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diff --git a/43350-0.txt b/43350-0.txt new file mode 100644 index 0000000..fb07099 --- /dev/null +++ b/43350-0.txt @@ -0,0 +1,14114 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 43350 *** + +Transcriber's notes: + +In this transcription, italic text is denoted by _underscores_ and bold +text by =equal signs=. Superscripts are indicated by ^ (e.g. Fig. 509, +_I^a_). + +The text contains numerous inconsistencies of hyphenation. A few have +been adjusted where there was clear evidence of a preferred style (e.g. +meso-colic-->mesocolic and meso-duodenum-->mesoduodenum) but most have +been left in their original format. + +A few spelling typos have been corrected silently (e.g. +improtant-->important, mecocolon-->mesocolon) and missing letters have +been inserted inside square brackets (e.g. junct[i]on, t[r]ansverse). +Some spelling inconsistencies probably represent contemporarily +acceptable spelling alternatives (e.g. coati/coaiti, mesal/mesial, +praecava/precava, hyaena/hyena). + +A small number of punctuation inconsistencies have been corrected +silently by insertion of missing punctuation or deletion of redundant +punctuation. + +The abbreviation viz. appears inconsistently in both roman and italic +font. + +Illustrations (not displayed in this transcription) were originally +grouped separately from the text. Their captions have been relocated +adjacent to their first mention in the text. Some illustrations +contained features labelled as X, A, B, 1, 2, etc. and when these are +cross referenced in the text they are sometimes inconsistently styled +(upper/lower case, roman/italic). + +The numbering system and font styling of the TOC is not consistent +within the TOC nor with the corresponding headings in the text, and some +TOC entries correspond to in-line text rather than to true headings. No +attempt has been made to remedy these inconsistencies. One missing TOC +entry has been inserted. + +The index is shown as it originally appeared - with some entries not in +correct alphabetic sequence. + +Footnotes are located below the paragraph of reference. + + + + + THE ANATOMY + OF THE + HUMAN PERITONEUM + AND + ABDOMINAL CAVITY + + CONSIDERED FROM THE STANDPOINT OF DEVELOPMENT + AND COMPARATIVE ANATOMY + + BY + + GEORGE S. HUNTINGTON, M.A., M.D. + + PROFESSOR OF ANATOMY, COLLEGE OF PHYSICIANS AND SURGEONS, + COLUMBIA UNIVERSITY, + NEW YORK CITY + + ILLUSTRATED WITH 300 FULL-PAGE PLATES CONTAINING + 582 FIGURES, MANY IN COLORS + + LEA BROTHERS & CO. + PHILADELPHIA AND NEW YORK + 1903 + + Entered according to the Act of Congress, in the year 1903, by + + LEA BROTHERS & CO., + + In the Office of the Librarian of Congress. All rights reserved + + + + +PREFACE. + + +In the following pages an attempt has been made to emphasize the value +of Embryology and Comparative Anatomy in elucidating the difficult and +often complicated morphological problems encountered in the study of +human adult anatomy. + +Moreover, in addition to the direct advance in the method and scope of +anatomical teaching afforded by these aids, it is further hoped that the +broader interpretation, both of structure and function, obtained by +ontogenetic and phylogenetic comparison, will impart an interest to the +study of adult human morphology, such as the subject, considered solely +in the narrow field of its own limitations, could never arouse. + +The book represents part of the course in visceral anatomy as developed +during the past fourteen years at Columbia University. The sections +dealing with the morphology of the vertebrate ileo-colic junction and +with the structural details of the human caecum and appendix are +considered somewhat more fully, as warranted by the extensive material +available. The illustrations are for the greater part taken from +preparations in the Morphological Museum of the University. Wherever +practicable the direct photographic reproduction of the actual +preparation is given. In the case of preparations not suitable for this +purpose, careful drawings have been made which offer in every instance a +faithful and correct interpretation of the conditions presented by the +actual object. A number of the embryonic illustrations are taken from +the standard text-books on the subject, due credit being given to their +source. I desire to express my sincere thanks to Dr. Edward Leaming, of +the Department of Photography and to Mr. M. Petersen, artist of the +Anatomical Department of the University, for their skilful and +thoroughly reliable work in the preparation of the illustrations. + + =George S. Huntington.