The Anatomy of the Human Peritoneum and Abdominal Cavity Considered from the Standpoint of Development and Comparative Anatomy

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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 th