from the point where the ventral longitudinal
muscular band turns around the caudal end of the pouch between the two
sacculations, and 3.7 cm. below the caudal margin of the ileo-colic
junction.

The dorsal vascular fold (2), forming the broad appendicular mesentery
(1), is well developed and free in its distal portion, extending, with
gradually diminishing width, to the apex of the appendix. The proximal
segment of this fold (between 1 and 2) descends over the dorsal surface
of the ileo-colic junction and meets (at 4) the intermediate
non-vascular fold (3) which extends between the ileum and caecum,
rounding out a crescentic ridge (4) which bounds the entrance into the
posterior ileo-caecal fossa (between 2 and 3). The influence of the folds
and of the blood vessels on the position and curves of the appendix is
quite apparent in this preparation.

The dorsal larger branch of the ileo-colic artery, supplying caecum and
appendix, passes over the dorsal surface of the ileo-colic junction (2)
where the same, as well as the adjacent dorsal surface of the colon, is
adherent to the parietal peritoneum. At the point where the dorsal
vascular fold intersects and fuses with the intermediate non-vascular
fold (4) the artery divides into a proximal and distal branch. The
former proceeds to the caecum and root of the appendix, reaching this
tube at the point marked 5. The latter continues (from 1 on) in the free
border of the appendicular mesentery to the beginning of the distal
third of the appendix, from which point on the fold extends as a narrow
reduplication to the tip of the tube. The segment of the appendix
situated between these two main arterial branches is thrown into several
coils, the expression of the continued growth between two points
relatively fixed by the accession of the two arterial branches. The
pathological significance of these bends is apparent when we consider
the effect which the kinking of the tube would have on catarrhal and
other inflammations accompanied by distension of the appendix.

Typical examples of the posterior ileo-caecal fossa and of the mutual
relationship of the limiting folds are seen in Figs. 565 and 566, both
taken from adult human subjects.

[Illustration: FIG. 565.--Human adult, Caecum and ileo-colic junction
with large intermediate non-vascular fold and deep posterior ileo-caecal
fossa. (Columbia University Museum, No. 1546.)]

[Illustration: FIG. 566.--Human adult. Ileo-colic junction and caecum.
(Columbia University Museum, No. 1659.)]

The significance and mutual relations of the folds seen in the
preparations just considered--which illustrate the typical adult human
arrangement of the structures--will perhaps be best understood by
comparison with an adult caecum in which the infantile condition, as seen
in Fig. 563, has become further developed.

[Illustration: FIG. 567.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Drawn from preparation in Columbia University, Study
Collection.)

1. Dorsal vascular fold, carrying the distal appendicular branch of the
dorsal caecal artery in the mesentery of the appendix.

2. Proximal branch of the same vessel, turning downward to caecum and
root of appendix.

3. Intermediate non-vascular fold.]

Fig. 567 shows the dorsal view of such a preparation. The caecum is
funnel-shaped with the apex, carrying the root of the appendix, turned
upward and to the left, the sacculation to the right of the ventral
muscular band being somewhat dilated. The appendix--7.2 cm. long--turns
sharply upward and to the left, closely applied to the left caecal
sacculation, passes dorsad to the ileo-colic junction and lies in its
terminal part under cover of the ileo-colic mesentery. The ventral
branch of the ileo-colic artery descends over the ileo-colic junction,
supplying the ventral wall of the caecum. The intermediate non-vascular
fold (3) is 3.9 cm. long and entirely free.

The dorsal vascular fold contains the large dorsal branch of the
ileo-colic artery, dividing into two main branches. The first of these
(1) passes distally in the free edge of the fold to the terminal part of
the appendix. The other proximal branch (2) turns downward to the root
of the appendix and the adjacent wall of the caecum, aiding materially in
holding the proximal upturned segment of the appendix in contact with
the left caecal sacculation.

The intermediate fold, short in its caecal attachment, does not meet the
dorsal vascular fold at any point, consequently the ileo-caecal fossa is
not limited caudad toward the root of the appendix. The conditions
presented by this specimen correspond exactly to those found in the
gorilla (Fig. 457) and in the human infantile preparation (Fig. 563).

In comparing Figs. 564 and 567 it will be noticed that the line of
fusion between the intermediate fold and the dorsal vascular fold (Fig.
564, 4) corresponds to the point where the dorsal ileo-caecal artery
divides into its proximal and distal branches (Fig. 567, angle between 1
and 2). Fig. 567 shows that the proximal arterial twig, even without
fusion with the intermediate fold, suffices to influence to a
considerable degree the curves and position of the appendix, inasmuch as
it serves to hold the proximal segment of the tube closely applied in
the erected position to the surface of the left caecal sacculation. The
intermediate segment of the appendix, between the points of accession of
the two arterial branches, is most prone to develop spiral
twists and bends, especially when the usual fusion of the two folds
takes place and still further fixes the parts, while the distal segment,
carrying the narrow crescentic terminal appendicular mesentery, remains
free.

[Illustration: FIG. 568.--Human adult. Ileo-colic junction and caecum.
(Drawn from preparation in Columbia University, Study Collection.)

1. Distal and

2. Proximal branch of dorsal ileo-caecal artery running in dorsal
vascular fold.

3. Intermediate non-vascular fold fusing with 2 and forming a narrow
caudal limit to the posterior ileo-caecal fossa.]

Finally, in a certain number of cases, an intermediate condition between
the types presented by Figs. 564 and 567 is encountered. In Fig. 568 the
general arrangement of the parts corresponds pretty accurately to that
seen in Fig. 566, but the transition from a completely free intermediate
non-vascular fold to one which has begun to fuse with the dorsal
vascular fold is evident. The caecum is bent upward and to the left, the
caput coli being formed by the right sacculation. The appendix, 7.8 cm.
long, takes a wide {~WREATH PRODUCT~}-shaped curve. The convexity of the proximal curve
corresponds to the point where the proximal appendicular artery (2)
passes to the tube. The non-vascular intermediate fold (3), measuring
2.2 cm., fuses with the dorsal vascular fold at this point.

The three preparations illustrate serially the share which the
peritoneal folds take in the formation of the posterior ileo-caecal
fossa.

In Fig. 566 the failure of the intermediate fold to meet and fuse with
the dorsal vascular fold has left the caudal boundary of the fossa
(between 2 and 3) incomplete, the ventral and dorsal walls being formed
by the folds in question. Fig. 568, in which fusion between the
non-vascular and the dorsal vascular folds has commenced, shows the
shallow form of the complete fossa under these conditions, while in Fig.
567, with extensive union of the folds, the fossa has correspondingly
increased in depth.

[Illustration: FIG. 569.--Human adult. Ileo-colic junction and caecum.
(Columbia University Museum, No. 1610.)]

[Illustration: FIG. 570.--Human infant, four days old. Ileo-colic
junction and caecum. (Columbia University Museum, No. 879.)]

[Illustration: FIG. 571.--Human infant. Ileo-colic junction and caecum.
(Columbia University, Study Collection.)]

A similar series is shown in Figs. 569, 570 and 571. In Fig. 569, taken
from an adult subject, the intermediate non-vascular fold is entirely
free, the dorsal branch of the ileo-caecal artery passes to caecum and
appendix in an area of adhesion between parietal peritoneum and the
intestine which includes the dorsal vascular fold. There is consequently
no caudal boundary to the ileo-caecal fossa. Figs. 570 and 571 are both
taken from infantile preparations.

In Fig. 570 the dorsal vascular and the intermediate folds nearly meet
at the root of the appendix. They serve to outline the fossa, which
appears completed in Fig. 571 by the actual meeting and fusion of the
folds.

_The Ileo-caecal Folds in the Anthropoid Apes.--(1) Chimpanzee,
Troglodytes niger._

The structures in a juvenile specimen of this animal are shown in Figs.
460 and 461.

The ventral vascular fold (Fig. 460, 3), containing epiploic fat,
descends over the ileo-colic junction nearly to the level of the lower
ileal margin. The intermediate non-vascular fold (Figs. 460 and 461, 2),
derived from the ileum opposite to the mesenteric border, passes to the
ventral and left aspects of the caecum and meets, near the root of the
appendix, the dorsal vascular fold (Fig. 461, 3) carrying the dorsal
caecal branch of the ileo-colic artery, which ramifies over the caecum and
supplies the appendix.

The appendix measures 12.3 cm. and presents a terminal hook, slightly
dilated.

The appendicular mesentery terminates within the concavity of this hook
and measures 1.5 cm. in width at the broadest part, about 4.5 cm. from
the root of the appendix.