= + + =Columbia University=, in the City of New York, + _December, 1902_. + + + + +CONTENTS. + + PAGE. + + INTRODUCTION 17 + + Development of Vertebrate Ovum 19 + + Development of Coelom and of Alimentary Canal 21 + + Development of Cloaca 24 + + Development and Divisions of the Peritoneum 32 + + Derivatives of Entodermal Intestinal Canal 34 + + Divisions of Alimentary Canal 38 + + + PART I. ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY 39 + + COMPARATIVE ANATOMY OF FOREGUT AND STOMACH 42 + + Morphological Types of Stomach 43 + + Development of the Intestine 51 + + I. Intestinal Rotation and Definition of Adult Segments of + the Intestinal Canal 58 + + Development of Aortal Arterial System 63 + + II. Demonstration of Intestinal Rotation in the Lower + Mammalia 67 + + Peritoneal and Visceral Relations in the Infra-colic + Compartment of the Abdominal Cavity in the Adult 74 + + + Part II. ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT + OF THE ABDOMEN 99 + + 1. STOMACH AND DORSAL MESOGASTRIUM 100 + + _a._ Changes in Position of Stomach 102 + + _b._ Changes in Direction and Extent of Dorsal Mesogastrium 103 + + _c._ Development of Spleen and Pancreas in the Dorsal + Mesogastrium and Changes in the Disposition of the Great + Omentum 108 + + 1. Development of Spleen 108 + + 2. Development of Pancreas 111 + + Development of Pancreas in Lower Vertebrates 115 + + Comparative Anatomy of Pancreas 116 + + Pyloric Caeca or Appendices 119 + + Peritoneal Relations of Pancreas 122 + + Comparison of Embryonal Stages during the Development of + the Human Dorsal Mesogastrium, Spleen and Pancreas with + the Permanent Adult Condition of the same Structures in + Lower Mammalia 126 + + 1. Spleen, Pancreas and Great Omentum of Cat 127 + + 2. Relation of Great Omentum to Transverse Colon, + Transverse Mesocolon and Third Part of Duodenum 129 + + 2. VENTRAL MESOGASTRIUM AND LIVER 140 + + I. _A._ Development of Liver 141 + + _B._ Comparative Anatomy of Liver 144 + + _C._ Development of Vascular System of Liver 145 + + Comparative Anatomy of the Hepatic Venous Circulation 154 + + II. Ventral Mesogastrium 163 + + Peritoneal Relations of Liver 167 + + Relation of Hepatic Peritoneum to the "Lesser Sac" 174 + + Caudal Boundary of Foramen of Winslow 178 + + Pancreatico-gastric Folds 181 + + + PART III. LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION + AND CAECUM 189 + + I. GENERAL REVIEW OF MORPHOLOGY AND PHYSIOLOGY OF THE + VERTEBRATE INTESTINE 190 + + I. Midgut or Small Intestine 192 + + Intestinal Folds 193 + + Divisions of Small Intestine 194 + + Structure of Small Intestine 194 + + 1. Secretory Apparatus 194 + + 2. Absorbing Apparatus 195 + + Valvulae Conniventes 196 + + II. Endgut or Large Intestine 198 + + II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED + STRUCTURES IN VERTEBRATES 200 + + I. Fishes 200 + + II. Amphibia 201 + + III. Reptilia 201 + + IV. Birds 203 + + V. Mammalia 204 + + Monotremata 204 + + Marsupalia 204 + + Edentata 206 + + Sirenia 208 + + Cetacea 209 + + Ungulata 209 + + Rodentia 211 + + Carnivora 212 + + Cheiroptera 212 + + Insectivora 213 + + Primates 213 + + III. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION AND CAECUM + IN THE VERTEBRATE SERIES 217 + + 1. Symmetrical Form of Ileo-colic Junction; Mid- and End-gut + in Direct Linear Continuity 221 + + 2. Asymmetrical Development of a Single Caecal Pouch, lateral + to the Ileo-colic Junction, Mid- and End-gut Preserving + their Linear Continuity 223 + + 3. Rectangular Ileo-colic Junction, with Direct Linear + Continuity of Caecum and Colon 225 + + IV. STRUCTURE OF CAECAL APPARATUS AND SPECIALIZED MORPHOLOGICAL + CHARACTERS OF COLON IN RODENTS AND UNGULATES 229 + + 1. Caecum Proper 229 + + 2. Structural Modifications of Proximal Segment of Colon + analogous in their Functional Significance to the Caecal + Apparatus 230 + + V. CAECAL APPARATUS AND COLON IN HYRAX. 