MEDICAL .SClnl®©L Gift Of- Lane iviedical Library t^-t-/ ^,i!,^>6c<..tz^Xx: Digitized by the Internet Archive in 2007 with funding from IVIicrosoft Corporation http://www.archive.org/details/anathumaperitoneumOOhuntrich THE ANATOMY HUMAN PERITONEUM AND ABDOMINAL CAVITY CONSIDERED FROM THE STANDPOINT OF DEVELOPMENT AND COMPARATIVE ANATOMY BY GEORGE S. gUNTINGTON, M.A., M.D. PROFBSSOR OF ANATOMY, COLLEGE OF PHYSICIANS AND SURGEONS, COLOMBIA UNIVERSITY, NEW YORK CITY ILLUSTRATED WITH 300 FULL-PAGE PLATES CONTAINING 682 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 \ 903. PREFACE. In the following pages an attempt has been made to empha- size 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 func- tion, 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 Univer- sity. 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 illustra- tions 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 Photog- raphy and to Mr. M. Petersen, artist of the Anatomical Depart- ment of the University, for their skilful and thoroughly reliable work in the preparation of the illustrations. George S. Huntington. Columbia Univkesity, in the City of New York, December, 190S. 3424 CONTENTS. Page, Introduction. . . . 17 Development of Vertebrate Ovum ........ 19 Development of Ccelom and of Alimentary Canal 21 Development of Cloaca 24 Development and Divisions of the Peritoneum 82 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 Intes- tinal Canal 58 Development of Aortal Arterial System 63 II. Demonstration of Intestinal Rotation in the Lower Mammalia . 67 Peritoneal and Visceral Relations in the Infracolic Compartment of the Abdominal Cavity in the Adult 74 Part II. Anatomy of the Peritoneum in the Supracolic Compart- ment 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 . Ill . 115 . 116 . 119 . 122 2. Development of Pancreas .... Development of Pancreas in Lower Vertebrates Comparative Anatomy of Pancreas . Pyloric Cseca or Appendices .... Peritoneal Relations of Pancreas 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 . 2. Relation of Great Omentum to Transverse Colon, Transverse Mesocolon and Third Part of Duodenum Ventral Mesogastrium and Liver I. A. Development of Liver B. Comparative Anatomy of Liver. . . . . . C. Development of Vascular System of Liver Comparative Anatomy of the Hepatic Venous Circulation 127 129 140 141 144 145 154 Vi CONTENTS. Page. II. Ventral Mesogastrium 163 Peritoneal Relations of Liver 167 Eelation of Hepatic Peritoneum to the " Lesser Sac " . . . 174 Caudal Boundary of Foramen of Winslow 178 Pancreatico-gastric Folds 181 Part HI. Large and Small Intestine, Ileo-colic Junction and C^cum. 189 I. General Review of Morphology and Physiology of the Verte- brate 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 Valvules 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. Phylogbny of the Types of Ileo-colic Junction and Cjecum in the Vertebrate Series 217 1. Symmetrical Form of Ileo-colic Juncton ; 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 Structure of C^cal 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 Csecal Apparatus . . . 230 Part IV. Morphology of the Human C^cum and Vermiform Appendix. 237 I. Development of the Csecum and Appendix 237 CONTENTS. VU Page. 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 in. 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 a Ileo-Csecal Folds and Fossae 260 PLATE I. ZONA PELLUCIDA CLEAR LAYER EPITHELIUM OF FOLLICLE GERMINAL VESICLE GERMINAL SPOT FIBRILLAR LAYER Fig. 1. — Human ovum, from a mature follicle, a sphere of about 0.2 mm. diameter. X 25. (KoUmann.) THE SMALLER BLASTOMERE ZONA PELLUCIDA THE LARGER BLASTOMERE 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. Fig. 3. — Segmentation of mammalian ovum. Fig. 4. — Ovum of rabbit, from terminal por- Four Vjlastomeres. (After E. von Beneden.) tion of oviduct. The zona pellucida appears thickened, and contains many spermatozoa which failed to penetrate the ovum. (After Bischofl".) f PLATE II. ALBUMINOUS ENVELOPE GERMINAL MEMBRANE OR BLASTODERM ZONA PELLUCIDA GERMINAL AREA CAVITY OF BLASTOSPHERC Fig. 5. — Blastodermic vesicle of rabbit. (After E. von Beneden.) SEGMENTATION CAVITY MARGINAL ZONE OF GERMINAL MEMBRANE INNER CELL-MASS FORMING GERMINAL AREA Fig. 6. — Blastodermic vesicle of Triton tseniatus. (Hertwig.) PRIMITIVE STREAK HEAD PROCESS NEURENTERIC CANAL Fig. 7. — Embryonic ana of rabbit embryo. (Heis- ler, after E. von Beuedeu.) The primitive streak begin- ning m the cell-proliferation known as the "node of Hensen." RAUBCR'S PROCHORION OR COVER-LAYER ZONA FCLUJCJOA Fig. 8. — Blastodermic vesicle of mammal. (E. von Bene- den.) The layer of cells lining the interior of the vesicle next to the zona pellucida forms Rauber's " Deckschichte " or pro- chorion. This is not the true ectoderm, since it does not par- ticipate in the formation of the embryo, which is entirely derived from the cells of the germinal area. ATTACHMENT OF AMNION AND CHORION DRAWN OUT TO A TIP ALLANTOIC STALK CAUDAL END OF EMBRYO PRIMITIVE SEGMENT. VITELLINE BLOOD-VESSELS YOLK-SAC VISCERAL ARCHES Fig. 9. — Human embryo with yolk-sac, amnion, and belly-stalk of fifteen to eighteen days. (Heisler, after Coste.) Fig. 1 0. — Embryonal area of sheep, composed of ectoderm and ento- derm. (After Bonnet.) ECTODERM MESODERM ENTODERM Fig. 11. — Blastodermic vesicle of rabbit. Section through embry- onic area at caudal limit of node of Hensen. (Rabl. ) i PLATE IV. WALL OF BLASTODERNirC VESICLE AREA PELLUCIDA EMBRYONAL SHIELD Fig. 12. — Oval oinbiyDiiic area of rabbit's egg, detached with part of wall of blastodermic vesicle. X 30. (Kollmann.) PARIETAL LAYER OF MESODERM ECTODERM PRIMITIVE GROOVE MESODERM BEGINNING OF AMNIOTIC FOLD PRIMITIVE CCZLOM VISCERAL LAYER OF MESODERM PRIMITIVE STREAK ENTODERMAL LINING OF ENTODERM INTESTINAL FURROW Fig. 13.— Transverse section of embryonic area of ovum of sheep of fourteen and a half days. (Heisler, after Bonnet.) WALL OF BLASTODERMIC VESICLE TERMINAL KNOB AREA OF MEDULLARY FOLDS PRIMITIVE FURROW Fig. 14. — Germinal luva i>l rabbit's ovum. (Kollmann.) PLATE V. MEDULLARY FOLOS I I MEDULLARY GROOVE PRIMITIVE STREAK AND GROOVE Fig. 15. — Surface-view of area pellucida of an eighteen-hour chick -embryo. (Balfour.) MEDULLARY AMNIOTIC MESODERM AMNIOTIC ECTODERM YOLK-SAC MESODERM ECTODERM MESODri:RM MESOUERMAL. CLEFT ENTODERM Fig. 16. — Transverse section of human embryo before develop- ment of protovertebrae or chorda dorsalis. (Keibel.) i AMNION MEDULLARY FURROW MESODERM PARIETAL MESODERM VISCERAL MESODERM PLEURO-PERICARDIAL CAVITY ENTODCRMAL LINING OF HEART \ PRIMITIVE HEAD-GUT PLATE PRIMITIVE ENDOCARDIUM EXTENSION OF CCELOM Fig. 17. — Transverse section of a sixteen and a half day sheep embryo. (Heisler, after Bonnet. PLATE VI. mi;dullary tube PROTOVERTEBR* WOLFFIAN DUCT PARIETAL MESODERM UNITING WITH ECTO- DERM TO FORM SOMATOPLEURE ENTODERM PARAXIAL MESODERM VISCERAL MESODERM UNITING WITH ENTO- DERM TO FORM SPLANCHNOPLEURE Fig. 18. — Embryo of bird, at begiiniiug of third day, with four blasto- dermic layers, resulting from the division of the mesoderm into parietal and visceral layers, separated by the ccelom cavity. Transverse section, X 170. (Kollmann.) SOMITE OR LATERAL PROTOVERTEBRA ZONE AXIAL ZONE MEDULLARY TUBE CAVITY WITHIN SOMITE ECTODERM LATERAL PLATES FOR BODY-WALL LATERAL PLATES FOR INTESTINAL CANAL PARIETAL MESODERM FUSED WITH ECTO- DERM PLEURO- PERITONEAL CAVITY VISCERAL MESODERM UNITED TO ENTO- DERM -VITELLINE VEIN INTESTINAL ENTODERM PRIMITIVE AORTiE Fig. 19.— Transverse section of a seventeen and a half day sheep embryo. (Bonnet.) PLATE VII. MEDULLARY PLATES PARIETAL. MESODERM VISCERAL MESODERM ENTODERM MEDULLARY GROOVE INTESTINAL TUBE YOLK-SAC Fig. 20. — Curves of blastodermic layers and division of mesoderm in amniote embryo. (KoUmaun.) CLOACA CLOACAL MEMBRANE. PROCTODCEAL INVAGINATION OF ECTODERM :ntestine CANAL OF ALLANTOIS CAUDAL END OF EMBRYO POST-ANAL GUT CHORDA DORSALIS MEDULLARY CANAL Fig. 21. — Sagittal section of caudal extremity of cat embryo of 6 mm. (Tourneux.) ECTODERM MEDULLARY FOLDS PRIMITIVE FURROW CLEFT OF MESODERM MEDULLARY GROOVE NEURO-ENTERIC CANAL PRIMITIVE RIDGES ALLANTOIC STALK Fig. 22. — Caudal half of human blastoderm measuring 3 mm., with open medullarj- groove. Dorsal view. X 30. (After Spee.) PLATE VIII. PROBE IN ABDOM- INAL OPENING OF RIGHT OVIDUCT CUT EDGE OF RIGHT MESOVARIUIM INTESTINE CLOACAL ORIFICE OF BLADDER CLOACAL ORIFICES OF OVIDUCTS LEFT OVARY URINARY PAPILLA WITH PROBES IN URETERAL 0R:FICES Fig. 23.— Goiiito-urinary tract and cloaca of /(/((rtim ^Hfte/'CK^rt^rt, 9. (Columbia University Museum, No. 1846.) 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 illus- trated 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 mammaUan ahmentary tract. For the sake of clearness and brevity all discussion of the theories! of peritoneal development has been designedly omitted. The as- sumption of peritoneal adhesion, and consequent obliteration of serous areas, offers many advantages in considering the adult human abdominal cavity, especially from the standpoint of com- parative 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 embry- ological 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 en- tirely. Many of the diagrams combine several successive devel- opmental stages, showing different degrees of development in different portions of the same drawing. Again it is frequently 2 17 18 * ABDOMINAL CAVITY AND PERITONEUM. necessary, for the sake of brevity and clearness, to actually depart from known embryological conditions. If, for example, the stom- ach 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 refer- ence 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 oppor- tunity 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 developniental 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 struc- tural conditions is immaterial. He can draw on all the verte- brate 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 illus- trate the significance of the complicated anatomical details pre- sented by the adult human abdominal cavity by reference to the simpler antecedent conditions encountered during the early de- INTRODUCTION. 19 velopmental stages of the higher forms and permanently in the structure of the lower vertebrates. While, as just stated, a complete presentation of the develop- ment 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 accom- plished by a very brief and condensed consideration of the car- dinal 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 dif- ferent component parts of the egg-cell have received special dis- tinctive 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 rela- tive 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 indi- vidual are derived, while the deuteroplasm serves for the nutri- tion of the ovum during the earHest stages of development. The nucleus of the egg-cell is distinguished as the germinal ves- icle, 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 pelludda, presenting a radially striated appearance. The ovum is contained in the cortical portion of the ovary, en- closed 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. 20 ABDOMINAL CAVITY AND PERITONEUM. 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 re- sulting from this process of segmentation forms the ground work of the future body. A vertebrate ovum in this stage of com- plete 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 blasto- dermic vesicle and the cellular membrane enclosing the segmenta- tion-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 blasto- sphere) 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 dis- appears. Transverse sections at right angles to the long axis of the em- br3'onic 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 — INTRODUCTION. 21 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.) The following changes in the ventral aspect lead to the forma- tion 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 lin- ing them become distinctly epithelial. This mesodermic epithe- lium 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 22 ABDOMINAL CAVITY AND PERITONEUM. 19). The former is closely applied to the ectoderm, forming with it the somatopleure or hody-wall. The latter, in close con- tact 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). 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 in- testine 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, pro- ceeding 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 somato- pleural 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 splanchopleural 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 ap- pears as a nearly straight cylindrical tube extending from the INTRODUCTION. 23 cephalic to the caudal end of the embryo. This primitive alimen- tary 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 provertebrse. 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 ento- derm. 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 aper- ture 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 aUmentary canal is called the ''post-anal gut" (Fig. 21). The post-anal gut and the neuro-enteric canal are better de- veloped 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 phylo- genetic history, they have lost among living vertebrates all physi- ological 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, how- ever, the ectoderm becomes invaginated at both extremities and 24 ABDOMINAL CAVITY AND PERITONEUM. finally perforates into the lumen of the intestine, thus establish- ing the oral and anal communications with the exterior. The anal ectodermal invagination (proctodseum) (Fig. 21), is smaller than the oral (stomadseum) (Fig. 27), but the intestinal tube forms an extensive pouch in the anal region which descends to meet the ectodermal invagination of the proctodseum. The de- tails 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 develop- ment 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 intes- tine receives firom above the stalk of the allantois, while the Wolffian duct, the canal of the embryonic excretory appa- ratus, 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 lin- ing of the blind pouch of the caudal gut. Ectoderm and en- toderm 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 en- larges 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). PLATE IX. ENTRANCE OF INTESTINE INTO CLOACA UPPER COMPARTMENT OF CLOACA (cOPROD/EUm) OPENINGS OF URETERS INTO CENTRAL COMPART- MENT OF CLOACA (uROD£UM) LOWER COMPARTMENT OF CLOACA (proctod;eum) ABDOMINAL OSTIUM OF OVIDUCT Fig. 24. — Geuito-urinary tract and cloaca of the hen, Gallus bankiva. versity Museum, No. 1208.) CLOACAL OPENING OF OVIDUCT INTO URODiEUM (Columbia Uni- PLATE X. VAS DEFERENS EPIDIDYMIS GENITO-UHINARY SINUS _PENIS ENVELOPED IN FIBROUS SHEATH CLOACA FORMED CONFLUENCE OF GENITO-URINARY SINUS AND RECTU OPENING IN VENTRAL —WALL OF CLOACA FOR EVERSION OF PENIS V'V CUM Fig. 31.— Section of pelvis of human foetus, showing atresia recti. (Esraarch.) PLATE XIII. MEDULLARY TUBE CHORDA DORSALIS MESOTHELIUM ENTODERMAL TUBE OF ALIMENTARY CANAL Fig. 32.— Schematic diaKnuiis. illustrating the vertebral nuseutery. A, ear- lier; B, later condition. (Minot.) MIDDLE LOBE OF THYROID GLAND LATERAL LOBE OF THYROID GLAND TRACHEA LUNG RIGHT LOBE OF LIVER OMPHALO- MESENTERIC DUCT SALIVARY GLANDS PHARYNGEAL POUCHES THYMUS GLAND STOMACH PANCREAS LEFT LOBE OF LIVER SMALL INTESTINE LARGE INTESTINE Fig. 33. — Schema of alimentary canal and accessory organs, derived from same. (After Bonnet.) YPOPHYSIS ALLANTOtS WOLFFIAN DUCT Fig. 34. — Eeconstruction of alimentary canal of human embryo of 4.2 mm. X 24. (After His.) PLATE XIV. CESOPHAGUS TRACHEA AND LUNG PANCREAS WOLFFIAN DUCT PRIMITIVE LARYNX PITUITARY FOSSA HEPATIC DUCT VITELLINE DUCT ALLANTOIS END-GUT Fig. 35.— Reconstruction of alimentary canal of human embrvo of 7 mm ("twentv- eight days). X 12. (After His.) • v J- CESOPHAGUS TRACHEA STOMACH PANCREAS- PITUITARY FOSSA TONGUE HEPATIC DUCT C>ECUM GENITAL EMINENCE ANAL INVAGINATION CAUDAL END OF EMBRYO five £ys (fc^ mm"''x t" (iS^'r ""' '' '""'" '""'"'" "' ""''^' PLATE XV. SEROUS NON-VASCU- LAR FOLD BETWEEN ILEUM AND DIVER- TICULUM ROOT OF MECKEL'S DIVERTICULUM OMPHALO -MESEN- TERIC ARTERY EX- TENDING UPON DIVERTICULUM Fig. 37. — Human adult ileum with Meckel's diverticulum. Ileo-diverticnlar serous fold and persistent omphalo-mesenteric artery. (Columbia University Museum, No. 1803.) Fig. 38. — Human adult ileum, with Meckel's diverticulum. Museum, No. 745.) (Columbia University O u S O > X w h Pu INTRODUCTION. 25 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 sur- rounded 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-uri- nary 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 compart- ment of the cloaca (coprodasum) from a lower space (urodseum) 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 open- ing of the right oviduct — by dividing the mesovarian peritoneal fold. 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 ahmentary canal, in entering the cloaca, forms an expanded upper cloacal compartment for the ac- cumulation of the excreta, called the coprodseum. 26 ABDOMINAL CAVITY AND PERITONEUM. It is separated by a prominent mucous fold from the central compartment, or urodxum which receives the terminations of the two ureters and of the single (left) oviduct. A second fold forms the distal limit of the urodseum and separates it from the lowest cloacal compartment, the prododseum. 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 em- bryos 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 en- trance into the cloaca of the intestine proper and of the allantoic stalk (urinary bladder). The growth of the mesoderm pushes INTRODUCTION. 27 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 perfora- tion is preceded by the formation of a transverse ectodermal redu- plication, producing a depression called the transverse anal fissure. This depression increases in depth until a distinct anal invagi- nation results, known as the proctodxum, which grows as a funnel- shaped fossa toward the blind termination of the endgut. In embryos of 25 mm. cervico-coccygeal measure (8i-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 forma- tion of the oral cavity by means of the ectodermal invagination called the stomadxum. 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 prolifer- ates 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 28 ABDOMINAL CAVITY AND PERITONEUM. 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 per- forated 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 proctodseum. These developmental stages in the formation of the end-gut are of importance because they offer the explanation of the patho- logical 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 mal- formation are observed. The layer separating the skin from the blind end of the rectum may be so thin that the meconium con- tained 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 me- chanical conditions which the above-outlined course of develop- ment 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 INTRODUCTION. 29 closure of the medullary plates and inclusion of the neural tul)e 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, Uke the two remaining layers of the blastoderm, the meso- derm splits early on each side of the chorda dorsalis into two layers, including between them spaces which after coalescence form the 'primitive 'pleuro-j)erit(me(il 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 sfplanchruypleure. 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-caWty (except in the region of the ventral mesogaa- trium). The splanchnopleure is not so wide as the somatopleure. As it closes in the ventral median line it includes the deepest or ento- dermal layer. It now forms a tube whose walls are composed superficially of mesoderm (splanchnopleure) while the lumen is lined by epitheUnm 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 ccslom. The walls of this space are largely made up of the skeletal and muscular ele- ments developed firom the mesoderm of the somatopleure, cov- 30 . ABDOMINAL CAVITY AND PERITONEUM, ered superficially by the common ectodermal investment of the body. It will be seen that the enteric tube thus becomes in- cluded 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 Kning of the coelom. The cells of the mesen- chyma, connected wath each other and with the mesothelial cells by protoplasmic processes, are not as close together as in an epi- thelium 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 meso- thelium and the ectoderm forms the mass of the skeletal, muscu- lar 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 en- tire 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 connec- tive tissue of the splanchnic mesoderm appears as an intermediate layer uniting the epithelial lining and the muscular walls. Situ- ated thus between the mucous and muscular coats of the intes- tine 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 INTRODUCTION. 31 mesoderm is the production of the serous membrane investing the body cavity and its contents from the mesotheUum Uning the primitive coelom. This mesotheUal 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 i^ubsequent 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 splanchno- pleure 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 me- sentery caudad of this point establishes the condition indicated in Fig. 32, B. The entodermal tube and the surrounding splanch- nic 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 32 ABDOMINAL CAVITY AND PERITONEUM. 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 meso- derm 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 con- tained 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 di- rection 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 abdom- inal 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 INTRODUCTION. 33 smooth muscular fibers derived from the mesoderm. These bands of smooth muscle fibers are also encountered, though less well de- veloped, 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 prseaortal 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-csecal junction (Luschka) and in the mesorectum — forrding 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 mesen- tery are derived, by further development, displacement and adhe- sion, 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 sus- pended by the mesentery and freely movable within the ab- dominal cavity, become later, by secondary adhesion, firmly con- nected 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 con- nection with the parietes being afforded by the blood vessels. This condition depends upon absorption of the original mesentery. 8 34 ABDOMINAL CA VITT AND PERITONEUM. A similar process, though much more circumscribed, is observed in the omenta of many mammals, which appear perforated at sev- eral points. Derivatives of the Entodennal 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 intes- tinal 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, pro- ceeding 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 prim- itive entoderm of the intestinal tube, except the epithelium of the salivary glands, which, being derived from the stomadseal invagi- nation, 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 implanta- tion 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 INTRODUGTJON. 35 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 MeckeVs 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 oblit- eration of the intestinal lumen and consequent obstruction at this point. The intestinal opening of the diverticulum is situated at a vary- ing 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 csecal 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. In order to complete the embryological history of the alimen- tary canal it is necessary to take brief account of another struc- ture 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. 36 ABDOMINAL CAVITY AND PERITONEUM. In the embryo, however, it has important nutritive and respira- tory functions. In the embryos of the higher vertebrates nutri- tion depends only in the earhest stages upon the yolk-sac of the ovum, over which a vascular network extends. Very soon the caudal portion of the primitive intestine devel- ops 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 ^ it is large and grows around the embryo. In those of the higher vertebrates which are developed within an egg (rep- tiles, 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 be- comes attached by vascular villi to the uterine wall and estab- Hshes 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 Aplacenialia 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 : ' In the embryos of reptiles, birds and mammals folds of the somatoplenre arise exter- nally 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. INTRODUCTION. 37 1. Organs of the adult body, derived by budding from the ah- mentary entodermal epithelium, in the form of pouch-like diver- ticula which follow the glandular type of development and be- come 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, I Connected by their ducts with the diges- The pancreas, tive canal. The lungs, which open by means of the trachea and the laryngeal aper- ture into the pharyngeal cavum. (6) 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 con- stricted off from the yolk-sac. The reduced channel of communi- cation 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 re- mains 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 em- bryonic intestinal canal of the higher vertebrates, performing im- portant 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 38 ABDOMINAL CAVITY AND PERITONEUM. into the urinary bladder. In mammals it loses its original con- nection 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 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 sepa- rated from both by ring-like aggregations of the circular muscu- lar 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 appre- ciation of the disposition of the chief viscera within the abdomi- inal cavity and of their mutual relations. In the second place the visceral vascular supply of the abdomen must be carefully con- sidered 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 re- productive 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 oesoph- agus — 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 apparatuis proper, succeeding to the oesophagus, is usually divisible into two sections differing in function and struc- ture. 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. 39 40 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 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 extend- ing 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 rudi- mentary 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 examin- ing directly, as macroscopic objects, structural conditions which are found only as temporary embryonal stages during the develop- ment of the higher mammalia (Fig. 43). In the early stages the alimentary tract of the mammalian em- bryo 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. Subse- quently 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, By 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 en- todermal tube, placed in the median plane, dorsad of the embry- onic outgrowth of the liver, between it and the oesophagus. PLATE XVIIT PERICARDIUM GASTRIC DILATATION INTESTINAL CANAL WITH SPIRAL FOLD OF MUCOSA Fig. 44. — Schematic diagram representing tliree stages in the differentiation of the mam- malian digestive tract: A. Early undiiieren- tiated stage, in which the entire canal ai)]>ears as a tube of uniform calibre. 15. Spindle- shaped gastric dilatation. C. Typical mam- malian gastric dilatation. Fig. 43. — Entire alimentary canal of the lamjircy. Petromyzoii marinus, below the pericardium. (Columbia University Museum, No. 1575.) PLATE XIX. BILIARY DUCT. WOLFFIAN DUCT CESOPHAGUS LUNG STOMACH PANCREAS VITELLINE DUCT ALLANTOIS WOLFFIAN BODY Fig. 45. — Eecoustruction of human embryo. 1, 2, 3, 4, Gill-pouches. (After Fol.) PRIMORDIAL CRANIUM YOLK-SAC UMBILICAL CORD CAUDAL GUT RENAL BUD FORE-GUT BRANCHIAL CLEFTS LUNG BUD COMMON DORSAL MESENTERY WOLFFIAN BODY Fig. 46. — Alimentary canal of human embryo of 5 mm. X 15. (Eecoustruction after His.) PLATE XX. RECTAL GLAND BENT PROBE PASSED INTO I NTE ST INAL OPENING OF RECTAL GLAND FORE-GUT DIVIDED VAS DEFERENS END-GUT WITH SPIRAL MUCOUS FOLD ROD PASSED INTO CLOACAL OPENING OF ALIMENTARY CANAL PROBE PASSED INTO OPENING OF GENITO- URINARY PAPILLA PROBE IN ABDOM- INAL PORE Fig. 47. — Gallns cnnin, dog-shark, ^. Genitourinary tract and cloaca hi situ. The fore-gut has been divided just caudad of the communication with the oral cavity. (Columbia University Museum, No. 1694.) PLATE XXI. ■XSOPHAGUS -OESOPHAGUS SLIGHT GASTRIC DILATATION -MID-GUT U Fig. 48. — Alimentary caual of Belone, pickerel. (Nuhn.) Fig. 51. — Alimentary canal of Proteus an- guineus. (Nuhn.) ENTRANCE TO MOUTH WITH OVERLYING BUCCAL CIRRI ENDOSTYLE HEPATIC CiECUM GONADIC POUCHES STOMACH -STOMACH -INTESTINE Fig. 52. — Alimentary canal of Coluber natrix. (Nuhn.) , SPLEEN NTESTINE PYLORODUODENAL JUNCTION »^ — ANUS Fig. 49. — Amphioxnti, dissected from the ventral side. The relatively enor- mous pharvnx occupies more than half the length ol' the body. The walls are seiiarali'd by the gill-clefts, and the parallel gill-bars abut at the midveutral line on the endosli/le. (Willey, after Rathke.) PANCREAS — MID-GUT END-GUT Fig. 50. — Kecturus rnacnlatus, mud-puppy. Ali- mentary canal and appendages. (Columbia Uuiversity Museum, No. 1454.) PLATE XXII. • (ESOPHAGUS OeSO PHAGE O- GAS- TRIC JUNCTION GASTRIC MUCOUS MEMBRANE Fig. 53. — Human adult. Mucous surface of cesophageo-j trie junction. (Columbia University Museum, No. 1842.) GASTRIC MUCOUb MEMBRANE THICKENED CIRCULAR MUSCULAR FIBRES OF PYLORIC VALVE DUODENUM Fig. 54. — Human adult. Pyloro-duodenal junction and jiyloric valve in section. (Columbia University Museum, No. 1842.) PLATE XXIII. -ZSOPHAGUS INTESTINE Fig. 56. — Alimentary canal of Scincns ocellatus. Pyloric extremity of the slightly marked gastric dilatation presents an angular bend. (Nuhn.) INTESTINE XSOPHAGUS Fig. 57. — Alimentary canal of Gohius niger. (Nuhn.) 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, ga.stric surface. 3. Adult pyloric valve and gastro-duodenal junction in section. 4. Fcetal gastro-duodenal junction in section. Entrance of biliary and pancreatic ducts on summit of papilla of duodenum. (Columbia University Museum, No. 1S51.) PYLORUS OESOPHAGUS Fig. 5S. — Alimentary canal of shark. (Nuhn.) PLATE XXIV. DUODENUM XSOPHAGUS Fig. 59.— Stomacli of Phocavitulina, harbor seal. (Columbia University Museum, No. GOO.) DUODENUM OESOPHAGUS CARDIAC END OF STOMACH Fig. 60.— Stomach of PseHilenu/s elegau.% pond turtle. (Columbia University Museum, No. 1710.) ANATOMY. 41 Th6 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 dor- sad, becomes in turn concave (Fig. 44, 6^). 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 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 abdom- inal 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 alimen- tary canal appears as a proximal or cephalic oesophageal and a distal or caudal intestinal opening. 42 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. COMPARATIVE ANATOMY OF FOREGUT AND STOMACH. A serial review of this portion of the ahmentary tract in verte- brates 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 func- tion and of the physiological demands on the structure of this portion of the digestive tract is strikingly illustrated by the numer- ous 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 demarca- tion (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 persist- ence of the early embryonic pregastric stage of the higher types, before the simple alimentary tube is differentiated by the appear- ance of the distinct gastric dilatation. . In the Cydostomata (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 Ammocostes the foregut begins behind the wide branchial basket, dorsad of the heart, with a narrow entrance, which is suc- ceeded by a dilated segment. The entrance of the hepatic duct separates fore- and midgut. In Amphioxus the branchial pouch passes with a slight constric- tion directly into the gut which extends through the body-cavity in a straight Hne. The narrow segment is usually regarded as the "oesophagus." This is followed by a slightly dilated segment, the " stomach," STOMACH. 43 into which a blind pouch enters. This csecal 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 separa- tion of fore- and midgut. The stomach, when it develops, is situ- ated cephalad of the entrance of the hepatic duct into the intes- tine. 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 as- signed 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 ali- mentary tube of Proteus anguineus and Necturus maculatus (Figs. 50 and 51). Since this condition is common to all verte- brates in the earliest foetal period it can be designated as the foetal or primitive stomach form. All others appear as secondary de- rivatives from this typical early condition. The influences which bring about such derivations and modi- fications 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. 44 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 ver- tebrates 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 Inflnence 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 dila- tation of the alimentary canal. Hence in the higher vertebrates generally the stomach appears as a large and more sac-like dilata- tion 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. 62). 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 peren- nibranchiates 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 con- stitutes a slight advance in development over the earliest stage represented, as we have seen, by the nearly uniform and undif- ferentiated alimentary tube of amphioxus and the cyclostomata. This transition of the foetal form to the more advanced secon- dary types of the stomach is marked by the development of two improtant 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 projec- tion of the mucous membrane into the lumen of the canal. It STOMACH FORMS. 45 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 65.) 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 af- forded 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. There is no aggregation of muscular circular fibers in this situa- tion and no valve. Superficially the external longitudinal mus- cular 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. (b) The pyloric end of the stomach makes an angular bend, while the rest of the organ remains in the original vertical posi- tion in the long axis of the body. An example of this condition is presented by the stomach of Sdncus 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 transi- tory 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. 46 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 60). In others the change in position occurs after the gastric dilatation has assumed the sac-Hke 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 sub- stances 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 pur- pose 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 acces- sory 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 uni- versal position of the glandular stomach of birds (Fig. 69). In STOMACH FORMS. 4:1 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. (6) 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 in- creases 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 in- creased and the action of the gastric juice is prolonged and ren- dered more intense. Such modifications of the structure of the stomach are encoun- tered 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. 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). 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 trans- versely 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 48 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. mammalia permits of the transverse position of the entire stomach and of the development of the diverticulum from its left ex- tremity. 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 Latra (Fig. 63), the cul-de-sac is almost wanting. In some forms, as the pig, the left extremity of the stomach carries a csecal appendix with a spiral valve in the interior sepa- rating its lumen from the general gastric cavity (Fig. 78). Others have two such csecal appendices added to the left end of the stomach (Peccary, Fig. 79). These csecal 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). 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. In many mammals reservoirs of similar import are added di- PLATE XXV. DUODENUM CE80PHAGUS Fig. fil. — Stomach of Cheh/dm serpentinu, snapping turtle. (Colum- bia University Museum, No. i852.) CESOPHAGUS AT JUNCTION WITH C A R D I A OF STOMACH STOMACH - DUODENUM PYLORUS WITH THICKENED RING OF CIRCULAR MUS- CULAR FIBRES Fig. 62. — Same in section. PLATE XXVI. PYLORUS CESOPHAGUS CESOPHACUS Fig. 63. — Stomach of Lutra vulgaris, otter. (Nuhu.) Fig. 64. — Stomach of Eqmis cahallus, horse. (Nuhn.) PYLORIC CiECA PYLORUS (ESOPHAGUS Fig. 65. — Stomach o{ Lnphius piscatorhis, angler. (Nnhii.) OESOPHAGUS STOMACH Fig. 66. — Stomach of Pipa rentcosa. (Nuhu. CZSOPHAGUS PROVENTRICULUS FUNDUS OF STOMACH Fig. 67.