Figs. 462 and 463 show the caecum of the adult chimpanzee in the ventral
and dorsal view. The ventral vascular fold (Fig. 462, 1) is well
developed, heavily fringed with epiploic appendages.

The non-vascular fold is extremely short and tense, fusing with the
short appendicular mesentery near the point where in the dorsal view
(Fig. 463) the appendix is seen bent at its origin sharply to the right.

Fig. 464, also taken from an adult specimen of the same animal, shows a
very well-developed dorsal vascular fold, which fuses with the
intermediate fold to limit a distinct ileo-caecal recess.

The chimpanzee, therefore, agrees closely with the human subject in the
arrangement of the folds.

(2) _Orang, Simia satyrus._

In Figs. 458 and 459 the arrangement of the folds in an adult specimen
of the orang is shown.

The ventral caecal artery (Fig. 458) is well developed, forming with the
peritoneal fold and epiploic appendages surrounding it, a sharp
sickle-shaped edge which descends over the ventral surface of the
ileo-colic junction following the curve of the left caecal margin, and
turning its concavity to the left toward the entering ileum.

The ventral caecal artery follows the left margin of the caecum below the
ileo-caecal junction and passes for 0.5 cm. upon the portion of the pouch
which turns up behind the terminal ileum.

The dorsal caecal artery is a vessel of large size, supplying branches to
the narrow appendicular mesentery which extends, with many epiploic
appendages, to within 9 mm. of the blunt apex of the appendix. 2.5 cm.
beyond the first bend in the appendix the fold is narrowed to a fringe
not more than 0.75 cm. wide. Up to this point the dorsal vascular fold
measures 1.5 cm. in width, and just where it narrows it is joined by the
intermediate non-vascular fold (Fig. 459), which forms a membranous
band, 3.3 cm. wide in the middle, spread out in the angle between the
lower and dorsal surfaces of the ileum and the dorsal surface of the
caecum which turns up behind the ileo-colic junction. Between this fold
and the dorsal vascular fold is seen the deep recess of the posterior
ileo-caecal fossa--which by reason of the sharp curve of the caecum looks
not only to the left but also upward and backward.

Direct comparison of the preparations of these two anthropoid apes just
described with the conditions found in many adult human caeca shows the
close correspondence in the arrangement of these folds and of their
influence on the configuration of the parts.

Figs. 572 and 573--taken from an adult human subject--show a caecum and
appendix which almost reproduces that of the chimpanzee illustrated in
Figs. 462 and 463 and closely resembles that of the orang.

[Illustration: FIG. 572. Human adult. Ileo-colic junction and caecum;
ventral view. (Drawn from preparation in Columbia University, Study
Collection.)

1. Ventral vascular fold.]

[Illustration: FIG. 573.--Dorsal view of the same preparation.

1. Appendix.

2. Intermediate non-vascular fold.]

Fig. 572, giving the ventral view, shows, by the course of the ventral
longitudinal muscular band, the turn of the caecum upwards and to the
left. The ventral caecal artery runs in a fold (1) loaded with epiploic
appendages.

The non-vascular intermediate fold (Fig. 573, 2) passes to the root of
the appendix, joining the proximal segment of the dorsal vascular fold
in which the dorsal branch of the ileo-colic artery runs to the tip of
the appendix. The distal two thirds of the appendicular mesentery are
free.

3. _Gibbon, Hylobates hoolock_ (Figs. 455 and 456).--In the gibbon the
folds appear well developed. The intermediate and dorsal vascular folds
are quite distinct structures, although fusion (Fig. 456) has begun at
one point, thus limiting a typical posterior ileo-caecal fossa.

4. _Gorilla, Gorilla savagei_ (Fig. 457).--Finally in the gorilla all
three folds appear quite distinct and separate from each other, the
dorsal vascular fold being especially well developed.

_Unusual and Aberrant Types of Ileo-caecal Folds and Fossae.--(A) Ventral
caecal artery larger than the dorsal, supplying the greater part of the
caecum and the appendix._

This condition is occasionally encountered. Dr. Martin, in a recent
examination of the vascular supply of caecum and appendix in one hundred
subjects, found it to obtain in six instances.

Apparently the dorsal wall of the caecum and of the proximal segment of
the ascending colon remains free in these cases and does not become
adherent to the parietal peritoneum. The shape of the pouch, moreover,
indicates a free and unimpeded embryonal caecal descent. The normal
relative size of the two vascular folds is reversed. A good example of
this variation, in the caecum of an infant, is seen in Fig. 516. The same
arrangement in an adult specimen is seen in Fig. 574.

[Illustration: FIG. 574.--Human adult. Caecum and ileo-colic junction;
well-developed ventral vascular fold, carrying appendicular artery.
(Columbia University Museum, No. 1613.)]

In the Slow Lemur (_Nycticebus tardigradus_) (Fig. 420) the ventral
artery is normally the larger of the two, extending in the ventral fold
to the tip of the reduced appendix of the caecal pouch.

(_B_) _Fusion of ventral vascular fold with the intermediate fold,
resulting in the production of a well-defined superior or
ventral ileo-caecal fossa._

Normally the reduced ventral artery crosses the ileo-colic junction in a
slightly developed ventral vascular fold, closely adherent to the
intestine, with a very narrow free margin. The superior or ventral
ileo-caecal fossa in these cases is very shallow and confined (Fig. 574)
to the ventral surface of the ileo-colic junction. Occasionally the fold
is better developed and fuses with the intermediate non-vascular fold,
producing a fossa of greater extent, which is bounded dorsad by the
ileum, ventrad and cephalad by the ventral fold, caudad by the fusion of
this fold with the intermediate reduplication, and to the right by the
left wall of the caecum. Figs. 576, 577, 578 and 579 show this aberrant
disposition of the structures in a series of adult human caeca.

[Illustration: FIG. 575.--Human foetus at term. Ileo-colic junction and
caecum; ventral view. (Columbia University Museum, No. 1715.)]

[Illustration: FIG. 576.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, No. 1614.)]

[Illustration: FIG. 577.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, No. 1657.)]

[Illustration: FIG. 578.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, No. 1856.)]

[Illustration: FIG. 579.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, Study Collection.)]

A corresponding arrangement is noted in the preparation of the caecum of
_Cercopithecus campbellii_ (Fig. 433). The large intermediate fold is
joined by the ventral vascular fold, thus defining the lower boundary of
ventral ileo-caecal fossa.

(_C_) _Union of both vascular folds with the intermediate non-vascular
fold._

I have encountered one instance of this arrangement in an infant, whose
caecum and ileo-colic junction is shown in Fig. 580. Both the ventral and
dorsal arteries in this case were equally developed, and shared equally
in the supply of caecum and appendix. Both vascular folds fused with the
intermediate fold, thus producing two typical ileo-caecal fossae, one
ventral, the other dorsal.

[Illustration: FIG. 580.--Human infant. Ileo-colic junction and caecum;
fusion of ventral and dorsal vascular folds, with intermediate fold.
(Columbia University Museum, No. 1663.)]

(_D_) _Abnormal positions of the appendix due to variations in the
arrangement and tension of the intermediate fold._

Fig. 510 shows a foetal caecum in the ventral view. The ventral vascular
fold (3) is well developed. The non-vascular fold is short, arising from
the ventral surface of the ileum, instead of from the free border of the
intestine opposite to the mesenteric attachment. It fuses with the
ventral vascular fold a short distance below the ileo-colic junction,
thus limiting a small ventral ileo-caecal fossa. The dorsal caecal artery
in this specimen was large, but the fold carrying it extremely narrow.

The preparation illustrates the type resulting from the reduction in
size and extent of the non-vascular and mesenteric folds. The
intermediate fold is reduced to a short and narrow band. Compared with
the usual infantile type the caecum lacks the characteristic turn upwards
and to the left, possibly in consequence of the slight traction caused
by the rudimentary intermediate fold. The pouch occupies a nearly
vertical pendent position, which the appendix, arising from the lowest
point of the caecal funnel, shares. The appendix is not drawn into the
retro-ileal position by the dorsal vascular fold, which is much reduced.

In Fig. 511, representing the caecum and appendix of a foetus at term, the
effect of the tense non-vascular intermediate fold (2) is seen in the
sharp turn to the left which it imparts to the nearly transversely
directed funnel-shaped caecum. The appendix (1) is coiled spirally for
13/4 turns behind the ileo-colic junction, with the tip directed upward
behind the mesentery of the terminal ileum. The non-vascular
intermediate fold (2) extends to the rest of the appendix. It appears
short in its caecal attachment, on account of the turn of the caecum
backwards and to the left and the close connection between the adjacent
margins of the ileum and caecum.

