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+*** START OF THE PROJECT GUTENBERG EBOOK 43350 ***
+
+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.
+
+The abbreviation viz. appears inconsistently in both roman and italic
+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).
+
+The numbering system and font styling of the TOC is not consistent
+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 colored 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_, negro 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 retard 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 retard 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 (negro). 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
+
+*** END OF THE PROJECT GUTENBERG EBOOK 43350 ***