— Stomach of (usior tihcr, ))faver. (Ntihii. PLATE XXVII. OECAL POUCHES CON- NECTED WITH STOM- ACH PYLORUS PYLORIC PORTION OF STOMACH (dices TIVE SEGMENT^ OESOPHAGUS PROVENTRIC- ULUS CARDIAC POR- TION OF STOMACH Fig. 68. — Stomach of Manatus americanns, manatee. (Nuhn.) PROYENTRICULUS OR GLANDULAR STOMACH DUODENUM PYLORUS ANO PYLORIC VALVE CESOPHAGUS VENTRICULUS OR MUSCU- LAR STOM- ACH Fig. (59. — Stomach of Phalacrocorax dilophiis, douhle-crested cormorant ; section. (Columbia University Museum, No. ifj^.) PLATE XXVIII. CESOPHAGUS DIVERTICULUM OF FUNDUS DUODENUM Fig. 70. — Stomach of Halmaturus derhyanus, rock kangaroo. (Colum- bia University Museum, No. 582.) INTERMEDIATE GAS- TRIC SEGMENT RE- SEMBLING HUMAN COLON IN STRUCT- URE SMOOTH-WALLED PY- LORIC SEGMENT SACCULATED DIVERTICULA OF FUNDUS Fig. 71.— Stomach of Semnopithems entellus, enteUus moukey. (Columbia University Museum, ^o. tI §5.) X^ m M "" oil 1 T STOMACH GASTRIC DIWER TICULUM Fig. 72. — Alimentary canal of Angiiilla angniUa, eel. (Colum- bia University Museum, No. 1271.) PLATE XXIX. CESOPHAGUS FUNDUS Fig. 73. — Stomach of Lepiis cunmihis, rabbit. (Nuhn.) Fig. 74. — Stomach of Nasua rufa, coati. (Nuhn.) (ESOPHAGUS Fig. 75. — Stomach of FeUs Jeo, lion. (Nuhn.) DUODENUM t* — CESOPHAGUS Fig. 7G.— Stomach of E.n,. University Museum, No. 358.) o, .v.,iv.ncan porcupine. (Columbia PLATE XXX. (ESOPHAGUS Fig. 77. — Stomach of Cercopithecus cephus, moustache mon- key. (Columbia University Museum, No. 158.) CESOPHAGUS CiCCAL APPENDIX OF FUNDUS Fig. 78. — Stomach of Sus serofa, pig. The fundus of the stomach car- ries a csecal appendage separated in the interior by a spiral fold of the mucous membrane from the gastric cavity. DORSAL C/ECAL POUCH OF FUN DUS STOMACH - PYLORIC^ antrum' ._ji^- CESOPHAGUS VENTRAL CjECAU POUCH OF FUN- DUS Fig. 79. — Stomach of DicolyJes forquatits, peccary. The fundus is a caiKK imis ikhkIi prolonged ventrally and dorsally into two caecal appendages resembling the single appendage of the pig's stomach. (Columbia University Museum, No. 1806.) PLATE XXXI. BUCCAL ORIFICE OF POUCH Fig. 80. — Macnens nemestrinns, pig-tail macaque monkey ; cheek-pouches. (From fresh dissection.) DUODENUM PYLORIC SEGMENT (digestive STO ACH proper) OESOPHAGUS CARDIAC PORTION OF STOMACH DE- VOID OF GASTRIC GLANDS AND FORMING A STOR- AGE CHAMBER Fig. 81. — Stomach of Cricetus vulgaris, hamster. (Nuhn.) PLATE XXXII. CEOSPHAGUS RETICULUM PSALTERIUM ABOMASUS Fig. 82. — Stomach of Ovis aries, sheep. (Columbia University Museum, No. 1807.) CESOPHAGUS 3D STOMACH (psalter lUM' 2d stomach (reticulum) 4th stomach (abomasus or digestive stom- ACH proper) DUODENUM 1st stomach (rumen) Fig. 83. — Scheme of ruminant compound stomach. (Nuhn. PLATE XXXIII. Fig. 84. — Mucous niembraue of stomach of Camelns dioniedarius, dromedary, showing •water- celts. (Cohimbia University Museum, No. 1123.) DUODENUM : — (XSOPHAGUS FiQ. 85. — Stomach of PAoca?Hfl, porpoise. (Xuhn.) PLATE XXXIV. DUODENUM CESOPHAGUS PROVENTRICULUS OR GLANDULAR STOMACH TENDON-PLATE OF MUSCULAR STOM- ACH MUSCULAR STOM- ACH OR VENTRIC- ULUS Fig. 86. — Stomach of I'rinatorimher, red-throated loon. (Columbia Uuiversity Museum, No. 1808.) OESOPHAGUS OESOPHAGUS DUODENUM proventriculus (glandular stom- ach) VENTRICULUS MUSCULAR stom- ach) Fig. 87. — Scheme of stomach of grauivorous bird. (Nuhu.^ PLATE XXXVI. DUODENUM PYLORUS MUSCULAR STOMACH ;. — CESOPHAGUS PROVEN- TRICULUS Fig. 90. — Stomach of owl sp. (Xuhu.) DUODENUM PY LO R I C STOMACH MUSCULAR STOMAC CESOPHAGUS PROVCNTRIC- ULUS Fig. 91. — Stomach of Ardea cinerea, herou. (Xuhn.) PYLORUS DUODENUM PYLORIC STOMACH XSCPHAGUS MUSCULAR STOM- ACH WITH TENDI- j NOUS CENTRE Fig. 92. — Stomach of crocodile. (Nnhn.) IS i '^ ci 6 c5 a" PLATE XXXVIII. (ESOPHAGUS PYLORUS OECAL APPENDAGE WITH LONGITUDI- NAL MUCOUS FOLDS, DEVOID OF GASTRIC GLANDS Fig. 95. — Stomach of Brady pus tridactylus, 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 hy 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. /V*>> DUODENUM THICKENED MUS- CULAR FIBRES OF PYLORIC SEGMENT Fig. 96.— Stomach of Tmnantua hirittata, collared ant-eater. (Columbia University Museum, No. xlis) PLATE XXXIX. PRIMORDIAL CRANIUM UMBILICAL CORD CAUDAL GUT- RENAL BUD FORE-GUT BRANCHIAL CLEFTS COMMON DORSAL MESENTERY WOLFFIAN BODY DUCT OF ALLANTOIS Fig. 97. — Alimenfary canal of liuman embryo of 5 mm. X 15. (Eeconstruction after His.) DUODENUM DESCENDING LIMB OF IN- T E ST I N A L LOOP VITELLO-INTES- TINAL DUCT ASCENDING LIMB OF INTESTINAL LOOP Fig. 98. — Schema of human embryonic intestinal canal, with intes- tinal umbilical loop, but before diflereutiation of the large and small intestine. PLATE XL, STOMACH SPLEEN VENTRAL WESO- GASTRIUM PYLORUS PANCREAS TRUNCUS ARTERIOSUS OSTIUM OF OVIDUCT LEFT LUNG LEFT OVIDUCT DORSAL MESO- GASTRIUM LEFT OVARY - INTESTINE Fig. 99. — Viscera of yectuncs macidatiis, mud-puppy, in situ. (Columbia University Museum, No. 1175.) STOMACH FORMS. 49 rectly 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 addi- tional reservoirs are in imagination removed and the digestive stomach proper restored so to speak to the type-form. The prox- imal 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 corre- sponding 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 col- lected, 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 sec- ond 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). The three stomachs of the cetaceans are similar to those of the ruminants (Fig. 85). The first is a crop-like reservoir for the re- ception 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 4 50 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 Froventriculus 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, func- tions 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 mu- cous 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). 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. INTESTINE. 51 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. (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 com- partment 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. INTESTINE. Continuing our consideration of the development of the ali- mentary 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 ex- ceeds relatively the increase in the length of the body cavity in which it is contained. 2. The differentiation into small and large intestine, the devel- opment of the caecum and ileo-csecal junction, and the development of the accessory digestive glands, liver and pancreas, by budding 52 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 devia- tion. 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, Nec- turus 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 DIFFERENTIATION OF INTESTINES. 53 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 ap- plied 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 cahber from pylorus to cloaca. 2. The differentiation of the small from the large intestine, marked by the appearance of the csecal 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 csecal bud appears in the 6th week as a plainly marked pro- tuberance, which grows very slowly in length and circumfer- ence. 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 descend- ing 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 con- volutions 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. 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 de- scending limb of the umbilical loop, develops into the duodenum. This portion of the small intestine is indicated early in embryos 54 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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). 2. The descending limb, the apex and a small part of the as- cending 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 alimen- tary tract, after difierentiation of the large intestine is well illus- trated by some of the lower vertebrates in which development never proceeds beyond this stage. Fig. 112 shows the entire ali- mentary canal of a teleost fish, the conger eel {Echelus conger) isolated. 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 difierentiated both by its caliber and by the formation of a pyloric ring valve. The midgut forms a simple loop with a descending and ascend- ing limb closely bound together by mesenteric attachment. Dif- ferent from the course of development followed in the human embryo is the situation of the ileo-colic junction. The same ap- pears in the terminal straight segment of the canal — correspond- ing to the human descending colon — while in the human embryo the difierentiation of small and large intestine takes place in the course of the ascending limb of the loop. This condition de- PRIMITIVE TYPES. 55 pends upon the relatively much shorter extent of the teleost endgut compared with the human large intestine. Other ex- amples 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 consid- erable 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 intes- tinal 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, Pseu- demys 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 trans- verse 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 mamma- lian embryonic umbilical loop. The well-marked ileo-colic junc- tion 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 isth- mus) forms one of the most important factors in the further development of the mammalian intestinal canal and will again be referred to below. From the ileo-colic junction the large intestine of the turtle con- tinues caudad to the cloaca in a nearly straight line. The same 56 ANAT03IY OF PERITONEUM AND ABDOMINAL CAVITY. 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. 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 at- tached border of the mesentery — consequently the membrane presents a pleated or crenated appearance. The csecum is well developed, the ileo-csecal junction being situated within the re- turning limb of the loop, a little distance from the apex. In Figs. 109 and 110, taken from the same specimens, the en- tire 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 por- tion 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. It will be observed, in reference to the change from the primi- tive 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. Ill, I.) the two segments of the loop are of equal length, parallel to one another, the distance be- tween the beginning and termination of the loop (1-2) being PLATE XLI. (XSOPKAGUS JEJUNO-ILCUM DE- VELOPED FROM DESCENDING LIMB OF INTES- TINAL LOOP ORIGINAL APEX OF LOOP (site of meckel's diverticulum) DUODENUM SEGMENT OF ascend- ing LIMB OF LOOP FURNISHING ASCEND- ING AND TRANSVERSE COLON DESCENDING COLON Fig. 100. — Schema of human embryonic intestinal canal after diflfereutiation of the large and small intestine. SEESSEL'S SAC HEAD-GUT CAUDAL GUT STALK OF ALLANTOIS STOMADSUM HEPATIC BUD YOLK-SAC —MID-GUT ENTRANCE INTO HIND-GUT ALLANTOIC DUCT Fig. 101.— Human embryo of 2.15 mm., twelve days old. Seessel's sac is the cephalic blind termination of the embryonic fore-gut before the communication with the ectodermal invagination of the stomadseum has been formed. (Reconstruction after His.) PLATE XLII. EPIGLOTTIS TONGUE HYPOPHYSIS VITELLINE DUCT CAUDAL END OF VERTEBRAL. COLUMN ALLANTOIC DUCT CAUDAL GUT LUNG VESICLE STOIVIACH PANCREAS URINARY BLADDER WOLFFIAN DUCT RENAL BUD Fig. 102.— Eepresentation of alimentary canal and appendages of human embryo of 4.1 mm. ; isolated. X 15. (Kollmann, after His.) EPIGLOTTIS- HYPOPHYSIS HCAD-GUT INTESTINAL- LOOP ALLANTOIC DUCT GENITAL PRO- TUBERANCE CAUDAL END OF VERTEBRAL COLUMN STOMACH PANCREAS WOLFFIAN DUCT URINARY BLADDER RENAL BUD CAUDAL GUT Fig. 103. — Alimentary canal and appendages of human em- bryo of 12.5 mm. X 12. (Kollmann, after His.) PLATE XLIII. CESOPHAGUS DORSAL MESOGASTRIUM VENTRAL MESOGASTRIUM DUODENUM SUPERIOR MESENTERIC ARTERY DUODENO-COLIC ISTHMUS ART. COLICA MEDIA JEJUNO-ILEUM C>ECUM INF MESENTERIC ART. VITELLINE DUCT AND OMPHALO - MESEN- TERIC ART. Fk;. 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. PLATE XLIV. I LEO-COLIC JUNCTION (XSOPHAGUS STOMACH PANCREAS DUODENUM Fig. 105. — Bona cafesMana, hnll-frog. Alimentary canal and append- ages. (Columbia University Museum, No. 1454.) RIGHT LOBE OF LIVER (LEO-COLIC JUNCTION SMALL INTESTINE (ESOPHAGUS LEFT LOBE OF LIVER Fig. 106.— Pseudemys elegans, pond turtle. Alimentary canal. (Columbia University Museum. No. 1437.) PLATE XLV. DUODENO COLIC ISTH MUS i SaH STOMACH PANCREAS TERMINAL BEND OF COLON ILEO-COLIC JUNCTION Fig. 107.— Abdominal viscera of Tamnmhia birittata. the little ant-eater, seen from the left, with the intestines turned to the right. (From a fresh dissection.) PLATE XLVI. PYLORUS — ^.-;-- DUODENO JEJUNAL TRANSI TION SMALL IN- TESTINE FORMING EFFERENT LIMB OF INTESTI- NAL LOOP tLEO-COLIC JUNCTION GREAT OMEN- TUM TURNED UP SUPERIOR M E S E N - TERIC AR- TERY IN DUODENO- COLIC ISTHMUS COLON AT TERMINA- TION OF AFFERENT LIMB OF INTEST I - NAL LOOP COLON : PROXIMAL SEGMENT FORMING AFFERENT LIMB OF INTESTI- NAL LOOP Fig. 108.— The same view, from another speeimeu. and compared together, as each supplements the otlier. Figures 107 and 108 should be studied PLATE XLVII. DUODE- NUM RIGHT KIDNEY BEND OF COLON TURNING CAUDAD INTO SHORT TERMINAL PORTION OF LARGE INTESTINE STOMACH DUODENO- COLIC ISTHMUS ILEO-COL1C JUNCTION Fig. 109. — Ahdomiiml viscera (if TitnimuUia hirittain, the little ant-eater, seen from the right, with the intestines turned to the left. (From a fresh dissection.) PLATE XLVIII. CAUDATE LOBE OF LIVER POST CAVA PROBE PASSED THROUGH FORAMEN OF WINSLOW R. KIDNEY COLON TURN' ING INTO TERMINAL SEGMENT PROXIMAL SEG- MENT OF CO- LON (affer ent limb of intesti n a l loop) --— DUODENUM PANCREAS DUODENO- COLIC ISTHMUS SMALL IN- TESTINE (efferent LIMB OF intestinal loop) ILEO-COLIC JUNCTION Fig. 110. — The same view, from another specimen. DVODENO-COLIC ISTUMU8. 57 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. Ill, II.) the increase in the length of the intestine is accompanied by a corresponding widen- ing 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 nar- rowing 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. Ill, 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 convo- luted, and the mesentery, as it is attached to the gut, of necessity follows all the twists and appears fluted or pleated in its distal at- tached portion. 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. Ill, I.) the umbilical loop begins and ter- minates. 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 in- testine. These two points mark the neck of the loop, which we can describe as the duodeno-colic neck or isthmus. 58 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. The same condition is well shown in the intestinal canal of the snapping turtle (Fig. 1 1 3). 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. 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 dis- position of the intestine in man and the higher mammals is the rotation of the portions developed from the two limbs of the primi- tive loop around an oblique axis drawn from the duodeno-colic isthmus to the apex of the loop. The portion of the large in- testine, 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 curva- ture 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 subse- quently reaches the ventral surface of the right kidney. As the result of this rotation the ileo-colic junction, caecum and succeed- ing 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 descend- ing 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 DISPOSITION OF TEE PRIMITIVE MESENTERY. 59 the duodenum will be best appreciated by considering in the first place the effect of the change on the arrangement of the prim- itive mesentery and the intestinal vessels, and secondly by re- peating actually the rotation in the intestinal tract of a mammal (cat) in which the adult arrangement of the intestine and peri- toneum permits us to perform the manipulations and note the result. 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. Ill, representing a profile view of the original ar- rangement, 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 ven- tral aspect of the vertebral column and aorta, through the narrow neck of the duodeno-colic isthmus, to expand in the manner al- ready 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, J? and 115, ^) 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 csecal bud turns its crescentic margin ventrad and to the right. 60 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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, rep- resenting 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. 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 disposi- tion 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 ocicupy 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. PLATE XLIX. 1. 11. 111. Fig. 111. — Schematic representation of the development of the mesentery of the umbilical loop. STOMACH ILEO-COLIC JUNCTION AND VALVE Fig. 112. — Alimentary canal, isolated and in section, of Echelus conger, the conger eel. (Columbia University Museum, No. 1812.) PLATE L. BEGINNING OF MID-GUT DUODCNO-COLIC ISTHMUS PANCREAS ILEO-COLIC JUNCTION MID-GUT FORM- ING APEX OF I N T E S T I N AL LOOP Fig. 113. — Chelydra serpenfina, snapping turtle; intestinal canal, pancreas, and spleen, isolated. (Columbia University Museum, No. 1369.) 2.3 to H ag lO 4) .-a o a < Sw U PLATE LII. PYLORO- DUODENAL JUNCTION PANCREAS R. LOBE OF LIVER R. KIDNEY3 ILEO-COLIC JUNCTION LEFT LOBE OF LIVER STOMACH GREAT OMENTUM MESODUO- DENUM WITH CONTAI NED PANCREAS DUODENO- JEJUNAL ANGLE SMALL INTESTINE Fig. 116. — Abdominal viscera of Tamandua hirittata, with the intestine rotated to correspond to the development in the human subject. (From a fresh dissection.) PLATE LIII. DESCENDING DUODENUM SLEO-COLIC JUNCTION STOMACH PANCREAS TERMINAL BEND OF COLON DUODENO- JEJUNAL ANGLE Fig. 117. — The same yiew as Fig. 116, from another specimen. PLATE LIV. DUODENUM SMALL INTESTINE STOMACH SPLENIC FLEXURE SEGMENT COR- RESPONDING TO TRANSVERSA COLON ASCENDING COLO\" ILEO-COLIC JUNCTION APPENDIX Fig. 118. — Abdominal viscera of adult human female, in a case of arrested rota- tion of the intestines. (Columbia University Museum, Study Collection.) DUODENUM PRIMITIVE PARIETAL PERI TO- NEUM OF RIGHT LUMBAR REGION PRIMITIVE MES- ENTERY COM- MON TO SMALL I N T E ST I N E AND NON-RO- TATED COLON ILEOCOLIC JUNCTION DORSAL SURFACE Fig. 119. — The same preparation with the intestinal coils displaced upward and to the left. PLATE LV. DUODENUM ASCCNDIN COLON STOMACH IRREGULAR TRANSVERSE COLON ILEOCOLIC JUNCTION DESCENDING COLON OMEGA LOOP Fig. 120.— Abdominal viscera of adult human male ; non-rotation of intestine. (Columbia University Museum, Study Collection.) DUODENUM SMALL INTESTINE STOMACH BEND OF COLON COR- RESPONDING TO HE- PATIC FLEXURE SPLENIC FLEXURE DUODENOCOLIC ISTHMUS ASCENDING COLON IRREGULAR TRANS- VERSE COLON DESCENDING COLON ILEOCOLIC JUNCTION Fig. 121. — Abdominal viscera of adult human male; non-rotation of intestine. (Columbia University Museum, Study Collection.) •%. »o NON-ROTATION OF INTESTINE IN ADULT. 61 Fig. 119, taken from the same specimen, shows the entire mass of intestines lifted up and turned to the left, exposing the back- ground 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 meso-duodehum with the jejuno-ileal mesentery is well shown. The primitive right leaf of the mesentery is turned to the observer. This laj^er after completed rotation would form the left layer of the adult mes- entery 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. Fig. 121, presenting the same adhesion of the duodenum, illus- trates 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. 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 hu- man infant at two years of age. 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, 62 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 csecum 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 diagrammatic- ally. 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 unques- tionably find their explanation in the failure of the intestine to completely rotate at the ileo-colic junction. 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. 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 circum- ference 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 as- cending 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 adhe- sions along the lateral border of the proximal segment of the colon. In the next place it is desirable to clearly understand the vas- cular 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 THE AORTAL SYSTEM. 63 Development of Aortal Arterial System. The thoracic and abdominal aortse are at first double, the first aortic arches continuing as so-called "primitive aortse" 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 pro- ceeds slowly caudad. In the rabbit the fusion of the primitive aortse 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). That the entire descending aorta in nian 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 aortse. After fusion of the primitive aortse to form the abdominal aorta the allantoic arteries, now passing as the um- bilical arteries to the placenta, appear as the branches of bifurca- tion 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 modi- fied in the following respect : When the allantois develops its arteries are in fact end- branches of the two primitive aortse. After their fusion and after the formation of the single aorta this vessel is continued be- 64 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. yond 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 ves- sels 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 intes- tinal 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 abdom- inal 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 com- mon dorsal mesentery develops, the two omphalo-mesenteric ar- teries 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 in- testine 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, sit- uated between the layers of the mesentery, gives numerous anas- tomosing 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 PLATE LVII. POSTCAVAL VEIN R. KIDNEY PYLORUS DUODENUM TERMINAL. ILEUM APPENDIX- BLADDER STOMACH PANCREAS TRANSVERSE COLON GREAT OMENTUM DESCENDING COLON DUODENOJEJ- UNAL ANGLE CUT MESENTERY OFSMALL INTES- TINE OMEGA LOOP Fig. 124.— Human foetus at term ; abdominal viscera, hardened in situ ; normal position of completely rotated caecum and appendix. (Columbia University Museum, No. 1814.) Fig. 125. — Just before final rotation of caecum Fig. 126. — Rotation completed. Con- and terminal ileum. Concavity of cajcum directed cavity of ca?cum turns caudad and to cephalad and to right. Terminal ileum enters left. Terminal ileum enters colon from colon from right to left. left to right. Figs. 125, 12(). — Schematic representation of final stages in rotation of caecum and large intestine. PLATE LVIII. TERMINAL ILEUM GREAT OMENTUM Fig. 127. — Adult human sulyect with non-rotated ciPcum. from right to left to enter right side of colon. The terminal ileum turns eaudad ASC. COLON APPENDIX AD HERENT TO LATERAL AS- PECT OF ASC. COLON AND TO ILIAC PA- RIETAL PERI- TONEUM GREAT OMENTUM TERM.NAL ILEUM Fig. 128. — Adult humun suhjcct with non-rotated ca'cuin. tlic ilcuin entering large intestine from the right and behind, and the appendix placed to the right of the ascending colon. '.From a fresh dissection.) PLATE LIX. PRIMITIVE, AORTiE TRUNCUS ARTERIOSUS PRIMITIVE VENTRICLE VENOUS END OF HEART. TUBE VITELLINE VEINS RE- TURNING BLOOD FROM VASCULAR AREA VITELLINE ARTERIES CARRYING BLOOD TO VASCULAR AREA CONTINUATION OF DOUBLE AORTA TO CAUDAL POLE OF EM- BRYO Fig. 129. — Diagrams illustrating the arrangement of the primitive heart and aortic arches. (After Heisler, modified from Allen Thompson.) GASTRIC (coronary] ART. PYLORIC ART. A- GASTRO-EPIPLOICA DEXTRA HEPATIC ARTERY A. PANCREATICO-DUODEN- ALIS SUP. A. PANCREATICO-DUODEN- ALIS INF. SUP. MESENTERIC ART. GIVING OFF VASA INTES- TINI TENUIS A. COLICA DEXTRA VITELLINE DUCT ILEOCOLIC ART. OECAL BUD A. GASTRO-EPIPLOICA SIN- ISTRA SPLEEN SPLENIC ART. ABDOMINAL AORTA A. COLICA MEDIA NF. MESENTERIC ART. A. SIGMOIDEA Fig. 130. — Diagrammatic representation of the arteries proceeding to the alimentary canal and appendages prior to rotation of intestine (stage of simple umbilical loop). PLATE LX. GASTRIC (coronary) ART. PYLORIC ART. HEPATIC ART. A. PANCREATICO- DUODENALIS SUP A. PANCREATICO- DUODENALIS INF ABDOMINAL AORTA A. GASTRO-EPIPLOICA SINISTRA SPLENIC ART. A. GASTRO-EPIPLOICA DEXTRA SUP. MESENTERIC ART. A. COLICA MEDIA A. COLICA DEXTRA INF. MESENTERIC ART. ILEOCOLIC ART. 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. PYLORIC ART HEPATIC ART. A PANCREATICO-DUOD. SUP. A. PANCREATICO-DUOD. INF A. COLICA MEDIA INF MESENTERIC ART GASTRIC ART. A. GASTRO-EPIPLOICA SINISTRA -SPLEEN A. GASTRO-EPIPLOICA DEXTRA SPLENIC ART. PANCREAS SUP. MESENTERIC ART. A. COLICA DEXTRA 1/ Fig. 132. — Arteries of alimentary canal in the later stages of intestinal rotation. PLATE LXI. AORTA GASTRIC ART. HEPATIC ART. WITH PYLORIC BRANCH DUODENUM A. PANCREATICO- DUODENALIS SUP. A. GASTRO-EPIPLO- ICA DEXTRA PANCREAS A. PANCREATICO- DUOOENALIS INF. A. COLICA MEDIA A. COLICA DEXTRA A. GASTRO-EPIPLO- ICA SINISTRA SPLENIC ART. SUP. MESENTERIC ART. VASA INTESTINI TENUIS A. COLICA SirriSTRA INF. MESENTERIC ART. A. SIGMOIDEA AND SUP. H>EMOR- RHOIDAL ART. Fig. 133.^ — Final arrangement of arteries of alimentary canal after completed rotation of the intestines. GASTRIC (coro- nary) ART. HEPATIC ART A. PANCREATICO- DUOD. SUP. A. GASTRODUOD. DUODENUM A. PANCREATICO- DUOD. INF. iUP. MESENTERIC ART. STOMACH A. GASTRO-EPi- PLOICA SINISTRA SPLENIC ART. PANCREAS AORTA A COLICA MEDIA A. COLICA DEXTRA INF. MESENTERIC ART. Fig. 134. — Schematic representation of intestinal arterial supply from superior mesenteric artery iu cases of arrested rotation of the intestine. PLATE LXir. GREAT OMENTUM TURNED UP LIVER (B) terminal bend of colon cor- responding to splenic flexure right kidney ILEOCOLIC JUNCTION PANCREAS SPLEEN (A) DUOOCNUM - MESENTERY Fig. 135. — Abdomiual viscera of cat; great omeutum raised; iutestiiies turned down and to left. (From a fresh dissection.) PLATE LXIII. GASTROHEPATIC OMENTUM HEPATIC ART. BILE-DUCT PORTAL VEIN PYLORUS, LINE OF DIVIDED GREAT OMENTUM DUODENUM PANCREAS SUP. MESENTERIC ART. ENTERING MESENTERY AT DUODENOCOLIC ISTHMUS (X.) SMALL INTESTINE LIVER, L. LOBE PYLORIC ART. SPIGELIAN LOBE STOMACH, DORSAL SUR. LINE OF CUT G. OMENTUM GASTRIC ART. SPLENIC ART. SPLEEN COLON, TERMI- NAL PORTION COLON, PROXI- MAL PORTION ILEOCOLIC JUNCTION OCCUM Fig. 136. — Abdominal viscera of cat, hardened ; omentum removed to di.splay 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.) PLATE LXIV. DUODENUM DUODENOCOLIC ISTHMUS WITH PANCREAS AND SUP. MESENTERIC ART RIGHT KIDNEY GREAT OMENTUM TURNED UP ' STOMACH SPLEEN TERMINAL BEND OF COLON L. KIDNEY LEOCOLIC JUNCTION Fig. 137. — Abdominal cavity of cat. (From a fresh dissection.) DEVELOPMENT OF THE INTESTINAL ARTERIES. 65 connective tissue 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. Hjrrtl has observed the same variation. An example of par- tial 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 repre- sents 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 duo- denum, 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 duo- denal vessels and the adult arrangement of the coeliac axis is thus obtained (Figs. 130, 131, 132 and 133). 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 supply- ing the duodenum, pancreas, the small and a part of the large in- testine 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 ar- rangement of the umbilical loop (Figs. 104 and 130) this vessel 5 66 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 por- tion 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. 138) 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 be- come the left or convex side of the vessel are the original lower right-hand vessels to the small intestine developed from the de- scending 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 INTESTINAL ROTATION IN THE CAT. 67 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 in- testinal arterial supply persists, as indicated schematically in Fig. 134. Not only the pancreatico-duodenalis inferior, but all the re- maining 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. 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 con- tents in an animal whose permanent adult arrangement corre- sponds 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 unsatis- factory unless the actual examination of the object itself is com- bined with their study. Fortunately, among the common do- mestic 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 arrange- ment of the human abdominal viscera and peritoneum in the light which the anatomy of this animal can shed on the compli- cated and obscure conditions encountered in the human subject. 68 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. The plan of having the opened abdominal cavity of the cat di- rectly 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 ab- dominal 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 examina- tion 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 ani- mal 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 abdo- men, 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, pan- creas and duodenum. They may be kept for permanent ref- erence 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 posi- tion, 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. 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 INTESTINAL CANAL OF CAT. 69 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 direc- tion, i. e., over to the right side. The arrangement of the intestinal tract after these manipula- tions should appear as shown in Figs. 136 and 137. 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 ven- tral surface of the vertebral column by the portion of the prim- itive 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 mil not be diflBcult to recognize in the numerous coils of small intestine which succeed to the duodeno-colic isthmus the results of the in- crease 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 duo- deno-colic isthmus will be reached; from here the short large in- testine of the carnivore descends straight into the pelvis, attached 70 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. to the ventral surface of the vertebral column by a meso-colon 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 ab- dominal 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 peri- toneum 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 peri- toneum, 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, in- vesting 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 peri- toneum 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. Figs. 138 and 139 shows the arrangement in the cat where a FIXATION OF DUODENUM AND PANCREAS. 71 free duodenum and mesoduodenum exists, with the pancreas included between its layers.^ 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 sec- ondary 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 ven- tral 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 transi- tion between primary and secondary parietal peritoneum occur- ring along the line marked X in Fig. 140. The opposed peri- toneal surfaces indicated by the dotted lines have become adher- ent 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. ^ The student should not be confused by the fact that a considerable portion of the pan- creatic gland in the cat will be found included between the layers of the great omentum, ex- tending over to the left aide 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 XKtncreas, situated close to the duodenum and included between the layers of the mesoduo- denum. 72 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. 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 be- tween its two layers at this point to supply the segments of the intestine forming the loop. In conformity with the greatly in- creased length of the intestine it will be found that the mesen- tery expands from the narrow pedicle at the neck in a fan-shaped manner in order to develop a sufficiently long margin for attach- ment 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 in- testine, partly mesocolon passing to the large intestine (csecum and proximal colon). This condition may be indicated schemat- ically in Fig. 141. 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 meso- colic segment of the primitive membrane. With the parts in their present position this line might be as- sumed 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 meso-colic segment would have been effected. PLATE LXV. DUODENUM HEAD OF PANCREAS, BETWEEN LAYERS OF MCSODUO- DENUM AORTA PRIMITIVE PARIETAL PERITONEUM L. KIDNEY POSTCAVA PRIMITIVE PARIETAL PERITONEUM R. KIDNEY Figs. 138-140. — Diagrammatic representation of three stages in the development of the meso- duodenum, duodenum, and pancreas leading to the secondary "retroperitoneal" position of these viscera. Fig. 138. — Free mesoduodennm in sagittal ijlane, including head of pancreas between right and left layers. VENTRAL (original LEFT) LAYER OF MESODUODENUM PANCREAS DUODENUM DORSAL (ORIGINAL RIGHT) LAYER OF MESODUODENUM PRIMITIVE PARIETAL PEHIT L. KIDNEY POSTCAVA PRIMITIVE PARIETAL PERITONEUM R. KIDNEY Fig. 139. — Mesoduodeuum folded to right; left leaf has become ventral; right dorsal, directed toward primitive prerenal parietal peritoneum. SECONDARY PARIETAL PER IT. COVERING VEN- TRAL SUR FACE O F PAN- CREAS DERIVED FROM LEFT LAYER OF MESO- DUODENUM DUODENUM POSTCAVA X.SECONDARY TRANSI- TION FR. VISCERAL TO PARIETAL PERITONEUM KIDNEY AREA OF ADHESION BETWEEN RIGHT SUB- SEQUENTLY DORSAL) LAYER OF MESODUO- DENUM AND PRIMITIVE PARIETAL PERI- TONEUM Fig. 140. — Fixation of head of pancreas and duodenum under cover of secondary parietal peritoneum by adhesion of apposed surfaces of mesoduodennm and primitive parietal peritoneum. PLATE LXVI. DUODENOCOLIC ISTHMUS SEGMENT OF PRIMI- TrVE COMMON MES- ENTERY OF INTESTI- NAL LOOP PRODUC- ING MESENTERY OF JEJUNO-ILEUM DORSAL MESO- GASTRIUM MESODUODENUM SEGMENT OF PRIM- ITIVE COMMON MES- ENTERY OF INTESTI- NAL LOOP FROM WHICH ASCENDING AND TRANSVERSE MESOCOLA ARE DE- RIVED DISTAL PORTION OF PRIMITIVE COMMON DORSAL MESENTERY FURNISHING MESO- COLA OF DESCEND- ING COLON AND OMEGA LOOP Fig. 141. — Schematic representation of mesentery of umbilical loop, common to small intestine and proximal portion of large intestine. DIVIDED VENTRAL MESOGASTRIUM STOMACH DORSAL MESOGASTRIUM INTESTINE Figs. 142-144.— Scliematic representation of three stages in the development of the mesentery of the umbilical intestinal loop. Fig. 142.— Early stage before differentiation of intestinal canal. PLATE LXVII. DIVIDED VENTRAL MESOGAS- TRIUM DUODENUM DESCENDING LIMB OF INTESTINAL LOOP mesentery 'primitive^ VITELLO- INTESTINAL DUCT STOMACH DORSAL MESO* GASTRIUM MESODUODENUM ASCENDING LIMB OF INTESTINAL LOOP TERMINAL SEGMENT OF COLON Fig. 143. — Stage of umbilical loop. Difl'erentiation of common dorsal mesentery of earlier stage into dorsal mesogastrium, mesoduodenum, primitive mesentery of umbilical loop, and descending mesocolon. DIVIDED ventral MESOGAS- TRIUM duodenum COMMON PERI- TONEAL PLATE OF MESENTERY AND ASCEND iNG AND TRANS VERSE MESO COLON DORSAL MESOGAS- TRIUM MESODUODENUM ASCENDING COLON DESCENDING COLON P'iG. 144. — Final stage. With complete diflerentiation of large and small intestine, the primitive mesentery of the umbilical loop contains not only the mesentery of the future jejuuo-ileum, but also the mesocola and the ascending and transver.se colon, developed from the ascending or afferent limb of the um- bilical loop. > PLATE LXIX. STOMACH DUODENO- JEJUNAL JUNCTION Fig. 147. — Human foetus, 6.6 cm., vertex-coccygeal measure ; liver removed, (Columbia University Museum, Study Collection.) X 4. POSTCAVA RIGHT ADRENAL SUPRACOLIC PART OF DUODENUM RIGHT KIDNEY COLON (A) TERMINAL ILEUM RIGHT OVARY SSCPHAGUS STOMACH LESSER CURVATURE PORTAL VEIN PYLORUS SPLENIC FLEXURE OF COLON (B) / 3D PART OF DUO- DENUM DESCENDING COLON OMEGA LOOP Fig. 148. — Abdominal viscera of human foetus of 12.5 cm., vertex-coccygeal measure, hardened in situ; transverse and ascending colon not yet differen- tiated. (Columbia University Museum, No. 1815.) Natural size. PLATE LXX. HEPATIC FLEXURE OF COLON ASCENDING COLON TERMINAL ILEUM STOMACH SPLEEN TRANSVERSE COLON Fig. 149. — Abdominal viscera of human foetus at term, hardened in situ; hepatic flexure formed and ascending and transverse colon differentiated. (Columbia University Museum, No. 1816.) UMBILICAL CORD ALLANTOIC DUCT WOLFFIAN DUCT VITELLINE DUCT MID-GUT WOLFFIAN BODY WOLFFIAN DUCT Fig. 150. — Caudal portion of human embryo of 5 mm., with the end- and caudal gut at the highest stage of its development. X 25. (Eeconstruction after His.) PLATE LXXI. HEPATIC FLEXURE OF COLON GREAT OMENTUM TURTLE O-UP TRANSVERSE MESOCOuON COVERING COILS OF SMALL IN- TESTINE TRANSV. COLON BENT IN V-SHAPE TO PUBES Fig. 1")!. — Abdominal viscera of Macacus rhesus, rhesus monkey, hard- ened m situ. (Columbia University Museum, No. 1817.) VISCERAL PERIT. OF DESCEND- ING DUO- DENUM DUODENUM Figs. l.')2-l.'J4. — Schematic representaticni of peritoneum in fixation of descending duodenum and formation of transverse colon and mesocolon. Fig. 152. — Sagittal section through right kidney and descending duodenum before adhesion of latter to parietal peritoneum. PLATE LXXII. ADHESION BETW. PRIMITIVE PARIETAL PERIT. AND DOR- SAL VISCERAL SEROSA OF DUODENUM R. KIDNEY TRANSITION OF PRIMITIVE INTO SECONDARY PARIETAL PERITONEUM DERIVED FROM VENTRAL VISCERAL SEROSA OF DUODENUM PARIETAL PERITONEUM VENTRAL VISCERAL SEROSA OF DUODENUM FORMING SECONDARY PARIETAL PERITONEUM DESCENDING DUODENUM LAYERS OF MESOCOLON Fig. 153.