[Illustration: FIG. 581.--Human foetus at term. Ileo-colic junction and
caecum. (Columbia University, Study Collection.)

1. Appendix, terminal portion turned ventrad of ileo-colic junction.

2. Intermediate non-vascular fold.]

[Illustration: FIG. 582.--Human infant. Ileo-colic junction and caecum;
ventral position of appendix. (Columbia University Museum, No. 693.)]

In Fig. 581--a foetal preparation at term--the caecum is turned to the
left, below and behind the terminal ileum. The non-vascular fold (2) is
well developed as regards _length_ of _ileal_ attachment, but is very
narrow and tense, passing between ileum and the proximal curve of the
caecum behind the ileo-colic junction, where it merges with the dorsal
vascular fold. The appendix takes a sudden turn caudad at this point and
then continues up _ventrad_ to the ileo-colic junction, the proximal
portion being kept firmly in contact with the dorsal and caudal
circumference of the ileum by the tension of the non-vascular band. It
is quite evident that this peculiar turn of the appendix is directly due
to the confining influence of the non-vascular band--which passes from
its ileal attachment almost directly dorsad to the point of fusion with
the dorsal vascular fold, causing the sharp downward and forward turn of
the proximal segment of the appendix. Similar cases with ventral
position of the appendix are shown in Figs. 545 and 582.




INDEX.


  Abdominal vein in Anure Amphibian, 158
      in Reptilia, 167
      in _Iguana_, 160
      in Urodele Amphibian, 157
    viscera of _Macacus rhesus_, 77

  Abnormal positions of appendix, 277

  Abomasum, 49

  _Accipenser sturio_, biliary ducts in, 145
      pyloric appendices in, 120
      ileo-colic junction of, 212

  Ailuroidea, ileo-colic junction of, 212

  Alimentary canal of _Ammocoetes_, 42
      of _Amphioxus_, 42
      of _Belone_, 40
      of _Chelydra_, 58
      of Cyclostomata, 40, 42
      derivation of epithelium, 30
        of muscular and connective tissue, 30
      differentiation from body-cavity, 21, 29
      divisions of, 38
      early developmental stages, 21, 29
      mammalian embryonal stages, 40
      of _Esox_, 40
      of _Echelus conger_, 54
      of _Necturus_, 40
      of _Petromyzon_, 200
      of _Proteus_, 40
      primitive type, 40, 42
      of _Pseudemys elegans_, 55
      of _Rana_, 55
      separation from yolk-sac, 22
    tract of _Necturus maculatus_, 52
      of _Tamandua_, 56

  Allantois in Amniota, 36
    arteries of, 63, 146
    derivation from alimentary canal, 35
    function of, 36
    relation to placenta, 36
      to primitive intestine, 24
      to urinary bladder, 24

  _Alligator mississippiensis_, ileo-colic junction of, 201
    stomach of, 51

  _Ammocoetes_, alimentary canal of, 42
    pancreas in, 117

  _Ammodytes_, pyloric appendix in, 120

  Amnion, definition of, 36

  Amniota, development of liver in, 143

  Amphibia, development of pancreas, 115
    folds of intestine in, 196
    ileo-colic junction of, 201

  Amphibians, biliary ducts in, 145
    intestinal canal of, 191

  _Amphioxus_, alimentary canal of, 42
    hepatic diverticulum of, 43
    intestinal canal of, 191

  Anthropoid apes, ileo-caecal folds of, 274

  Anthropoidea, ileo-colic junction of, 213

  Anthropomorpha, ileo-colic junction of, 216

  _Anguilla anguilla_, stomach of, 47

  Anure Amphibian, abdominal vein of, 158
      cardiac vein in, 158
      musculo-cutaneous vein in, 158
      pelvic vein in, 158
      post-cava in, 158
      pre-cava in, 158
      venous system in, 158

  Aorta, early condition of intestinal branches, 32

  Aortal arterial system, development of, 63

  Aplacentalia, definition of, 36

  Appendix, abnormal positions of, 277
    absence of, 249
    influence of dorsal vascular fold on shape of, 271
    origin of, and shape of caecum, 245
    position and peritoneal relations of, 250
    variations of peritoneal relations, 258

  Arctoidea, ileo-colic junction of, 212

  _Arctopithecus marmoratus_, ileo-colic junction of, 208

  Arctopithecini, ileo-colic junction of, 214

  Arrest of development before intestinal rotation, 60

  Arteries, of allantois, 64, 146

  Artery, caudal, 64
    ileo-colica, 66
    colica dextra, 66
      media, 66
    coronary, 181
    external iliac, 64
    gastro-epiploica sinistra, 108
    hepatic, 65, 179
    ileo-colic, 262
    inferior mesenteric, 67
    internal iliac, 64
    pancreatico-duodenalis inferior, 66
    omphalo-mesenteric, 64, 146
    sacralis media, 64
    splenic, 65, 108
    superior mesenteric, 64, 65
    umbilical, 64
    vitelline, 64, 146

  Artiodactyla, ileo-colic junction of, 209

  _Arvicola pennsylvanicus_, ileo-colic junction and caecum of, 211

  Asymmetrical type of ileo-colic junction, 223

  _Ateles_, ileo-caecal folds of, 261
    _ater_, ileo-colic junction and caecum of, 214

  Atresia ani, 24, 28

  Axial mesoderm, connection with splanchnic and somatic mesoderm, 22


  _Bassaris astuta_, ileo-colic junction of, 212

  Batrachians, stomach of, 44, 46

  _Belone_, alimentary canal of, 40

  Biliary ducts in _Accipenser_, 145
      in Amphibians, 145
      arrangement of, 145
      in birds, 145
      in _Buceros_, 145
      in calf, 145
      in dog, 145
      in _Galeopithecus_, 145
      in _Lophius_, 145
      in _Lutra_, 145
      in Monotremes, 145
      in _Phoca_, 145
      in Reptilia, 145
      in sheep, 145
      in _Tarsius_, 145
      in _Trigla_, 145
      in _Xiphias_, 145

  Birds, folds of intestine in, 196
    glandular stomach of, 50
    biliary ducts in, 145
    ileo-colic junction of, 203
    muscular stomach of, 50
    venous system of, 161

  Blastoderm, 20
    layers of, 21
    primitive, 20

  Blastodermic vesicle, 20

  Blastomeres, 20

  Blastula, 20

  Blastosphere, 20

  Body-cavity, development of, 21
      primitive condition of, 29

  Body-wall, 22

  _Boselaphus tragocamelus_, ileo-colic junction and caecum of, 210

  _Bos indicus_, ileo-colic junction and caecum of, 210
      spiral colon of, 233

  _Bradypus marmoratus_, ileo-colic junction of, 208
      stomach of, 51

  Brunner's glands, 194

  _Buceros_, biliary ducts in, 145

  Bursa epiploica in lower forms, 187


  Caeca of the anthropoidea, compared with the human, 247

  Caecal gastric appendices of _Dicotyles_, 48

  Caecum and appendix, changes in position during development, 239
      development of, 237
      morphology of, 237
      variations of, 244
    descent of, 76, 243
    of embryo, shape of, 245
    first appearance in human embryo of, 53
    function of, 219
    non-descent in adult, 75
    persistent subhepatic position in adult, 75
    in the Rodentia, 229
    shape of, and origin of appendix, 245
    types of, 245
    in the Ungulata, 229

  Calf, biliary ducts in, 145

  Camel, gastric water-cells, 49

  Canal, medullary, 21, 28
    neuro-enteric, 23

  _Canis familiaris_, ileo-colic junction and caecum of, 212

  _Capra aegagrus_, ileo-colic junction and caecum of, 209

  Cardiac vein in Anure Amphibian, 158

  Cardinal veins, anterior, 147
      posterior, 147

  Carnivora, gastric diverticula of, 48
    ileo-colic junction of, 212
    stomach of, 46, 47

  Carnivorous birds, stomach of, 50

  _Casuarius_, duodenum, biliary and pancreatic ducts of, 115
    intestinal villi of, 195

  _Castor fiber_, ileo-colic junction and caecum of, 211
    stomach of, 46

  Cat, development of pancreas, 115
    dorsal mesogastrium, spleen and pancreas, 126
    lesser peritoneal sac, 128
    spleen, pancreas and great omentum, 127