234 + + + PART IV. MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX 237 + + I. Development of the Caecum and Appendix 237 + + II. Changes in the Position of the Caecum and Appendix during + normal Development, depending upon the Rotation of the + Intestine and the subsequent Descent of the Caecum 239 + + III. Variations of Adult Caecum and Appendix 244 + + _A._ Shape of Caecum and Origin of Appendix. Types and + Variations of Adult Caecum and Appendix 245 + + _B._ Position and Peritoneal Relations of Appendix 250 + + _C._ Ileo-Caecal Folds and Fossae 260 + + + + +INTRODUCTION. + + +In considering the anatomy of the human abdominal cavity and peritoneum +in the following pages the explanation of the adult conditions +encountered is based upon the development of the parts, and the +successive human embryonal stages are illustrated by the examination of +the lower vertebrates presenting permanent adult structural conditions +which appear as merely temporary embryonal stages in the development of +the higher mammalian alimentary tract. + +For the sake of clearness and brevity all discussion of the _theories of +peritoneal development_ has been designedly omitted. The assumption of +peritoneal _adhesion_, and consequent obliteration of serous areas, +offers many advantages in considering the adult human abdominal cavity, +especially from the standpoint of comparative anatomy. The same has +consequently been adopted without reference to divergent views and +theories. + +In studying the descriptive text and the diagrams the student should +remember that the volume offers in no sense a complete or detailed +account of the development of the abdominal cavity and its contents. The +purpose is not to present the embryology of this portion of the +vertebrate body, but to _utilize_ certain embryological facts in order +to _explain_ the complicated adult conditions encountered. To avoid +confusion, and to bring the salient points into strong relief, the +majority of the diagrams illustrating human embryonal stages are purely +schematic. + +Moreover, in order to avoid confusing and unnecessary details it is +often desirable to disregard developmental chronology entirely. Many of +the diagrams combine several successive developmental stages, showing +different degrees of development in different portions of the same +drawing. Again it is frequently necessary, for the sake of brevity and +clearness, to actually depart from known embryological conditions. If, +for example, the stomach and liver are treated as if they were from +their inception abdominal organs, the student of systematic embryology +will recall the fact that this position is only _obtained after_ their +primitive differentiation by growth and migration. + +Again the mesenteries are treated here as if they formed definite and +well-defined membranes from the beginning--without reference to the +abdominal organs with which they are associated. We speak of the liver +as growing into and between the layers of the ventral mesogastrium, +because this conception offers the opportunity of more clearly +explaining the adult condition. Actually, however, the membrane +develops, as a new structure, after the first differentiation of liver +and stomach, as these organs descend into the abdominal cavity. + +Similar discrepancies between fact and schema are encountered +throughout. Consequently, while the purpose of the volume is to +facilitate the study and comprehension of the _adult_ peritoneal cavity +and its contents, the reader should guard against receiving the +developmental illustration as a correct successive and detailed account +of the _embryology_ of the parts concerned. + +In like manner the comparative anatomical facts adduced form in no sense +even approximately a complete serial morphological account of the +vertebrate alimentary tract. + +To the student of human anatomy the zoological position of the forms +which help him to understand complicated human structural conditions is +immaterial. He can draw on all the vertebrate classes independently of +their mutual relations. Hence neither ontogeny nor phylogeny are here +introduced, except as aids to the study of adult human anatomy. The +following pages offer neither an embryology nor a comparative