— Adhesion of descending duodenum to primitive parietal peritoneum, mesocolon after rotation of the intestine, but before adhesion. Colon and ADHESION BETWEEN PRIMITIVE PARIETAL PERITONEUM AND DORSAL .VISCERAL SEROSAOF DUODENUM POINT WHERE MESO- COLON BEGINS TO RE- PLACE PRIMITIVE PRE- RENAL PARIETAL PERI- TONEUM ADHESION BETWEEN PRIMITIVE PARIETAL PERIT. AND DORSAL LAYER OF MESOCOLON VENTRAL VISCERAL SEROSA OF DUODENUM FORMING SEC- ONDARY PARIETAL PERITO- NEUM COVERING SUPRACOLIC PORTION OF DUODENUM HEPATIC FLEXURE OF COLON beginning oftransv. colon secondary parietal perit. derived from mesocolon covering infracolic seg- ment of duodenum area of adhesion between mesocolon and visceral serosa of duodenum secondary parietal perit. (mesocolic) covering R. KIDNEY Fig. 154. — Adhesion of mesocolon to duodenum and primitive parietal peritoneum, resulting in formation of root of transverse mesocolon. PRIMITIVE MESENTERY OF UMBILICAL LOOP. 73 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 ap- parent 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 meso-colon 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 fac- tor is of importance in correctly estimating the value of the human adult conditions. 3. A brief consideration of the mechanical conditions and com- parison 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 be- come differentiated into the subsequent segments, and before the umbilical loop has been formed (Fig. 142), with the later stages represented by the intestines of the cat as now arranged (Figs. 143 and 144), it will be seen that the vertical line of attachment to the ventral surface of the vertebral column, between the points a and b corresponds in the advanced stages to the interval ab separating the two points of the duodeno-colic isthmus ; also that the entire mesenteric peritoneal surface beyond the isthmus is the result of drawing out and lengthening the intestinal tract. Con- sequently folding or overlapping of this extensive membrane affords opportunities for adhesions between its own serous surfaces or between it and the remaining visceral and parietal peritoneum of the abdomen. Moreover, it will be appreciated that the entire extensive coil of intestines extending between the two boundaries of the duodeno- colic isthmus (a, b) is suspended from the back part of the abdo- 74 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. men by a narrow pedicle and that consequently rotation will readily occur around the axis drawn through the neck of the isthmus. Now proceed to illustrate on the cat the result of the rotation as it occurs normally during the development of the primate in- testinal tract. Take the caecum and commencement of the colon and draw the same over to the right across the duodeno-colic isthmus and the duodenum. Twist or rotate the entire mass of small intestines around the isthmic pedicle, so that the original left leaf of the mesentery will look to the right and vice versa (Fig. 145). The conditions thus established will be found to correspond to the schemata shown in Figs. 114 and 115. The main features of the intestinal tract in the rearranged position will be as follows : 1. The two points, a and 6, of the duodeno-colic isthmus (Fig. 145) are still close together, but reversed in position, h is in front and to the right, a behind and to the left, whereas before the rota- tion h was situated below and to the left, a above and to the right (Fig. 135). 2. The direction of the ileo-colic entrance is reversed, the ileum now entering the large intestine from below and the left upwards and to the right, instead of from right to left. 3. The descending duodenum is now situated dorsad to the colon. 4. The original left leaf of the mesentery has become the right, and vice versa. 5. The superior mesenteric artery crosses over the transverse portion of the duodenum, and with the exception of the inferior pancreatico-duodenal artery the original right-sided branches now arise from the left side of the vessel and v'we versa. It is now time to compare the conditions established in the cat by the manipulations just detailed with the arrangement of the adult human intestinal tract and peritoneum below the level of the transverse colon and mesocolon. I. The shortness of the large intestine in the cat will require careful manipulation in order to produce a disposition in con- formity with the arrangement of this portion of the human intes- RELATIONS OF COLON. 76 tinal tract. By stretching the gut somewhat and pulling it well out of the pelvis sufficient length will be obtained to establish an ascending, transverse and descending colon. Move the caecum from the subhepatic position which it occupies immediately after rotation (Fig. 145) down to the lower and right-hand corner of the abdomen. Pull the distal portion of the large intestine well out of the pelvis and obtain thus sufficient length to establish an ascending, transverse and descending division each provided with a free mesocolon (Fig. 146). In the formation of the three definite main segments of the human large intestine, ascending, transverse and descending colon, the following stages may be recognized : 1. Immediately after rotation the large intestine lies trans- versely along the greater curvature of the stomach, with the caecum on the nght side in front of the duodenum and closely applied to the caudal surface of the right lobe of the liver (Fig. 147). Persistence of Subhepatic Position of CiEcuM in Adult. — The period at which thje caecum descends into the iliac fossa is liable to a considerable range of variation. Treves found in two foetus, measuring respectively 4 J" and 5 J", the caecum on a level with the caudal end of the right kidney, while in several individuals at full term the caput coli was still placed immediately below the liver, with no large intestine in the place of the ascending colon. This condition is well illustrated in the foetus shown in Fig. 124. The caecum may remain undescended throughout life. Treves, in an examination of 100 bodies, found this condition in two sub- jects, both females, one 41, the other 74 years of age. Both cases presented an identical disposition. There was no large intestine in the place of the ascending colon. The caecum was placed on the right side, immediately underneath the Uver, just to the right of the gall-bladder ; it was quite horizontal in position, continu- ing the long axis of the transverse colon and included between the layers of the transverse mesocolon. From the extremity of 76 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. the csecum a horizontal fold was continued to the abdominal parietes and upon it the edge of the liver rested. In one of these instances the colon from the tip of the cascum to the splenic flexure measured 38". The great omentum was attached only to the left half of this portion. The descending colon was very long, measuring 15". In the other case the distance from the tip of the caecum to the splenic flexure was 27", the great omentum commencing 5" from the former point. The descending colon was of normal length. In both bodies the remaining viscera were normal. 2. The caecum next descends ventrad of right kidney to the iliac fossa. The future ascending colon is at this time placed very obliquely on account of the large size of the foetal liver, and passes without a marked angle into the transverse segment. Thus in Fig. 148, from a foetus 5" in length, the descending colon is verti- cal and the splenic flexure well marked, forming the highest point of the colic arch. There is no hepatic flexure, and no ascending and transverse colon, but instead of these an oblique segment pass- ing upwards and to the left between caecum and splenic flexure. This disposition, due to the large size of the liver, is still marked at times in the foetus at term, and occasionally even in children np to 2 or 3 years of age. 3. The ascending colon is subsequently differentiated from the transverse segment and the hepatic flexure formed consequent upon the diminution of the relative size of the liver, which per- mits the foetal oblique segment of the colon extending in the earlier stages between the right iliac fossa and the spleen to be- come divided by a right-angled (hepatic) bend or flexure into an ascending and a transverse segment (Fig. 149). 4. The splenic flexure develops early and is well marked. It indicates the point of transition of the original ascending limb of the umbilical loop into the remaining vertical median segment of the large intestine, from which the descending colon is formed. In the adult the ascending and descending portions of the colon are vertical. The transverse colon is not quite horizontal DIFFERENTIATION OF ASCENDING AND TRANSVERSE COLON. 77 since the splenic flexure is higher and placed more dorsally than the hepatic flexure. In the embryo the rapidly-growing coils of the small intestine push the descending colon to the left and dorsad into close contact with the dorsal abdominal wall. A small bend which appears about the middle of^the third month in the left iliac fossa indicates the rudiment of the future sigmoid flexure or omega loop. The rest of the endgut follows the body wall in a well-marked curve, whose termination lies within the concavity of the caudal portion of the embryo (Fig. 150). From this terminal part the rectum develops after the division of the cloaca and the union of the proctodseum with the entodermal intestinal pouch has taken place as detailed above. The early position of the colon produced by the large size of the foetal liver, and before the descent of the caecum has occurred, is shown in Fig. 124. In Fig. 123, where the liver has regained its normal proportions with reference to the abdominal cavity and viscera, and the caecum has descended into the right iliac fossa, the hepatic flexure is well marked and the first segment of the colon has acquired the vertical position on the right side, the single obliquely transverse segment of Fig. 124, having become divided into an ascending and a transverse colon. [Fig. 124. Early stage. Liver relatively large. Proximal portion of the colon extends obliquely between the right lumbar region and the spleen. The caecum has not yet descended. Fig. 123. Later stage. The caecum occupies the right iliac fossa. Relative reduction in the size of the liver allows the colic segment to be divided by the hepatic flexure into an ascending colon and a transverse colon.] At times the transverse colon, whose normal average length in the adult is 20", greatly exceeds this measurement and forms an arch which hangs down or makes a well-marked V-shaped bend with the apex directed toward the pubes. This is the normal arrangement of this portion of the large intestine in many of the lower primates. Fig. 151 shows the abdominal viscera of 78 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. Macacus rhesus, hardened in situ, seen from the front and the right side, with the omentum turned up over the stomach. The transverse colon forms an extensive V-shaped bend, whose apex reaches to the pubes, from which point the large intestine turns again cephalad and dorsad to form the splenic flexure and then descends to the pelvis. The average length of the ascending colon in the adult, meas- ured from the tip of the caecum to the hepatic flexure, was found by Treves in his series of 100 bodies to be 8", while the descend- ing colon, from the splenic flexure to the beginning of the sigmoid loop, measured 8i". The descending colon may at times be much longer, up to 15", and become convoluted. II. In the next place, in order to understand the arrangement of the peritoneum in this lower larger compartment of the abdo- men, disregard for the present the peritoneal connections of the stomach, liver, pancreas and spleen, and the folds of the great omentum entirely. This latter membrane is adherent in the adult human subject by its dorsal surface to the upper margin of the transverse colon, so that in turning the omentum up over the ventral chest wall the transverse colon will be carried with the omentum and the lower layer of the transverse mesocolon will be put upon the stretch. This membrane forms in adult man by its transverse attachment to the abdominal backgi-ound the cephalic limit of the larger lower compartment of the abdomen, which is framed laterally by ascending and descending colon, con- tinuous below with the pelvic cavity and occupied chiefly by the freely movable coils of the jejuno-ileum. Remember that the duodenum starting from the pyloric ex- tremity of the stomach first turns cephalad and dorsad in contact with the caudal surface of the right lobe of the liver, forming the first portion or hepatic angle of the duodenum ; that in the next place the second or descending portion of the duodenum passes down in front of the medial part of the ventral surface of the right kidney and the inferior vena cava, but behind the right ex- COURSE OF DUODENUM, THE MESOCOLA AND THE MESENTERY. 79 tremity (hepatic flexure) of what after rotation and formation of the ascending colon appears as the transverse colon ; that conse- quently the descending duodenum is divided by its intersection, with the transverse colon into a cephalic supra-colic and a caudal infra-colic segment. Also remember that the second angle of the duodenum (transi- tion between the descending and transverse portions) is conse- quently situated to the right of the vertebral column below the level of the transverse colon and the secondary attachment pres- ently to be considered of the transverse mesocolon to the back- ground of the abdominal cavity. The third portion of the duodenum extends from this point more or less transversely — depending upon the type — to the left, across the vertebral column and aorta. This transverse portion, after the rotation of the primitive loop at the duodeno-colic angle, is crossed in the direction caudad and ventrad by the superior mesenteric vessels, which hence divide this portion of the intes- tine into a right and left segment. The latter turns cephalad and ventrad on the left side of the vertebral column (4th or ascending portion) to become continuous at the duodeno-jejunal angle with the free or floating small intes- tine (jejunum). If we imagine in the cat the duodenum anchored or fixed by adhesion of the dorsal (originally right) leaf of the mesoduode- num and of its own dorsal visceral peritoneum to the abdominal parietal peritoneum in the manner above indicated (p. 70) as far as the duodeno-jejunal angle we will have conditions estab- lished which correspond to those found in the human adult ab- dominal cavity. III. It is next necessary to study carefully the disposition of the primitive dorsal mesentery connected after rotation with the dif- ferent segments of the intestinal tube, ascending, transverse and descending colon and free small intestine. In order to obtain in the cat a cephalic limit to the region now under consideration which will correspond to the arrangement of 80 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. the adult human peritoneum, we will begin with the peritoneal membrane attached to the portion of the colon which in the re- arranged intestinal tract represents the human transverse colon. This transverse segment of the large intestine is now made to ex- tend directly across the abdomen from the liver to the spleen. The two layers composing the transverse mesocolon are an upper or cephalic and a lower or caudal layer. Now it will be seen in the cat that the upper or cephalic layer of the transverse mesocolon thus established^ is continuous on each side with the dorsal (originally right) leaf of the ascending and with the dorsal (originally left) leaf of the descending meso- colon, which peritoneal layers are in direct opposition to the parietal lumbar and prerenal peritoneum. On the other hand, the inferior or ventral layer of the transverse mesocolon is con- tinuous on each side of the median line with the ventral (origi- nally respectively left and right) leaves of the same mesocola, while at the site of the duodeno-colic isthmus the two layers of the transverse mesocolon are continuous as originally with the two layers of the mesentery of the jejuno-ileum (Fig. 146). Now fix the transverse mesocolon firmly against the back- ground of the abdomen and place the ascending and descending colon as far as possible over to the right and left side respectively. We will assume a line of secondary adhesion between the trans- verse mesocolon and the parietal peritoneum investing the dorsal abdominal wall. Along this line the upper or cephalic surface of the transverse mesocolon would become continuous with the dorsal parietal peritoneum, while the lower or caudal layer would still be continuous with the left leaf of the ascending and the right leaf of the descending mesocolon. We have already seen that the duodenum and mesoduodenum become anchored in the sub- hepatic region and that the visceral ventral peritoneum of the gut and the original left leaf of the mesoduodenum appear then as secondary parietal peritoneum. Hence a sagittal section through the right lumbar region, right kidney and descending duodenum would, immediately after rotation and establishment PLATE LXXIII. GREAT OMENTUM RAISED HEPATIC FLEXURE OF COLON TRANSVERSE COLON PANCREAS L. KIDNEY FREE DE- SCENDING MESOCOLON Fig. 155. — Abdominal viscera of Macacus cynomolgiis, Kra monkey, liardened in situ. (Colum- bia University Museum, No. 1801.) PLATE LXXIV NON-PERITONEAL SURFACE OF RIGHT ADRENAL ADHER- ENT TO LIVER PERITONEAL SUR- FACE OF RIGHT AD- PERITONEAL (he PATIC) AREA OF R KIDNEY COLON AT HE- PATIC FLEXURE PERITONEAL (meSO- COLIC) AREA OF R. KIDNEY DESCENDING DUODENUM TRANSVERSE MESOCOLON Fig. 156. — Schema of visceral and peritoneal relations of ventral sur- face of right Itidney. DESC. COLON DESC. MESOCOLON PRIMARY PARIE- TAL PERIT. t T S^ SMALL NTESTINE MESENTERY ASC. COLON ASC. MESO- COLON PRIMARY PARIETAL PERIT. Fig. 157. SECOrlDARY PARIETAL PERIT. FROM R. LAYER DESC. MESO- COLON DESC. COLON AREA OF ADHESION BETW. L. LAYER DESC. MESOCOLON AND PRI- MARY PARIETAL PERIT. L. KIDNEY c =^ S r Ka/' SMALL INTESTINE MESENTERY SECONDARY PARIETA5- PERIT. FROM LEFT LAYER OF ASC. MESOCOLON ASC. COLON AREA OF ADHESION BETW R LAYER ASC. MESOCOLON AND PRIMARY PARIETAL PER'T R. KIDNEY Fig. 158. Figs. 157, 158. — Schema showing peritoneal arrangement in transection of infracolic compart- ment of abdomen before and after fixation of ascending and descending colon. PLATE LXXV. R. KIDNEY DUODENUM MESENTERY OFJEJUNO-ILEUNI CROSSING TRANSV. DUOD. ASC. COLON ASC. MESOCOLON ORIGINAL LEFT, NOW VENTRAL LAYER FORMING SECONDARY PARIE- TAL PERITONEUM GREAT OMENTUM TURNED UP TRANSVERSE COLON B CAUDAL LAYER OF TRANSV. MESOCOLON L. KIDNEY DESC. COLON DESC. MESOCOLON, ORIGINAL RIGHT, NOW VENTRAL LAYER FORMING SECONDARY PARIE- TAL PERITONEUM C OMEGA MESOCOLON OMEGA LOOP INTERSIGMOID FOSSA Fig. 159. — Schematic figure to show lines of mesocolic adhesion, formation of root of trans- verse mesocolon and root of mesentery of jejuno-ileum in human subject. SMALL INTESTINE GREAT OMENTUM TURNED UP MESOCOLON Fig. 160. — Abdominal cavity of cat, with intestines everted and elevated to show duodenal fold. (From a fresh dissection.) PLATE LXXVI. PORTAL VEIN POSTCAVA R KIDNEY DUODENAL FOLD PANCREAS STOMACH DUODENUM ILEO-COLIC JUNCTION Fig. 161.— Abdominal viscera of Nasua rufa, brown coaiti. (From a fresh dissection.) DUODENUM R KIDNEY ILEO-COLIC JUNCTION STOMACH V HEPATIC ^ FLEXURE OF COLON Fig. 162. — Abdominal viscera of Hapale vulgaris, the marmoset. (Colum- bia University Museum, No. 1818.) PLATE LXXVII. GREAT OMENTUM MESOCOLON CORRESPONDING TO ASCENDING AND TRANS- VERSE HUMAN SEGMENTS DUODENAL FOLD BOUNDING SUP. DUODENAL FOSSA DESC. MESOCOLON TERMINAL PART OF DUOD DESCENDING COLON Fig. 163.— Abdominal viscera of cat ; intestines rotated and turned to the riglat to show duodenal fold. (From a fresh dissection.) DUODENUM PANCREAS I LEO-COLIC JUNCTION OMENTUM TURNED UP SUP. MESENTERIC ART DORSAL MESENTERY LEFT KIDNEY TERMINAL PART OF DUODENUM COLON DUODENAL FOLD SMALL IN- TESTINE AND MESENTERY, WITH ORIG- INAL RIGHT LAYER TURNED TO LEFT BY ROTATION OF INTESTINE AT DUODENO-COLIC ISTHMUS Fig. 164. — Abdominal viscera of Nasua rufa, the brown coain, snowin<^ the position of the duodenal fold after rotation of the intestine. (From a fresh dissection.) PLATE LXXVIII. DUODENO- JEJUNAL ANGLE INF. MESENT. VEIN IN MAR- GIN OF FOLD BOUNDING DUODENAL FOSSA OMEGA LOOP STOMACH PANCREAS CUT END OF TRANSVERSE COLON DESC. COLON Fig. 165. — Abdominal viscera of human foetus at term, arranged to show duodenal folds and fossa. The jejuno-ileum, ascending and transverse colon have been removed. (Columbia University Museum, No. 1819.) R. KIDNEY CUT EDGE OF MESENTERY iLEO-COLIC JUNCTION TRANSV. COLON TRANSV. MESO- COLON DUODENO-JEJUNAL ANGLE INF. MESENT. VEIN AND ART. COLICA SINISTRA DUODENAL FOLD OMEGA LOOP Fig. 166.— Abdominal viscerii t.l' liuniMii lii'ius at term. (I'olumbia Uni- versity Museum, No. 1820.) PLATE LXXIX. ASC OUOD ENUM GREAT OMENTUM SUP. DUOD< ENAL FOLD INF. MESENT. , i/EIN AND ART. COLICA SINISTRA INF. DUOD- ENAL FOLD Fig. 1(}7. — Abdominal viscera ui ;i(lult human subject, showing duodenal folds and fossa. (From a fresh dissection.) PLATE LXXX. TERMINATION OF ASCENDING DUODENUM INTERMEDIATE DUOD. FOLD GREAT OMENTUM TURNED UP DUODENO- JEJUNAL OR MCSO- COLIC FOLD SUP. DUOD- ENAL FOLD INF. MESEN- TERIC V. INF. DUOD- ENAL FOLD /i:' Fig 168.— Abdominal viscera of adult human subject, showing duodenal folds and fossa. (b rom a iresn dissection.) BRONCHUS CESOPHAGUS SMALL INTESTINE ^. Figs. 169, 170.— Two front views of the entodermal canal. (Minot. after His.) ' Fig. 169.— Embryo Sch. 1 of His. Fig. 170.— Embryo Sch. 2 of His. PERITONEAL RELATIONS OF DESCENDING DUODENUM IN ADULT. 81 of the transverse mesocolon, show the peritoneal arrangement indicated in Fig. 153. After adhesion of the transverse mesocolon continuity would be estabhshed between its upper or cephalic layer and the secondary parietal peritoneum investing the supra- colic portion of the descending duodenum (Fig. 154) while its caudal layer becomes continuous with the secondary parietal peritoneum covering the infra-colic segment of the duodenum and the lower portion of the ventral surface of the right kidney. Reference to the schematic Figs. 152, 153 and 154, will show that the adult duodenum becomes fixed to the posterior parietes of the abdomen by adhesion of its visceral serous covering and of the dorsal layer of the mesoduodenum to the primitive parietal peritoneum. The supra-colic segment of the adult descending duodenum lies under cover of a single peritoneal layer, derived from its own visceral investment and appearing as secondary parietal peritoneum by continuity laterad along the line of adhesion with the primitive parietal peritoneum covering the upper part of ventral surface of right kidney, while mesad, the layer covering this segment of the duodenum, is con- tinued into the secondary parietal peritoneum derived from the left or ventral leaf of the mesoduodenum and covering the ven- tral surface of the pancreas (cf Figs. 138-140). On the other hand, the infra-colic segment of the descending duodenum, as well as the lower and mesal angle of the ventral surface of right kidney, between ascending and transverse colon, is covered by a layer of secondary parietal peritoneum derived from the ventral layer of the ascending mesocolon and continu- ous with the caudal layer of the transverse mesocolon. Beneath this secondary parietal peritoneum are two obliterated layers, on the one hand the dorsal layer of the mesocolon, on the other the visceral infra-colic duodenal serosa and the primitive prerenal parietal peritoneum. In the further development of the adult human arrangement the changes below the level of the transverse colon and meso- colon result in the fixation of the ascending and descending colon 6 82 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. to the background of the right and left lumbar regions. The opposed serous surfaces of the ascending and descending mesocola and of the dorsal parietal peritoneum adhere and the process also usually involves the dorsal visceral peritoneum of the ascending and descending colon, so that these portions of the gut obtain a fixed position. Adhesion of the mesocolon to the dorsal body wall (parietal peritoneum) does not occur at all points at the same time. Usually adhesion proceeds from the midline laterad. The fixation of the ascending colon in the human embryo begins about the fourth month. In the descending segment by the same time adhesion has usually proceeded nearly up to the descending colon, but the in- testine itself is as yet free. In the fifth month the descending colon has usually become fixed between the splenic flexure and the beginning of the sigmoidea. In the latter region a free meso- colon usually persists throughout life. Differences in the rate of growth between the length of the body wall and the length of the mesocolon may play an impor- tant part in the production of peritoneal fossas, small pouches which in some regions of the abdomen may assume considerable proportions. Such fossae are found around the duodeno-jejuneal angle, the caecum and appendix, and the sigmoid flexure. They will be considered more in detail with these respective regions, especially in reference to their relation to retro-peritoneal hernia. In a certain proportion of cases adhesion between the parietal peritoneum and the ascending and descending mesocolon is incom- plete or entirely wanting, resulting in the formation of a more or less completely free ascending and descending mesocolon. Treves, in an examination of 100 bodies, obtained the following figures : In 52 subjects there was neither an ascending nor a descending mesocolon, the intestine being fixed in the manner which is re- garded as normal. In 22 there was a descending, but no trace of an ascending mesocolon. ASCENDING AND DESCENDING MESOCOLA IN ADULT. 83 In 14 a mesocolon was found in both the ascending and de- scending segments of the large intestine. In 12 there was an ascending mesocolon, but no corresponding fold on the left side. Hence from this series a mesocolon may be expected on the left side in 36 per cent., on the right side in 26 per cent. ^ Both development and comparative anatomy would lead us to expect that the descending mesocolon would be found more fre- quently than the ascending. In the lower animals the descending mesocolon is always an extensive and conspicuous membrane. It is well developed in all monkeys and the anthropoidea, as the remains of the primary vertical fold of the dorsal mesentery, while the ascending meso- colon is a secondary production, acquired during the development of the bowel by rotation. In most of the lower monkeys the ascending mesocolon is also largely or entirely free. The descending mesocolon can always in these animals be reflected to the median line (cf Fig. 155). The line of attachment in man of the descending mesocolon is usually along the lateral border of the left kidney and vertical, while the line of attachment of the ascending mesocolon is usually less vertical, crossing the caudal end of the right kidney obliquely from right to left and with an upward direction (Fig. 156). In Uke manner when both the ascending and descending meso- cola are absent as free membranes the left or descending colon is adherent along the lateral border of the kidney to the abdominal parietes, while the ascending colon is fixed at the hepatic flexure a little obliquely across the ventral surface of the caudal end of the corresponding gland ascending toward the medial margin. Treves found in the cases of persistent ascending mesocolon in the adult that the membrane varied in breadth from 1" to 2" while the persistent fold on the left side varied between 2" and 3" in breadth. In the foetus, up to 5"-6" in length, the descending mesocolon is usually an extensive fold. Its attachment is vertical, but nearer to the median line than in the adult, usually along the medial 84 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. border of the left kidney. It is at times found attached along this line in the adult. An ascending mesocolon is rare even in the foetus. The caecum and beginning of the ascending colon are complete^ invested by peritoneum, but above the parts so invested the colon is usually adherent along an oblique line to the ventral and medial aspect of the right kidney. In the foetus at full term, if the caecum is still undescended and in contact with the liver, it is not uncommon to find the cephalic portion of the descending colon provided with a mesocolon, while the caudal part of the descending colon is fixed by adhesion to the ventral surface and lateral border of the left kidney. This free membrane is then really a part of the transverse mesocolon. Where the caecum descends to the iliac fossa the portion of the foetal descending colon so invested is drawn over to the right and incorporated in the transverse colon. Treves in two out of 100 bodies found the caecum in the right iliac region, but both it and the whole of the ascending colon were entirely free from any peritoneal connections with the dorsal parietes of the abdomen. The gut from the tip of the caecum to the hepatic flexure was entirely invested by peritoneum continuous with the mesentery. The ascending colon was covered in the same manner and by the same fold as the small intestine. The segment of large intestine thus free measured 8" in both instances. The mesentery lacked the usual attachment to the dorsal ab- dominal wall and its root was represented by the interval between the duodenum and the transverse colon. The membrane had no other than its original primary attachment, and small intestine and ascending colon formed together a loop that practically represented the condition of the great primary intestinal loop. (Compare p. 73.) The arrangement presented in these two subjects corresponds to that met in many animals, such as the cat. A cross section of the cat's abdomen arranged as above would POSITION OF COLIC SEGMENTS IN ADULT. 85 show the following disposition of the peritoneum, corresponding to the stage in the human development preceding the fixation of the two vertical colic segments (Fig. 157). It will be seen that the right and left mesocola can be reflected to the median line where they become continuous ventrad of the vertebral column and aorta with the mesentery of the small intestine. The ventral surfaces of both kidneys are seen to be covered by the primitive parietal peritoneum of the abdominal cavity. Fig. 158 shows the adult human arrangement of the same parts, after fixation of the vertical colic segments by adhesion of the opposed surfaces of their mesocola and the primitive parietal peri- toneum. The background of the abdomen is now seen to be covered by a layer of secondary parietal peritoneum, viz., the original left leaf of the ascending and right leaf of the descending mesocolon, continuous above with the lower or caudal layer of the transverse mesocolon. This adhesion is so complete that the original condition is dis- regarded in adult descriptive anatomy. The layer which has ad- hered to the parietal peritoneum can no longer be recognized and the other has assumed the rdle of parietal peritoneum. The connection of the transverse mesocolon with the dorsal lamella of the great omentum will be considered below. The course of the vessels in the ascending and descending meso- cola is not altered by the secondary adhesions. These vessels are in the adult situated behind the secondary parietal peritoneum derived from the mesocola. The origin of the transverse mesocolon obtains by the fixation of the hepatic and splenic flexures high up in the abdomen a transverse course, and the transverse growth of the abdomen holds the membrane in this position cephalad of the duodeno- jejunal flexure, so that on elevating the transverse colon the mesocolon appears as separating the upper from the lower ab- dominal compartment. This posterior line of attachment or so- called "root of the transverse mesocolon," is nothing more than the upper limit of the area of adhesion between the primitive 86 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. parietal peritoneum and the opposed surfaces of the ascending and descending mesocola. Reference to the abdominal cavity of the cat after complete rotation (Fig. 146) will show the original continuity of the three mesocola very clearly. A secondary con- nection is established along the lateral border of ascending and descending colon (Fig. 158), between the primitive parietal peri- toneum and the ventral visceral peritoneal investment of the large intestine. Both of the vertical segments of the colon now appear fixed. Their dorsal surfaces are uncovered by peritoneum and can be reached in the lumbar region, laterad of the kidney, without opening the peritoneal cavity (lumbar colotomy). The caudal portions of both kidneys are covered, beneath the secondary parietal peritoneum, by a layer of loose connective tissue representing the result of obliteration by adhesion of the first and second of the original three layers of prerenal peritoneum, viz., the primitive parietal (1) and the two layers of the mesocola (2 and 3). Line of Attachment of the Mesentery of the Jejuno-ileum. — Examination of the caudal surface of the transverse mesocolon in the cat, with the parts in the above outlined position, will show how and why in the adult human abdomen the duodeno-jejunal angle appears to dip out from beneath the transverse mesocolon, becoming gradually more and more free until complete transition to the mobile jejunum is obtained. From this point, situated to the left of the second lumbar vertebra, the dorsal attachment of the adult human mesentery of the jejuno-ileum extends some- what obliquely caudad and to the right to terminate in the right iliac fossa at the ileo-colic junction. Returning to the conditions presented by the cat's intestines to obtain an explanation of this line of fixation we must recall the fact that in the peritoneum included within the limits of the umbilical loop, after differentiation of small and large intestine, but before rotation, we have both the elements of the mesentery of the small intestines and of the ascending and transverse meso- colon combined (Fig. 141). For it will be seen that this mem- MESENTERY OF JEJUNO-ILEUM. 87 brane carries at this time vessels both to the jejuno-ileum and to the segments of the large intestine (caecum, ascending and trans- verse colon). This fact will be at once recognized if the cat's in- testines are arranged to correspond to the primitive condition (Fig. 136) and the mesentery examined. After rotation and differentiation of the colic segments and after the adhesion of the ascending and descending colon in man, the course of the main trunk of the superior mesenteric artery passes, after crossing the third portion of the duodenum, down and to the right to terminate near the ileo-colic junction by anastomosis with its ileo-colic branch. The adhesion of the right and left meso- cola to the dorsal parietal peritoneum proceeds mesad as far as this line, leaving free the mesentery of the small intestines, which contains the vasa intestini tenuis derived from the left side of the main vessel. The secondary line of attachment of the mesentery to the abdominal background is therefore along this line. To obtain a clear idea of these processes of development in man as- sume that in the cat, after rotation and establishment of the three divisions of the colon, the two vertical (ascending and descending) mesocola become adherent to the dorsal parietal peritoneum, leav- ing the mesentery of the small intestine free. Fig. 159 illustrates schematically the area of mesocolic adhe- sion in the human subject after complete rotation, and the Hne of the mesentery of jejuno-ileum. Fixation of the ascending and descending cola and of their mesocola proceeds cephalad as far as the line AB, which thereby constitutes the root of the free transverse mesocolon. The secondary parietal peritoneum derived from the ventral layer of the ascending mesocolon covers the lower and inner por- tion of the ventral surface of the right kidney, the infra-colic division of the descending and the dextro-mesenteric segment of the transverse duodenum, while along the root of the jejuno-ileal mesentery it becomes continuous with the right layer of that mem- brane. The secondary parietal peritoneum derived from the ven- tral layer of the descending colon covers the lower part of the ven- 88 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. tral surface of the left kidney and the sinistro-mesenteric segment of the transverse duodenum and becomes continuous along the mesenteric radix with the left layer of the jejuno-ileal mesentery. Caudad the adhesion of the descending colon and mesocolon to the parietal peritoneum proceeds only to the point C, following the dotted line mesad and resulting in the formation of the free mesocolon of the sigmoid flexure. Resume of the Adult Arrangement of the Human Peritoneum in the Lower Compartment of the Abdomen, Below the Level of the Trans- verse Colon and Mesocolon. — ^We should now consider the arrange- ment of the human peritoneum in the adult below the dorsal attachment of the transverse meso-colon in the light of the em- bryological and comparative anatomical facts just stated. In doing this it will be advisable to study both the actual conditions encountered and their significance in the sense of determining the derivation of the peritoneal layers from the primitive dorsal mesentery. Open the abdominal cavity in the usual manner by a cruciform incision. Turn the great omentum up on the chest wall, exposing the underlying intestines. This manipulation, as already stated, will cause the omentum to carry the transverse colon with it, on account of the adhesion, in the adult, of the gut to the dorsal layer of the omentum. Hence the cephalic or upper layer of the transverse mesocolon will not be seen at this stage because the omental adhesion just referred to prevents us from passing be- tween the greater curvature of the stomach and the transverse colon without tearing peritoneal layers. It will, however, be pos- sible to trace on the right side the duodenum from the pylorus down ventrad of the right kidney until the descending portion disappears behind the hepatic flexure of the colon. With the omentum and transverse mesocolon turned up, as stated, and the transverse mesocolon put upon the stretch, it will be seen that the abdominal space now overlooked is bounded cephalad by the lower layer of the transverse mesocolon and its attachment to the dorsal abdominal wall. The lateral limits of the space are given ADULT PERITONEAL RELATIONS IN INFRA-COLIC COMPARTMENT. 