  Caudal artery, 64
    vein in Selachian, 154
    in Urodele Amphibian, 156

  Caudate lobe, 170

  Cebidae, ileo-colic junction of, 214

  _Cebus leucophaeus_, ileo-colic junction and caecum of, 216
    _monachus_, ileo-colic junction and caecum of, 216

  Cell-body, 19

  Cellulae coli, 199

  _Ceratodus_, spiral intestinal valve in, 119

  _Cercoleptes caudivolvulus_, ileo-colic junction of, 212

  _Cercopithecus campbellii_, ileo-colic junction and caecum of, 214
    _pogonias_, ileo-colic junction and caecum of, 214
    _sabaeus_, ileo-caecal folds of, 264
      ileo-colic junction and caecum of, 214

  _Cervicapra_, intestinal folds of, 196

  _Cervus sika_, ileo-colic junction and caecum of, 210
      spiral colon of, 233

  Cetacea, ileo-colic junction of, 209

  Cetaceans, stomach of, 49

  Changes in position during development of caecum and appendix, 239

  Cheek pouches, 48
      of _Macacus nemestrinus_, 48

  Cheiroptera, ileo-colic junction of, 212

  Chelonians, liver of, 144
    stomach of, 45, 46

  _Chelydra_, alimentary canal of, 58
    pancreas of, 117
    _serpentaria_, ileo-colic junction of, 201

  Chick, development of liver in, 143
    development of pancreas in, 115

  _Chlamydophorus_, ileo-colic junction of, 207

  _Choloepus didactylus_, ileo-colic junction of, 207

  _Chrysothrix sciureus_, ileo-colic junction and caecum of, 214

  Cleft, uro-genital, 27

  Cloaca, development of, 24
    division of, in higher vertebrates, 27
    in human embryos, 26
    in _Platypus anatinus_, 26
    structure of, in lower vertebrates, 25
      in _Iguana tuberculata_, 25

  Cloacal membrane, 24
      anal segment, 28
      uro-genital segment, 28

  Coeliac axis, 65

  Coelom, composition and derivation of walls, 29
    development of, 21
    primitive condition of, 29

  Colic bend of the Manidae, 234
    loop in _Phascolarctos_, 234

  Colico-phrenic ligament, 109

  Colon, ascending, adhesions of, 81
      position of, in foetus, 84
    and caecum of _Lagomys pusillus_, 232
    descending, adhesion of, 81
      relation of, to left kidney, 83
    position as influenced by foetal liver, 77
    spiral coil of, 233
    structural modifications of, 230

  _Coluber natrix_, stomach of, 44

  Common bile duct, 145

  Comparative anatomy of hepatic venous circulation, 154
      of liver, 144

  Comparison of human and anthropoid caeca, 247

  Connective tissue and muscular fibers, derivation of, 30

  Coprodaeum, 25

  Coronary artery, 181
    ligaments of liver, 173

  _Corvus_, caeca of, 203

  Costo-colic ligament, 109

  Crocodiles, stomach of, 46, 51

  Crop, 48

  _Cryptobranchus alleghaniensis_, ileo-colic junction of, 201

  Cyclostomata, divisions of alimentary canal of, 40, 42
    intestinal canal of, 191
    spiral intestinal valve of, 119

  _Cyclothurus didactylus_, ileo-colic junction and caeca of, 207

  _Cyclura teres_, ileo-colic junction and caecum of, 202

  _Cynocephalus anubis_, ileo-colic junction and caecum of, 214
    _babuin_, ileo-colic junction and caecum of, 214
    _porcarius_, ileo-colic junction and caecum of, 214
    _sphinx_, ileo-colic junction and caecum of, 214

  Cynoidea, ileo-colic junction of, 212

  Cynomorpha, ileo-colic junction of, 213

  _Cyprini_, stomach of, 44

  Cystic duct, 146
      development of, 142

  Cysto-enteric duct, 145


  _Dasyprocta agouti_, ileo-colic junction and caecum of, 211
      spiral colon of, 234

  _Dasypus sexcinctus_, ileo-colic junction and caeca of, 207

  _Dasyurus viverinus_, ileo-colic junction of, 206

  Descent of caecum, 243

  Derivatives of entodermal intestinal tube, 34

  Deuteroplasm, 19

  Development of caecum and appendix, 237
    of cystic duct, 142
    of gall-bladder, 142
    of liver, 141
      in amniota, 143
      in chick, 143
      in Elasmobranchs, 143
      in Teleosts, 143
    of portal circulation, 147
    of spiral colon, 233
    of transverse colon, 244
    of vascular system of liver, 145

  _Dicotyles_, caecal gastric appendices of, 48
    _torquatus_, ileo-colic junction and caecum of, 209

  _Didelphis_, ileo-caecal folds of, 263
    _virginiana_, ileo-colic junction and caecum of, 205

  Digitiform gland of Selachians, 201

  Dipnoeans, intestinal canal of, 191
    spiral intestinal valve in, 119

  Diverticulum caecum vitelli in birds, 35
      in _Urinator imber_, 35
        _lumme_, 35
    vateri, 114

  Dog, biliary ducts in, 145

  Dorsal mesentery, early condition and derivation, 32
      in lower vertebrates, 32
      smooth muscular fiber of, 33
    mesogastrium, area of adhesion to parietal peritoneum, 106
      developmental changes in direction and extent, 103
      definition of, 100, 101
      gastro-splenic segment, 108
      redundant omental growth, 105
      spleen and pancreas in cat, 126
      vertebro-splenic segment, 108
    vascular ileo-caecal fold, 262
    fold, influence on shape of appendix, 271

  Double caecal pouches of birds, 203

  Ducts of Cuvier, 147
      in Selachian, 155
      in Urodele amphibian, 156
    omphalo-mesenteric, 22
    of Santorini, 111
    vitello-intestinal, 22
    of Wirsung, 111
      development of, 112

  Ductus venosus, 149
      changes after birth in, 152

  Duodenal antrum, 194
    fold of cat, 92
      of _Hapale vulgaris_, 93
      inferior, 95
      of _Nasua rufa_, 92
      superior, 95
    fossae, 92
      superior, 94
      vascular relations, 95, 96
    loop, 54

  Duodeno-colic neck, 57

  Duodeno-jejunal fossa in the cat, 93

  Duodenum, adhesion of, 67
    development of, 53
    peritoneal relations of infra-colic segment, 81
      of supra-colic segment, 81
    suspensory muscle of, 33
    with biliary and pancreatic ducts, of _Casuarius_, 115


  _Echidna hystrix_, ileo-colic junction and caecum of, 204

  _Echelus conger_, alimentary canal of, 54
      ileo-colic junction of, 200
      intestinal mucosa of, 197
    endgut of, 199
    pyloric appendix of, 120

  Ectoderm, 21

  Edentata, ileo-colic junction of, 206
    types of ileo-colic junction and caecum in, 218

  Egg, development of, 20
    structure of, 19

  Elasmobranchs, development of liver in, 143

  _Elephas indicus_, ileo-colic junction and caecum of, 210

  Embryonal intestinal hernia, 52

  Embryonic shield, 20

  Embryo, separation of, 20

  Endgut of _Echelus_, 199
    extent and contained segments, 38
    function of, 198
    in lower vertebrates, 199

  Enteric canal, primitive condition of, 29

  Entoderm, 21
    derivatives of, 28

  Entodermal intestinal tube, derivatives of, 34

  Epiblast, 21

  Epiploic bursa, 107
      early stages of, 104

  Epithelium of alimentary canal, derivation of, 30

  _Erethizon dorsatus_, ileo-colic junction and caecum of, 211

  _Esox_, alimentary canal of, 40

  _Eunectes marinus_, ileo-colic junction and caecum of, 203

  External iliac artery, 63
    perineal folds, 28


  Foetus at term, venous system of, 162

  Falciform ligament, as part of ventral mesogastrium, 165

  _Felis_, ileo-colic junction and caecum of, 212
    _leo_, ileo-colic junction and caecum of, 212

  Fish, development of pancreas, 115
    folds of intestine, 196
    ileo-colic junction of, 200

  Fissipedia, ileo-colic junction of, 212

  Fissure, transverse anal, 27

  Folds, ileo-caecal, 260

  Follicles, solitary, 196

  Foramen of Winslow, 174
      boundaries in adult human subject, 184
      caudal boundary, 178
      in lower mammals, 183
      relation to duodenal adhesion, 184
      in _Tamandua bivittata_, 183

  Foregut, comparative anatomy of, 42
    divisions of, 191
    extent and contained segments, 38