anatomy of +the alimentary tract, but an attempt has been made in them to illustrate +the significance of the complicated anatomical details presented by the +adult human abdominal cavity by reference to the simpler antecedent +conditions encountered during the early developmental stages of the +higher forms and permanently in the structure of the lower vertebrates. + +While, as just stated, a complete presentation of the development of the +abdominal cavity is not required, yet the student will find it of +advantage to rehearse the main facts of vertebrate embryology, for the +purpose of bringing a clear understanding of the manner in which the +vertebrate body is built up to bear upon the problems which the special +organs and structures of the body-cavity present for his consideration. +This purpose can be accomplished by a very brief and condensed +consideration of the cardinal facts. + +The entire vertebrate body is the product of developmental changes +taking place after fertilization in a single primitive CELL, the EGG or +OVUM (Fig. 1). + +[Illustration: FIG. 1.--Human ovum, from a mature follicle, a sphere of +about 0.2 mm. diameter. x 25. (Kollmann.)] + +In structure the ovum corresponds to other animal cells. On account of +their special significance during development the different component +parts of the egg-cell have received special distinctive names. The +_cell-body_ is known as the _vitellus_ or _yolk_. It is composed of two +substances, the _protoplasm_ or formative yolk and the _deuteroplasm_ or +nutritive yolk, which vary in their relative proportions in the ova of +different animals. + +The protoplasm represents the material from which in the course of +development the cells forming the body of the individual are derived, +while the deuteroplasm serves for the nutrition of the ovum during the +earliest stages of development. + +The _nucleus_ of the egg-cell is distinguished as the _germinal +vesicle_, and its _nucleolus_ as the _germinal spot_. + +The _cell-body_ or _vitellus_ is surrounded by a condensed portion of +the cell contents to which the name of the vitelline membrane has been +applied, which in turn is enclosed by a transparent and elastic cover, +the _zona pellucida_, presenting a radially striated appearance. + +The ovum is contained in the cortical portion of the ovary, enclosed in +the _Graafian follicle_, a vesicle 4-8 mm. in diameter, whose fibrous +walls are lined by several layers of epithelial cells, which surround +the ovum, forming the _discus proligerus_. + +After impregnation the egg-cell, by a process of repeated division or +cleavage, undergoes _segmentation_, the cell-body being divided +successively into two, four, eight, sixteen, thirty-two, etc., _cells_, +called _blastomeres_ (Figs. 2 and 3). The mass of cells finally +resulting from this process of segmentation forms the ground work of the +future body. A vertebrate ovum in this stage of complete segmentation is +called the _morula_ from its resemblance to a mulberry (Fig. 4). + +[Illustration: FIG. 2.--Segmentation of mammalian ovum (bat). (After E. +von Beneden.) Two blastomeres, each with a nucleus, shown in lighter +color. The dark bodies are yolk-granules.] + +[Illustration: FIG. 3.--Segmentation of mammalian ovum. Four +blastomeres. (After E. von Beneden.)] + +[Illustration: FIG. 4.--Ovum of rabbit, from terminal portion of +oviduct. The zona pellucida appears thickened, and contains many +spermatozoa which failed to penetrate the ovum. (After Bischoff.)] + +After segmentation is completed a cavity filled with fluid and +surrounded by the developing cells is gradually formed in the interior +of the mass. This cavity is known as the _segmentation-cavity_. The egg +is now called the _blastula_, _blastosphere_ or _blastodermic vesicle_ +and the cellular membrane enclosing the segmentation-cavity forms the +_germinal membrane_ or _blastoderm_ (Figs. 5 and 6). The cells of the +blastoderm become aggregated at one point on the circumference of the +vesicle (dorsal pole of blastosphere) forming, when viewed from above, a +thickened biscuit or disk-shaped opaque area. This is known as the +_germinal area_, or _primitive blastoderm_ or _embryonic shield_ (Figs. +7 and 12). + +[Illustration: FIG. 5.