89 by the ascending and descending colon respectively. The attach- ment of the mesentery of the small intestine to the oblique line extending from the left of the vertebral column at about the level of the second lumbar vertebra to the right iliac fossa sub- divides the entire space into a secondary right and left compart- ment. Begin by following the caudal layer of the transverse meso- colon dorsad on the right side. In the angle between ascending and transverse colon (hepatic flexure) pressure will locate the caudal portion of the ventral surface of the right kidney. Re- member that the peritoneum touched in these procedures appears in the adult as parietal prerenal peritoneum, but that in reality it is the left leaf of the originally free ascending mesocolon, whereas the original right leaf of this membrane and the primitive parietal peritoneum have, by adhesion of their serous surfaces, been con- verted into the loose subserous connective tissue covering the ventral aspect of the kidney beneath what now appears as parietal peritoneum. Mesad of the resistance offered to the finger by the right kidney the caudal (infra-colic) portion of the descending duodenum and the angle of transition between it and the third or transverse portion will be found, invested in the same way by secondary (mesocolic) parietal peritoneum. It will be seen, especially if the duodenum is injected or inflated, that the hepatic flexure of the colon lies ventrad of the vertical descending second portion of the duodenum, so that one part of this intestine is situated cephalad the other caudad of the colon. (Supra- and infra-colic segments of descending duodenum.) Individual differences are observed in the area of colic attach- ment to the duodenum. Usually the two intestines are in con- tact with each other and adherent over a considerable surface. Exceptionally the transverse mesocolon extends across to the right so as to include the hepatic flexure. . In this latter case the uncovered non-peritoneal surface of the descending duodenum is small, represented by the interval between the layers of the trans- 90 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. verse mesocolon, and the hepatic flexure is then not directly ad- herent to the gut. If we now trace the transverse duodenum from right to left w^ will encounter the right layer of the root of the jejuno-ileal mes- entery. The caudal layer of the transverse mesocolon, the right leaf of the mesentery and the secondary parietal peritoneum invest- ing the ventral surface of the transverse duodenum all meet at this point. Surround the mesentery of the free small intestine with the fingers of one hand so that the entire mass of intestinal coils can be swung alternately from side to side. Turning them over to the left, as already stated, the proximal portion of the transverse duodenum can be traced from right to left as far as the root of the mesentery. Here the peritoneum investing the ventral surface of the duodenum becomes con- tinuous with the right leaf of the mesentery. Now swing the whole mass of small intestines over to the right, exposing the parietal peritoneum in the space to the left of the vertebral column, between the attachment of the mesentery to the median side, the root of transverse mesocolon cephalad and the descend- ing colon to the left. Remember that the same significance at- taches to this secondary parietal peritoneum as on the right side. It appears in the adult as parietal peritoneum, but is in its deri- vation the original right leaf of the descending mesocolon. Close to the root of the mesentery the continuation from the right side of the transverse duodenum will be seen, crossing the median line from right to left ventrad of aorta and vertebral column and usu- ally turning cephalad on the left side of the lumbar vertebrae, as the fourth or ascending duodenum, to reach the caudal surface of the transverse mesocolon near its attachment, where the gut turns ventrad to form the duodeno-jejunal angle and become continuous with the free small intestine. From the fact that the transverse duodenum is thus seen on each side of the root of the mesentery it will be recalled that after rotation of the primitive intestine the superior mesenteric artery crosses the transverse portion of the duodenum to reach its ADULT PERITONEAL RELATIONS IN INFRA-COLIC COMPARTMENT. 91 distribution between the leaves of the mesentery. Hence this portion of the small intestine consists of a dextro- and sinistro- mesenteric segment. This intersection of mesentery and duode- num marks the site of the primitive duodeno-colic -isthmus through which the superior mesenteric artery passed to reach its distribution to the gut composing the embryonic umbilical loop. To the left of the ascending duodenum a portion of the caudal surface of the pancreas will be seen, covered by the continuation of the caudal leaf of the transverse meso-colon into the parietal peritoneum. The consideration of this relation of peritoneum and pancreas will profitably be deferred until we have studied the developmental changes in the region of the dorsal meso- gastrium and great omentum. In the angle between termination of the transverse colon and proximal part of descending colon (splenic flexure) the caudal part of the ventral surface of the left kidney will be felt. The disposition of the peritoneum and its significance is the same as on the right side. Inasmuch as we have already seen that the sec- ondary parietal peritoneum covering the dorsal abdominal wall on each side of the small intestine's mesenteric attachment is de- rived from the primitive ascending and descending mesocolon, it will be readily understood why the blood vessels supplying the ascending and descending colon (arteria ileo-colica, a. colica dextra, a. colica sinistra) are placed beJmid the parietal peritoneum, while the colica media, supplying the transverse colon, runs between the layers of the transverse mesocolon. Originally the same condition obtained for the two vertical colic segments, but with the anchor- ing of these portions of the large intestine and the adhesion of their mesocola to the parietal peritoneum the blood vessels which for- merly ran between the two layers of the membrane, as long as it re- mained free, now appear as retroperitoneal vessels placed beneath the parietal peritoneum derived secondarily from the mesocola. This fact must be borne in mind in studying the arrangement of certain folds and fossae of the parietal peritoneum which are now to be considered. 92 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. Duodenal Fossae. Fossa of Treitz and Retro-peritoneal Hernia The peritoneal cavity of the cat can be used to great advantage in order to obtain a clear idea of the formation of these folds and fossae, whose relation to the so-called " retro-peritoneal hernia " has led to an exaggerated elaboration of minute detail and a somewhat puzzling terminology in human descriptive anatomy. Directions for Examining the Folds and the Formation of the Duo- deno-jejunal Fossa in the Cat — ^Turn the omentum and the coils of the small intestine cephalad out of the abdomen until they rest upon the ventral thoracic wall. Press the large intestine over to the left side, putting the mesocolon on the stretch until the parts are arranged as shown in Fig. 160. The loop of the duodenum with the head portion of the pancreas will be seen caudad of the liver and ventrad of the right kidney. A well-marked peritoneal fold, somewhat sickle-shaped, with the concavity of the free edge directed caudad and to the right, will be seen extending from the convex border of the duodenum, directly opposite the mesenteric or attached margin, to the right leaf of the mesocolon. This fold indicates the beginning adhesion of the duodenum to the mesocolic peritoneum, the first step toward the subsequent com- plete fixation of the gut as it is found in man. Fig. 161 shows the abdominal cavity of Nasua rufa, the brown Coati-mundi, a South American arctoid carnivore, with the in- testines everted and turned to the left side. In this animal the large intestine is very short, there is no caecum, the ileo-colic junction is only marked on the surface by a pyloric-like constric- tion of the tube and in the interior by the projection of a ring- valve (Fig. 408). The duodenal fold is very well developed, passing between the convex surface of the duodenal loop and the adjacent right leaf of the short mesocolon. In Primates, in which complete rotation of the intestine, on the plan of the human development, takes place, still further and more extensive agglutination of the serous surface of the duode- num to the peritoneum of the mesocolon occurs. Fig. 162 shows DUODENO-JEJUNAL FOLD AND FOSSA IN THE CAT. 93 the condition in Hapale vulgaris, one of the marmosets. The as- cending and descending mesocola and the mesoduodenum of this animal are still free, but the surface of the duodenum has become fastened to the opposed mesocolon. With fixation of the hepatic flexure and adhesion of the ascending colon, such as occurs in man, the duodenum is carried dorsad against the ventral surface of the right kidney, and now anchoring of the duodenum, by ob- literation of the mesoduodenum and adhesion to the prerenal parietal peritoneum, takes place as already detailed above. To return now to the formation of the duodeno-jejunal fossa by means of this fold, as illustrated in the cat. Perform the manipu- lations already described in rotation of the intestine. The appear- ance of the parts then will be as shown in Fig. 163. The large intestine is drawn over so as to represent the human ascending and transverse colon in one segment, the descending colon in the other, and the mesocolon appears correspondingly as transverse and descending. In other words the cat's intestines as arranged in the figure would represent the stage in the human develop- ment in which caecum and beginning of large intestine are still subhepatic in position ventrad of the right kidney, before differen- tiation of ascending and transverse colon by descent of caecum into right iliac fossa. In the human subject, as we have seen, the transverse meso- colon obtains a secondary attachment to the background of the abdominal cavity, its caudal surface remaining free. The descending mecocolon turns its original right leaf ventrad, its left leaf dorsad, and the latter adheres to the primitive parietal peritoneum covering the left lumbar region and ventral surface of left kidney. This area of adhesion extends up to and usually involves the dorsal surface of the descending colon, anchoring the same in the left lumbar region, down to the point where the sigmoid flexure begins and where the original mesocolon again appears free. In the cat, therefore, with the intestines arranged to correspond to the course of the human large intestine after rotation has been 94 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. accomplished, the Unas representing the peritoneal human adhe- sions should be fixed, as shown in the schema, Fig. 159 : ab, Une of secondary attachment after rotation resulting in the formation of the " root " of a free transverse mesocolon, bc, line of limit of secondary adhesion to the original parietal peritoneum involving the entire left (now dorsal) layer of the descending mesocolon and the dorsal surface of the descending colon, resulting in the fixa- tion of the latter part of the large intestine. This establishes, as already stated, a secondary parietal peri- toneal surface in the left lumbar region derived from the original right leaf of the descending mesocolon. Inasmuch as the infe- rior mesenteric vessels originally passed to the descending colon between the layers of the mesocolon they will now apparently be placed beneath the (secondary) parietal peritoneum of the left lumbar region. If now the duodenal fold in the cat be examined after rotation of the intestine it will be found presenting the original relations (Figs. 160 and 163), viz., passing from the convex margin of that portion of the duodenal loop which would correspond to the human fourth or ascending portion, to the original right layer of the meso- colon, which in man becomes secondarily converted into the parietal peritoneum of the left lumbar region. Hence the connec- tions of the fold are as follows : On the right : ventral surface of the ascending duodenum. On the left : right layer of mesocolon (secondary lumbar parietal peritoneum in the adult human subject). Cephalad it abuts against the caudal layer of the transverse meso- colon along the line which would correspond to the root of the mesocolon in the adult human subject. The concave caudal edge is free and bounds the entrance into a fossa, the "superior duodenal fossa" of anthropotomy. This fossa opens caudad and extends cephalad to the root of the transverse mesocolon. The ventral and left wall of the fossa is formed by the fold in question, its background by the mesocolon (right leaf) ; to the right the left circumference of the ascending duo- DUODENO-JEJUNAL FOLDS AND FOSSM IN MAN. 95 denum enters into the formation of the fossa, and its fundus is formed by the confluence of the fold and of the caudal layer of the transverse mesocolon. The inferior mesenteric vessels are found near the left margin of the entrance into the fossa. Fig. 164 shows the appearance of the fold in Nasua rufa after rotation of the intestine. The short course of the large intestine in this animal, and the consequent reduction of the mesocolon, brings the fold much below the level which it occupies in the cat. If we now look for the corresponding structures in man we will find certain modifications depending chiefly upon still closer ad- hesion between duodenum and the mesocolon which is destined to become the left parietal peritoneum after anchoring of the descending colon. We have already encountered an example of such closer connection in the marmoset shown in Fig. 162. In all cases the "superior duodenal" fold, corresponding to the fold just encountered in the cat, is the original condition, and the duodenal fossa consequently opens caudad. In many instances this will be the only fold and fossa encountered in the adult human subject. In other instances more extensive duodeno-meso- colic adhesions result in the addition of an "inferior fold," bounding a fossa the entrance into which is directed cephalad to- ward the transverse mesocolon. Such a condition is seen in Fig. 165 taken from a foetus at term. The duodenal fossa in this case is bounded by an " upper " and "lower " duodenal fold continuous with each other on the left side, but separated on the right at their attachment to the duodenum. It will be seen that the inferior mesenteric vein runs in the left margin of the fold, following along the left border of the entrance into the fossa. A segment of the colica sinistra artery may occupy the same position. This position of the vein, or artery, or of both vessels, is not the cause leading to the formation of the duodenal fossa, but is more or less accidental and variable. In many cases the vessels run at some distance from the folds bounding the fossa. In some subjects the "inferior" fold is the only one found, and the only duodenal fossa then encountered looks cephalad. 96 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. This condition, when associated with the course of the inferior mesenteric vessels in the free edge of the fold, constituted the classical "fossa duodeno-jejunalis" of Treitz, and is described as "Treitz's fossa." Fig. 166 shows the condition in which only a small inferior fold attaches itself to the termination of the transverse duodenum. There is practically an entire absence of duodenal or duodeno- jejunal folds and fossse. The inferior mesenteric vessels course under cover of the mesocolic secondary parietal peritoneum, but do not produce a fold. Fig. 167, from an adult human subject, illustrates the further development of the fossa from the foetal conditions shown in Fig. 165. The well-marked duodenal fossa is bounded by a superior and inferior duodenal fold, uniting laterally in a crescentic mar- gin containing a segment of the inferior mesenteric vein and colica sinistra artery. The lower division of the peritoneal recess thus produced corresponds to the typical (vascular) "fossa of Treitz." Mesally the projection of the fourth portion of the duo- denum bounds the fossa. In Fig. 168, also taken from an adult human subject, an exten- sive duodenal recess is bounded in the same way by a superior and inferior duodenal fold. In the interior of the fossa a third duodenal reduplication of the peritoneum ("intermediate duodenal fold") is seen, as is also the trunk of the inferior mesenteric vein, while the main trunk of the colica sinistra artery courses laterally behind the secondary mesocolic parietal peritoneum near the margin of the descending colon. It will be seen that the freedom of the ascending or fourth por- tion of the duodenum depends largely upon the disposition and extent of these folds. Inasmuch as they are the product of vary- ing degrees of adhesion of this segment of the intestine they are subject to great individual variations and have given rise to an unnecessary and complicated classification of the duodenal folds arid fossae. The close relation maintained between the duodeno- jejunal angle and the caudal layer of the transverse mesocolon FOSSA INTERSIQMOIDEA. 97 near its root at times leads to the production of a peritoneal fold connecting this membrane with the duodeno-jejunal knuckle of intestine (duodeno-jejunal or mesocolic fold) and may result in the formation of a duodeno-jejunal or mesocolic fossa of the peri- toneum. An instance of this fold is seen in Fig. 168. The importance of the duodenal fossae, and of similar peritoneal recesses in other parts of the abdominal cavity, is founded on the fact that by gradual enlargement they may lodge the greater part of the movable small intestine in their interior, leading to the formation of intra- or retro-peritoneal hernise.^ Fossa Intersigmoidea. — A second peritoneal pocket or fossa is encountered in the region of the sigmoid flexure and its meso- colon. The formation of this fossa is closely associated with the adult disposition of the sigmoid mesocolon as part of the original primitive vertical dorsal mesentery. In the typical arrangement of the parts the sigmoid or omega loop of the large intestine has a free mesocolon. The adhesion of the descending mesocolon to the parietal peritoneum usually ceases along a line drawn hori- zontally from the lateral margin of the left psoas at a level with the crest of the ilium to the medial side of the iliac vessels. This line, along which the mesocolon ceases to be adherent to the pa- rietal peritoneum, joins the attachment of the distal portion of the sigmoid mesocolon, which partially retains its primitive ver- tical origin to the dorsal midline, at a right angle. This angle is the site of the intersigmoid fossa, the entrance into which is seen usually as a round opening of variable size on elevating the sigmoid flexure and putting its mesocolon on the stretch. Fig. 159 shows the area of adhesion between the primitive descending mesocolon and the parietal peritoneum (from c mesad) which re- sults in the formation of a free mesocolon for the sigmoid flexure. Frequently in the angle formed by the horizontal and vertical line of attachment of the sigmoid mesocolon a non-adherent strip of the primitive mesocolon roofs in a more or less extensive ^ For fnll details of the anatomical and i>atbological conditions involved consult B. G. A. Moynihan "On Retro-peritoneal Hernia" — London, 1899. 7 98 ANATOMY OF PERITONEUM AND ABDOMINAL CAVITY. intersigmoid fossa, whose fundus is directed upwards and in- wards. Caecum, Appendix and Ileo-colic Junction. — Several peritoneal fossae and folds are found in the ileo-colic region in connection with the csecum, appendix and termination of the ileum. The practical importance of this portion of the intestinal tract and the great morphological interest which attaches to the same make it worth while to consider its anatomy in a separate chapter. PART II. ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT OF THE ABDOMEN. We have already seen that the transverse colon and meso- colon effect a general division of the adult human abdominal cavity into a cephalic supra-colic compartment, situated between the diaphragm and the level of the transverse colon and meso- colon, comprising in general the hypochondriac and epigastric regions, and a larger caudal infra-colic space which includes the entire rest of the abdominal cavity and is continued caudad into the pelvic cavity. The arrangement of the peritoneum and viscera in this latter space has just been considered. The fact will be recalled that the second or descending portion of the duodenum, passing dorsad of the hepatic colic flexure, forms so to speak the visceral connection between the portions of the alimentary tube situated in the supra-colic compartment and those situated in the infra-colic space. The fixation of this segment of the duodenum and its consequent secondary retroperitoneal position in the adult human subject masks this continuity of the alimentary canal to a certain extent so that it requires more than a superficial exami- nation in order to trace correctly the course of the duodenum from the pylorus to the duodeno-jejunal angle, dorsad of the colon, root of transverse mesocolon and mesentery, and under cover of the secondary parietal peritoneum. We have now to turn our attention to the viscera contained in the cephalic or supra-colic compartment of the abdomen and to consider the disposition of the serous membrane investing them and connecting them with each other and with the abdominal parietes. The visceral contents of the supra-colic compartment comprise 99 100 ANATOMY OF THE PERITONEUM. the liver, pancreas, spleen, stomach and the proximal portion of the duodenum, including the hepatic angle and the supra-colic part of the descending duodenum. Less directly the cephalic portions of the right and left kidney and the corresponding supra- renal capsules belong to this visceral group. In this region of the abdomen we meet with the most exten- sive modifications of the primitive dorsal peritoneal membrane, producing conditions which, considered without reference to devel- opment and comparative anatomy, are complex and difficult of comprehension. These changes lead to the formation of the so- called " lesser sac," a term which in some respects is unfortunate as it implies a more complete degree of separation from the gen- eral peritoneal cavity or " greater sac " than actually exists. In order to clearly understand the adult arrangement of the peritoneum in this region it is advisable to consider the subject in two distinct subdivisions, dealing successively with the two cardinal facts which contribute to effect the change from the sim- ple primitive to the complicated adult condition. These two main elements are : 1. Developmental changes in the position of the stomach, al- terations in the disposition of the proximal part of the primitive dorsal mesentery attached to the stomach, and the development of pancreas and spleen in connection with this membrane. 2. The development of the liver and the successive stages in the production of the final adult vascular and serous relations of this organ. 1. Stomach and Dorsal Mesogastrium. — We have already consid- ered the early stages in the differentiation of the stomach from the primitive intestinal tube of uniform caliber (p. 40). It will be recalled that the stomach at a certain period, while it already presents the main structural features familiar in the adult organ, occupies a vertical position in the abdominal cavity, turning its concave margin (lesser curvature) ventrad, while the convex dor- sal border (greater curvature) is directed toward the vertebral column, being attached to the same by the layers of the proximal STOMACH AND DORSAL MESOGASTRIUM. 101 part of the primitive dorsal mesentery. At this time the stomach presents right and left surfaces, and the oesophageal entrance is at the highest or cephalic point of the organ, while the pyloric transition to the small intestine occupies the distal caudal ex- tremity. ~^ " The primitive dorsal mesentery, as already stated, passes as a thin double-layered membrane between the ventral surface of the vertebral column and the dorsal border of the stomach, which, as we will presently see, becomes during the later stages of develop- ment the caudal (lower) margin or greater curvature. It will be seen that the embryonic differentiation of the intes- tinal tract into successive segments justifies the application of a terminology based on this differentiation to the corresponding portions of the primitive common dorsal mesentery. Thus the proximal portion extending between the vertebral column and the dorsal border or greater curvature of the stom- ach becomes the mesogastrium ; we differentiate this portion still further as the "dorsal mesogastrium" to distinguish it from a "ventral mesogastrium" which we will presently encoun- ter in considering the development of the liver and the con- nected peritoneum. In the same way the section of the primitive common dorsal mesentery attached to the duodenal loop becomes the mesoduo- denum, that connected with the mobile part of the small intestine (jejuno-ileum) the mesentery proper, while the portion passing to the colon forms the mesocolon, to be subsequently still further subdivided, after the different segments of the large intestine have become mapped out, as the ascending, transverse and descending mesocolon, the mesosigmoidea and the mesorectum. In tracing the development of the adult human peritoneum it is well to consider certain stages, which we will find illustrated by the permanent conditions presented by some of the lower ver- tebrates : These stages comprise : (a) Changes in the position of the stomach. 102 ANATOMY OF THE PERITONEUM. (6) Changes in the direction and extent of the dorsal meso- gastrium. (c) Development of the pancreas and spleen in connection with the mesogastrium. A. Changes in the Position of the Stomach. The primitive position of the organ above outlined (p. 41) is changed during the course of further development by a twofold rotation. 1. The primitive vertical position, in which the oesophageal en- trance occupies the highest cephalic extremity, while the pyloric opening is at the opposite caudal end, is exchanged for one directed more transversely, approximating the two gastric orifices to the same horizontal level. In human embryos of 13.9 mm. the fundus has already descended, the pylorus moving cephalad and to the right, while the cardia becomes shifted more to the left. At the same time the greater growth and prominence of the convex border or greater curvature becomes marked in comparison with the rela- tively short extent of the opposite margin or lesser curvature. 2. Coincident with this change in position is a rotation around the vertical axis, by means of which the original left side of the stomach is turned ventrad, becoming the ventral or " anterior " surface, while the original right surface of the organ now looks dorsad toward the vertebral column, becoming the dorsal or " posterior " surface of human anatomy. The oesophageal or cephalic end is placed to the left of the median line, while the caudal or pyloric end is situated on the right side (Figs. 169 and 170). The original ventral border, now the " lesser curvature " or "upper border," looks cephalad and to the right, toward the caudal surface of the liver, while the original dorsal border, as the " greater curvature " or ''lower border" is directed in the main caudad and to the left. The prominence of this border is still further increased by the greater development of the stomach to the left of the oesophageal CHANGES IN POSITION OF STOMACH. 103 entrance resulting in the formation of the "fundus" or "great cul-de-sac." This rotation of the stomach explains the asymmetrical position of the vagus nerve in the adult, the left side of the embryonic stomach, innervated by the left vagus, becoming the "anterior " surface of adult descriptive anatomy and vice versa. It will be readily appreciated that a comparatively flat organ like the stomach, will, as long as it occupies a sagittal position, with right and left surfaces, help to divide the upper part of the abdominal cavity to a certain extent into a right and left half, even if the peritoneal connections of the organ are left out of considera- tion. As soon, however, as the above-described changes in posi- tion take place and the surfaces of the stomach are directed ven- trad and dorsad, the relative arrangement and extent of this right and left abdominal space becomes altered by the different disposition of the septum, i. e., the stomach. The original right side of the organ is now directed dorsad, and the rotation of the organ has created a space between this dorsal or "posterior" surface of the stomach and the background of the abdominal cavity, which is the inception of the " lesser peritoneal cavity " or retrogastric space. We will find that this space becomes well defined and circumscribed by the peritoneal connections of the stomach, but we will realize, even at this stage, that the dorsal surface of the stomach will form a part of the general ventral wall of the lesser peritoneal space. On the other hand, the partial division of the abdomen into a right and left half, effected by the stomach in its primitive sagit- tal position, disappears after rotation of the organ. We now pass uninterruptedly from left to right across the ventral (original left) surface of the stomach. B. Changes in the Direction and Extent of the Dorsal Mesogastrium. The effects of the altered position of the stomach on the dispo- sition of the abdominal space have just been considered in rela- tion to the organ itself, without reference to its natural connec- 104 ANATOMY OF THE PERITONEUM. tions with the parietes and with adjacent viscera. Their true sig- nificance and their influence on the adult anatomical arrange- ment of the abdomen is, however, only appreciated when the changes in the arrangement of the peritoneal membrane which they involve, are taken into account. The dorsal mesogastrium changes more than any other portion of the peritoneum in the course of development. It not only becomes displaced and altered in direction by the rotation of the stomach, but in addition it grows so extensively that it finally hangs down like an apron over the entire mass of small intestines, forming the great omentum. If we begin with the primitive disposition of the sagittal stomach and dorsal mesogastrium shown in Fig. 171 it will be observed that both structures together actually divide the dorsal portion of the abdominal cavity into symmetrical right and left halves (Fig. 172). After rotation of the stomach (Fig. 173) the mesogastrium loses its original sagittal direction. It follows the altered position of the original dorsal border of the stomach, which has now become the caudal margin or "greater curvature," by turning caudad and to the left, being at the same time considerably elongated. This occurs during the second month. Hence the dorsal mesogastrium, after leaving the vertebral column, turns ventrad and to the left to reach its gastric attachment along the greater curvature. This is the first indication of the formation of the great omental or epiploic bursa. The stomach is here considered as developing in situ and as in- fluencing by its growth and change of position the arrangement and direction of the peritoneal layers with which it is connected. As a matter of fact it is well to note that the stomach at first lies above the primitive diaphragm or septum transversum, migrating, however, at an early period into the subhepatic abdominal posi- tion. This migration produces a corresponding increase in the length of the oesophagus (Fig. 34) and the stomach, in conse- quence of this change in position, acquires its ventral and dorsal mesogastrium. For the purpose of explaining the adult peri- PLATE LXXXI. STOMACH DORSAL ME^OGAS- TRIUM PRIMITIVE PARIETAL PERITONEUM KIDNEY Fig. 171.— Schematic representation of dorsal mesogastrium before rotation of stomach. VENTR MESOGA TRIUM - f^^ P^ X $^ ::> Fig. 175. — After adhesion over area of dotted line between dorsal mesogastrium and ])rimitive parietal jjeritoneum. Secondary line of transition from dorsal mesogastrium to parietal peritoneum at X. PLATE LXXXIII. MESODUOD- ENUM DORSAL MESOGAS- TRIUM VENTRAL MESOGAS- TRIUM Dt- VIDED ALONG LESSER CURVA- TURE OMENTAL BURSA Fig. 17G. — Schematic ventral view of stomach, duodenum, and dorsal mesogastrium, after rotation of stomach and ex- tension of omental bursa caudad beyond greater curvature of stomach. The ventral mesogastrium is detached along the lesser curvature. DORSAL MESO- GASTRIUM VENTRAL MESOGAS- TRIUM FORMING GASTRO-HEPATIC OMENTUM HEPATIC DUCT MESODUODCNUM WITH PANCREATIC BUDS BETWEEN LAYERS TRANSV. COLON ASC. MESOCOLON ASC. COLON APPENDIX VITELLO- INTESTINAL CESOPHAGUS DORSAL MESO- GASTRIUM FORMING GREAT OMENTUM DUODENO- JEJUNAL JUNCTION OESC. COLON DESC. MESO- COLON Fig. 177. — Semi-diagrammatic representation of peritoneal membrane in human embryo. (After KoUmann.) PLATE LXXXIV. STOMACH DORSAL MESOGAS- TRIUM FORMING VENTRAL LAYERS OF OMENTUM CAVITY OF OMENTAL BURSA PRIMITIYE PARIETAL PERITONEUM DORSAL MESOGAS- TRIUM FORMING DORSAL LAYERS OF OMENTUM Fig. 178. — Schematic sagittal section through stomach and dorsjil mesogastrium, after rotation and formation of omental bursa. SECONDARY PARIETAL PERIT. DERIVED FROM ORIGINAL RIGHT LAYER OF DORSAL MESOGAS- TRIUM DORSAL MESOGAS- TRIUM FORMING VENTRAL LAYERS OF OMENTUM CAVITY OF OMENTAL BURSA AREA OF ADHESION BETW. PRIMITIVE PARIETAL PERIT. ANDORIGINAL LEFT LAYER OF DORSAL MESOGASTRIUM KIDNEY DORSAL MESOGAS- TRIUM FORMING DORSAL LAYERS OF OMENTUM Fig. 179. — Schematic sagittal section through stomach and dorsal meso- gastrium after adhesion to prerenal parietal xieritoneum. PLATE LXXXV. STOMACH GASTHO-SPLENIC SEGMENT OF DORSAL MESOGASTR. WITH A. GAGTRO-EPIPLOICA SINISTRA SPLEEN VERTEBRO-SPLENIC SEGMENT OF DORSAL MESOGASTR. WITH SPLENIC ART. Fig. 180. — Schematic transverse section of the abdomen, showing early stage of development of spleen from extreme left end of dorsal mesogastric pouch. SPLEEN GASTRO-SPLENIC OMENTUM WITH GASTRIC BRANCHES OF SPLENIC ART. SECONDARY PARIETAL PER IT. DERIVED FROM MESOGASTR LIENO-RENAL PERIT. FOLD PARIETAL PERIT. STOMACH AORTA AND ROOT OF DORSAL MESOGAS- TRIUM R. KIDNEY Fig. 181. — Schematic transverse section of the abdomen, showing later stage of development of spleen and arrangement of peritoneum after adhesion of dorsal layer of mesogastrium and primitive prerenal parietal peritoneum. CUT EDGE OF GRE*T OMENTUM DUODENUM Fig. 182. — Part of the abdominal viscera of child, two years old, hardened in situ and removed |from body. The great omentum has been detached along the line of the transverse colon. (Columbia University, Study Collection.) PLATE LXXXVI. SECONDARY PARIETAL PERIT. OF LESSER SAC DERIVED FROM MESO- GASTRIUM TWO LAYERS OF GAS TRO-SPLENIC OMENTUM CUT EDGE OF LIENO- RENAL FOLD TAIL OF PANCREAS CUT EDGE OF GREAT OMENTUM GASTRIC SURFACE OF SPLEEN SURFACE COVERED BY LESSER SAC GASTRO-SPLENIC OMENTUM REFLECTION TO DOR- SAL WALL OF LESSER SAC REFLECTION TO KIDNEY AND DIAPHRAGM (LIENO- RENAL LIG.) TAIL OF PANCREAS RENAL SURFACE OF SPLEEN Fig. 183.