  Formative yolk, 19

  Fossa duodeno-jejunalis, 96
    ileo-caecal, 260
    intersigmoidea, 97
    of Treitz, 92, 96

  Function of caecum, 219
    of pyloric appendices, 221
    of pyloric caeca, 221
    of spiral fold of intestinal mucous membrane, 220

  Furrow, primitive intestinal, 22


  _Gadus callarias_, ileo-colic junction of, 201
    pyloric appendices in, 120

  _Galeopithecus_, biliary ducts in, 145
    ileo-colic junction and caecum of, 213

  Gall-bladder, development of, 142
      occurrence of, 144

  Gastric diverticula of Carnivora, 48
      of Herbivora, 48
      of Omnivora, 48

  Gastro-hepatic omentum, as part of ventral mesogastrium, 165

  Gastro-splenic omentum, 109

  Genito-urinary sinus, 27
      tract, male, in _Platypus anatinus_, 26

  Germinal area, 20
    membrane, 20
    spot, 19
    vesicle, 19

  Glands of Lieberkuehn, 194

  Glandular stomach of birds, 50

  _Gobius_, stomach of, 45

  _Gorilla savagei_, ileo-colic junction and caecum of, 216

  Graafian follicle, 19

  Greater curvature, first appearance of, 41

  Groove, medullary, 21
    primitive intestinal, 22


  _Halicore_, ileo-colic junction of, 208

  _Halmaturus derbyanus_, ileo-caecal folds of, 263
      ileo-colic junction and caecum of, 205
      stomach of, 47

  _Hapale jacchus_, ileo-colic junction and caecum of, 214
    _vulgaris_, duodenal fold, 93

  _Heloderma suspectum_, ileo-colic junction of, 211

  Hepatic antrum of lesser sac, 170
    artery, 65
      development of, 179
      in relation to foramen of Winslow, 180
      relation to duodenal adhesion, 182
      relation to primitive dorsal mesentery, 182
    cylinders, 143
    ducts, 145
    flexure, formation of, 76
    recess of lesser sac, 177
    ridge, 142
    veins, 148
    venous circulation, comparative anatomy of, 154
      direction of current, 152
      summary of development, 153

  Hepatic-portal system in Selachian, 155
      in Urodele Amphibian, 157
    vein in _Iguana_, 160

  Hepato-cystic duct, 145

  Hepato-enteric duct, 145

  Herbivora, gastric diverticula of, 48
    stomach of, 46, 47

  Herbivorous birds, stomach of, 50

  Herons, caecum of, 204
    stomach of, 50

  _Hippopotamus_, ileo-colic junction of, 209

  Human caeca compared with those of the Anthropoidea, 247

  _Herpestes griseus_, ileo-colic junction and caecum of, 212
    _ichneumon_, ileo-colic junction and caecum of, 212

  _Hyaena striata_, ileo-colic junction and caecum of, 212

  _Hylobates hoolock_, ileo-colic junction and caecum of, 216

  Hypoblast, 21

  Hyracoidea, ileo-colic junction of, 210

  _Hyrax capensis_, ileo-colon, ileo-caecum and colic caeca of, 210
    large intestine and caeca of, 234


  Iguana, abdominal vein of, 160
    caecal pouch and valves of, 202
    hepatic-portal vein of, 160
    post-cava of, 159
    renal-portal system of, 159
    sciatic vein of, 160
    segmental veins of, 161
    _tuberculata_, ileo-colic junction and caecum of, 201
      cloaca in, 25
    ventral mesogastrium of, 166

  Ileo-caecal folds, aberrant types of, 276
      of the anthropoid apes, 274
      of _Ateles_, 261
      of _Cercopithecus sabaeus_, 264
      of _Didelphis_, 263
      and fossae, 260
      of _Halmaturus derbyanus_, 263
      of _Mycetes fuscus_, 264
      smooth muscular fibers of, 33
    fossa, anterior, 267
      posterior, 271
    fossae, aberrant types of, 276

  Ileo-colic artery, 262
    junction of _Accipenser sturio_, 201
      of _Alligator mississippiensis_, 201
      of Amphibia, 201
      of the Arctoidea, 212
      of the Ailuroidea, 212
      of _Arvicola pennsylvanicus_, 211
      of the Anthropoidea, 213
      of the Anthropomorpha, 216
      of the Artiodactyla, 209
      of the Arctopithecini, 214
      of _Arctopithecus marmoratus_, 208
      of _Ateles ater_, 214
      of _Bassaris astuta_, 212
      in birds, 203
      of _Boselaphus tragocamelus_, 210
      of _Bos indicus_, 210
      in the Carnivora, 212
      of _Canis familiaris_, 212
      of _Capra aegagrus_, 209
      in cases of arrested intestinal rotation, 241
      of _Castor fiber_, 211
      of the Cebidae, 214
      of _Cebus_, 215
        _leucophaeus_, 216
        _monachus_, 216
      of _Cercoleptes caudivolvulus_, 212
      of _Cercopithecus campbellii_, 214
        _pogonias_, 214
        _sabaeus_, 214
      of _Cervus sika_, 210
      of the Cetacea, 209
      of _Chlamydophorus_, 207
      of Cheiroptera, 212
      of _Chelydra serpentaria_, 201
      of _Choloepus didactylus_, 207
      of _Chrysothrix sciureus_, 214
      of _Corvus_, 203
      of _Cryptobranchus alleghaniensis_, 201
      of _Cyclothurus didactylus_, 207
      of _Cyclura teres_, 202
      of _Cynocephalus_, 213
        _anubis_, 214
        _babuin_, 214
        _porcarius_, 214
        _sphinx_, 214
      of the Cynoidea, 212
      of the Cynomorpha, 213
      of _Dasyprocta agouti_, 211
      of _Dasypus sexcinctus_, 207
      of _Dasyurus viverinus_, 206
      of _Dicotyles torquatus_, 209
      of _Didelphis virginiana_, 205
      of _Echelus conger_, 200
      of _Echidna hystrix_, 204
      of the Edentata, 206
      effect of rotation on position of, 59
      of _Elephas indicus_, 210
      of _Erethizon dorsatus_, 211
      of _Eunectes marinus_, 203
      of _Felis_, 212
        leo, 212
      in fish, 200
      of the Fissipedia, 212
      of _Gadus callarias_, 201
      of _Galeopithecus_, 213
      of _Gorilla savagei_, 216
      of _Halicore_, 208
      of _Halmaturus derbyanus_, 205
      of _Hapale jacchus_, 214
      of _Heloderma suspectum_, 204
      of the herons, 204
      of _Herpestes ichneumon_, 212
      of _Hippopotamus_, 209
      of _Hyaena striata_, 212
      of _Hylobates hoolock_, 216
      of _Hyracoidea_, 210
      of _Hyrax capensis_, 210
      of _Iguana tuberculata_, 202
      of the Insectivora, 213
      of _Lagothrix humboldtii_, 215
      of Lamellirostra, 203
      of _Lemur macaco_, 213
        _mongoz_, 213
      of the Lemuroidea, 213
      of _Lepus cuniculus_, 211
      of _Macacus_, 214
        _cynomolgus_, 214
        _ochreatus_, 214
        _pileatus_, 214
        _rhesus_, 214
      of mammalia, 204
      of _Manatus americanus_, 208
      of _Manis longicauda_, 208
      of Marsupialia, 204
      of Monotremata, 204
      of _Midas geoffrei_, 214
        _ursulus_, 214
      of _Monodon_, 209
      of _Mustela_, 212
      of _Mycetes cabaya_, 214
        _fuscus_, 215
      of _Myoxus_, 211, 212
      of _Myrmecophaga jubata_, 207
      of _Nasua rufa_, 212
      of _Necturus maculatus_, 201
      non-vascular serous folds, 262
      of _Nycticebus tardigradus_, 213
      of _Nyctipithecus commersonii_, 214
      of _Ornithorhynchus anatinus_, 204
      of _Orycteropus_, 208
      of _Oryx leucoryx_, 210
      of _Otolicnus crassicaudatus_, 273
      of _Paradoxurus typus_, 212
      of _Perameles nasuta_, 206
      of the Perissodactyla, 210
      of _Phascolarctos cinereus_, 205
      of _Phascolomys wombat_, 206
      of _Phocaena_, 209
      of _Phoca vitulina_, 212
      of _Physeter_, 209
      of the Pinnipedia, 212
      of the piscivorous divers, 203
      of _Pithecia satanas_, 215
      of _Pleuronectes maculatus_, 201
      of the Primates, 213
      of the Proboscidea, 210
      of _Proteles lalandii_, 212
      of _Pseudemys elegans_, 201
      of _Pteropus medius_, 212
      of _Rana catesbiana_, 201
      of the Ratitae, 203
      of Reptilia, 201
      of the Rodentia, 211
      serial review in Vertebrata, 200
      of the Sirenia, 208
      of _Simia satyrus_, 216
      of _Strix_, 203
      of _Struthio africanus_, 204
      of _Sus scrofa_, 209
      asymmetrical type, 223
      symmetrical type, 221
      of _Tamandua bivittata_, 208
      of _Tapirus americanus_, 210
      of _Tarsius spectrum_, 213
      of _Tatusia novemcincta_, 207
      of _Taxidea americana_, 212
      of _Tolypeutes_, 207
      of _Trichosurus vulpinus_, 205
      of _Troglodytes niger_, 217
      types of, and caecum, 217
        in Edentata, 218
        in Marsupialia, 218
      of _Ursus_, 212
      of the Ungulata, 209
      of _Vulpes fulvus_, 212
      vascular mesenteric folds of, 262
      of _Xenurus_, 207
      of _Zalophus gillespiei_, 212

