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 cæcum 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.

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 zoölogical 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).

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).

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]).

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.

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

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.)

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 cœlom cavity. Transverse section. × 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 cœlom or body-cavity, which becomes subdivided in the adult mammal into the pleural, pericardial and abdominal cavities.

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

The development of the cœlom 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).

Fig. 19.—Transverse section of a seventeen and a half day sheep embryo. (Bonnet.)

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 provertebræ.

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).

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 (proctodæum) (Fig. 21), is smaller than the oral (stomadæum) ([Fig. 27]), but the intestinal tube forms an extensive pouch in the anal region which descends to meet the ectodermal invagination of the proctodæum. 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.

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).

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]).

Fig. 23.—Genito-urinary tract and cloaca of Iguana tuberculata, ♀. (Columbia University Museum, No. 1846.)

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 cloacæ). 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.

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 (coprodæum) from a lower space (urodæum) which receives in its dorsal wall the openings of the two oviducts and immediately above them—upon two papillæ—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.

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 coprodæum.

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

Fig. 28.—Reconstruction of caudal end of human embryo of 11.5 mm. (four and a half weeks), showing pelvic structures. × 40. (After Keibel.)

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 vertebræ, 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 proctodæum, which grows as a funnel-shaped fossa toward the blind termination of the endgut. In embryos of 25 mm. cervico-coccygeal measure (8½-9 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 stomadæum.

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 proctodæum.

Fig. 31.—Section of pelvis of human fœtus, showing atresia recti. (Esmarch.)

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 proctodæum 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.

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 cœlom. 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 cœlom 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 cœlom. 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 cœlom. 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 cœlom the great serous membranes of the adult, the pleuræ, pericardium and peritoneum, are developed from it.

Fig. 32.—Schematic diagrams, illustrating the vertebral mesentery. A. earlier; B. later condition. (Minot.)

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 cœlom of one side communicates ventrad of the intestine with the cœlom 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 cœlom 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.

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 cœlom 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 præaortal tissue around the origin of the superior mesenteric artery and cœliac 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-cæcal 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.

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 stomadæal 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, œsophagus, 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.

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 cæcal pouch (diverticulum cæcum 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.

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.

Fig. 41.—Diagram illustrating the later stages in the formation of the mammalian fœtal membranes. (Heisler, modified from Roule.)

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.

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 fœtal 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 fœtus 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.

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,		Connected by their ducts with the digestive canal.
The liver,
The pancreas,
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.

Fig. 42.—Genito-urinary tract and cloaca of Iguana tuberculata, ♀. (Columbia University Museum, No. 1846.)

(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.)

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 œsophagus.
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 œsophagus 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 œsophagus—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 œsophagus, 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]).

A distinct digestive segment may even be entirely wanting, owing to its failure to differentiate from the œsophagus 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.

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 œsophagus. The appearance of this dilatation marks the separation of the proximal cephalic part (pharynx and œsophagus) 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).

Fig. 46.—Alimentary canal of human embryo of 5 mm. × 15. (Reconstruction after His.)

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.

It will, however, be noted that the œsophageal 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 œsophageal 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 œsophageal 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.

Fig. 47.—Gallus canis, dog-shark, ♂. 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.)

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.

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 Ammocœtes 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 “œsophagus.” This is followed by a slightly dilated segment, the “stomach,” into which a blind pouch enters. This cæcal pouch is usually considered as a hepatic diverticulum ([Fig. 49]).

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 fœtal 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 fœtal period it can be designated as the fœtal 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.

Fig. 52.—Alimentary ca­nal of Coluber natrix. (Nuhn.)

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 fœtal 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 œsophagus 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.

This transition of the fœtal 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 œsophageal extremity of the stomach is less well differentiated from the afferent tube of the œsophagus.

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

(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]).

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.

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).

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.

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.

Fig. 69.—Stomach of Phalacrocorax dilophus, double-crested cormorant; section. (Columbia University Museum, No. 67/1804.)

(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 œsophagus 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.

(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).

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.

Fig. 72.—Alimentary canal of Anguilla anguilla, eel. (Columbia University Museum, No. 1271.)

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 cæcal gastric pouch lies in the long axis of the body, opposite the entrance of the œsophagus. A marked example of this arrangement is seen in the stomach of the eel, Anguilla anguilla (Fig. 72).

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 œsophageal 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.

In some forms, as the pig, the left extremity of the stomach carries a cæcal appendix with a spiral valve in the interior separating its lumen from the general gastric cavity (Fig. 78). Others have two such cæcal appendices added to the left end of the stomach (Peccary, Fig. 79). These cæcal 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]).

Fig. 79.—Stomach of Dicotyles torquatus, peccary. The fundus is a capacious pouch prolonged ventrally and dorsally into two cæcal 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 œsophagus, 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.

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]).

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 œsophageal 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.

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).

Fig. 86.—Stomach of Urinator imber, red-throated loon. (Columbia University Museum, No. 1808.)

Fig. 87.—Scheme of stomach of granivorous bird. (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 œsophagus, forming the Proventriculus or glandular stomach.

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

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).

Fig. 92.—Stomach of crocodile. (Nuhn.)

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).

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.

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.

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 œsophagus.

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 œsophageal gutter directly with the œsophagus. 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.

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 cæcum and ileo-cæcal junction, and the development of the accessory digestive glands, liver and pancreas, by budding from the proximal portion of the primitive entodermal intestinal tube.

Fig. 97.—Alimentary canal of human embryo of 5 mm. × 15. (Reconstruction after His.)

Fig. 98.—Schema of human embryonic intestinal canal, with intestinal umbilical loop, but before differentiation of the large and small intestine.

Fig. 99.—Viscera of Necturus maculatus, mud-puppy, in situ. (Columbia University Museum, No. 1175.)

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.”

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.

The stomach is tubular, not distinctly differentiated from the œsophagus, 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.