— The same preparation with the spleen removed, showing lines uf peritoneal reflec- tion on mesial surface of the organ. DORSAL SURFACE OF STOMACH LEFT PANCREATICO- GASTRIC FOLD SEPA- RATING HEPATIC AN- TRUM OF LESSER SAC FROM BURSA OMEN- TALIS R KIDNEY HEPATIC SURFACr COLIC AREA or DESC. DUODENUN^ COLIC AREA OF R. KIDNEV MESOCOLIC AREA OF R. KIDNEY INFRACOLIC SEG- MENT OF DESC. DUODENUM DEXTRO-MESEN- TERIC SEGMENTOF TRANSV. DUOD. LEFT EXTREMITY OF LESSER PERITONEAL SACTOUCHING HILUS OF SPLEEN SPLEEN VENTRAL SURFACE OF PANCREAS ATTACHMENT OF TRANSV MESOCOLON TO VENTRAL MARGIN OF PANCREAS CAUDAL SURFACE OF PANCREAS LEFT KIDNEY ASC. DUODENUM MESENTERY CROSSING TRANSV. DUODENUM ATTACHMENT OF DESC. COLON SI NISTRO- MESENTERIC SEGMENT OF TRANSV DUODENUM Fig. 184.—Upper abdominal viscera of adult human subject, hardened in situ, with liver and coJon removed and stomach turned up. (Columbia University, Study Collection.) PLATE LXXXVII. LEFT HEPATIC DUCT RIGHT HEPATIC DUCT GALL-BLADDER DORSAL PANCREAS VENTRAL PANCREAS DUODENUM Fig. 185. — Pancreatic and hepatic buds of human embryo of four weeks. (Kollmann.) VENTRAL PANCREAS DORSAL PANCREAS STOMACH Fig. 186. — Pancreatic buds of human embryo of five weeks. (Kollmann, after Hamburger.) DUODENUM BILE-DUCT VENTRAL PANCREAS AND PAN- CREATIC DUCT DUCT OF SANTORINI FUSION OF THE TWO PANCREATIC BUDS CORRESPONDING TO ADULT CONNEC- TION OF DUCT OF SANTORINI AND MAIN PANCREATIC DUCT DORSAL PANCREAS STOMACH Fig. 187. — Pancreatic buds of human embryo of six weeks. (Kollmann, after Hamburger.) > X X X < DIRECTION AND EXTENT OF THE DORSAL MESOGASTRWM. 105 toneal relations of the organ it is, however, more convenient to regard the stomach as an abdominal organ from the beginning and to deal with the subsequent changes in position from this^ standpoint. The inaccuracy is slight and renders the -comprehen- sion of the succeeding stages easier. It will be noticed (Fig. 173) that the rudimentary retro-gastric space or "lesser peritoneal sac" is bounded ventrally by the dor- sal (the primitive right) surface of the stomach, while its dorsal boundary is furnished by the ventral (originally right) layer of the dorsal mesogastrium. In the primitive condition, therefore, dorsal mesogastrium and stomach form together a straight line sagittal in direction and placed in the median plane of the body. As the result of the developmental changes above outlined this straight line becomes bent at the point where the mesogastrium reaches the stomach (Fig. 173, x). The two component elements of the line (stomach and mesogastrium) hinge on each other here, and the angle which they form opens to the right. The changes which are to be observed in the later stages depend principally upon a peculiar feature characteristic of the develop- ment of the dorsal mesogastrium. This feature consists in the extreme redundancy of the membrane which grows out of pro- portion to the requirements of its visceral connections, and to a certain extent becomes independent of the direct mechanical pur- pose of carrying blood vessels to the viscera. Hence in a trans- verse section at this period (Figs. 174 and 175) the mesogastrium no longer passes in a direct line between its points of attachment, viz. the greater curvature of the stomach and the vertebral col- umn, but extends beyond the stomach to the left. We will appreciate the significance of this extensive growth of the meso- gastrium especially in considering the development of the spleen and pancreas. For the present it will suffice to note (Figs. 174 and 175) that the growth has carried the mesogastrium well to the left of the stomach, consequently the retrogastric space is now bounded toward the left by the bend which the original right leaf 106 ANATOMY OF THE PERITONEUM. of the primitive sagittal mesogastrium takes in order to reach its gastric attachment. The retrogastric space therefore terminates toward the left in a blind pocket formed by this reduplication of the mesogastrium. One more factor is to be taken into consideration, namely the tendency, already noted, of peritoneal surfaces to become adherent to each other. Such adhesion involves the apposed surfaces of the mesogastrium and of the primitive parietal peritoneum to the left of the vertebral column. The dorsal (original left) layer of the mesogastrium adheres to the parietal peritoneum covering the left side of the abdominal background and the cephalic por- tion of the ventral surface of the left kidney up to the end of the blind pouch which forms the extreme left limit of the retro- gastric space. Hence, after this process of adhesion is completed^ the dorsal wall of the retrogastric space is lined by secondary parietal peritoneum covering the left kidney (original right leaf of primitive mesogastrium) (Fig. 175). We obtain (Fig. 175 at x) an apparent continuity of the parietal peritoneum with that portion of the mesogastrium which, derived from the original left layer of the membrane, appears now to extend, as the ven- tral one of two layers, between the stomach and the abdominal parietes near the lateral border of the left kidney. (Primitive gastro-splenic omentum.) It should be remembered that the disposition of the peritoneum just indicated is modified by the development of the pancreas and spleen, both of which organs are intimately associated with the mesogastrium. The foregoing statements and diagrams are therefore merely given for the purpose of affording a general view of the extent, growth and changes of the dorsal mesogastrium before proceeding to consider the development of the pancreas and spleen in and from the membrane itself In the view directly from in front the redundancy of the peri- toneum forming the mesogastrium is shown in Figs. 176 and 177. Just as the membrane extends further to the left than required by its visceral connection with the stomach, so the downward growth DIRECTION AND EXTENT OF THE DORSAL MESOGASTRIUM. 107 exceeds the demand made by the rotation of the attached border (greater curvature) caudad and to the left. The mesogastriura, forming, as it now does, the great omentum, enlarges in descend- ing toward the transverse colon (Fig. 177). The bag thus formed can be distended with air in a foetus of from 8 to 9 cm. vertex- coccygeal measure, as shown in the figure. Consequently in sagit- tal section the membrane is seen to extend caudad beyond the level of the greater curvature, and must turn on itself and pass again cephalad in order to reach the stomach (Fig. 178). By reason of this excessive growth the limits of the primitive retro- gastric space are enlarged, not only toward the left, but more especially in the caudal direction. The bend made by the meso- gastrium in returning to the stomach forms the blind extremity of a pouch which continues the retrogastric space caudad beyond the stomach, and whose dorsal and ventral walls are formed by the reduplicated mesogastrium. This pocket or pouch consti- tutes the omental or epiploic bursa of the lesser peritoneal cavity, for the great omentum is the direct product of this redundant growth of the mesogastrium caudad. It will be observed that the great omentum is made up of four peritoneal layers, the fold- ing of the double-layered mesogastrium naturally producing tliis result. The first or ventral and the fourth or dorsal layer are derived from the original left layer of the primitive sagittal meso- gastrium ; the intermediate second and third layers, separated from each other at this stage by the cavity of the omental bursa, are products of the primitive right leaf of the mesogastrium. Since the entire retrogastric space with its extension^ becomes the " lesser cavity " of the human adult peritoneum, it will be seen that its serous membrane is derived from the original right leaf of the mesogastrium (second and third omental layers). After the above-described adhesion of the mesogastrium to the parietal peritoneum overlying the ventral surface of the left kidney, the membrane would be traced in sagittal section (Fig. 179) from the dorsal surface of the stomach caudad, lining the interior of the omental bursa (second layer) to the turn or blind end of the 108 ANATOMY OF THE PERITONEUM. pouch ; thence cephalad as the third omental layei*, forming the dor- sal wall of the epiploic bursa, to invest, as secondary parietal perito- neum, the cephalic segment of the ventral surface of the left kidney. 0. Development of Spleen and Pancreas in the Dorsal Mesogastrinm and Changes in the Disposition of the Great Omentum. In order to obtain a correct conception of the adult human conditions it is finally necessary to consider the development of the spleen and pancreas in their connection with the dorsal meso- gastrinm and to note the changes which are produced by adhesion of portions of the great omentum to adjacent serous surfaces. It will be advisable to discuss these subjects at first separately, and to subsequently combine all the facts in an attempt to gain a cor- rect impression of their share in determining the disposition of the adult human peritoneum. 1. Development of Spleen. — The spleen develops from the meso- derm between the layers of the dorsal mesogastrinm, near its point of accession to the greater curvature, in the region of the subse- quent fundus. It has therefore, like the stomach, originally free peritoneal surfaces. After rotation of the stomach the organ lies between the two layers of the membrane at the extreme left end of the retrogastric space (Fig. 180). Vascular Connections. — The splenic artery accedes to the mesal sur- face of the spleen from the vessel which originally passed directly to the dorsal border (subsequent greater curvature) of the stomach, between the layers of the mesogastrinm. With the further growth of the spleen the segment of this vessel situated between its origin from the coeliac axis and the hilum of the spleen becomes relatively larger, forming the adult splenic artery, while the continuation of the original vessel to the greater curvature of the stomach appears now as a branch of the splenic artery, viz., the arteria gastro-epiploica sinistra. Through the development of the spleen the dorsal mesogastrinm has been subdivided into a proximal longer vertebro-splenic, and a distal shorter gastro-splenic segment. The former, as we have VASCULAR CONNECTIONS. 109 seen, loses its identity as a free membrane in the human adult, by fusing with the parietal peritoneum investing the ventral surface of the left kidney. Hence, after this adhesion hasltaken place, the splenic artery courses from the coeliac axis to the spleen behind peritoneum which functions as part of the general parietal mem- brane, but which is derived from the original right leaf of the proximal vertebro-splenic segment of the primitive mesogastrium (Fig. 181). On the other hand the distal segment of this mem- brane, beyond the spleen, remains free, carrying, as the gastro- splenic omentum, the left gastro-epiploic artery between its layers from the splenic artery to the greater curvature of the stomach. The lateral limit of the area of adhesion between mesogastrium and parietal peritoneum is situated along the lateral border of the left kidney. Hence, in the final condition of the parts, the main splenic vessels at the hilum are situated between two peritoneal layers of which the ventral (Fig. 181) appears as the parietal peritoneum forming the dorsal wall of the retro-gastric space, while the dorsal layer (Fig. 181) forms a reflection from the mesal surface of the spleen, along the dorsal margin of the hilum, to the adjacent lateral border of the left kidney (lieno-renal ligament) and to the diaphragm. At this point of adhesion subsequently firmer strands of connective tissue develop in the serous redupli- cation forming the ligamentum phrenico-lienale of systematic anat- omy. This process of adhesion takes place during the second half of intra-uterine life. A connection with the colon, produced by adhesion of the mesogastrium to the splenic flexure of the large intestine, forms the adult lig. colico-lienale, while a similar adhesion between great omentum, transverse mesocolon and phrenic parietal peritoneum just caudad of the spleen, gives rise to the colico-phrenic or costo-colic " supporting " ligament of the spleen. On the other hand, the ventral one of the two layers constitut- ing the gastro-splenic omentum and including between them the left gastro-epiploic artery, is formed by the distal part of the primitive left layer of the mesogastrium, while the dorsal layer of the same fold is the portion of the primitive right layer be- 110 ANATOMY OF THE PERITONEUM. yond the spleen, which has not been converted into secondary parietal peritoneum, but forms now part of the ventral wall of the lesser peritoneal sac between the spleen and the stomach (Fig. 181) (lig. gastro-lienale). Since, therefore, the gastro-splenic omentum is a specialized part of the fully-developed dorsal meso- gastrium, and since we have seen that the great omentum is formed directly by the excessive growth of this membrane cau- dad, it is not difficult to understand why in the adult human subject the ventral layer of the gastro-splenic omentum is di- rectly continuous with the ventral layer of the great omentum along the greater curvature of the stomach to which both are attached. The dorsal layer of the gastro-splenic omentum would, in the same wa}^ be continuous with the second layer of the great omentum, lining the ventral wall of the omental bursa, if it were not for the fact that in the adult adhesions usually obliterate the cavity of the bursa. Fig. 182 shows the stomach, left kidney, spleen and splenic flexure of the colon hardened in situ and removed from the body of a two-year-old child. The great omentum has been divided along the line of adherence to the transverse colon. In Fig. 183 the spleen has been removed from the preparation by division of its peritoneal and vascular connections, and is shown in its mesal aspect (gastric and renal surfaces, intermediate margin and hilum). It will be seen that the peritoneal reflections are arranged in the form of two concentric elliptical lines. The two ventral lines form the gastro-splenic omentum and correspond to the reflection of the peritoneum from spleen to left end of stomach carrying the gastric branches derived from the splenic artery. The third line from before backwards results from the division of the secondary parietal peritoneum of the lesser sac, covering splenic artery, and ventral surface of pancreas and derived from the dorsal mesogastrium ; while the most dorsal fourth line represents the divided reflection of the peritoneum from the renal surface of spleen to lateral border of left kidney and diaphragm (lig. lieno-renale). DEVELOPMENT OF THE PANCREAS. Ill Between the second and third lines of peritoneal reflection ap- pears the portion of the mesal surface of the spleen in contact with and invested by the extreme left end of the lesser peritoneal sac. Fig. 184, taken from an adult human subject with -the viscera hardened in situ, shows the left or splenic extension of the lesser peritoneal cavity. 2. Development of the Pancreas. — The pancreatic gland is derived from the hypoblast of the enteric tube. The secreting epithelium and that lining the ducts of the adult gland is formed by bud- ding and proliferation of the intestinal epithelium. The gland develops primarily from two outgrowths which are at first sepa- rate and distinct from each other. 1. The proximal and dorsal bud grows directly from the hypo- blast lining the duodenum immediately beyond the pyloric junc- tion. In embryos of 8 mm. (four weeks) (Fig. 185) it appears as a small spherical outgrowth connected by a slightly narrower stalk with the epithelial intestinal tube. 2, The distal and ventral outgrowth is separated from the pre- ceding and is from the beginning closely connected with the similar embryonic outgrowth from the enteric tube which is to form the liver. This portion of the pancreas is, strictly speak- ing, derived primarily from the epithelium of the primitive hepatic duct and not directly from the duodenum. This pri- mary arrangement of the gland, being formed of two main col- lections of budding hypoblastic cells, corresponds to the adult system of the pancreatic excretory ducts. The proximal or dor- sal outgrowth furnishes that portion of the head of the gland whose excretory system terminates in the secondary pancreatic duct or duct of Santorini, while the distal (ventral) outgrowth includes within its area the termination of the principal pancre- atic duct or canal of Wirsung, which is closely connected with the end of the common bile-duct at the intestinal opening com- mon to both (Figs. 186-187). The method of union of the two pancreatic outgrowths and their respective share in building up 112 AMATOMY OF THE PERITONEUM. the adult gland explains the usual adult arrangement of the excretory system and its variations. In the embryo of five weeks (Fig. 186) the two portions have grown in length. The dorsal or proximal outgrowth, developing between the layers of the mesoduodenum, is at this time the larger of the two, composed of a number of glandular vesicles clustered around the stalk represented by the parent duct. The distal or ventral pancreatic growth, connected with the liver di *-, is as yet small and presents only a few vesicular ap- pendages. The duct of this portion empties in common with the hepatic duct into the duodenum. In embryos of the sixth to seventh week (Fig. 187), the two glandular outgrowths have become connected with each other at a point which corresponds exactly to the divergence of the duct of Santorini from the main pancreatic duct (canal of Wirsung) in the adult gland (Fig. 188). The secondary pancreatic duct (of Santorini) of the adult corre- sponds to that section of the proximal or larger embryonic out- growth situated between the intestine and the point where the two glandular diverticula fuse with each other. Hence the canal of Wirsung in the adult is a compound product. It includes the duct system developed, in connection with the bile duct, in the head of the gland, forming the intestinal termination of the main duct. Its distal body portion on the other hand is derived from the duct system of the originally larger proximal outgrowth, in- cluding the entire peripheral portion which has become secondarily added to the duct of tne ventral outgrowth to form together with it the canal of Wirsung. On the other hand the proximal portion of the duct system of this originally larger part becomes second- arily differentiated as the duct of Santorini. Fig. 188 shows the normal adult arrangement of the pancreatic and biliary ducts in a corrosion preparation of the canal. The duct of Santorini in this case opened by a separate orifice into the duodenum above the common opening of the biliary and pancreatic ducts (cf p. 113). PLATE LXXXIX. DUODENUM COMMON BILE-DUCT DIVERTICULUM VATERI 'COM- K'.CN DUODENAL OPENING OF BILIARY AND PANCREATIC DUCTS) DUCT OF SANTORINI MAtN PANCRE- ATIC DUCT CANAL OF WIRSUNG) Figs. 1 89-192.— Series of schemata showing normal and variant adult types of biliary and pancreatic ducts. Fig. 189. — Usual human adult type. DUODENUM DUCT OF SANTORIN ACTING AS MAIN PAN CREATIC DUCT COMMON BILE-DUCT PANCREATIC DUCT DEVEL- OPED FROM DISTAL EM- BRYONIC BUD Fig. 190. — Persistence of early embryonal type. PLATE XC. DUODENUM COMMON BILE-DUCT DIVERTICULUM VATERI WITH DUODENAL OPENING OF BILIARY AND PANCREATIC DUCTS DUCT OF SANTORINl WITHOUT DUODENAL OPENINC main pancre- atic duct (canal of WIRSUNG) Fig. 191. — Duct of Santoriiii has no duodenal orifice. DUODENUM DUCT OF SANTORINl FORMING SOLE PAN- CREATIC DUCT COMMON BILE-DUCT Fig. 192. — Duct of Santorini forms the only pancreatic duct. Separate duodenal openings of biliary and pancreatic ducts, resulting from failure of development of distal embryonal pancre- atic bud. PLATE XCI. PROBE IN DUODENAL OPENING OF DUCT OF SANTOBINI PROBE IN OPENING OF DIVERTICULUM VATERI (COM- MON OPENING OF BILIARY AND PANCRE- ATIC DUCTSJ Fig. 193.^Mucous surface of human duodenuuj, showing entrance of biliary and pancreatic ducts and diverticulum Vateri. (Columbia University Museum, No. 1842.) Fig. 194. — Adult human subject. Mucous membrane of pyloro-duodeual junction and of duodenum. (Columbia University Museum, No. 1840.) PLATE XCII. PANCREATIC DUCT COMMON BILE-DUCT DIVERTICU- LUM VATERI Fig. 195. — Duodenum, with entrance of pancreatic and biliary ducts and well-developed diverticulum Vateri in the cassowary, Casuarms caauarius. (Columbia Uni- versity Museum, No. 1821.) PLATE XCIII, OAI 1 - BLADDER HEPATIC DUCT PANCREAS C^^i COMMON BILE-DUCT RECEIVING PANCRE ATIC DUCTS / MID-GUT 1 ' S^ V PYLORIC C>ECA— C n -STOMACH Fig. 196.— a portion of alimentary canal of Pleurmiectes maculatus, the flounder, with pancreas attached to biliary duct and concealed in the substance of the liver, which has been removed. (Columbia Uni- versity Museum, No. 1491.) DUODENUM LOBES OF LIVER COMMON BILE-DUCT PANCREATIC DUCT Fig. 197.— Pancreas and biliary ducts of Bana esculenta, frog. (Wieder- sheim, after Parker; both from Ecker.) PLATE XCIV. CESOPHAGUS STOMACH POSTCAVA DIVIDED AT ENTRANCE INTO SINUS VENOSUS OF RIGHT AURICLE VENTRAL f ^ — BODY- WALL VENTRAL MESOGAS- TRIUM POSTCAVA DIVIDED AT ENTRANCE INTO LIVER SPLENIC V. PANCREAS GALL- BLADDER abdominal v. in free edge of ventral meso- gastrium hepatic portal 'mesenteric Fig. 198. — Necturus maculatus, mud puppy. Dissection of intestinal canal, liver, pancreas, and spleen, with blood-vessels injected. (Colum- bia University Museum, No. 18G3.) PLATE XCV. SMALL PROXI MAL PANCREATIC DUCT JOINING HEPATIC DUCT DISTAL MAIN PANCREATIC DUCT OPENING INTO ASC. LIMB OF DUODENAL LOOP Fig. 199. — Pancreas aud pancreatic ducts of rabbit. (Nuhn.) HEPATIC DUCT PROXIMAL SMALLER PANCREATIC DUCT OECUM DISTAL LARGER PANCREATIC DUCT Fig. 200. — Abdominal viscera of dog, showing arrangement of pancreatic ducts. (Nuhu.) PLATE XCVI. PROBE IN ORIFICE OF MAIN PANCRE- ATIC DUCT DUODENAL MUCOSA PROBE IN ORIFICE OF BILI- ARY DUCT OESOPHAGUS Fig. 201.— Section of dog's stomach, and proximal portion of duodenum, -with entrance of biliary and pancreatic ducts. (Columbia University Museum, No. 1822.) PLATE XCVII. PANCREAS MID-GUT: WITH SUB- INTESTINAL VEIN AND SPIRAL VALVE FORE-GUT STOMACH CONTRACTED PYLORIC SEG- MENT OF STOMACH Fig. 202. — Alimentary tract witli spleen and pancreas of Squalus acanthias, the dog-fish. (Columbia University Museum, No. 1405.) PLATE XCVIII. STOMACH PANCREAS SPIRAL INTES- TINAL VALVE Fig. 203. — Alimentary canal of Galeus canis, dog-siark, in section, showing spiral intestinal valve. (Columbia Univer- sity Museum, No. 1429.) PLATE XCIX. Fig. 204. — Alimentary canal with spiral valve of Ceratodus forsteri, the Australian lung-fish (Barramunda). (Columbia Uni- versity Museum, No. 1645.) o Oh ^ M S--S S § ._5 ft U) 0. u 1- tt) 1 1 1 1 1 1 1 1 g c !2; ^ a j3 (A, s< «t-l s hS 2 c-- '^ ^?-- r%> PLATE CI. PYLORUS — — — Fig. 207. — Stomach, duodenum, and pyloric cseca of Lophius piscatorius, angler. (Columbia University Museum, No. 1824.) MID-GUT PYLORIC C>eCA BILE-DUCT CESOPHAGUS STOMACH PYLORUS Fig. 208. — Pleuronecfes maculatus, window-pane. Stomach and mid-gut with pyloric cseca and hepatic duct. (Columbia UuivLr.sity Museum, No. 1432.) ORIFICE OF BILE-DUCT Fig. 209. — Pleuronectes maculatus, window-pane. Stomach and mid-gut with pyloric caeca, in section. (Columbia University Museum, No. 1433.) PLATE CII. Fig. 210. — Paralichthys dentatus, summer flounder. Stomach and mid-gut with pyloric cseca and liver. (Columbia University Museum, No. 1431.) COLLECTING TUBULES FORMED BY JUNCTION OF PYLORIC C/ECA A B Fig. 211. — Pyloric cffica of Gadns callarias, codfish. (Columbia University Museum, No. 182.5.) A. Bound together by connective tissue and blood-vessels. B, Dissected to show contiuence of cseca to form a smaller number of terminal tubes of larger calibre entering the intestine. PLATE cm. PYLORIC APPENDICES ILEO-COLIC JUNCTION STOMACH — - SPLEEN Fig. 212. — Alimentary canal of Accipenser sturio, sturgeon. Numerous pyloric cseca are bound together to form a gland-like organ. (Columbia Uni- versity Museum, Nos. 1826, 1827, and 1828.) In the smaller upper figure on the left the stomach, mid-gut, and pyloric ca?ca are seen in section, showing the lumen of the latter and their openings into the mid-gut. The lower left-hand figure shows the mid- and end-gut in section, the latter provided with a spiral mucous valve. PLATE CIV. Fig. 213. — Melamgrammus xglifinm, haddock. Stomach, mid-gut, and pyloric cseca ; spleen. (Columbia University Museum, No. lo98.) PYLORIC OECA PROBES PASSED INTO INTESTINAL ORIFICES OF PYLORIC CJECA STOMACH Fig. 214. — Stomach and mid-gut of Gadiis callarias, cod- fish, in section, showing intestinal openings of pyloric caeca. (Columbia University Museum, No. 1830.) ADULT ARRANGEMENT OF HUMAN PANCREATIC DUCTS. 113 Explanation of Adult Arrangement of Human Pancreatic Ducts and Their Variations Dependent Upon the Embryonic Development. — The smaller distal embryonic outgrowth is, as we have seen, from its inception in close connection with the duodenal end of the common bile- duct (Fig. 185). The proximal outgrowth, situated nearer to pylorus and de- rived directly from the duodenal epithehum, is the larger and forms the greater part of the bulk of the adult pancreas (Figs. 186, 187). If, notwithstanding this primitive arrangement, the distal duct (canal of Wirsung) appears as the main pancreatic duct in the adult, while the proximal (duct of Santorini) is secondary, this depends upon a union of the products of the two outgrowths in such a manner that the greater part of the duct system of the proximal and larger portion is transferred to the distal duct to form the adult canal of Wirsung, while the smaller segment of the proximal duct, between its opening into the duodenum and the point of fusion of the two outgrowths, forms the adult secon- dary duct of Santorini. This duct opens usually into the duo- denum upon a small papilla situated about 2.5 cm. above the common duodenal termination of the bile-duct and canal of Wir- sung (papilla Vateri) (Fig. 193). The duct of Santorini usually tapers toward the duodenal opening from its point of departure from the main duct, its caliber gradually diminishing in the di- rection indicated, so that it is smaller at the duodenal opening than at the point of confluence with the main duct (Fig. 189). Hence the secretion from the proximal head portion of the pan- creas, conveyed by this duct and its tributaries, passes usually into the main pancreatic duct and not directly into the intestine through the duodenal opening of the duct of Santorini. The latter is, however, thus enabled to vicariously take upon itself the conduct of the pancreatic secretion in cases of obstruction or obliteration of the main duct (calculi, ulcers, cicatrices, etc.). In these cases of obstruction of the main duct the duct of Santorini enlarges and performs its functions. 8 114 ANATOMY OF THE PERITONEUM, Occasionally, without obstruction of the main duct, the duo- denal opening of the duct of Santorini is large, and the flow of secretion evidently the reverse of the usual, i. e., directly into the intestine. In other cases, also without pathological conditions, the prox- imal duct is the larger of the two and serves as the principal channel of pancreatic secretion, the canal of Wirsung being small. This is evidently a persistence and further development of the early embryonic relative condition of the two outgrowths above described (Fig. 190). On the other hand the duct of Santorini may not open at all into the duodenum, terminating in small branches which drain the proximal part of the head of the gland Fig. 191). Schirmer has examined the arrangement of the pancreatic ducts in 105 specimens. In 56 of these the duct of Santorini passed from the main duct into the duodenum, opening upon a papilla situated 2.5 cm. above the common opening of the bile duct and canal of Wirsung. In 19 the duct of Santorini was well developed but did not open into the duodenum. In but 4 cases the duct of Santorini formed the only pancreatic duct, the lower opening being occupied by the bile duct alone (Fig. 192). We may assume in these cases failure of develop- ment of the distal outgrowth connected with the primitive hepatic bud, leaving only the proximal duodenal outgrowth to form the entire adult gland. Figs. 188 and 189 show the normal arrangement of the duo- denal openings of the biliary and pancreatic ducts. Figs. 190 to 192 show schematically the variations in the rela- tive development and the adult arrangement of the pancreatic ducts. Diverticulum and Papilla Vateri — From what has been said re- garding the embryonic union of the distal pancreatic outgrowth with the hepatic bud it will be easy to recognize the corresponding features in the arrangement of the adult duodenal termination of DEVELOPMENT OF PANCREAS IN LOWER VERTEBRATES. 115 the common bile-duct and canal of Wirsung. The dilated interior of the duodenal papilla (diverticulum Vateri) corresponds to the embryonic segment between the intestinal opening of the primi- tive liver duct and the point when this duct gives off the distal larger pancreatic outbud (Figs. 186, 187, 188, 193 and 194). The union of the pancreatic and biliary ducts to form the recess of the diverticulum Vateri, which then opens by a single common orifice into the duodenum, is better marked in some of the lower vertebrates than in man. Fig. 195 shows the proximal portion of the duodenum of the cassowary (Casuarius casuarius) with the biliary and pancreatic ducts and the diverticulum at their confluence in section. The development of these two main digestive glands as diver- ticula from the intestinal canal also explains the direct continuity of the mucous membrane of their ducts with that lining the duo- denum, a fact which is of considerable importance in the patho- logical extension of mucous inflammations from the intestine to the duct system of the glands. Development of the Pancreas in Lower Vertebrates. — In the em- bryo of the sheep two pancreatic buds are found, but the duct of the dorsal (proximal) outgrowth (duct of Santorini) subsequently fuses entirely with the main duct. In the cat there are likewise two pancreatic outgrowths. In the chick three pancreatic buds are visible about the fourth day. Amphibia likewise present three embryonic pancreas buds. The ventral (distal) outgrowth is double, the two portions pro- ceeding symmetically from each side of the hepatic duct. The single dorsal outgrowth is derived directly from the duodenal epithelium. Later on all these outgrowths fuse to form the single adult gland. Fish also possess several (up to four) embryonic pancreatic out- growths. Recently in human embryos of 4.9 mm. cervico-coccygeal meas- ure three pancreatic outgrowths have been observed, all entirely 116 ANATOMY OF THE PERITONEUM. distinct from each other, one dorsal, budding from the epithelium of the primitive duodenum and two ventral, proceeding from the grooved gutter which represents the primitive ductus choledochus at this period. In embryos of from 6 to 10 mm. the two ventral outgrowths have already fused, hence only two buds, a single ventral and a dorsal, are now encountered.^ These observations place the development of the human pan- creas in line with the triple pancreatic outgrowths, two ventral and one dorsal characteristic of the majority of the lower verte- brates, which have been hitherto carefully examined. The ven- tral or distal bud is probably double in the majorit}'' of vertebrates. The two segments fuse, however, so early that the derivation of the pancreas from a double outgrowth, as described above for the human embryo, practically obtains. In forms in which the adult gland presents a number of separate openings into the duo- denum (cf p. 118), the development would probably show mul- tiple embryonic outgrowths from the intestinal hypoblast. In any case the dorsal pancreatic bud appears to have developed in the vertebrate series before the ventral outgrowth and to be hence phylogenetically the older structure. COMPARATIVE ANATOMY OF THE PANCREAS. With the exception of Amphioxus and probably also of the Cyclostomata, the gland appears to be present in all vertebrates, varying, however, much in size, shape and relation to the intes- tinal tube. Usually it appears as an elongated, flattened, more or less distinctly lobulated organ, in close apposition to the duo- denum between the layers of the mesoduodenum. In all forms in which the gland is found it is connected with the post-gastric intestine and marks the beginning of the midgut. In structure the gland is usually acinous, resembling the salivary glands. It is well developed in the selachians, forming a triangular body connected with the beginning of the midgut (Fig. 202). In some instances the gland elements do not extend beyond the in- ' lankelowitz, Arch. f. Mikr. Anat., Bd. 46, 1895. COMPARATIVE ANATOMY OF THE PANCREAS. 117 testine itself, but remain imbedded in the wall of the midgut, as in Protopterus. In certain adult teleosts the pancreas is sur- rounded by the liver (Fig. 196), in others it does not appear as a compact gland but is distributed in the form of finely scattered lobules throughout the mesentery between the two layers of this membrane. On account of this concealed position of the gland it was formerly believed that the adult teleosts did not possess a pancreas. The pyloric caeca (cf p. 119) found in these forms were consequently considered to be homologous with the pancreas of the higher vertebrates. In Myxinoids a peculiar lobulated glandular organ is found imbedded in the peritoneal coat of the intestine near the entrance of the bile-duct, into which its lobules open separately. This organ possibly corresponds to the higher vertebrate pancreas. An organ which may represent a dorsal pancreas is also devel- oped in Ammocostes (larva of Petromyzon), but its exact homol- ogy is still doubtful. It is possible that a true pancreas has not yet developed in the cyclostomata. In Amphioxus no trace of a pancreas is found. In all other vertebrates the gland is present. In certain amphibians, as the frog, the single pancreatic duct opens into the common bile duct (Fig. 197). In lacertilians and in some chelonians a lateral offshoot of the pancreas is directed transversely and is adherent to the spleen. Fig. 113 shows the gland in Chelydra serpentaria. While the gland usually has a single duct, yet two ducts are found in a number of animals (many mammals, birds, chelonians and crocodiles). At times three ducts are encountered, as in the chicken and pigeon. The arrangement of the pancreatic duct system among mam- malia presents the following variations : 1. Mammals with one pancreatic duct, either connected with the bile-duct or entering the intestine independently : Monkeys, most rodents (except the beaver), marsupials, car- nivora (except dog and hyena), many ungulates (pig, peccary, hyrax, etc.), most ruminating artiodactyla. 118 ANATOMY OF THE PERITONEUM. (a) The pancreatic duct joins the common bile-duct before en- tering the duodenum in the monkeys, marsupials, carnivora, in the sheep, goat and camel. The point of entrance of the combined duct into the intestine varies. In some forms it is near the pylorus, in others at some distance from the same. The common opening is situated li" to 2" beyond the pylorus in carnivora, and one foot behind the same point in the goat and sheep. (6) The pancreatic duct does not join the bile-duct, but empties separately into the intestine, in most rodents and in the calf and pig. In the calf the pancreatic duct opens into the duodenum 15' beyond the bile-duct and 3' beyond the pylorus. In the pig the pancreatic opening is 5"-7" beyond that of the bile-duct and 6"-8" behind the pylorus. 2. Mammals with two pancreatic ducts, of which one usually joins the bile-duct : perissodactyla (except the ass according to Meckel), elephant, beaver, several carnivora, dog, hyena, and according to Bernard the cat. In the perissodactyla the prox- imal of the two pancreatic ducts empties, either combined mth the bile-duct, or separate from it, but very close to it, 3"-4" be- hind the pylorus. The second distal duct is smaller and opens several inches further down. In most rodents the pancreatic entrance is placed at some dis- tance from the pylorus. Fig. 199 shows the arrangement of the parts in the rabbit, in which animal the main distal pancreatic duct empties at a distance of 13"-14" from the pylorus into the end of the duodenum, which intestine forms a very long loop, while the biliary duct, receiving the smaller proximal pancreatic duct, opens near the pylorus. In the beaver the smaller proximal duct joins the bile-duct or even enters the duodenum anterior to the bile-duct, nearer the pylorus, while the distal larger pancreatic duct opens into the in- testine 16"-18" behind the biliary duct. Of the two ducts found in the dog (Fig. 200) the smaller proximal either joins the bile- duct or opens into the intestine close to it, T'-li" beyond the PYLORIC CJECA OR APPENDICES. 