  Iliac vein in Urodele Amphibian, 157

  Inferior mesenteric artery, 67

  Infra-colic compartment, secondary parietal peritoneum of, 85, 86

  Insectivora, ileo-colic junction of, 213

  Intermediate duodenal fold, 96
    non-vascular ileo-caecal fold, 262

  Internal iliac artery, 63
    perineal folds, 27

  Intestinal blood vessels, effect of intestinal rotation on, 59
    canal of _Amphioxus_, 191
      of Amphibians, 191
      of Cyclostomata, 191
      diverticula, 193
      of Dipnoeans, 191
      of Teleosts, 191
      folds of mucosa, 193
      non-differentiated, of lower vertebrates, 191
    folds in Amphibia, 196
      in birds, 195
      in fish, 196
    furrow, primitive, 22
    glandular apparatus in lower vertebrates, 195
    groove, primitive, 22
    juice, function of, 194
    mucous membrane of _Cervicapra_, 196
      of _Echelus conger_, 197
      _Lophius_, 197
      lymphoid tissue, 196
      of _Phocaena_, 196
      of _Thalassochelys_, 197
    rotation, 58
      arrest of development, 60
      demonstration in cat, 67
    spiral fold, function of, 193
    vascular supply, 63
      in cases of non-rotation, 67
    villi of Carnivora, 195
      of _Casuarius_, 195
      of Ophidia, 195
      of _Ursus maritimus_, 195

  Intestine in early human embryo, 52
    general consideration of, 51
    large, and caeca of _Hyrax_, 234
      functions of, 198
      length of, 199
      of monkeys, 199
      of rodents, 199
      width of, 199
    small, 192
      absorbing apparatus, 195
      divisions of, 194
      length of, 192
      secretory apparatus, 194
      structure of, 194
      villi, 195

  Isthmus, duodeno-colic, 57


  Jejuno-ileum, development of, 54


  _Labrus_, stomach of, 44

  _Lagomys pusillus_, colon and caecum of, 232

  _Lagothrix humboldtii_, ileo-colic junction and caecum of, 215

  Lamellirostra, ileo-colic junction and caeca of, 203

  Lateral vein in Selachian, 155

  _Lemur macaco_, ileo-colic junction and caecum of, 213
    _mongoz_, ileo-colic junction and caecum of, 213

  Lemuroidea, ileo-colic junction of, 213

  _Lepus cuniculus_, ileo-colic junction and caecum of, 211
      saccus lymphaticus of, 211

  Lesser curvature, first appearance of, 41

  Ligament, colico-lienale, 109
    of ductus venosus, 152
    gastro-lienale, 110
    lieno-renale, 109
    phrenico-lienale, 109

  Ligamenta coli, 199

  Liver in Chelonians, 144
    comparative anatomy of, 144
    derivation of, 34
    development of, 141
    function of, 195
    lobation of, 144
    in Ophidia, 144
    peritoneal lines of reflection in embryo, 167
      in foetus at term, 171
    peritoneal relations of, 167
    of _Petromyzon_, 141
    unilobar type, 144

  _Lophius_, biliary ducts in, 145
    _piscatorius_, mucosa of midgut, 197
    pyloric appendices in, 120
    stomach of, 46

  Lungs, derivation of, 34

  _Lutra_, biliary ducts in, 145
    stomach of, 48

  Lymphoid tissue of intestinal mucosa, 196


  _Macacus cynomolgus_, ileo-colic junction and caecum of, 214
    descending mesocolon of, 140
    lesser omentum of, 176
    mesosigmoidea in, 140
    _nemestrinus_, cheek-pouches of, 48
    _ochreatus_, ileo-colic junction and caecum of, 214
     peritoneum of infra-colic compartment, 136
    _pileatus_, ileo-colic junction and caecum of, 214
    relations of spleen and great omentum, 139
    _rhesus_, abdominal viscera, 77
      ileo-colic junction and caecum of, 214

  Mammalia, ileo-colic junction of, 204
    pancreatic ducts in, 117

  _Manatus americanus_, ileo-colic junction and bifid caecum of, 208
      stomach of, 48

  _Manis longicauda_, ileo-colic junction of, 208

  Manidae, colic bend of, 234

  Marsupalia, ileo-colic junction of, 204
    types of ileo-colic junction and caecum in, 218

  Meckel's diverticulum, 35
    serous folds connected with, 262, 263

  Medullary canal, 21
    groove, 21
    plates, 21

  Membrane, cloacal, 24

  Mesenchyma, 30

  Mesenteric peritoneum, definition of, 32

  Mesentery, absorption of, 33
    definition of, 101
    jejuno-ileal, line of attachment, 86
    of umbilical loop, development of, 56
      relation to adult mesocolon and mesentery, 72

  Mesoblast, 21

  Mesocola in cat, 86

  Mesocolic fossa, 97

  Mesocolon, ascending, adhesion of, 82
      in monkeys, 83
      relation of, to right kidney, 83
    definition of, 101
    descending, adhesion of, 82
      in foetus, 83
      line of attachment of, 83
      in lower mammals, 83
      in _Macacus_, 140
      in monkeys, 83
    transverse, root of, 85

  Mesoderm, 21
    derivatives of, 28

  Mesoduodenum, adhesion of, 67
    definition of, 101

  Mesorectum, definition of, 101

  Mesosigmoidea, definition of, 101
    in _Macacus_, 140

  Mesothelium, 21

  Metanephros, 24

  _Midas geoffrei_, ileo-colic junction and caecum of, 214
    _ursulus_, ileo-colic junction and caecum of, 214

  Midgut, 192
    extent of, 38

  _Monodon_, ileo-colic junction of, 209

  Monotremata, ileo-colic junction of, 204

  Monotreme, structure of penis, 26

  Monotremes, biliary ducts in, 145

  Morphology, general, of vertebrate intestine, 190
    of human caecum and appendix, 237

  Morula, 20

  _Moschus_, stomach of, 49

  Muscular stomach of birds, 50

  Musculo-cutaneous vein in Anure Amphibian, 158

  _Mustela_, ileo-colic junction of, 212

  _Mycetes cobaya_, ileo-colic junction and caecum of, 214
    _fuscus_, ileo-caecal folds of, 264
      ileo-colic junction and caecum of, 215

  _Myoxus_, alimentary canal of, 211, 212
    stomach of, 46

  _Myrmecophaga jubata_, ileo-colic junction of, 207

  _Myxinoids_, pancreas in, 117


  _Nasua rufa_, duodenal fold of, 92
      ileo-colic junction of, 212
      pancreatico-gastric folds of, 181

  _Necturus_, alimentary canal of, 40
    _maculatus_, alimentary tract of, 52
      ileo-colic junction of, 201
      stomach of, 43
      venous system of, 158

  Neuro-enteric canal, 23

  Non-vascular ileo-caecal folds, 262

  Nucleolus, 19

  Nucleus, 19

  Nutritive yolk, 19

  _Nycticebus tardigradus_, ileo-colic junction and caecum of, 213
      spiral colon of, 234

  _Nyctipithecus commersonii_, ileo-colic junction and caecum of, 214


  OEsophageal gutter of ruminants, 49

  OEsophageo-gastric junction, 45

  Omega loop, development of, 77

  Omental bursa, 107
      early stages of, 104

  Omentum, great, 107
      layers of, 107
      peritoneal adhesions in adult, 131
      relation of, to transverse colon, transverse colon and duodenum,
        129
    lesser, as part of ventral mesogastrium, 165
      divisions of, 172
      in _Macacus_, 176