119 pylorus. The larger distal duct opens into the duodenum V'-lh" behind the bihary duct. Fig. 201 shows the dog's stomach and proximal portion of the duodenum in section. The proximal smaller pancreatic duct here joins the biliary duct, and opens with it by a single orifice into the duodenum. The distal larger pan- creatic duct opens independently into the intestine further caudad. The parts in Hysena present a similar arrangement. Bernard always found two pancreatic ducts in the cat, one large principal duct and a second smaller accessory duct. Of these, the one situated nearest to the pylorus always united with the bile- duct. The pancreatic duct thus joining the bile-duct was some- times the main duct, sometimes the accessory smaller duct. Since the main function of the pancreatic juice is the conver- sion of starch into sugar, the gland appears better developed in general in herbivora than in carnivora, without, however, disap- pearing in the latter. In fact it is of considerable size in the carnivora, because the secretion also acts on the albuminous food substances and, though to a lesser degree, on the fats. PYLORIC GMGA OR APPENDICES. In the Cyclostomata and Selachians the intestinal canal is in the main free from csecal appendages, while a large portion of the tube is provided with a special fold of the mucous membrane which projects into the lumen of the gut (spiral valve). Fig. 43 shows the straight intestinal tract with the spiral valve of the longer distal segment in a cyclostome, Petromyzon marinus or lamprey. In Figs. 202 and 203 the selachian (shark) intestine is represented in two examples, while the similar spiral valve in a Dipnoean or lung fish, Ceratodus, is seen in Fig. 204. On the other hand in the Ganoids and in many Teleosts longer or shorter finger-shaped diverticula of the midgut are found im- mediately bej^ond the pylorus in the region of the bile-duct. These pouches or diverticula of the intestine form the so-called pyloric caeca or appendices of these fish. They vary very much in length, diameter and number in different forms. 120 ANATOMY OF THE PERITONEUM. Thus but a single diverticulum appears in Folypterus and Ammo- dytes (Fig. 205). Rhombus maximus and Echelus conger(Figs. 112 and 206) have two, and the same number appear in Lophius piscatorius (Fig. 207). Perca has three and the Pleuronedidxhsiye three to five. Fig. 208 shows the stomach and the beginning of the midgut with four pyloric caeca in Pleuronectes maculatus, and Fig. 209 the same parts of this animal in section. Fig. 210 shows the stomach and midgut of Paralichthys dentatus, the summer flounder, with three well-developed conical pyloric caeca. On the other hand in some forms the number of pyloric appendices is enormously increased, while their caliber dimin- ishes. Thus 191 csecal appendages are found surrounding the beginning of the midgut in Scomber scomber. A well-marked example of prolific development of the pyloric appendages is furnished by the common cod, Gadus callarias (Fig. 211). The appendices are in the natural condition bound together by connec- tive tissue and blood vessels, so as to form a compact organ, re- sembling a gland (Fig. 21 1, A), and a similar arrangement is found in Thynnus vulgaris and alalonga, Pelamys and Accipenser (Fig. 212). In some Teleosts (Siluroidea, Labroidea, Cyprinodontia, Plecto- gnathi and Leptobranchiates) the appendices are entirely wanting. If there are not more than 8-10 appendices they usually surround the gut and empty into the same in a circle. In other cases they are arranged in a single line, or in a double row, opposite to each other (Fig. 213). Each appendix may open into the intestine independently, this especially where the number is limited and the individual pouches large (cf Figs. 206-210), or several may unite to form a common duct. Fig. 211, B, shows the appendices in Gadus callarias, the cod, freed by dissection from the investing connective and vascular tissue. It will be noticed that a considerable number of the tubes unite to form ducts of larger caliber which open into the intestine, as seen in the section shown in Fig. 214. The pyloric appendices apparently have the same significance as the spiral intestinal fold of the Selachians, Cyclostomes and Dip- PLATE CV. VENTRAL MESOGAS- TRIUM DUODENUM DORSAL MESO- GASTRIUM FORM- ING OMENTAL POUCH STOMACH CEPHALIC PRO- TON OF PANCREAS MESODUOOENUM CAUDAL PROTON OF PANCREAS Fig. 215.— Cephalic segment of primitive mesentery in sclicmatic profile view. VENTRAL MESO GASTHIUM DUODENUM DORSAL MESO- GASTRIUM DISTAL PORTION OF PANCREAS, DEVEL- OPING BETWEEN LAYERS OF DORSAL MESOGASTRIUM MARGIN OF OMENTAL BURSA MESOOUODENUM PROXIMAL PORTION (head) OF PANCREAS DEVELOPING BE- TWEEN LAYERS OF MESODUOO- ENUM Fig. 216.— Schematic profile view of i)rimitive mesenteries with formation of omental Dursa and developing spleen and pancreas. PLATE CVI. CESOPHAGUS DUODENUM STOMACH OMENTAL BURSA SPLEEN PANCREAS DUODENO- JEJUNAL FLEXURE MESENTERY OF SMALL INTESTINE Fig. 217.— Sos scrofa fmt., foetal pig. Portions of tlioracic and abdominal viscera hardened in situ. (Columbia University Museum, No. 1449.) DORSAL MESOGAS- TRIUM FORMING OMENTAL BURSA STOMACH DUODENUM ILEO-COLIC JUNCTION PRIMITIVE DOR- SAL MESEN- TERY OF INTES- TINAL LOOP FORMING MESEN- TERY OF SMALL INTESTINE AND ASCENDING MESO- COLON SMALL INTESTINE MESODUODENUM TRANSVERSE COLON SPLENIC FLEXURE INFRACOLIC SEGMENT OF DUODENUM DESCENDING COLON PRIMITIVE DORSAL MESENTERY FORMING OESC MESOCOLON i Fig. 218.— Schematic view of primitive mesentery after intestinal rotation and incipient formation of omental bursa from dorsal mesogastrium. PLATE CVII. STOMACH GASTRO-SPLENIC OMENTUM PANCREAS AORTA GIVING OFF SPLENIC ARTERY Figs. 219, 220. — Schematic transection of dorsal mesogastrium, pancreas, spleen, and stomach. Fig. 219. — Before adhesion to primitive parietal peritoneum (arrow indicates the direction in which the adhesion takes place). GASTRO-SPLENIC OMENTUM CONTAINING A. GASTRO-EPI- PLOICA SINISTRA SPLEEN PANCREAS SECONDARY LINE OF TRANS- ITION BETW. VISCERAL AND parietal peritoneum s. lieno-renale, lig. lieno-phrenicum) L KIDNEY AREA OF ADHESION BETWEEN PRIMITIVE PARIETAL PERITONEUM AND DORSAL MESO- GASTRIUM, CARRYING SPLENIC ART. AND IN- VESTING DORSAL SURFACE OF PANCREAS Fig. 220. — After adhesion and formation of secondary line of transition between mesogastrium and parietal peritoneum (lieno-reiiiil ligament). PLATE CVIIl. PANCREAS GREAT OMENTUM DORSAL MCSO- GASTRIUM PARIETAL PERITONEUM Figs. 221, 222. — Schematic sagittal sections through stomach, pancreas, great omentum, and left kidney. Fig. 221. — Before adhesion between dorsal and mesogastrium and parietal peritoneum. AREA OF ADHESION BETWEEN PARIETAL PERITONEUM AND DORSAL MESOGAS- TRIUM Fig. 222.— After adhesion. PLATE CIX. CESOPHAGUS STOMACH SPLEEN GASTRIC ARTERY CCELIAC AXIS SPLENIC ART BODY AND TAIL OF PAN CREAS INCLUDED IH. ^^ DORSAL MESOGASTRIUM~Tf; SUP MESENTERIC ART LINE OF ATTACHMENT OF DORSAL MESO-' GASTRIUM POSTCAVA SI^TGELIAN LOBE PORTAL VEIN HEPATIC ARTERY -^GALL-BLADDER DUODENUM HEAD OF PANCREAS, INCLUDED IN MESO- DUODENUM Fig. 223. — Abdominal viscera of cat, hardened and removed from body, showing relation of pancreas to mesoduodenum and dorsal mesogastrium, respectively. (Columbia University Museum, No. 728.) DIAPHRAGM GASTRO-HEPATIC OMENTUM STOMACH PANCREAS DUODENUM TRANSV . COLON JEJUNO-ILEUM Fig. 224.— Schematic sagittal section of abdominal viscera of cat, after the intestines have been rotated to correspond to the adult human disposition, to show- lines of peritoneal reflection before adhesion. PLATE ex. DIAPHRAGM GASTRO-HEPATIC OMENTUM STOMACH PANCREAS DUODENUM TRANSVERSE COLON SMALL INTESTINE Fig. 225. — The same figure indicating the areas of adhesion and peri- toneal obliteration (shaded) which produce the arrangement of the adult human peritoneum. 1. Area of adhesion between opposed surfaces of great omentum and trans- verse mesocolon and colon. 2. Area of adhesion between parietal peritoneum, duodenum, and caudal layer of transverse mesocolon. 3. Adhesion of opposed walls of omental bursa leading to obliteration of distal portion of pouch and producing " gastro-colic " ligament of adult human subject. PANCREAS CAUDAL LAYER TRANSV. MESOCOLON DUODENUM TRANSV. COLON MESENTERY SMALL INTESTINE 1 2 Fig. 226. — Schematic sagittal section of adult human peritoneum. i PLATE CXI. GREAT OMENTUM' VENTRAL BORDER OF PANCREAS, WITH AT- TACHMENT OF RE- CURRENT LAYERS OF GREAT OMENTUM ASC. MESOCOLON SUP. MESENTERIC A. ASC. COLON TRANSV. COLON CUT EDGE OF MESE SMALL INTESTINE C OUS WITH ASC. MESOCOLON ILEO-COLIC JUNCTION NTERY OF_ ^^s •^'•lA CONTINU- ''^r~ "^'A* LEFT LOBE OF LIVER STOMACH PANCREAS, CAUDAL SURFACE CUT END OF DUODENUM L. KIDNEY BEHIND PRIMARY PARIETAL PERITONEUM CEPHALIC LAYER OF TRANSVERSE MESOCOLON DESCENDING MESOCOLON OMEGA LOOP r-crr. ^^^" ^?J,-T'^¥.""H"^^ ^^^■^*^' °^ Macacus rhesus, Rhesus moukey, with the small intestine removed. (Columbia University Museum, No. tHt-) PLATE CXIl. DIAPHRAGM STOMACH PANCREAS DORSAL MESOGASTRIUM Figs. 228-232. — Scheniatic sagittal sections of dorsal mesugas- tritim and omental bur.sa, in man, monkey, and cat. Fig. 228. — Common embryonal condition, as illustrated by cat, after rotation and formation of omental bursa. STOMACH PANCREAS KIDNEY AREA OF ADHESION BETWEEN DORSAL MESOGASTRIUM AND PRIMITIVE PARIETAL PERITONEUM Fig. 229. — Area of adhesion between dorsal mesogastriiim and jirimitive parietal peritoneum in Macacus, producing condition shown in Fig. 230. PYLORIC GjECA or APPENDICES. 121 noeans, i. e., the production of an increase in the area of the diges- tive and absorbing surfaces of the intestinal mucous membrane. Hence, as stated, the appendices and the spiral fold are found to vary in inverse ratio to each other. Thus, for example, Polyp- terus (Fig. 205) still has a fairly well developed spiral fold and only a single pyloric appendix, while Lepidosteus, with but slightly developed spiral fold, has numerous appendices. It was formerly held that the pyloric caeca and the pancreas were mutually in- compatible structures, and that where one is found the other will be wanting. Hence the appendices were regarded as homologous with the pancreas of the higher forms. Recent observations have shown that this view is not strictly and entirely correct, while at the same time it merits consideration in several respects. It is true that the pancreas in certain teleosts is now known to be present although concealed from observation in the liver or scattered in the form of small lobules between the layers of the mesentery (cf p. 117), and that in a number of fish, such as Salmo salar, Clupea harengus, Accipenser sturio, both the appendices and the pancreas are encountered. Consequently these structures are not identical or even completely homologous, since they occur side by side in the same form. On the other hand Krukenberg has demonstrated that the ap- pendices pyloricae may function physiologically as a pancreas by yielding a secretion which corresponds to the pancreatic juice in its digestive action. In the majority of forms, however, they ap- parently merely increase the intestinal absorbing surface, secret- ing only mucus. These structures are nevertheless very interesting and instruc- tive since they furnish a perfect gross morphological illustration of the embryonal stages just considered in connection with the development of the mammalian pancreas. In the adult ganoid or teleost these blind diverticula or pouches, varying greatly in .shape, number and size, protrude from the intestine immediately beyond the pylorus, usually in close connection with the duo- 122 ANATOMY OF THE PERITONEUM. denal entrance of the bile-duct. Two or more of these pouches may unite to form a common duct or canal opening into the intestine. These forms, therefore, offer direct and valuable morphological illustration of the manner in which the pancreas of the higher vertebrates develops, i. e., as a set of hollow outgrowths or diverticula from the hypoblast of the primitive enteric tube. We can establish a consecutive series, beginning with forms in which only one or two diverticula are found, and extending to types in which the number of the little cylindrical pouches reaches nearly two hundred and in which they are bound to- gether by connective tissue and blood vessels so as to closely resemble the structure of a glandular pancreas. This is one of the most striking instances in which the minute embryological stages of the higher types are directly illustrated by the permanent adult conditions found in the lower vertebrates. [The same statement, as we will see, holds good in reference to the develop- ment of the liver.'] RELATION OF THE PANCREAS TO THE PERITONEUM. The gland becomes very intimately connected with the serous layers of the primitive dorsal mesentery. In order to clearly comprehend the adult serous relations it is necessary to make a distinction between two divisions or portions of the gland, based upon the altered relations of the primitive dorsal mesentery which result from the differentiation of the primitive simple intestinal tube into stomach and duodenum. 1. The primary outgrowth of the pancreatic tubules from the duodenum, i. e., the part which is to form the "head" of the adult gland, is situated between the two layers of that division of the primitive dorsal mesentery which forms, after differen- tiation of stomach and small intestine, the mesoduodenum. Coincident with the rotation of the stomach, as we have seen, the duodenum and mesoduodenum exchange their original sag- ittal position in the median plane of the body for one to the right RELATION OF THE PANCREAS TO THE PERITONEUM. 123 of the median line, balancing, so to speak, the extension of the stomach to the left (Fig. 218). The original right layer of the mesoduodenum and the right surface of the duodenum now look dorsad and rest in contact with the parietal peritoneum investing the right abdominal back- ground and the ventral surface of the right kidney and inferior vena cava. We have already seen that the descending portion of the duodenum in man becomes anchored in this position by ad- hesion of these apposed peritoneal surfaces. This fixation in- cludes, of course, the structures situated between the layers of the mesoduodenum, i, e., the head of the pancreas. Consequently, after rotation and adhesion, this portion of the gland turns one surface ventrad, invested by secondary parietal peritoneum, origi- nally the left leaf of the free mesoduodenum, while the original right surface of the gland has become the dorsal and has lost its mesoduodenal investment by adhesion to the primary parietal peritoneum. 2. In order to understand the way in which the body and tail of the pancreas obtain their final peritoneal relations it is neces- sary to consider the development of the doi«al mesogastrium to form the omental bag. If we regard the primitive dorsal mesen- tery in the profile view from the left side (Fig. 215) it will be seen that, as already stated, the mesoduodenum is the first part of the membrane to be invaded by the pancreatic outgrowth from the intestine. Cephalad of the mesoduodenum the primitive dorsal mesogastrium (Fig. 215) is seen to protrude to the left and cau- dad to form, as already explained, the cavity of the omental bursa of the retrogastric space ("lesser peritoneal sac"). The further growth of the pancreas carries the developing gland from the district of the mesoduodenum into that portion of the dorsal mesogastrium which now forms the dorsal wall of the omental bursa (Fig. 216). This double relation of the pancreas to the mesoduodenum and to the mesogastrium forming the omental bursa is well seen in foetal pigs between two and three inches in length (Fig. 217). 124 ANATOMY OF THE PERITONEUM. The head portion of the pancreas is seen developing between the layers of the mesoduodenum, while the body and tail of the gland, extending to the left, grows between the two dorsal layers of the omentum bursa towards the spleen, which organ is found connected with the left and dorsal extremity of the omental sac derived from the dorsal mesogastrium. Before the growth of the great omentum is pronounced the continuity of the mesoduodenum and dorsal mesogastrium can be readily appreciated (Fig. 218). But after the redundant growth of the membrane has carried the great omentum further caudad, the stomach and the two omental layers attached to the greater curvature lie in front of the structures included between the two dorsal layers and conceal them from view (Fig. 177). In sagittal sections to the left of the median line (Figs. 221 and 222) the pancreas now appears included between the layers of the great omentum near their point of departure from the vertebral column. (This point is of course identical with the prevertebral attachment of the primitive dorsal mesogastrium from which the omentum is developed.) The foregoing considerations will, therefore, lead to the conclu- sion that the pancreas presents, in regard to its peritoneal rela- tions, two distinct segments : 1. The portion adjacent to duodenum (head and neck of the gland) is developed between the layers of the mesoduode- num. 2. The distal portion of the gland, comprising the body and tail, develops between the layers of the great omentum (dorsal segment), derived from the primitive dorsal mesogastrium. The transections of the dorsal mesogastrium shown in Figs. 180 and 181 will now have to be amplified by the introduction of the body of the pancreas between the two layers of the vertebro- splenic segment, in addition to the splenic artery (Figs. 219 and 220). Hence the following facts will be understood : RELATION OF THE PANCREAS TO THE PERITONEUM. 125 1. In the adult the splenic artery supplies a series of small branches to the pancreas as it courses along the cephalic border of the gland on its way to the spleen. 2. After the above-described adhesion of the original: left leaf of the dorsal mesogastrium (vertebro-splenic segment) to the parietal peritoneum (Fig. 220), the dorsal surface of the body of the pancreas loses its peritoneal investment and becomes attached by connective tissue to the ventral surface of the left kidney. 3. The ventral surface of the body of the pancreas is in the adult lined by peritoneum of the "lesser sac" ; in other words the organ has practically assumed a '' retro-peritoneal " position, its ventral peritoneal covering appearing now as the dorsal parietal peritoneum of the retro-gastric space. 4. When completely developed the extreme end (tail) of the pancreas extends to the left, following the splenic artery, until it touches the mesal aspect of the spleen at the hilus. 5. If we, therefore, leave out of consideration for the moment the transverse colon and duodenum, which will be taken up presently, and confine ourselves to the arrangement of the stomach, pancreas and great omentum, a sagittal section to the left of the median line would result as shown in Fig. 222, after the adult condition of adhesion has been estab- lished. The same process of fixation, which resulted in the anchoring of duodenum and head of pancreas, extends^ to the body of the gland and the investing omentum. The peritoneum lining the original left, now the dorsal surface of the gland, fuses with the primitive parietal peritoneum covering the diaphragm and the left kidney. The main body of the pancreas in the adult appears prismatic, giving a triangular sagittal section. The dorsal surface is adherent to the ventral surface of the left kidney ; the ventral surface is covered by the secondary parietal peritoneum (original right layer of mesogastrium) which lines the dorsal wall of the retrogastric space and omental bursa (lesser peritoneal, sac). The great omen- 126 ANATOMY OF THE PERITONEUM. turn now appears to take its dorsal point of departure along the sharp margin which separates this ventral surface of the pancreas from a third narrower surface directed caudad. This surface, under the conditions which we are at present examining, would be hned by the peritoneum continued onto it from the dorsal layer of the great omentum. This peritoneum merges along the dorsal margin of this caudal surface of the pancreas with the general parietal peritoneum covering the left lumbar region and the caudal part of ventral surface of the left kid- ney. We have, therefore, along this line a secondary tran- sition from visceral to parietal peritoneum, obtained by the obliteration of the original visceral peritoneum investing the dorsal surface of the pancreas before adhesion to the parietal peritoneum. The pancreas assumes, therefore, in the adult a secondary retro- peritoneal position, covered on its ventral surface by peritoneum of the "lesser sac," while the caudal surface is lined by part of the general peritoneal membrane of the "greater sac." The dor- sal surface, denuded of serous covering by obliteration, is adherent to the crura of the diaphragm, the aorta and the ventral surface of the left kidney. It is now proper to compare the conclusions just derived from the study of the development of the human dorsal mesogastrium and connected structures (spleen and pancreas) with the condi- tions presented by the corresponding parts in one of the lower mammalia, which illustrate some of the human embryonal stages. Here again the abdominal cavity of the cat forms an instructive object of study. The purpose of the following comparison should be twofold : I. The mesogastrium, spleen and pancreas in the cat will clearly illustrate the process of human development above out- lined. II. The abdominal viscera of the cat, if properly arranged, will enable us to complete the consideration of this region by in- cluding the very important relations which the transverse colon SPLEEN, PANCREAS AND GREAT OMENTUM OF CAT. 127 and third portion of the duodenum bear in man to the great omentum and pancreas. I. SPLEEN, PANCREAS AND GREAT OMENTUM OF CAT. After opening the abdominal cavity it will be seen that the great omentum can be lifted up, exposing the subjacent coils of the small and large intestine, to which it adheres at no point. In other words the entire dorsal surface of that part of the original mesogastrium which forms the great omentum is free. It will be remembered that this is not the case in the adult human subject, because here the dorsal surface of the great omentum adheres to the transverse colon. Consequently in man only that portion of the dorsal surface of the omentum can be seen which extends between the transverse colon and the caudal free edge of the membrane. It will be noted that on the left side the spleen is connected by its mesal surface to the omentum and through it with the stomach (gastro-splenic omentum). In other words the cat illustrates the human embryonal stage in which the spleen has appeared between the layers of the dorsal mesogastrium at the extreme left or blind end of the retrogastric pouch formed by the rotation of the stomach and elongation of the mesogastric membrane, but before the ad- hesion has taken place between the original left (now dorsal) layer of the vertebro-splenic segment of the mesogastrium and the primitive parietal peritoneum apposed to it (Fig. 219). Con- sequently the dorsal wall of the "lesser" sac in the cat is still composed of the two layers of the free vertebro-splenic segment of the mesogastrium, the primitive right (now ventral) layer not having been converted, as is the case in man, into secondary parietal peritoneum by adhesion of the original left (now dorsal) layer to the primitive prerenal parietal peritoneum. If we now examine the relation of the pancreas to the perito- neum we can estabHsh the following facts : 1. The portion of the gland adjacent to the duodenum, corre- sponding to the " head " of the human organ, is included between 128 ANATOMY OF THE PERITONEUM. the two layers of the mesoduodenum. This membrane is free, so that the dorsal surface of this portion of the pancreas is seen to be invested by the dorsal layer of the mesoduodenum (Fig. 223). The duodenum and the mesoduodenum, the latter con- taining the head of the pancreas between its layers, can be turned toward the median line, so as to expose the entire ventral surface of the post-cava and right kidney. To illustrate the arrange- ment which is found in the adult human subject the descending duodenum and pancreas should be allowed to fall over to the right so as to cover the vena cava and the mesal part of the ventral surface of right kidney. The adult human condition will now be produced if we assume that the structures are fixed in this posi- tion by the obliteration of the apposed serous surfaces, viz., the parietal peritoneum over kidney and vena cava on the one hand and the right layer of the mesoduodenum and the dorsal visceral peritoneum of the duodenum on the other. 2. In following out the pancreas of the cat in its entire extent, proceeding to the left of the pylorus, it will be seen that the body of the gland has extended between the two dorsal layers of the great omentum (primitive dorsal mesogastrium) over to the spleen (Fig. 223). Consequently the arrangement in the cat corresponds to the stage in the human development shown in Fig. 219 and Fig. 221 in which adhesion of the dorsal surface of the pancreas to the parietal peritoneum has not yet taken place. It will be quite easy to reconstruct from the facts as demon- strated by the arrangement of the parts in the cat, the stage in the development of the lesser peritoneal sac in which the dorsal wall of the space is still formed by the proximal portion of the free dorsal mesogastrium (great omentum) and the structures in- cluded between its two layers. It must then become apparent that the entire serous surface which in the adult human subject we regard as " parietal perito- neum of the lesser sac" lining the dorsal wall of the retrogastric space is derived from what originally was the right layer of the primitive sagittal dorsal mesogastrium. OBEAT OMENTUM, TRANSVERSE COLON AND DUODENUM. 129 II. RELATION OF GREAT OMENTUM TO TRANSVERSE COLON, TRANSVERSE MESOCOLON AND THIRD PART OF DUODENUM. The second purpose to be accomplished by the study of the cat's abdominal cavity at this stage is the correct appreciation of the adult human conditions which are produced by areas of ad- hesion between the transverse colon, transverse mesocolon and third part of the duodenum on the one hand, and the dorsal mesogastrium, as great omentum, with the structures contained betAveen its layers, on the other. Perform the manipulations of the large and small intestine in the cat (see p. 67) which are required in order that the tract may be arranged so that it will correspond in general to the topo- graphical conditions presented by the adult human subject. Locate the transverse colon and mesocolon and the third portion of the duodenum produced by these manipulations in imitation of the corresponding human structures. Then proceed to plot the different parts out successively as they would appear in a sagittal section (Fig. 224). The following facts are to be noted and indicated on the plan of the section : 1. The great omentum is free, hanging down from the greater curvature of the stomach over the coils of intestine. Turning the omentum up it will be observed that the body of the pancreas is included between the two dorsal layers of the membrane. 2. The omentum, containing the pancreas, can be lifted up, exposing the next succeeding structure, viz., the transverse colon and mesocolon. In the cat the large intestine has been brought over, by the manipulations above indicated, into a transverse posi- tion so as to represent the human transverse colon and its meso- colon. It is therefore necessary to remember that in this mammal the fixation of the transverse mesocolon in the position indicated, by adhesion of ascending and descending mesocola to the parietal peritoneum of the abdominal background, has not yet occurred. Consequently the membrane must be held in the transverse posi- tion in order to represent the human arrangement. 9 130 ANATOMY OF THE PERITONEUM. It^vill of course be observed that both surfaces of the transverse mesocolon estabhshed in this way are free, not adherent to either omentum or pancreas on the one hand, nor to the transverse duo- d^enum on the other. 3. The third or transverse portion of the duodenum is seen to be attached by the distal part of the mesoduodenum, both of the serous surfaces of the membrane being free. The duodenum hav- ing been brought from right to left transversely across vertebral column and aorta, underneath the superior mesenteric artery, the mesoduodenum, in the segment corresponding to the transverse duodenum, exchanges its original sagittal position for one in a horizontal plane, with cephalic (primitive left) and caudal (primi- tive right) surfaces. Now compare the above arrangement of the intestines and peritoneum in the cat at once with the conditions presented in the adult human subject, reserving certain intermediate stages, as ex- hibited by some of the lower monkeys, for subsequent study. The examination of a similar sagittal section representing sche- matically the adult human arrangement of the parts (Fig. 225) will reveal the following points of difference as compared with the cat : 1. The peritoneum covering the dorsal surface of the pancreas, derived from the primitive dorsal mesogastrium, has become adherent to the parietal peritoneum, as previously described. 2. The cephalic surfaces of the transverse colon and meso- colon fuse with the corresponding area of the dorsal (4th) layer of the great omentum (dorsal mesogastrium). In the human foetus in the 4 th month the connection is still so slight that the omentum can readily be separated from the transverse colon and mesocolon. Further dorsad the cephalic layer of the transverse mesocolon adheres to the serous investment of the caudal surface of the pancreas, derived, as we have seen, from the same dorsal layer of the great omentum. 3. The duodenum and mesoduodenum are fixed by adhesion on the one hand to the parietal peritoneum, on the other to the OREAT OMENTUM, TRANSVERSE COLON AND DUODENUM. 131 caudal layer of the transverse mesocolon near the root of that membrane. 4. The cavity of the omental bursa is usually obliterated in the adult caudad of the level of the transverse colon, by adhesion of the apposed surfaces of the two intermediate omental layers. We have therefore three general areas of secondary peritoneal adhesion to deal with (Fig. 225), viz : 1. Dorsal layer of primi- tive mesogastrium (great omentum) including the serous investment of the dorsal and caudal surfaces of the pancreas (Fig. 225, 1). 2. Transverse duodenum and mesoduodenum (Fig. 225, 2). to Parietal peritoneum, ce- phalic layer of transverse mesocolon and cephalic sur- face of transverse colon. to Parietal peritoneum and caudal layer of transverse mesocolon. 3. Between the apposed serous surfaces of the intermediate omental layers (Fig. 225, 3). These areas of adhesion result naturally in the production of secondary lines of peritoneal transition as follows : 1. Figs. 225, 1 ; 226, 1, from the omentum, dorsal layer, to the caudal surface of transverse colon, caudal layer of transverse meso- colon and caudal surface of the pancreas. 2. Figs. 205, 2; 226, 2, from the caudal layer of the trans- verse mesocolon across the transverse portion of the duodenum to the parietal peritoneum and mesentery of the jejuno-ileum. 3. Figs. 225, 3; 226, 3, between the intermediate omental lay- ers, forming the secondary caudal Hmit of the lesser sac. These changes consequently result in the rearrangement of the adult human peritoneum in accordance with the following schema (Fig. 226) : We trace the peritoneum as the ventral or superficial layer of the great omentum from the greater curvature of the stomach caudad around the distal free edge of the omentum and cephalad, as the dorsal layer, to the ventral border of the transverse colon. 132 ANATOMY OF TEE PERITONEUM. Here apparently this layer is continued across the caudal surface of the large intestine and beyond as the caudal layer of the transverse mesocolon. While this condition obtains practically in the adult it is to be remembered that the adhesion (at 1 in Fig. 225) pre- vents us from lifting the omentum away from the colon, and that consequently the apparent continuity of the dorsal layer of the great omentum with the caudal layer of the transverse mesocolon is the result of this peritoneal fusion. Near the dorsal attachment or " root " of the transverse meso- colon the caudal layer of the membrane becomes continuous with the parietal peritoneum investing the transverse portion of the duodenum on its ventral aspect, which peritoneum in turn passes into the free mesentery of the jejuno-ileum (Fig. 225, 2). Comparison with the previous figures will show that we are deal- ing here with another area of secondary peritoneal fusion. If we now open the " lesser peritoneal cavity " by dividing the two layers of the omentum attached to the greater curvature of the stomach (Figs. 225 and 226 in direction of arrow) we will apparently reach the upper or cephalic surface of the transverse mesocolon. This layer can be followed dorsad to the sharp border which separates the ventral and caudal surfaces of the pancreatic body and the membrane can be traced thence over the ventral surface of the gland to the diaphragm. (The connections with the liver and stomach shown schematically in the diagram (Fig. 225) are to be considered in detail subsequently.) In the adult the peritoneal surface just described appears as the cephalic layer of the transverse mesocolon and its continuation dorsad. From the facts previously considered it will be at once apparent that we are really dealing here with a part of the third layer of the primitive omentum. We do not see the original cephalic layer of the transverse mesocolon. This membrane has become fused with the fourth omental layer, and its free serous surface obliterated in the stretch between the vertebral column and the transverse colon. Hence the human adult transverse mesocolon is apparently composed of two layers ; the cephalic of RELATIONS OF GREAT OMENTUM. 133 these layers appears as peritoneum of the "lesser sac," in conform- ity with its derivation from the original third omental layer lining the interior of the omental bursa. The caudal layer, on the other hand, is a part of the general or ''greater" peritoneal membrane. The entire adult transverse mesocolon, hence, comprises /our peri- toneal layers, of which only two remain as permanently free serous surfaces. These differ in their derivation, the cephalic layer being a part of the primitive dorsal mesogastrium (third omental layer), while the caudal layer is part of the primitive mesocolon. Between these two layers of the adult transverse mesocolon are included the two obliterated embryonic mem- branes, viz., the fourth omental layer and the original dorsal layer of the transverse mesocolon. Caudad the two layers of the adult transverse mesocolon sur- round the transverse colon and are continuous along the ventral margin of the intestine with the layers of the great omentum. Toward the vertebral column these layers again diverge. The cephalic layer, lining the "lesser peritoneal cavity " invests the ventral surface of the pancreas. The caudal layer continues over the caudal surface of the body of the gland and transverse portion of the duodenum into the parietal peritoneum and the free mes- entery of the jejuno-ileum. Consequently the returning layers of the great omentum are said to surround the transverse colon and unite along the dorsal border of the intestine to form the transverse mesocolon, which membrane is continued dorsad to- ward the vertebral column as two layers. At the "root" of the transverse mesocolon these layers are then described as diverging, the cephalic passing up to line the ventral surface of the pancreas, while the caudal continues over the caudal surface of the pancreas and third portion of the duodenum into the parietal peritoneum and mesentery. Wherever in this discussion of the transverse mesocolon the transition between the caudal layer of the membrane and the "parietal" peritoneum is referred to it is necessary to remember that this "parietal" peritoneum is the secondary investment of 134 ANATOMY OF THE PERITONEUM. the abdominal background, formed by the surface of the ascend- ing and descending mesocolon which remains free after the oppo- site surface and the vertical segments of the large intestine have been anchored by adhesion to the primary parietal peritoneum (cf. p. 81, Fig. 158). A summary at this point of the course of the dorsal mesogas- trium, in forming the great omentum and its subsequent connec- tions, would show us that the membrane first enlarges and de- scends towards the transverse colon (Fig. 177). The omental bag is formed by the descending or superficial segment (starting from the greater curvature of the stomach), turned toward the observer in the figure, and by the ascending or deep layer which is attached above to the dorsal abdominal wall, in front of the vertebral column and aorta along the original line of origin of the dorsal mesogastrium. Gradually growing and descending further, the deep segment becomes attached to the transverse colon. It also becomes connected, especially on the left side, with the diaphragmatic peritoneum (phrenicocolic lig.), so that its original starting point is no longer distinct. Finally the devel- opment of the spleen and pancreas between the layers of the dor- sal segment and their subsequent connections obscure the origi- nal conditions. Fig. 297 shows the primitive condition at a time when the con- nection with the transverse colon and mesocolon has not yet taken place. The omental bag or bursa epiploica develops in the region of the dorsal mesogastrium and the viscera included between its layers, by changes in the position and extent of the membrane which finally result in placing a part of the right half of the primitive ccelom cavity behind the stomach. Up to the sixth week the line of origin of the dorsal mesogastrium is from the mid- dorsal line of the abdomen. It deviates from this origin to the left because the great curvature of the stomach to which it is attached turns in this direction. On this account, and because of the rapid growth of this portion of the mesogastrium, a bag or SUMMARY OF CHANGES IN DORSAL MESOGASTRIUM. 135 space is formed behind the stomach. The entrance into this space is situated to the right of the lesser curvature, behind the peri- toneal layers connecting the same with the liver (lesser or gastro- hepatic omentum and hepato-duodenal ligament). The ventral wall of this space is formed by the dorsal surface of the stomach itself, the dorsal wall by the mesogastrium, turning to the left and presenting its original right surface, now directed ventrad. The caudal limit of the retro-gastric space is given by the turn of the mesogastrium to reach its attachment along the greater cur- vature of the stomach (rudiment of great omentum). The stomach, in contributing to produce these changes, passes from the vertical to the oblique and finally into the transverse position. The pylorus, formerly directed caudad, passes up and to the right. The fundus develops and the original left side of the stomach becomes the ventral, the right side the dorsal. The original dorsal border, now the greater curvature, moving caudad, carries the attached dorsal mesogastrium with it into its new position. The mesogastrium now pouches to form the great omentum and rapidly enlarges. At first hardly projecting be- yond the greater curvature, it increases in length until it forms a four-layered apron which hangs down as a loose sac over the transverse colon and the coils of the small intestine (Fig. 177). In the foetus of six months the cavity of the omental bag ex- tends caudad as far as the lower edge of the omentum. Later adhesions between the peritoneal surfaces lining the interior of the bursa limit this. extension. The omental bursa is therefore formed by a ventral lamella, consisting of two peritoneal layers, which hangs down from the greater curvature of the stomach and passes around the caudal free edge of the omentum into the double-layered dorsal lamella, which ascends, over the transverse colon, to the original starting point of the dorsal mesogastrium along the front of the vertebral column and aorta. Hence the " great omentum " is originally composed of four layers of peritoneum. The dorsal double lamella becomes adherent over a consider- 136 ANATOMY OF THE PERITONEUM. able area to the parietal peritoneum of the dorsal abdominal wall. In this way the organs developed between the two layers of the lamella obtain their final fixed position. The pancreas becomes anchored and appears in the adult as a '' retro-peritoneal " struc- ture, while the spleen is attached by the " phrenico-lienal liga- ment " to the diaphragm. In addition the dorsal omental lamella adheres in the fourth month to the cephalic layer of the transverse mesocolon and to the transverse colon. Important illustrations of some of the intermediate stages in the human development of this portion of the peritoneal tract are afforded by the permanent adult conditions found in the abdom- inal cavity of some of the lower primates, notably certain of the cynomorphous monkeys. Fig. 227 shows the abdominal cavity and disposition of the peritoneum in a macaque monkey {Macacus rhesus, ^ ) in the ven- tral view, with the coils of small intestines removed and the omen- tum lifted up and reflected upon the ventral body wall. The fol- lowing important points of difference from the arrangement in the cat on the one hand, and in man on the other, are to be noted : 1. The large intestine presents the typical primate course, with an ascending, transverse and descending colon. The ileo-csecal junction is situated in the right iliac fossa. 2. The ascending and descending mesocola are still free, not having become adherent to the parietal peritoneum along the dorsal abdominal wall. Hence the caudal portions of the ventral surfaces of the two kidneys are still covered by the primitive parietal peritoneum. 3. The great omentum is not yet adherent to the transverse colon and mesocolon except for a short distance on the extreme right. At this point the dorsal layer of the omentum has begun to contract adhesions to the hepatic flexure of the colon and as- cending colon, but the rest of the transverse colon is free. Dif- fering from the human arrangement is a line of adhesion, uniformly present in these monkeys, between the dorsal surface PLATE CXIII. PANCREAS TRANSV. COLON Fig. 230. — Arrangement of great omentum as found in Mamma rhcsHf!, shown without reference to areas of peritoneal obliteration. STOMACH PANCREAS Fig. 2.'!1.— Corresponding section of human adult peritoneum showing, along dotted lines, area of peritoneal adhesion. \ PLATE CXIV. DIAPHRAGM STOMACH PANCREAS Fig. 232. — Section showiug human adult peritoneum without reference to area of adhesion. PANCREAS Figs. 233-235. — Series of schematic sagittal sections through left kidney and adrenal, pancreas, and transverse colon, to show develop- ment of adult peritoneal relations. Fig. 233. — Embryonic condition, as illustrated by cat, after rotation of intestine. Pancreas free between dorsal layers of great omentum. Tansverse colon and mesocolon free. Kidney behind primitive parietal peritoneum. PLATE CXV. PANCREAS Fig. 234. — Area of adhesion between : 1. Primitive parietal perito- neum. 2. Mesogastrium forming great omentum. 3. Colon and mesocolon. Fig. 235. — Adult human arrangement, shown without reference to obliterated areas. PLATE CXVI. GREAT OMENTUM RAISED HEPATIC FLEXURE OF COLON TRANSVERSE COLON PANCREAS L. KIDNEY FREE DE- SCENDING MESOCOLON Fig. 236.— Abdominal viscera of Macacus cyuomolgus, Kr:i monkey. Museum, No. 1801.) (Columbia Uuiversity PLATE CXVII. ECTODERM NOTOCHOR^ HEPATIC POUCH ORAL PLATE Fig. 237. — Longitudinal section of an embryo of Petromyzon planeri, four days old. (Minot, after KupflPer.) RIGHT LOBE OF LIVER ILEO-COLIC JUNCTION SMALL INTESTINE CESOPHAGU8 STOMACH LEFT LOBE OF LIVER Fig. 238. — Fseudemys elegans, pond turtle. Alimentary canal. (Columbia University Museum, No. 1437.) PLATE CXVllI. GALL-BLADDER STOMACH -_ MID-GUT Fig. 239. — Stomach, mid-gut, pancreas, and liver of Boa constrictor, boa. (Columbia University Mu- seum, No, 1832.) PLATE CXIX. Fig. 240. — Liver of Macacus cynomolgm, Kra monkey. (Columbia University Museum, No. xf 1^-) Fig. 241.— Liver of Pleuronectes maculatus, flounder. (Columbia University Museum, No. 1679.) PLATE CXX. COMMON BILE-DUCT DUODENUM Fig. 242. — Schema of hepatic and cys- tic ducts. (Nuhn.) COMMON DUCT DUODENUM Fig. 243. — Schema of hepatic and cystic ducts. (Nuhn.) GALL- BLADDER COMMON DUCT HEPATICO- CYSTIC DUCT DUODENUM Fig. 244. — Schema of hepatic and cys- tic ducts. (Nuhn.) CYSTICO- ENTERIC DUCT HEPATICO- CYSTIC DUCT HEPATICO- ENTERIC DUCT DUODENUM Fig. 24.5. — Schema of liepatic and cystic ducts. (Nuhn.) PRECARDINAL VEIN DUCT OF CUVIER SUBCLAVIAN VEIN VITELLINE VEINS VITELLINE ARTERIES allantoic (hypogastric) arteries CAROTID ARTERIES BRANCHIAL ARCHES AORTIC ROOT HEART SUBCLAVIAN ARTERY POSTCARDI NAL VEIN COMMON ILIAC ART EXT. ILIAC ART. CAUDAL ART Fig. 246. — Diagram of erahryonic vascular system, without the portal circulation. (Parker, after Wiedersheim.) The dorsal aorta is formed by the junction of the right and left aortic roots arising from the confluence of the branchial arterial arches. PERITONEUM IN MACAGUS RHESUS. 137 of the omentum along its right edge and the ventral surface and right border of the csecum and ascending colon, parts which normally are not adherent to the omentum in man. 4. Hence in tracing the omentum to the left of the limited adhesion to the hepatic flexure and ascending colon, i. e., nearly throughout the entire extent of the transverse colon, we find the membrane passing freely without adhesion over the cephalic surface of the transverse mesocolon, which preserves its original free condition, independent of the omentum. This arrangement is shown in the schematic sagittal section in Fig. 230. 5. Tracing the omentum dorsad beyond the transverse colon and mesocolon the pancreas is reached. Here we encounter the first extensive area of omental or mesogastric adhesion. The omental peritoneum continues over the ventral and caudal sur- faces of the gland, investing the same, but the dorsal surface has lost its serous covering and is anchored to the ventral surface of the left kidney. Hence a sagittal section would show the arrange- ment of the monkey's omentum as indicated in the schematic Figs. 229 and 230. Making now a general comparison of the' peritoneal membrane of this animal with that of man, and of both with the preceding common embryonal condition, we can draw the following conclusions, indicated schematically in the five figures 228-232. 1. The dorsal layer of the monkey's omentum in its proximal segment behaves in the same way as in man, i. e., it becomes ad- herent to the primitive parietal peritoneum down as far as the caudal margin of the dorsal surface of the pancreas included be- tween the primitive mesogastric layers forming by their further growth the omental apron. Therefore we find, as in the human subject, (a) The pancreas adherent to the ventral surface of the left kidney. (6) A portion of the ventral surface of the kidney, cephalad of the pancreas, and the dorsal wall of the retrogastric (lesser peri- toneal) space lined by secondary parietal peritoneum derived from 138 AJSTATOMY OF THE PERITONEUM. the third layer of the omentum (original right layer of dorsal mesogastrium). 2. The monkey differs from adult man in the behavior of the dorsal omental layer in relation to the cephalic surface of the transverse mesocolon. The adhesion, which in the human subject fuses this layer with the transverse colon and mesocolon, does not occur in the monkey. Hence we have in this animal the following conditions : (a) The omentum is non-adherent to the transverse colon and transverse mesocolon. (6) The caudal surface of the pancreas is lined by its original mesogastric peritoneum. (c) The transverse mesocolon is formed by the original two layers of the primitive dorsal mesentery ; hence its cephalic layer is not '' peritoneum of the lesser sac " as is the case in man. (d) The caudal part of the ventral surface of the left kidney below the pancreas, is covered by the original parietal peritoneum. (e) Only one point or line of secondary peritoneal transition exists, where the dorsal layer of the omentum in the adult becomes con- tinuous with the parietal peritoneum covering the caudal surface of the pancreas and the ventral surface of the left kidney. Note: In the schematic sections shown in Figs. 228 to 232 the transverse I colon is represented as far removed from the ventral sur- face of the left kidney, in order to make the peritoneal lines of the mesocolon more clear. Actually a sagittal section which would divide the kidney would cut the transverse colon at its extreme left end, where it turns close to the ventral surface of the left kid- ney and then follows its lateral border to form the splenic flexure (Fig. 235). The caudal part of the ventral surface of the left kidney in the adult human subject is covered by the peritoneum which, as secondary parietal peritoneum, is derived from the upper part of the right leaf (later ventral leaf) of the descending mesocolon. Hence it should be remembered that these diagrams present combinations of sections. A section which will show the full development of the transverse mesocolon is mesad of the SPLEEN AND OMENTUM IN MACAGUS RHESUS. 139 kidney ; while a section through the kidney would be too far lat- erad to show the transverse mesocolon. Figs. 233, 234 and 235 show sagittal sections through the left kidney with the adult arrangement of the peritoneum and colon and the embryonic and adhesion stages leading to the same. It will be observed that in all the schematic sections of the early embryonic stages the two layers of the transverse mesocolon are shown without dorsal attachment, as turning with the forma- tion of a fold (Fig. 228 at x) into two layers descending ven- trad of the parietal peritoneum. This is because the dorsal at- tachment of the mesocolon is at this stage still in the median Hne and would hence not be encountered by a sagittal section through the kidney, and because the two layers of the transverse meso- colon, immediately after rotation of the large intestine, are still directly continuous with the two layers of the descending meso- colon. That is to say, the cephalic layer of the transverse meso- colon is continuous with the dorsal (originally the left) layer of the descending mesocolon, and the caudal layer of the transverse mesocolon with the ventral (originally the right) layer of the descending mesocolon, which is, in the human subject, to assume subsequently the character of parietal peritoneum after the dorsal layer and the primitive parietal peritoneum have become oblit- erated by adhesion (Fig. 235). Fig. 236 shows this continuity of the descending and transverse mesocolon as a permanent adult condition in the macaque. The fold of transition between the two is seen at x in Fig. 228. It will be noticed that the ventral surface of the left kidney, caudad of the adherent pancreas, is covered by the primitive parietal peritoneum, corresponding to section in Fig. 230. RELATIONS OF SPLEEN AND OMENTUM IN MACACUS BHESUS. The spleen in this animal has not contracted any extensive adhesions to the parietal peritoneum (the phrenico-lienal lig. of anthropotomy is not developed). It can be turned mesad so as to expose the lateral border and an adjacent segment of the 140 ANATOMY OF THE PERITONEUM. ventral surface of the left kidney, as well as the dorsal surface of the tail of the pancreas at its tip, still covered by mesogastric peritoneum. Hence in the monkey the adhesion of the original vertebro-splenic segment of the mesogastrium, including the pan- creas, to the primitive parietal peritoneum is less complete than in man. MEDIAN ATTACHMENT OF DESCENDING MESOCOLON AND ITS RELATION TO THE MESOCOLON OF THE SIGMOID FLEXURE IN THE MACAQUE. Fig. 236 shows the abdominal viscera, hardened in situ, of Macacus cynomolgus, the Kra monkey, in the ventral view and from the left side. The great omentum is lifted up, the pancreas is adherent to the ventral surface of the left kidney, the caudal portion of which is covered by the primary parietal peritoneum, which can be ex- posed by turning the still free descending mesocolon mesad. The mesocolon retains its primitive attachment to the median line ventrad of the large prevertebral blood vessels. It is readily seen that adhesion between the left leaf of this free descending meso- colon and the parietal peritoneum down to the level of the iliac crest would produce the conditions found in the human adult, with an attached descending colon and a free sigmoid flex- ure ; also that limited adhesion of the mesocolon of the sigmoid flexure to the parietal peritoneum would produce, as previously explained (cf p. 97), the intersigmoid peritoneal fossa. 2. Ventral Mesogastrium and Liver. — The peritoneal reflections from the stomach to the liver, and the arrangement of the membrane in connection with the latter organ, remain for con- sideration. Certain complicated adult conditions, encountered in this part of the abdominal cavity, make it desirable to arrange the subject for purposes of study under the following subdivisions : I. The development of the liver and of its vascular system, and the significance of the adult circulation of the liver and of the foetal remnants connected with the organ. DEVELOPMENT OF THE LIVER. 141 II. The anatomy of the ventral mesogastrium and the changes produced in the arrangement of the membrane by the develop- ment of the liver. I. A; Development of the Liver — The liver, like the pancreas, is developed from the duodenum as an outgrowth from the hypo- blast lining the enteric tube. As we have previously noted, the first outgrowth of the hepatic diverticulum is closely associated with the distal pancreatic outbud ; in fact the latter arises as a derivative from the hepatic duct rather than as a distinct outbud from the intestinal tube. (This close association of the hepatic duct with the pancreas is well seen in the arrangement of the concealed pancreas of some teleosts (cf p. 117, Fig. 196).) In point of time the liver is the first accessory structure to develop by budding from the primitive alimentary canal, the pancreas and lung following. In the primitive type of development, as seen in Petromyzon and in the Amphibia, the liver appears very early, as a divertic- ulum of the embryonic intestinal tube, near its cephalic ex- tremity, projecting on the ventral aspect down into the mass of yolk-cells (Fig. 237). The short stretch of the primitive alimen- tary canal cephalad of the hepatic diverticulum corresponds to the foregut. With the development of the heart the primitive foregut becomes divided into pharynx and post-pharyngeal seg- ment (oesophagus and stomach). The hepatic diverticulum then lies immediately dorsad of the caudal or venous extremity of the heart. Hence it is probable that the liver is an older organ in the ancestral history of the vertebrates than the pharynx or even the heart. The liver diverticulum lies in close connection with the omphalo-mesenteric veins which return the blood from the yolk-sac to the heart. In the course of further development, as will be seen below, the liver comes into very intimate relations with the venous circulation. In human embryos of 3.2 mm. the primitive hepatic duct ap- pears as a wide hollow pouch composed of hypoblast cells, grow- ing between the two layers of the ventral mesogastrium, which 142 ANATOMY OF THE PERITONEUM. membrane, extending between the ventral border of the primitive stomach and the ventral abdominal wall, will be subsequently considered in detail. The liver, in developing between the layers of the ventral mesogastrium, approaches very early the septum transversum or rudimentary diaphragm and becomes connected with the same. A mass of • mesodermal cells, derived from the mesogastrium and from the primitive mesodermal intestinal wall surrounding the hypoblastic lining of the tube, covers the csecal termination of the primitive hepatic duct, forming the so- called embryonic hepatic ridge. This mesodermal tissue accom- panies the duct in its further growth and branching, forming the connective tissue envelope, known in the adult as the capsule of Glison. The primitive hepatic duct is directed cephalad in the me- sogastrium between the vitelline duct and the stomach (Fig. 101). In embryos measuring 4,25 mm. the duct is 0.24 mm. long. Later (in embryos of 8 mm.) the primitive single duct divides into two secondary branches, indicating, even at an early stage, the adult arrangement of the duct, as formed by the union of the right and left hepatic ducts (Fig. 185). The gall-bladder in embryos of this size (8 mm.) is a well- defined csecal diverticulum, branching caudad from the main hepatic duct. The vesicular mucous surface is thus derived from the enteric hypoblast in the same way as the epithelial lining of the bile- ducts and capillaries. The external muscular and fibrous coats of the gall-bladder are developed from the mesoderm of the mesogastrium. It is to be noted that at an early stage the gall-bladder is de- rived from the main duct close to the intestine, the latter duct being very short. Later on the common duct grows in length, making the liver more and more a gross anatomical organ dis- tinct from the intestine. The cystic duct develops as the result of a similar increase in length of the cystic diverticulum. The two principal secondary branches of the hepatic duct give origin to sprouts or buds. These are derivatives of the hypoblastic DEVELOPMENT OF THE LIVER. 143 cells of the larger ducts and may from the beginning be hollow, possessing a lumen continuous with that of the parent duct (Selachians, Amphibians). In warm-blooded animals these sprouts are at first solid, forming the s. c. hepatic cylinders, and only subsequently become hollowed out with the further devel- opment of the biliary duct system of the liver. The rapid growth of the organ leads to a great increase in the number of the hepatic cylinders. They spread out on all sides, finally coalescing with adjacent buds so as to form an interlacing network whose meshes are filled by blood vessels. After the hepatic cylinders have become canalized they preserve the same arrangement, hence the resulting biliary capillaries of the adult form an anas- tomosing network. Amphioxus and the amphibians have a single hepatic outgrowth (Fig. 49). In the Selachians the liver arises as a ventral outgrowth at the hinder end of the foregut immediately in front of the vitelline duct, thus bringing the liver from the beginning into close prox- imity with the vitelline veins entering the heart. Almost as soon as formed the outgrowth develops two lateral diverticula, open- ing into a median canal. The two diverticula are the rudimentary lobes of the liver and the median canal uniting them is the rudiment of the common bile-duct and gall-bladder. In the Teleosts the liver arises quite late (in the trout about the 25th day) as a solid outgrowth from the intestinal canal close to the heart. In the Amniota the liver arises in the same position as in the Anamnia, but, at least in birds and mammals, shows its bifurcation almost, if not quite, from the start. The two forks embrace between them the omphalo-mesenteric or vitelline veins just before the}'- empty into the sinus venosus of the heart. In the chick the liver appears between the 56th and 60th hour, the right fork being always of greater length but less diameter than the left. The hepatic outbud in the rabbit appears during the 10th day, and during the 1 1th day begins to send out branches. In man, as above stated, the bud appears well marked in em- bryos of 3 mm. 144 ANAT03IY OF THE PERITONEUM. [Certain adult variations make it appear possible that there are two human embryonic hepatic buds, a cranial and a caudal, as is the case in birds.] I. B. Comparative Anatomy of the Liver. — The liver, phylogenet- ically a very old organ, occurs in all vertebrates, for the csecal diverticulum of the intestine of amphioxus (Fig. 49) has prob- ably the significance of a hepatic outbud. The primitive form of the liver is symmetrically bilobed, a type which is seen well in the chelonian organ (Fig. 238). In size the liver is subject to great variations. It is usually larger in animals whose food contains much fat. Hence carnivora in general have a larger liver than herbivorous animals. Its shape also varies considerably, depending on the form of the body cavity and on the amount and disposition of the available space. Hence in the snakes the organ appears long drawn out, flattened, almost ribbon-like (Fig. 239), while the relatively very large coronal diameter of the body cavity in the turtles permits the liver to expand transversely (Fig. 238). In general, when the liver is large and the available space for its reception limited, it is usually split into several (two to seven) lobes, which permit, by mutual displacement, the accommodation of the organ to varying space-conditions of the body cavity (Fig. 240). Under the opposite circumstances, on the other hand, even the primitive bilobed character may disappear and the liver is then unlobed (Fig. 241). The presence or absence of a gall-bladder depends appar- ently largely on the character of the food and on the habitual type of digestion. In many vertebrates digestion is carried on nearly continuously, without marked interruption, especially in many ungulates, ruminants and rodents. In such animals the gall- bladder is absent. It is also absent in several birds (most Parrots, Doves, Ostrich, Rhea americana, the Cuculidse, Rhamphastos, etc.). This variability emphasizes the morphological fact that the biliary bladder is only a modified portion of the hepatic duct system, as shown by the development above outlined. PLATE CXXI. SINUS TER- MINALIS 2D, 3D, AND 4TH AORTIC ARCHES VITELLINE VEINS R VITELLINE A POSTCAR- DINAL V. L. VITELLINE A Fig. 247.— Diagram of the circulation of the yolk-sac at the end of the third day of incuba- tion in the chick. (After Balfour.) The median portion of the first aortic arch has disappeared ; but its proximal end forms the external, its distal the internal carotid arteries. The whole blas- toderm has been removed from the egg and is viewed from below. Hence the left appears on the right, and vice versa. Arteries in black. Veins in outline. L. VITELLINE V SINUS VENOSUS R. VITELLINE V. YOLK-SAC AND VITELLINE CA- PILLARY NET- WORK Fig. 248. — Schema of vitelline veins. PLATE CXXII. SINUS VENOSUS L PRECAHDINAL Y L. DUCT OF CUVIER L. POSTCARDINAL V. VV. HEPATIOE REVEHENTES-C HEPATIC CAPIL- LARY SYSTEM L. UMBILICAL V. VV HEPATICjE advehentes-C L. vitelline v. UMBILICAL VEIN EN- TERING UMBILICAL CORD WITH THE ARTERIES R. PRECARDINAL V. R. DUCT OF CUVIER R POSTCARDINAL V. R. UMBILICAL V. R. VITELLINE CSUB- INTESTINAL) V. UMBILICAL OR HYPO- 'GASTRIC ARTERIES Fig. 249.— Schema of umbilical veins, early stage. L. PRECARDINAL V L. DUCT OF CUVIER' L. POSTCARDINAL V HEPATIC CAPIL LARY SYSTEM- L. UMBILICAL V L. VITELLINE V R. PRECARDINAL V. R. DUCT OF CUVIER R. POSTCARDINAL V. )-VV. HEPATIC/E REVEHENTES DUCTUS VENOSUS R. UMBILICAL V. VV. HEPATIC/E ADVe- HENTES 'PRIMITIVE PORTAL VEINS) TRANSV. ANASTOMOSIS OF VITELLINE OR OM- PHALO-MESENTERIC VEINS L. VITELLINE V. Fig. 250. — Schema of primitiv'e portal circulation. PLATE CXXIII. L. PRECARDINAL V.' L. POSTCARDINAL V L. DUCT OF CUVIER L. UMBILICAL V COMMUNICATION OF L UMBILICAL V. WITH INTRA HEPATIC CAPILLARY SYS TEM BY BRANCH TO LEFT' V. HEPATICA ADVEHENS AND DUCTUS VENOSUS DUODENUM OMPHALO- MESENTERIC (VITELLINE) V. R. PRECARDINAL V. R. POSTCARDINAL V. R DUCT OF CUVIER VV. HEPATIOE REVEHENTES R. UMBILICAl^^. _ DUCTUS VENOSUS COMMUNICATION OF R. UMBILICAL V. WITH INTRAHEPATIC CAPIL- LARY SYSTEM VV. HEPATIOE ADVe- HENTES (PORTAL V.) 1. PROXIMAL PERIDUODENAL ANNULAR ANASTOMOSIS OF OMPHALO- MESENTERIC (vitelline) VEINS DISTAL PERIDUODENAL ANNULAR ANASTOMOSIS OF OMPHALO -MESENTERIC (VITELLINE) VEINS R. OMPHALO-MESENTERIC (VITELLINE' V. Fig. 251. — Schema of further development of portal circulation and connection of same with umbilical veins in early stages. L. PRECARDINAL V L. POSTCARDINAL V L. DUCT OF CUVIER. PROXIMAL END OF L. UMBIt.lCAL VEIN- INTERMEDIATE SEGMENT OF L. UMBILICAL VEIN TAKEN INTO HEPATIC CIR- CULATION DISTAL ENLARGED SEG- MENT OF L. UMBILICALV. REDUCED LEFT HALF OF PROXIMAL ANNULAR ANASTOMOSIS OF OM- PHALO-MESENTERIC V. PORTAL VEIN PRECARDINAL V. R. POSTCARDINAL V. R. DUCT OF CUVIER PROXIMAL END RIGHT UMBILICAL VEIN JVV. HEPATIOE REVEHENTES DUCTUS VENOSUS RIGHT UMBILICAL V. ANASTOMOSIS OF R. UM- BILICAL V. WITH INTRA- HEPATIC CIRCULATION OBLITERATED RIGHT HALF OF DISTAL ANNULAR ANAS- OMOSIS OF OMPHALO- MESENTERIC V. DUODENU^ PORTAL VEIN FORMED BY FUSION OF OMPHALO- MESENTERIC VEINS Fig. 252. — Second stage in development of circulation through portal and umbilical veins. The proximal segment of the main portal vein is formed by the persistence of the left half of the distal and right half of the proximal periduodenal vascular ring of the omphalo-mesenteric veins. The distal segment of the main jiortal vein is the product of the fusion of the omphalo-mesenteric veins, and becomes connected with the veins of the intestinal canal, i)ancreas, and spleen. The proximal terminal segment of both umbilical veins becomes included in the system of the ven?e hepaticse revehentes. PLATE CXXIV. LEFT PRECARDINALV LEFT POSTCARDINAL V. LEFT VEN/E HEPATIC/E REVEHENTES, INCLUD- ING PROXIMAL SEG- MENT OF LEFT UMBILI- CAL VEIN RIGHT PRECARDINAL V. RIGHT POSTCARDINAL V. RIGHT DUCT OF CUVIER RIGHT VEN£ HEPATlOe REVEHENTES, INCLUD- ING PROXIMAL SEGMENT OF RIGHT UMBILICAL V. m W4w ...... .jw^mn, N (LEFT V. HE- \ 2i»* _,^^ ' LEFT BR TAL VEI PATICA ADVEHENS) DISTAL SEGMENT OF LEFT UMBILICAL VEIN MAIN TRUNK OF PORTAL V. DUODENUM -DUCTUS VENOSUS OBLITERATED INTER- MEDIATE SEGMENT OF RIGHT UMBILICAL VEIN RIGHT BRANCH OF POR- TAL VEIN (RIGHT V. HE- PATICA ADVEHENS) OBLITERATED LEFT HALF OF PROXIMAL PERIDUOD- ENAL VASCULAR RING OBLITERATED RIGHT HALF OF DISTAL PERIDUODENAL VASCULAR RING DISTAL SEGMENT OF RIGHT UMBILICAL V. ALMOST EN- TIRELY OBLITERATED Fig. 253.— Third stage in development of jiortal and umbilical veins during the placental period. ENTERIC V. Fig. 254. — Corrosion preparation showing cour.sc of jiortal vein and tributaries in relation to duodenum. (Columbia University Museum, No. 1857.) PLATE CXXV. R PRIMITIVE JUGU- LAR VEIN (PRECAR- DINAL) B. DUCT OF CUVI ER SINUS VENOSUS R. HEPATIC V PORTAL V. — R. UMBILICAL V UMBILICAL CORD L. PRIMITIVE JUGU- LAR VEIN (PRECAR- DINAL^ POSTCARDINAL V. L. DUCT OF CUVICR L. HEPATIC V. DUCTUS VENOSUS L. UMBILICAL V. LOWER EXTREMITY Fig. 255.— Human embryo of 10 mm. cervico-coccygeal measure. Heart and ventral body wall removed to show sinus venosus and entering veins. (Kollmann, after His.) RIGHT AURICLE LEFT HEPATIC VEIN- LEFT BRANCH OF PORTAL VEIN UMBILICAL VEIN 1 V. HEPATICA COMMUNIS HEPATIC SEGMENT OF POSTCAVA RIGHT HEPATIC VEIN DUCTUS VENOSUS IGHT BRANCH OF PORTAL VEIN PORTAL VEIN Fig. 25f).— Final stage of development of portal and umbilical veins in the placental period. PLATE CXXVI. POSTCAVA L. HEPATIC V DUCTUS VENOSUS' Fig. 257. — Schema of relation of postcava to hepatic veins and ductus venosus. POSTCAVA JUNCTION OF DUCTUS VENOSUS AND L. HE- PATIC VEIN L. HEPATIC V. DUCTUS VENOSUS L. BRANCH PORTAL BRANCH FR. UMBI CAL V. TO LEFT LO INTRAHEPATIC SE MENT OF UMBILICAL SUPPLYING LEFT QUADRATE LOB BRANCH FROM UMB ICAL VEIN TO L. LO R. HEPATIC V. R. BRANCH OF PORTAL V. MAIN TRUNK OF PORTAL V. BRANCH FROM UMBILICAL V. TO QUADRATE LOBE Fig. 258.— Corrosion preparation of venous system of human liver in foetus at term. (Columbia University Museum, No. 1834.) POSTCAVA WITH OPENINGS OF HEPATIC V. FALCIFORM LIG., CON- TINUED RIGHT AND LEFT INTO CEPHALIC LAYER OF CORONARY LIGS. L. TRIANGULAR LIG FISSURE FOR DUC' TUS VENOSUS SPIGELIAN LOBE' TRANSV. FISSURE LEFT LOBE UMBILICAL VEIN VENTRAL ABDOM- INAL WALL' PORTION OF DIAPHRAGM ADRENAL IMPRESSION R. TRIANGULAR LIG. POSTCAVA CAUDATE LOBE PORTAL VEIN R. LOBE ~- -G ALL-BLADDER QUADRATE LOBE UMBILICAL ARTERIES UMBILICAL CORD Fig. 259.— Injected and hardened human liver from foetus at term. (Columbia University Museum, No. 1853.) PLATE CXXVII. L. HEPATIC V, DUCTUS VENOSUS BRANCHES TO LEFT LOBE R HEPATIC V PO^TCTIVA — L. BRANCH OF POR- TAL V. JOINING RIGHT DIVISION OF UMBILI- CAL VEIN RIGHT BRANCH OF PORTAL VEIN PORTAL VEIN BRANCH TO QUAD- RATE LOBE UMBILICAL VEIN Fig. 260. — Diagram of intrahepatic fcetal venous circulation. L. HEPATIC VEINS OBLITERATED DUC- TUS VENOSUS LEFT BRANCHES OF PORTAL VEIN OBLITERATED SEGMENT OF UMBILICAL V. POSTCAVA R HEPATIC VEIN RIGHT BRANCH OF PORTAL VEiN PORTAL VEIN BRANCH TO QUAD RATE LOBE Fig. 261. — Diagram illustrating the changes in the intrahepatic venous circulation resulting from the cessation of the placental circulation at birth. PLATE CXXVIII. PRECARDINAL. (jugular' V HEART DUCT OF CUVIER SUBCLAVIAN V CARDINAL SINUS LATERAL VEIN GENITAL VEINS ADVEHENT RENAL- PORTAL VEINS POSTCARDINAL VEINS ADVEHENT RENAL PORTAL VEIN DE- RIVED FROM BIFUR- CATION OF CAUDAL VEIN VEINS OF PELVIC FIN CAUDAL VEIN INF. JUGULAR VEIN HEPATIC SINUS HEPATIC POR- TAL VEIN .CESOPHAGEAL VEIN STOMACH AND GASTRIC VEINS REVEHENT RENAL- PORTAL VEINS MID-GUT AND IN- TESTINAL VEINS LATERAL VEIN PERICLOACAL NETWORK CUTANEOUS VEIN OF TAIL Fig. 262. — Diagram of the veins of a selacliian. (Wiedersbeini, after Parker.) The lateral vein arises from a venous network surrounding the cloaca, receiving one or more cutaneous veins of the tail, veins of the body-wall, and veins of the pelvic fins. The caudal vein divides at the posterior end of the kidney into the two renal-portal veins, from which the advehent veins of the renal-portal system are derived. The revehent renal-por- tal veins join to form the posterior cardinal veins, which, after dilating enormously to form the cardinal sinuses, join with the anterior jugular, subclavian, and lateral veins to form the ducts of Cuvier. The latter receive the inferior jugular veins, from the deep parts of the head and neck and the terminations of the hepatic portal system (hepatic sinus). The hepatic portal vein is formed by the veins of the oesophagus, stomach, and intestines. After traversing the capillary vessels of the liver, the revehent hepatic veins unite to form an extensive hepatic sinus before entering the heart. m COMPARATIVE ANATOMY OF LIVER. 145 A great variety is observed in the arrangement of the biliary ducts, through which, at the period of intestinal digestion, bile passes from the liver and gall-bladder into the intestine, while in the intervals of digestion the secretion is only carried from the liver to the bladder. The following main types of the biliary duct system may be recognized : 1. The hepatic duct joins the cystic to form the common bile- duct, entering the duodenum by passing obliquely through the intestinal wall (Fig. 242). This form is encountered in man and in most mammals. It is also found in some birds {Buceros), many amphibians, and in some fish {Lophius). Instead of one hepatic duct two may join the cystic duct separately to form the common bile duct (Phoca litorea), or the number of hepatic ducts may be further increased. The separate hepatic ducts then unite successively with the cystic duct. This occurs in many mammals ( as Tardus, Galeopithecus, monotremes) and in some fishes {Xiphias, Trigla, Accipenser) (Fig. 243). 2. Of two hepatic ducts only one helps to form with the cystic duct the common duct, while the other leads from the liver trans- versely into the bladder, especially into the neck, forming the hepatico-cystic duct (Fig. 244). This arrangement is found in several mammals (calf, sheep, dog). 3. No common bile-duct is formed. The hepatic and cystic ducts each empty separately into the intestine (hepato-enteric and cysto-enteric ducts), while a hepato-cystic duct carries the bile directly from the liver to the gall-bladder (Fig. 245). Lnitra vulgaris among mammalia, the majority of the birds and several reptilia present this type. When the gall-bladder is absent a single large hepato-enteric duct is found, or instead a number of smaller ducts which enter the intestine successively. I. C. Development of Vascular System of Liver. — In order to com- prehend the peritoneal relations of the adult liver it is absolutely necessary to have a clear understanding of the development of the vascular system in connection with the gland. 10 146 ANATOMY OF THE PERITONEUM. For our purpose, in the first place, a serial consideration of the successive stages, illustrated by schematic diagrams, will prove most practicable. These diagrams represent the structures in the dorsal view, i. e., in the position which they would occupy in the adult liver with the gland resting on its upper or convex surface and with the ventral sharp margin turned toward the beholder (see Fig. 259). The development of the venous system, especially in connection with the liver, presents a somewhat complicated series of succes- sive conditions. After having become familiar with the principal typical embryonal stages, as shown in the following diagrams, the student is strongly recommended to cement this knowledge by the comparative examination of the venous system. The per- manent veins of the lower vertebrates, while in many cases not strictly homologous to those of the higher forms, yet are excellent objects for study, since they serve to illustrate temporary stages in the development of the mammalian venous system, and to that extent are of aid in comprehending one of the most difiicult and important chapters in human anatomy. At the conclusion of the diagrammatic consideration of the mammalian development a number of comparative facts will be put together for this purpose. 1. Early Stage. — In the earlier developmental stages in mamma- lian embryos the primitive dorsal aorta extends caudad along the ventral aspect of the vertebral axis, giving off paired vitelline or omphalo-mesenteric arteries to the yolk-sac and allantoic arteries to the embryonic urinary bladder or allantois (Figs. 246 and 247). The blood is returned from the vascular area of the yolk-sac by two vitelline or omphalo-mesenteric veins, which unite near the heart to form a common trunk, continued as the sinus venosus into the caudal or auricular extremity (venous end) of the primitive tubular heart (Figs. 246, 247 and 248). 