  Omnivora, gastric diverticula of, 48

  Omphalo-mesenteric arteries, 64, 146
    artery, persistence of rudiments of, 65
    duct, 22
    veins, 146

  Ophidia, intestinal villi of, 195
    liver in, 144
    stomach of, 44, 46

  Oral pouches, 48

  _Ornithorhynchus anatinus_, ileo-colic junction and caecum of, 204

  _Orycteropus_, ileo-colic junction and caecum of, 208

  _Oryx leucoryx_, ileo-colic junction and caecum of, 210
    spiral colon of, 233

  _Otolicnus crassicaudatus_, ileo-colic junction and caecum of, 213

  _Ovis aries_, spiral colon of, 233

  Ovum, structure of, 19

  Owl, stomach of, 50


  Pancreas, adhesion of, 67
    adhesion of mesoduodenal segment, 123
    in _Ammocoetes_, 117
    in Chelydra, 117
    comparative anatomy of, 116
    concealed, of Teleosts, 117
    derivation of, 34
    development of, in Amphibia, 115
      of, in cat, 115
      of, in chick, 115
      of, in fish, 115
      of, in lower vertebrates, 115
      of, in man, 111, 115
      of, in sheep, 115
    in foetal pig, 123
    function of, 195
    in _Myxinoids_, 117
    peritoneal relations, 122
      vascular and visceral relations of adult, 125
    in _Protopterus_, 117
    relation to dorsal mesogastrium, 123
      to mesoduodenum, 122
      to omental bursa, 123
    in Selachians, 116

  Pancreatic ducts in Mammalia, 117
      normal arrangement, 113
      secondary, 111
      variations, 114

  Pancreatico-gastric folds in man, 187
      in _Nasua rufa_, 181

  Papilla Vateri, 114

  _Paradoxurus typus_, ileo-colic junction of, 212

  _Paralichthys_, pyloric appendices in, 120

  Parietal mesoderm, 21
    peritoneum, definition of, 32

  _Pelamys_, pyloric appendices in, 120

  Pelvic vein in Anure Amphibian, 158

  _Perameles nasuta_, ileo-colic junction and caecum of, 206

  _Perca_, pyloric appendices in, 120

  Perennibranchiates, stomach of, 44, 46

  Perissodactyla, ileo-colic junction of, 210

  Peritoneal cavity, lesser, summary and development of structure, 180
    relations and position of appendix, 250
      of appendix, variations of, 258
    sac, lesser, of cat, 128

  Peritoneum, arrangement in infra-colic compartment, 78
    of cat, compared with human arrangement, 73
    of infra-colic compartment in adult human subject, 88
    lesser cavity of, in relation to liver, 174
    of liver, in relation to lesser sac, 174
    secondary lines of reflection, 73
    of supra-colic compartment, general considerations, 99

  _Petromyzon_, alimentary canal of, 200
    liver of, 141
    spiral intestinal valve of, 119

  Peyer's patches, 196

  _Phascolarctos cinereus_, ileo-colic junction and caecum of, 205
    colic loop in, 234

  _Phascolomys wombat_, ileo-colic junction, caecum and appendix of, 206

  _Pteropus medius_, ileo-colic junction of, 212

  _Phoca_, biliary ducts in, 145
    _vitulina_, ileo-colic junction and caecum of, 212
      stomach of, 45

  _Phocaena communis_, ileo-colic junction of, 209
    intestinal folds, 196

  Phylogeny of types of ileo-colic junction and caecum, 217

  _Physeter_, ileo-colic junction of, 209

  Physiology of vertebrate intestine, 190

  Pickerel, pyloric valve of, 45
    stomach of, 44

  Pinnipedia, ileo-colic junction of, 212

  _Pipa_, stomach of, 46

  Piscivorous divers, caeca of, 203

  _Pithecia satanas_, ileo-colic junction and caecum of, 215

  Placental circulation, 146

  Placentalia, definition of, 36

  Plates, medullary, 21, 28

  _Platypus anatinus_, male genito-urinary tract and cloaca, 26

  _Pleuronectes maculatus_, ileo-colic junction of, 201
    pyloric appendices in, 120

  Pleuro-peritoneal cavity, 28

  Plicae coli, 199

  _Polypterus_, pyloric appendix in, 120

  Portal circulation, development of, 147
    vein, development of, 148

  Position and peritoneal relations of appendix, 250

  Post-anal gut, 23

  Post-cardinal veins in Urodele Amphibian, 157

  Post-cava in Anure Amphibian, 158
    in _Iguana_, 159
    in Urodele Amphibian, 157

  Post-caval vein, 151

  Pre-cava in Anure Amphibian, 158

  Primates, ileo-colic junction of, 213
    types of ileo-caecal folds in, 265

  Primitive aortae, 63
    common dorsal mesentery, 33
    dorsal mesentery after rotation, 79
    mesentery, effect of intestinal rotation on, 59
      mesenteric segment, 72
      mesocolic segment, 72
    jugular veins, 147

  Proboscidea, ileo-colic junction of, 210

  Proctodaeum, 24, 26
    in human embryos, 27

  Protoplasm, 19

  _Protopterus_, pancreas in, 117

  _Proteus_, alimentary canal of, 40
    _anguineus_, stomach of, 43

  _Proteles lalandii_, ileo-colic junction and caecum of, 212

  Proventriculus, 50

  Psalterium, 49

  _Pseudemys elegans_, alimentary canal of, 55
      ileo-colic junction and caecum of, 201

  Pyloric appendices, 119
      function of, 221
      in _Accipenser_, 120
      in _Gadus_, 120
      in _Lophius_, 120
      in _Paralichthys_, 120
      in _Pelamys_, 120
      in _Perca_, 120
      in _Pleuronectes_, 120
      in _Rhombus_, 120
      in _Scomber_, 120
      in _Thynnus_, 120
      relation to pancreas, 121
      significance of, 120
    appendix in _Ammodytes_, 120
      in _Echelus_, 120
      in _Polypterus_, 120
    caeca, 119
      function of, 221
    stomach, 50
    valve, 44
      in fishes, 45
      of loon, 45


  _Rana_, alimentary canal of, 55
    _catesbiana_, ileo-colic junction of, 201
    _esculenta_, venous system of, 158

  Ratitae, ileo-colic junction and caeca of, 203

  Rectal gland of Selachians, 201

  Rectangular ileo-colic junction, 225

  Recto-coccygeal muscles, 33

  Recto-uterine muscles, 33

  Rectum, development of, 54
    separation from genito-urinary sinus, 27

  Renal-portal circulation in Urodele Amphibian, 156
      system in Selachian, 154
        in _Iguana_, 159

  Reptilia, abdominal vein, 167
    biliary ducts in, 145
    ileo-colic junction of, 201

  Retro-gastric peritoneal space, boundaries of, 175
    space, rudimentary form of, 105

  Retro-peritoneal hernia, 92

  Reticulum, 49

  _Rhombus_, pyloric appendices in, 120

  Rodentia, caecal pouch of, 229
    compound stomach of, 49
    ileo-colic junction of, 211
    spiral colic valve of, 231

  Rodents, saccus lymphaticus of, 196

  Round ligament of liver, 152

  Rumen, 49

  Ruminantia, structure of stomach in, 49


  Saccus lymphaticus of _Lepus cuniculus_, 211
      of Rodents, 196

  _Salamandra maculosa_, venous system of, 158

  Salivary glands, derivation of, 34

  Saurians, stomach of, 44, 46

  Sciatic vein in _Iguana_, 160

  _Scincus ocellatus_, stomach of, 45

  _Scomber_, pyloric appendices in, 120

  Segmental veins in _Iguana_, 160

  Segmentation, 20

  Segmentation-cavity, 20

  Selachian, caudal vein in, 154
    digitiform gland of, 201
    duct of Cuvier, 155
    hepatic portal system of, 155
    lateral vein of, 155
    pancreas in, 116
    rectal gland of, 201
    renal portal system of, 154
    spiral intestinal valve in, 119
    venous system, 154

  _Semnopithecus_, stomach of, 47

  Septum urogenitale, 27
    transversum, 142

  Serous folds in cases of Meckel's diverticulum, 262, 263
    membrane, derivation of, 31