2. Development of Allantois. Stage of Placental Circulation. — The pla- cental circulation, replacing the temporary vitelline circulation of the earliest stages, is inaugurated by the appearance of two DEVELOPMENT OF HEPATIC VASCULAR SYSTEM. 147 umbilical veins, which pass cephalad, imbedded in the tissue of the ventral mesogastrium, to empty into the sinus venosus near the vitelline veins (Fig. 249). The umbilical veins return the oxygenated blood from the placenta to the embryo. At first the right umbilical vein is the larger of the two. The sinus venosus at this time also receives two large veins, transversely directed, called the ducts of Cuvier, which are formed near the heart by the union of the anterior cardinal (primitive jugular) and posterior cardinal veins, draining respectively the head end of the embryo, and the body walls and Wolffian bodies. The vitelline veins are placed on each side of the primitive small intestine, and become connected with each other by a broad anastomotic branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the vitelline veins send out branches which break up into a wide-meshed capillary network in the mesodermic tissue enveloping the hepatic cylinders. Hence at this period the circulation in the vitelline veins is made up of three districts : (a) Distal segment of veins, coursing along duodenum, and joined by a transverse anastomosis, before reaching the liver bud (subintestinal veins). (6) Middle segment, from which capillary vessels are derived, ramifying upon and between the developing hepatic cylinders. (c) Proximal segment, formed by the continuation of the prox- imal part of the vitelline veins into the sinus venosus of the heart. 3. Formation of Portal Circulation. A. — With the further develop- ment of the liver the direct connection of the distal segment of the vitelline veins with the sinus venosus becomes lost, the in- termediate segment being entirely broken up into an intrahepatic network (Fig. 250). Hence all the blood brought to the liver by the vitelline veins (vense hepaticse advehentes) passes through the hepatic capillary circulation, before it is carried by the proxi- mal segment of the vitelline veins (venae hepaticse revehentes) into the sinus venosus. The amount of this blood increases with new connections which the vitelline veins make with the venous radicles developing in the intestinal tract and its appendages. 148 ANATOMY OF THE PERITONEUM. In proportion as, with the development of the placenta and re- duction of the yolk-sac, the original significance of the vitelline veins as nutritive and respiratory vessels disappears, this second- ary connection of the vitelline veins with the veins of the ali- mentary tract becomes more and more important, until finally the original vitelline veins, now properly called omphalo-mesen- teric veins, return the blood from the intestinal tube, pancreas and spleen to the liver. The vense hepaticse advehentes, becoming connected in this way with the developing intestine, pancreas and spleen, form the rudiments of the future portal system, while the venae hepaticse revehentes are prototypes of the hepatic veins of the adult circu- lation. B. Development of the Portal Vein. — The distal subintestinal seg- ments of the vitelline veins are early united by a transverse anas- tomotic branch. The section of the veins above this anastomosis is seen already in Fig. 250 to have assumed an annular shape, while the veins below the primary anastomosis are approaching each other to form a second ring-like junction. In Fig. 251 the subintestinal segments of the two vitelline veins are seen to have communicated with each other by transverse anastomotic branches around the duodenum, two of these branches being situated ventrad and one dorsad of the intestinal tube. These branches, and the portions of the primitive vitelline veins between their points of derivation, form two vascular loops or rings, encircling the primitive duodenum (Fig. 251). The distal portions of the vitelline veins, before reaching the caudal annular duodenal anastomosis, next fuse into a single lon- gitudinal vessel which also receives the veins from the stomach, intestine, spleen, and pancreas, and forms the beginning of the portal vein. By atrophy of the right half of the lower, and of the left half of the upper duodenal venous ring (Figs. 252 and 253), the proxi- mal portion of the portal vein is formed as a single vessel, taking a spiral course around the duodenum (Fig. 256). Hence in the FINAL ARRANGEMENT OF THE UMBILICAL VEINS. 149 adult the portal vein and its principal branch (the superior mesen- teric vein) crosses over the ventral Surface of the duodenum (third portion), turns along the mesal side of the second portion, and then continues to the liver along the dorsal aspect of the first portion (Fig. 254). Note — In comparing Fig. 254 with the sche- matic figures it should be noted that the same presents the parts in the ventral view, while the schemata offer the dorsal aspect. 4. Changes Leading to the Final Arrangement of the Umbilical Veins. — A very important rearrangement of the umbilical veins takes place. These veins originally course in the lateral abdominal wall, close to the fold of the amnion (Fig. 255), and then turn cephalad of the developing liver along the septum transversum to empty into the sinus venosus at each end (Figs. 249 and 250). The right umbilical vein is at first the larger. This symmetrical arrangement, and the direct connection of the umbilical veins with the sinus venosus, now becomes lost by the occurrence of the following changes : 1. At first (Fig. 249) all the blood carried to the liver by the omphalo-mesenteric veins passes through the hepatic capillary network before being conducted by the venae revehentes to the sinus venosus. Very early, however, a new intrahepatic channel develops, the ductus venosus (Figs. 250-253), which passes ob- liquely between the entrance of the left omphalo-mesenteric vein into the capillary system (1. v. advehens) and the termination of the right omphalo-mesenteric vein (r. vena revehens) in the sinus venosus. In human embryos of 4 mm. the ductus venosus can already be distinguished, and in embryos of 5 mm. the vessel has assumed considerable proportions. 2. A communication is next established on both sides between the capillary hepatic network in the portion of the Uver nearest to the abdominal wall and the umbilical veins as they ascend imbedded in the abdominal wall (Fig. 251). This connection is usually from the start larger on the left side and connects with the left omphalo-mesenteric vein just at the 150 ANATOMY OF THE PERITONEUM. point where the same is about to be continued into the ductus venosus. This connection becomes rapidly larger, so that the ductus venosus, which at first appeared merely as an anastomotic channel between the left omphalo-mesenteric vein and the termi- nal portion of the right omphalo-mesenteric vein, now forms the main continuation of the left umbilical vein. This vessel grows very rapidly up to its connection with the ductus venosus and soon exceeds the right umbilical vein in size (Fig. 252). Beyond the ductus venosus on the other hand the proximal segment of the left umbilical vein diminishes in size, and loses its indepen- dent character by incorporation in the hepatic circulation. Only its terminal portion, emptying into the sinus venosus, is pre- served. This is surrounded by the growing masses of hepatic cylinders and is converted into a vena revehens. The connection of the right umbilical vein with the liver ves- sels is at first symmetrical to that on the left side, but less strongly developed. The effect of this connection is to reduce in the same way the proximal segment of the right umbilical vein and to convert its termination into a vena revehens. With the great development of the left vein, however, the vein on the right side gradually diminishes and finally loses its connection with the intrahepatic circulation altogether. The right umbil- ical vein is now reduced to a vessel of the ventral abdominal wall, which carries blood in the reverse of the original direction, i. e., from the abdominal wall caudad into the left umbilical vein (Figs. 253 and 255). The connection thus established between the umbiHcal vein and the portal circulation results in the formation of a single large (the original left) umbilical vein which, throughout the remainder of foetal life, returns all of the placental blood (Fig. 253). The newly developed hepatic portion of the left umbilical vein becomes, however, not only connected with the ductus venosus, but also with the right part of the upper venous ring, derived from the right omphalo-mesenteric vein (Fig. 253). This con- FINAL ARRANGEMENT OF THE UMBILICAL VEINS. 151 nection forms the left portal vein of the adult, and enlarges rapidly. The terminations of the ductus venosus and of the^vense he- paticse revehentes undergo a number of secondary changes in relative position. The left hepatic vein loses its direct connec- tion with the sinus venosus, and now opens into the termination of the ductus venosus, into which the right hepatic vein also empties. This common vessel (v. hepatica communis) subse- quently forms the proximal segment of the postcava when this vessel develops (Fig. 256). The blood, therefore, returned to the liver by the left umbilical vein divides at the transverse fissure into three streams. Two of these pass through the connection with the portal vein and through branches developed from the hepatic part of the umbilical vein into the capillary system of the right and left lobe. The third continues through the ductus venosus to the common hepatic vein and sinus venosus (Fig. 256). The ductus venosus thus becomes the chief vessel returning arterialized placental blood to the heart. When the postcava develops fully the hepatic segment of this vessel also joins the terminal part of the ductus venosus (Fig. 256) and gradually replaces the same as the main returning venous channel, the proximal part of the ductus venosus being incorporated in the vena cava (Fig. 257). The postcava then receives the right hepatic veins separately, while the left hepatic veins and ductus venosus open together into the main vein. This condition obtains up to the time of birth and the consequent interruption of the placental circulation. While at first the ductus venosus communicates throughout its entire length with the meshwork of the hepatic capillary system, a separation into two segments, i. e., ductus venosus proper and intrahepatic segment of umbilical vein, is established after the free communication with the left umbilical vein takes place. This condition is exhibited in Fig. 258, which represents the corroded venous system of the foetal liver, and in Fig. 259, showing an injected liver in the fcetus at term. 152 ANATOMY OF THE PERITONEUM. It will be observed that the umbilical vein on entering the liver gives off a large branch to the left lobe, and a smaller branch on the right side to the quadrate lobe, which act as the main vense advehentes of these portions of the liver. Arrived at the trans- verse fissure the umbilical vein divides into three branches, at right angles to each other. The left branch enters the left lobe, the right branch becomes directly continuous with the left main division of the portal vein, while the central branch, continuing the direction of the umbilical vein, passes dorsad, as the ductus venosus proper, to join the left hepatic vein close to its entrance into the postcava. 5 . Changes Conseqiuent upon tlie Establishment of Pulmonary Respiration. — After birth the umbilical vein and its continuation, the ductus- venosus, become obliterated, the former constituting the round ligament of the liver, the latter the ligament of the ductus venosus, both structures imbedded in corresponding portions of the sagit- tal fissure on the caudal and dorsal surfaces of the adult liver (Figs. 284 and 286). The lateral branches of the umbilical vein, however, in its course from the ventral margin of the liver to the transverse fissure (Fig. 258), remain pervious and are transferred to the portal circulation. It will be noticed, in reference to the direction of the blood cur- rent, that at birth a sudden reversal takes place in the right ter- minal branch of the umbilical vein at the transverse fissure (Figs. 260 and 261). Before birth the blood current of the umbilical vein divides into three streams, right, left and central. The latter enters the ductus venosus. The left enters the liver directly, the right traverses, from left to right, the segment between the ter- mination of the umbilical and the bifurcation of the portal vein. This segment in the adult carries blood from right to left, as left branch of the portal vein. In the foetus, however, the blood traverses this segment from left to right, in passing from the umbilical to the right branch of the portal vein. The blood entering the liver through the portal vein passes chiefly into the right division of that vessel (Fig. 260). PLATE CXXIX. PRECAROINAL (jugularI V SUBCLAVIAN V. HEPATIC VEIN PROXIMAL SEGMENT OF POSTCAVA postcaroinal (azygos) vein VEINS FROM CLOACA, BLAD- DER, AND END- GUT DISTAL SEGMENT OF POSTCAVA HEVEHENT RENAL- PORTAL VEINS DUCT OF CUVIER NTESTINE HEPATIC POR- TAL SYSTEM HEPATIC PORTAL V. ABDOMINAL VEIN ADVEHENT RENAL- AORTAL VEIN ILIAC VEIN CAUDAL VEIN Fig. 263. — Diagram of the veins of urodele amphibian {Salamandra maculosa). (Wiedersheim.) The caudal vein bifurcates at the posterior extremity of the kidneys to form the afferent trunks of the renal-portal system along the lateral border of the kidneys, from which the adve- hent veins of the renal-portal system are derived. The iliac or femoral vein divides into an anterior and a posterior branch, the latter opening into the afferent renal-portal vein, while the former, uniting with the one of the opposite side, forms the abdominal vein, and receives vessels from the bladder, cloaca, and end-gut. The revehent veins of the renal-portal system, emerging upon the ventral surface of the kidneys, empty into a single median vessel, the distal or renal section of the postcava or vena cava inferior. Proceeding cephalad, the proximal or hepatic sec- tion of this vessel, after traversing the liver and receiving the revehent hepatic veins of the hepatic portal system, empties into the sinus venosus of the heart. Previous to entering the liver the postcava gives off the two posterior cardinal or azygos veins, which continue cephalad, receiv- ing tributary segmental veins from the body-walls and reach the sinus venosus by joining the subclavian veins. These latter uniting with the anterior cardinal (jugular) veins form the ducts of Cuvier (precaval veins). The abdominal vein continues cephalad in the ventral mesogastrium to the liver, giving off a number of smaller branches, which enter the hepatic i)ortal circulation by penetrating the ventral surface of the liver between the layers of the ventral mesogastrium, while the main continuation of the vessel joins the hepatic portal vein at its point of entrance into the liver. The hepatic portal vein is formed by tributaries returning the blood from the digestive tract (intestinal canal, spleen, pancreas). The blood, after traversing the hepatic portal circulation, is conducted by the hepatic revehent veins to the proximal section of the postcava. A number of secondary or accessory portal veins pass from the anterior portion of the intestinal canal (oesoph- agus, stomach) directly to the liver. PLATE CXXX. I i /^9. ABDOMINAL VEIN JOINING HEPATIC PORTAL VEIN ABDOMINAL VEIN STOMACH DIS- PLACED CAUDAD ^^"^ (azygos) veins POSTCAVAL VEIN PANCREAS EPATIC PORTAL V. KIDNEY INTERRENAL SEG- MENT OF POSTCAVA ILIAC VEIN Fig. 264.— Dissection of veins of Xecturus maculatus, mud-puppy. (Columbia University Museum, No. 1835.) The postcava has been divided at the cephalic end of the liver just before entering the sinus venosus, and the postcardinals have been cut prior to their junction with the subclavian veins. The stomach has been turned caudad. The abdominal vein has been divided after the com- mon trunk has been formed by branches from the iliac veins. The latter are seen entering the afferent renal-portal vein, derived from the bifurcation of the caudal vein, along the lateral border of the kidneys. The junction of the main trunk of the abdominal vein with the hepatic portal vein takes place close to the liver under cover of the pancreas. A series of accessory portal veins continuous with the abdominal vein enter the ventral surface of the liver between the layers of the ventral mesogastrium. The inter-renal segment of the postcava receives the revehent renal-portal veins. The iliac vein enters the advehent renal-portal veins derived from the caudal vein. PLATE CXXXI. PULMONARY V CUTANEOUS V. CARDIAC V. KIDNEY, WITH REVE HENT RENAL-PORTAL VEINS EMPTYING INTO POSTCAVA ABDOMINAL V. EXT. JUGULAR V. BRACHIO-CEPH. V. SUBCLAV. V. SINUS VENOSUS POSTCAVA HEPATIC V. PORTAL V. INTESTINE f ADVEHENT RENAL- l PORTAL VEINS SCIATIC VEIN FEMORAL VEIN Fig. 265. — Venous system of Rana esculenta, frog. (Ecker.) PLATE CXXXII. R. INT. JUGULAR V. (PRECARDINAL V.) R. AORTIC ARCH WITH CAROTID TRUNK R. AURICLE R. SUBCLAVIAN A. PROXIMAL SEGMENT OF POSTCAVA ENTERING RIGHT AURICLE VERTEBRAL (AZYGOS) V. COMMUNICATING WITH INTRAHEPATIC SEG- MENT OF POSTCAVA POSTCAVA DIVIDED AT ENTRANCE INTO LIVER ABDOMINAL VEIN DIVISION OF ABDOMI- NAL V. ON bladder' DIVIDED ENDS OF RIGHT BRANCH OF ABDOMINAL VEIN AFFERENT RENAL-PORTAL V. FROM ABDOMINAL V. END-GUT RIGHT PENIS EVERTED EXT. JUGULAR V. L. INT. JUGULAR V (PRECARDINAL V.) AORTIC ARCH -^— L. SUBCLAVIAN V, VERTEBRAL (aZYGOS^ V JOINING SUBCLAVIAN V PULMONARY A. VENTRICLE POSTCARDINAL ANASTOMOSIS L. POSTCARDINAL V. VAS DEFERENS AFFERENT RENAL- POR- TAL V. FROM ABDOMI- NAL VEIN LEFT KIDNEY UROGENITAL ORIFICES IN DORSAL WALL OF CLOACA Fig. 266. — Systemic veins of Ljuana tnbercnlata. The alimentary canal and appendages, together with the hepatic portal vein and the intrahepatic segment of the postcava, have been removed. The liver occupies the space between the divided ends of the postcava. The vertebral vein represents tln' rudimentary proximal segment of the postcardinal vein corresponding to the mammalian azygos vein. (Columbia University Museum, No. 1.320.) PLATE CXXXIII. AORTA R. AORTIC ARCH WITH CAROTID TRUNK SINUS VCNOSUS L AORTIC ARCH R. AORTIC ARCH WITH SUBCLAVIAN ARTERIES POSTCAVA- VERTEBRAL (aZYGOS^ V. CONNECTING WITH INTRAHEPATIC SEG- MENT OF POSTCAVA EXT. JUGULAR V. .NT. JUGULAR iPRECARDINALJ V. PULMONARY A. L. AORTIC ARCH L. SUBCLAVIAN V L SUBCLAV.AN A. L. VERTEBRAL (azygos) V. VERTEBRAL (aZY- GOS) VEINS Fig. 267. — Veins of Iguana tnberculata. Connection of systemic veins with sinus venosus of heart. The rudimentary system of the vertebral (azygos) veins and their proximal connection with the subclavian vein are shown. (Columbia University Museum, No. 1859.) ABDOMINAL V HEPATIC PORTAL V POSTCAVA ENTERING SINUS VENOSUS POSTCAVA ENTER- ING LIVER Fig. 2(^. — Corrosion preparation of venous system of liver in Iguana tubereulata. The hepatic portal system and its connection with the abdominal vein, as well as the relation to the postcava, are shown. The preparation supplements Fig. 266, showing the parts which have been removed in the latter. (Columbia University Museum, No. 1860.) PLATE CXXXIV. POSTCAVA DI- VIDED AT EN- TRANCE INTO LIVER TRANSV. ANASTOMO- SIS BETW. POSTCAR- DINAL VEINS L. POSTCARDINAL V L DIVISION OF ABDOMINAL V. BLADDER CAUDAL V, R POSTCARDINAL V AFFERENT RENAL- PORTAL VEiN Fig. 269. — Iguana tuberculata, (J. Genito-urinary tract, dorsal view, with renal-portal, post- cardinal, and postcaval veins. (Columbia University Museum, No. 1862.) PLATE CXXXV. R. BRACHIAL V R. PECTORAL V R. PRECAVA- R. AURICLE- HEPATIC VV P08TCAVA R. COMMON ILIAC V R. KIDNEY' R. FEMORAL V. R. RENAL V. divided' sciatic v. AFFERENT RENAL V. PELVIC V. ENTER- ING RENAL-POR- TAL SYSTEM R. INT. ILiAC V. RENAL-PORTAL V. CAUDAL V. POST. MESENTERIC V. L. JUGULAR PRECARDINAL) V. L. JUGULAR V L. BRACHIAL V. L. AURICLE L. VENTRICLE EPIGASTRIC V. EFFERENT RENAL V. FEMORAL V L. RENAL V. EFFERENT RENAL V. RENAL-PORTAL V. COCCYGEO-MES- ENTERIC VEIN Fig. 270. — Veins of pigeon, Columba Uvia. (Modified from Parker and Haswell.) The renal- portal vein of the right side is supposed to be dissected to show its passage through the right kidney. PLATE CXXXVI. R. COMMON CAROTID A. R. SUBCLAVIAN A. PRECAVA AZYGOS R. PULMONARY A. R. PULMONARY V. SINUS VENOSUS OF R. AURICLE R. HEPATIC V. R PORTAL V R. HEPATIC V R. RENAL V L. COMMON CAROTID A. L. BRACHIO- CEPHALIC V. PULMONARY A. AORTA DUCTUS VENOSUS JOINING L. HEPATIC VEIN L. PORTAL V. ABDOMINAL AORTA L. RENAL V. UMBILICAL V. UMBILICAL (HYPO- GASTRIC) ARTERIES MEETING UMBILI- CAL V. AT CORD Fig. 271. — Human foetus at term. Corrosion preparation of lieart and vascular system. (Columbia University Museum, No. 1858.) SUMMARY OF HEPATIC CIRCULATION. 153 After birth all the venous blood entering the liver passes through the portal vein. In the right division the direction of the current is the same as in the foetus. ~ ^ On the left side, however, the current is now from right to left, from the bifurcation of the portal into the channels of the left lobe formerly connected with the umbilical vein (Fig. 261). Hence the direction of the current in this segment is reversed at birth. SUMMARY OF HEPATIC CIRCULATION. The foregoing consideration of the development shows us that the hepatic circulation presents successively three main stages : 1. Omphalo-mesenteric or Vitelline Stage, which results in the laying down of the primary capillary circulation of the liver and in the establishment of its connection with the developing veins of the alimentary tract (primitive portal channels). 2. Umbilical or Placental Stage, in which the greater part of the blood circulating through the liver is oxygenated blood re- turned from the placenta by the umbilical vein, accounting for the rapid growth and relatively large size of the organ during foetal life. The placental blood uses the preformed capillary channels of the vitelline or primitive portal system in the liver, and t he same rapidly extend and enlarge with the accelerated growth of the gland. During this stage venous blood is also returned from the alimentary tract to the liver by the portal vein, produced by fusion of the distal segments of the primitive vitelline veins and their secondary connection with the mesenteric, splenic and pan- creatic veins (omphalo-mesenteric development of primitive vitel- line veins). 3. Adult or Portal Stage. — With the interruption of the placental circulation the portal vein assumes again its original position as the only vein carrying blood to the liver. With the establish- ment of intestinal digestion and absorption this vessel grows rapidly in size. 154 ANATOMY OF THE PERITONEUM. COMPARATIVE ANATOMY OF THE HEPATIC VENOUS CIRCULATION. For the purpose of fixing the main facts in connection with the development of the higher mammahan hepatic circulation, and in order to obtain a demonstration of the cycle through which the different veins pass, the student is recommended to examine, pref- erably by personal dissection, a limited series of lower vertebrates which can be readily procured and easily injected. The follow- ing series has been selected, but it will be understood that other forms can be substituted, according to the local conditions which govern the supply of the material. 1. Fish. A Selachian, the common skate {Raja ocellata) or dog- fish {Acanthias vulgaris). 2. Amphibian. (a) Urodele. Nedurus maculatus. (6) Anura. The common /ro^. 3. Reptile. Preferably, on account of the ease of injection, one of the larger lizards, as Iguana tuberculata. The turtles, although somewhat more difficult objects to pre- pare, can be substituted. 4. Bird. The common fowl. 5. Human foetus at term. 1. Fish. — The venous system can be injected by tying a canula in the lateral vein, and injecting both cephalad and caudad, or by injecting cephalad through the caudal vein. The injection of the systemic veins can also be made caudad through one of the ducts of Cuvier, combined with an injection cephalad of the caudal vein. The following main facts are to be noted in the venous system of the Selachian (Fig. 262) : 1. There are Two Portal Systems, (a) Renal Portal System. — The cau- dal vein divides near the vent into two branches which course along the lateral border of the kidneys, sending afferent or adve- COMPARATIVE ANATOMY OF VENOUS CIRCULATION. 155 hent veins into the organ. The blood traverses the renal capil- laries and is gathered together by the efferent or revehent veins, which empty into median paired vessels, the posterior cardinals. (6) Hepatic Portal System. — The veins of the digestive tract and appendages unite to form a hepatic portal vein. The blood after traversing the capillary system of the liver is collected by hepatic veins, which form a dilated hepatic sinus emptying into the sinus venosus of the heart. 2. The middle segment of the intestine, presenting a spiral valve in the interior, gives rise to a vein emptying into the portal vein which corresponds to the subintestinal vitelline vein of the mammalian embryo (Fig. 202). 3. The posterior cardinal veins, also greatly dilated and forming the posterior cardinal sinus, join, near the heart, the veins return- ing blood from the head, the anterior cardinal or jugular, to form a transversely directed trunk, the duct of Cuvier, which empties into the sinus venosus at the auricular extremity of the heart. Into the duct of Cuvier empties on each side a lateral vein return- ing the blood from the body walls. This vein can be considered, for our present purpose, as representing in general the abdominal vein of amphibians and reptiles, and the umbilical vein of the mammalian embryo. The adult selachian venous system is therefore to be consid- ered as illustrating the following conditions above encountered in our study of the embryology of the mammalian venous system. 1. The heart illustrates excellently the stage in the mammalian development, in which auricular and ventricular segments have dilBferentiated, but before the division of the cavities into a pul- monary and systemic portion by the development of the auric- ular and ventricular septa and the division of the arterial trunk into pulmonary artery and aorta. The sinus venosus still exists, as an ante-chamber to the auric- ular cavity proper, receiving on each side the ducts of Cuvier, which represent the fusion product of the systemic veins, ante- rior and posterior cardinal. 156 ANATOMY OF THE PERITONEUM. 2. The hepatic portal circulation corresponds to the mamma- lian stage in which the vitelline veins have become omphalo- mesenteric by joining the intestinal veins. The spiral vein remains as a portion of the original vitelline vein corresponding to the subintestinal segment of the mamma- lian embryo (cf Figs. 248 and 249). The selachian portal vein represents the united vitelline veins, into which the veins of the digestive tract open. In the liver we find a simple system of vense advehentes, de- rived from the branching of the portal vein, a hepatic capillary network, and vense revehentes, the proximal remnants of the original vitelline veins which carry the liver blood to the sinus venosus. The condition of the hepatic circulation corresponds therefore to the stage shown in Fig. 250 of the mammalian de- velopment. There is as yet no association of the hepatic venous system with the representative of the umbilical vein ( the lateral vein of the selachian). 3. The lateral veins, which we can, as stated, regard for pur- poses of illustration, without prejudging their genetic significance, as representing the mammalian embryonic umbilical veins, still present the condition corresponding to the early mammalian embryonal stage shown in Fig. 250. They are veins of the body walls, emptying cephalad of the liver, directly into the ducts of Cuvier, and through them into the sinus venosus of the heart. Fig. 262 shows the arrangement of the venous system in a typical selachian diagrammatically. 2. Amphibian, (a) Urodele. — The following points are to be noted in comparison with the preceding form : 1. The two ducts of Cuvier entering into the sinus venosus are formed by the anterior cardinal and subclavian veins, which latter, having appeared with the full development of an anterior extremity, receives the posterior cardinal veins, representing the mammalian azygos system. 2. The renal portal circulation persists. The caudal vein is, however, no longer the only afferent vein of this system. With COMPARATIVE ANATOMY OF VENOUS CIRCULATION. 157 the full development of a posterior extremity an iliac vein re- turns the blood from the same and gives a large branch (afferent to the portal renal system), while the trunk continues cephalad as an anterior abdominal vein, corresponding to the lateral sela- chian vein, emptying in the hepatic portal vein. 3. The efferent veins of the renal portal system no longer unite to form the posterior cardinal, as in the Selachian, but empty into a new median vessel, the inferior vena cava, or postcava, which has replaced the distal segments of the posterior cardinal veins. The postcava now carries the blood from the kidneys directly to the heart. The original posterior cardinal veins still persist in their proximal segments, as smaller trunks connecting the distal part of the postcava with the ducts of Cuvier through the sub- clavian veins. The ducts of Cuvier represent the precavse (vense cavse superiores) of mammalia and the postcardinals the mam- malian azygos veins. 4. The hepatic portal system differs in two respects from the Selachian type. (a) The blood returned to the liver from the digestive tract by the portal vein becomes mixed before entering the gland with the blood returned from the posterior extremities and abdominal walls by the abdominal vein. This vein, paired below and continuous with the lateral of the two branches into which the iliac vein divides, becomes united into a single trunk above and empties into the portal vein. The abdominal vein represents the lateral vein of the Selachian and corresponds to the umbilical vein of the higher vertebrates. (h) The vense hepaticse revehentes do not empty directly into the sinus venosus, but into the proximal portion of the postcava. Hence the adult urodele venous system illustrates, in reference to the mammalian development, these stages : 1. The umbilical (abdominal) vein has lost its direct connection with the sinus venosus. The proximal segment, cephalad of the liver, has disappeared, and its blood now passes directly into the hepatic circulation by its union with the portal vein. 158 ANATOMY OF THE PERITONEUM. (Cf. stage schema Figs. 251 and 252.) 2. The postcaval vein has made its appearance, largely replac- ing the posterior cardinal veins, whose proximal segments became converted into secondary vessels (azygos) uniting the system of the postcava with that of the duct of Cuvier (mammalian prae- cava), while their distal segments are transformed into the distal portion of the postcava. The postcava, therefore, is made up of two districts : {a) The proximal portion is a new vessel, developed in connec- tion with the hepatic venous system. (6) The distal portion is derived from the distal segments of the original posterior cardinal veins. The termination of the hepatic veins in the postcava corre- sponds to the stage shown in schema Fig. 256. Fig. 263 gives a schematic representation of the arrangement of the venous system in a typical urodele amphibian {Salamandra maculosa). In Fig. 264 the dissected venous system of Necturus rrMculatvs, the mud puppy, is shown in an injected preparation. (6) Anure. — The venous system of Rana esculenta is shown in Fig. 265. Comparison with venous system of urodele : 1. The abdominal vein, corresponding to the mammalian um- bilical vein, has assumed a greater importance in reference to the hepatic circulation. It is a large trunk, continuous below with the pelvic vein, terminating above in two branches, which enter the liver as afferent veins, being joined just prior to the division by the hepatic portal vein. 2. A small cardiac vein, coming from the heart, empties into the angle of bifurcation of the abdominal vein. 3. The postcava is well developed, formed by large efferent renal veins. It entirely replaces the posterior cardinal veins which are absent in the adult animal. 4. A right and left prsecaval vein is formed by the union of two jugular trunks with the vein of the anterior extremity an da large musculo-cutaneous vein. COMPARATIVE ANATOAfY OF VENOUS CIRCULATION. 159 Comparison with the mammaHan development : the venous system of this amphibian can be used to illustrate the mamma- lian embryonal stage shown in schema Fig. 252, in which the ab- dominal or umbilical vein has become the most important vessel in the afferent hepatic venous system. The communication existing by means of the cardiac vein be- tween the heart and the hepatic afferent system may suggest, but purely for illustrative purposes, the direct connection of the um- bilical vein with the heart by the ductus venosus in the mam- malian embryo (cf. schema Figs. 250-256). 3. Reptile. — In Iguana the renal portal system is well developed. The caudal vein, returning the blood from the tail and the cavern- ous tissue of the genital organs, continues for a short distance upon the fused caudal end of the two kidneys (Fig, 269) and then > divides into two afferent renal veins which ascend on the ventral surface of the glands, giving branches to the renal cap- illary system. About the middle of the kidney each afferent vein is joined by a large transverse branch from the abdominal vein (Fig. 266). The renal efferent system begins by a number of interrenal anastomoses which unite along the mesal border of the right kid- ney into a large ascending trunk, while the corresponding vessel of the left side, starting from the same anastomosis, is consider- ably smaller (Figs. 266 and 269). Each of these vessels also re- ceives blood from the testis, epididymis, vas deferens and adrenal body in the male, and from the ovary and oviduct in the female. They represent, in fact, the distal functional part of the right and left embryonic postcardinal vein. Just caudad of the left testis the vein of the left side crosses obliquely ventrad of the aorta and joins the right vessel to form the trunk of the postcava, which enters, immediately beyond the cephalic pole of the right testis, the prolonged caval lobe of the liver (Figs. 266 and 269). Ascending in the substance of this gland and receiving the afferent hepatic veins (Fig. 268), the vena cava emerges from the cephalic surface of the liver greatly enlarged and proceeds to the right auricle. 160 ANATOMY OF THE PERITONEUM. The abdominal vein divides below into two branches which pass caudad on each side of the bladder, receiving tributaries from the same, to the lateral border of the kidneys (Figs. 266 and 269). Here the vessel is connected by the transverse branch above described with the afferent renal portal system derived from the caudal vein. At the same point it receives the sciatic vein, the principal venous vessel of the posterior extremity. Above, the main abdominal vein, resulting from the union of the two branches referred to, ascends on the dorsal surface of the ventral abdominal wall, receiving a few twigs from the ventral mesogastrium within whose free caudal edge the vessel runs. Just before reaching the liver the abdominal vein turns dorsad on the caudal surface of the gland and joins the hepatic portal vein (Figs. 268 and 275). Several accessory veins, two or three in number, belonging to the system of the abdominal vein, pass above this point from the ventral body wall between the layers of the ventral mesogastrium, to enter the liver separately on its convex ventral surface, above the fusion of the main abdominal vein with the portal vein. These additional branches on entering the liver join the portal system, forming a set of ventral accessory portal veins. The hepatic portal vein derives its principal tributaries from the splenic, gastric, pancreatic and intestinal veins. One or two additional branches (accessory vertebral portal veins), as above stated, connect the system of the segmental and vertebral veins with the portal circulation, entering the liver separately. In like manner one or two gastric veins (accessory gastric portal veins) enter the dorsal aspect of the liver separately, passing from the stomach to the gland between the layers of the gastro-hepatic omentum (Fig. 275). Compared with the development of the mammalian type, the venous system of Iguana serves to illustrate the stage in the his- tory of the umbilical vein (represented by the abdominal vein of the reptile) in which the connection of the vessel with the portal vein has been formed and transmits the greater part of the blood PLATE CXXXVII. R. INT. JUGULAR V INF. THYROID V R. EXT. JUGULAR V INF. THYROID V. R. SUBCLAVIAN V. R. INT. IMAMMARY V. PRECAVA SUP. INTER- COSTAL V. COMMON AZYGOS V V. AZYGOS MAJOR POSTCAVA DIVIDED AT ENTRANCE-f^ INTO LIVER R. RENAL V QUADRATUS LUMBORUM R. SPERMATIC V L. INT. JUGULAR V. INF. THYROID V. L. EXT. JUGULAR V. L. SUBCLAVIAN V. L. BRACHIO- CEPHALIC V. L. INT. MAMMARY V. L. SUP. INTER- COSTAL V. L. INTERMEDIATE AZYGOS FROM 5TH, 6TH AND 7TH SPACES V. AZYGOS MINOR — L. RENAL V -r t