  Shape of caecum and origin of appendix, 245
    of embryonic caecum, 245

  Sheep, biliary ducts in, 145
    development of pancreas, 115

  Sigmoid flexure, development of, 54, 77

  _Simia satyrus_, ileo-colic junction and caecum of, 216

  Sinus venosus, 146

  Sirenia, ileo-colic junction of, 208

  Soft palate, 42

  Somatic mesoderm, 21

  Somatopleure, 22, 29

  Spigelian lobe, boundaries of, 170
      development of, 169
    recess of lesser sac, 177

  Spiral coil of colon, 233
    colic valve of Rodentia, 231
    colon of _Bos indicus_, 233
      of _Cervus sika_, 233
      of _Dasyprocta agouti_, 234
      development of, 233
      of _Nycticebus tardigradus_, 234
      of _Oryx leucoryx_, 233
      of _Ovis aries_, 233
    fold of intestinal mucous membrane, function of, 220
    intestinal valve in _Ceratodus_, 119
      in Cyclostomata, 119
      in Dipnoeans, 119
      in _Petromyzon_, 119
    valve of gastric diverticulum in _Sus_, 48

  Splanchnic mesoderm, 21

  Splanchnopleure, 22, 29

  Spleen, development and relation to dorsal mesogastrium, 108
    and great omentum in _Macacus_, 139
    pancreas and great omentum in cat, 127
    peritoneal relations, 110
    vascular connections, 108

  Splenic artery, 65, 108
    flexure, development of, 54, 76
    vessels, peritoneal relations, 109

  Stomach of _Alligator_, 51
    assumption of special functions modifying form of, 48
    of _Anguilla anguilla_, 47
    of Batrachians, 44, 46
    of _Bradypus_, 51
    caecal diverticula of, 47
    of Carnivora, 46, 47
    of carnivore birds, 50
    of _Castor_, 46
    cellular structures connected with, 47
    of Cetaceans, 49
    changes in position during development, 102
    of Chelonians, 45, 46
    of _Coluber natrix_, 44
    comparative anatomy of, 42
    of Crocodiles, 46, 51
    of the _Cyprini_, 44
    definition of, as segment of foregut, 43
    embryonic borders and surfaces, 41
    factors modifying form of, 43
    first differentiation in human embryos, 40
    further development in human embryos, 40
    glandular, of birds, 46
    of _Gobius_, 45
    of _Halmaturus_, 47
    of heron, 50
    of Herbivora, 46, 47
    of herbivorous birds, 50
    influence of habitual amount of food on form of, 44
      of size and shape of abdominal cavity on form of, 46
      of volume and character of food on form of, 46
    of Teleosts, 46, 47
    of _Labrus_, 44
    of _Lophius_, 46
    of _Lutra_, 48
    of _Manatus americanus_, 48
    of _Moschus_, 49
    masticating surfaces of, 48
    of _Myoxus_, 46
    of _Necturus maculatus_, 43
    of Ophidia, 44, 46
    of owl, 50
    of Perennibranchiates, 44, 46
    of _Phoca vitulina_, 45
    of the pickerels, 44
    of _Pipa_, 46
    of _Proteus anguineus_, 43
    relation to vagus nerve, 43
    ruminant type of, 43
    of Saurians, 44, 46
    of _Scincus ocellatus_, 45
    of _Semnopithecus_, 47
    storage compartments of, 48
    structural modifications of, increasing action of gastric juice, 46
    of _Tamandua_, 51
    transverse position of, 45
    type-form of, 43

  Stomadaeum, 24
    in human embryos, 27

  _Strix_, caeca of, 203

  Structural modifications of colon, 230

  _Struthio africanus_, ileo-colic junction and caeca of, 204

  Sturgeon, pyloric valve of, 45

  Subintestinal veins, 147

  Submucosa, derivation of, 30

  Superior mesenteric artery, 64, 65
      relation to umbilical loop, 66

  Suspensory ligament of liver, derived from ventral mesogastrium, 165

  _Sus scrofa_, ileo-colic junction and caecum of, 209
    spiral valve of gastric diverticulum in, 48

  Symmetrical type of ileo-colic junction, 221


  Taenia coli-, 199

  _Tamandua_, alimentary tract of, 56
    _bivittata_, foramen of Winslow in, 183
      ileo-colic junction and caeca of, 208
    stomach of, 51

  _Tapirus americanus_, ileo-colic junction and caecum of, 210

  _Tarsius_, biliary ducts in, 145
    _spectrum_, ileo-colic junction and caecum of, 213

  _Taxidea americana_, ileo-colic junction of, 212

  _Tatusia novemcincta_, ileo-colic junction of, 207

  Teleosts, anal and genito-urinary orifices in, 25
    concealed pancreas of, 117
    development of liver in, 143
    gastric diverticula of, 47
    intestinal canal of, 191
    stomach of, 46, 47
    without pyloric appendices, 120

  _Thalassochelys_, intestinal folds of, 197

  Thymus, derivation of, 34

  _Thynnus_, pyloric appendices in, 120

  Thyroid, derivation of, 34

  _Tolypeutes_, ileo-colic junction of, 207

  Transverse anal fissure, 27
    colon, development of, 54, 244
      differentiation of, 76
    mesocolon, development of, 80

  _Trichosurus vulpinus_, ileo-colic junction and caecum of, 205

  _Trigla_, biliary ducts in, 145

  _Troglodytes niger_, ileo-colic junction and caecum of, 217

  Types of ileo-caecal folds in Primates, 265
    ileo-colic junction and caecum, phylogeny of, 217


  Umbilical arteries, 63
    hernia of embryo, 52
    loop, derivation of adult intestinal segments from, 53
      divisions of, 52
      of embryonic intestine, 52
      relation of vitello-intestinal duct to, 52
    veins, 147
      changes after birth in, 152
      final arrangement, 151
      further changes in, 149
      intra-hepatic distribution, 152
    vesicle, 22

  Umbilicus, 21

  Ungulata, caecal pouch of, 229
    ileo-colic junction of, 209

  Urinary bladder, in human embryos, 27
      relation to allantois, 24

  _Urinator imber_, diverticulum caecum vitelli, 35
    _lumme_, diverticulum caecum vitelli, 35
    pyloric valve of, 45

  Urodaeum, 25

  Urodele Amphibian, abdominal vein in, 157
      caudal vein in, 156
      ducts of Cuvier in, 156
      hepatic-portal system of, 157
      iliac vein in, 157
      post-cardinal veins in, 157
      post-cava in, 157
      renal-portal system in, 156
      venous system of, 156

  Uro-genital cleft, 27

  _Ursus_, ileo-colic junction of, 212
    _maritimus_, intestinal villi, 195

  Uvula, 42


  Vagus, gastric distribution of, 43

  Valves of Kerkring, 196, 197

  Valvulae conniventes, 196, 197

  Variations of caecum and appendix, 244
    in the peritoneal relations of the appendix, 258

  Vasa intestini tenuis, 66

  Vascular mesenteric folds of ileo-colic junction, 262
    system of liver, development of, 145

  Vein, portal, development of, 148

  Veins, anterior cardinal, 147
    hepatic, 148
    omphalo-mesenteric, 146
    posterior cardinal, 147
    primitive jugular, 147
    subintestinal, 147
    umbilical, 147
    vitelline, 146
    hepaticae advehentes, 147
      revehentes, 147

  Venous system in Anure Amphibian, 158
      of bird, 161
      of human foetus at term, 162
      of _Necturus maculatus_, 158
      of _Rana esculenta_, 158
      in Selachian, 154
      of _Salamandra maculosa_, 158
      of Urodele Amphibian, 156

  Ventral mesentery, early condition and derivation, 31
    mesogastrium, 163
      in _Iguana_, 166
      and liver, 140
      relation to duodenum, 164
      relation to liver, 105
      to umbilical vein, 165
    vascular ileo-caecal fold, 262

  Vertebrate intestine, general morphology and physiology, 190

  Visceral mesoderm, 21
    peritoneum, definition of, 32

  Vitelline arteries, 64, 146
    membrane, 19
    sac, 20
    veins, 146
      anastomosis of, 147

  Vitello-intestinal duct, 22

  Vitellus, 19

  _Vulpes fulvus_, ileo-colic junction and caecum of, 212


  Water-cells of camel's stomach, 49

  Wolffian duct, relation to primitive intestine, 24


  _Xenurus_, ileo-colic junction of, 207

  _Xiphias_, biliary ducts in, 145


  Yolk, 19

  Yolk-sac, 20


  _Zalophus gillespiei_, ileo-colic junction and caecum of, 212

  Zona pellucida, 19







End of the Project Gutenberg EBook of The Anatomy of the Human Peritoneum
and Abdominal Cavity, by George. S. Huntington

*** 