--Blastodermic vesicle of rabbit. (After E. von +Beneden.)] + +[Illustration: FIG. 6.--Blastodermic vesicle of _Triton taeniatus_. +(Hertwig.)] + +[Illustration: FIG. 7.--Embryonic area of rabbit embryo. (Heisler, after +E. von Beneden.) The primitive streak beginning in the +cell-proliferation known as the "node of Hensen."] + +[Illustration: FIG. 12.--Oval embryonic area of rabbit's egg, detached +with part of wall of blastodermic vesicle. x 30. (Kollmann.)] + +This is the first indication of the coming division of the entire +egg-cell into the _embryo proper_ and the _vitelline_ or _yolk-sac_ +(Figs. 8 and 9). The entire future individual develops from the cells of +the germinal area. This area comprises both the embryo proper and the +region immediately surrounding it. + +[Illustration: FIG. 8.--Blastodermic vesicle of mammal. (E. von +Beneden.) The layer of cells lining the interior of the vesicle next to +the zona pellucida forms Rauber's "Deckschichte" or prochorion. This is +not the true ectoderm, since it does not participate in the formation of +the embryo, which is entirely derived from the cells of the germinal +area.] + +[Illustration: FIG. 9.--Human embryo with yolk-sac, amnion, and +belly-stalk of fifteen to eighteen days. (Heisler, after Coste.)] + +The remainder of the ovum, serving temporary purposes of nutrition and +respiration, gradually becomes absorbed and disappears. + +[Illustration: FIG. 10.--Embryonal area of sheep, composed of ectoderm +and entoderm. (After Bonnet.)] + +[Illustration: FIG. 11.--Blastodermic vesicle of rabbit. Section through +embryonic area at caudal limit of node of Hensen. (Rabl.)] + +Transverse sections at right angles to the long axis of the embryonic +area show that the single layer of cells composing the primitive +germinal membrane becomes differentiated first into two (Fig. 10) and +subsequently into three layers of cells (Fig. 11). At the margins of the +germinal area these layers are of course continuous with the rest of +yolk-sac wall. From their position in reference to the center of the +cell the three layers of the blastoderm are described as-- + + 1. The outer, Epiblast or Ectoderm. + 2. The middle, Mesoblast or Mesoderm. + 3. The inner, Hypoblast or Entoderm. + +The central nervous system (brain and spinal cord) is derived from the +ectoderm by the development of a groove in the long axis of the +embryonic area (Figs. 13, 14, 16 and 17), and by the subsequent union in +the dorsal midline of the ridges bounding the groove to form a closed +tube (Fig. 18). (Medullary groove, plates and canal.) + +[Illustration: FIG. 13.--Transverse section of embryonic area of ovum of +sheep of fourteen and a half days. (Heisler, after Bonnet.)] + +[Illustration: FIG. 14.--Germinal area of rabbit's ovum. (Kollmann.)] + +[Illustration: FIG. 15.--Surface-view of area pellucida of an +eighteen-hour chick-embryo. (Balfour.)] + +[Illustration: FIG. 16.--Transverse section of human embryo before +development of protovertebrae or chorda dorsalis. (Keibel.)] + +[Illustration: FIG. 17.--Transverse section of a sixteen and a half day +sheep embryo. (Heisler, after Bonnet.)] + +[Illustration: FIG. 18.--Embryo of bird, at beginning of third day, with +four blastodermic layers, resulting from the division of the mesoderm +into parietal and visceral layers, separated by the coelom cavity. +Transverse section. x 170. (Kollmann.)] + +The following changes in the ventral aspect lead to the formation of the +alimentary canal and body-cavity: + +The developing embryo at first lies flat on the subjacent yolk-mass, and +subsequently becomes gradually separated more and more from the rest of +the blastoderm by grooves or furrows which develop along the sides and +at the cephalic and caudal extremity of the embryo. The folds resulting +from these furrows indent the yolk more and more as development proceeds +and tend to approach each other at a central point, the future +_umbilicus_. + +In the meanwhile changes in the region of the mesoderm have led to +conditions which produce a differentiation of the ventral portion of the +embryo into two tubes or cylinders, the _alimentary_ or _intestinal +canal_ and the _general body-cavity_, the former being included within +the latter. + +Early in the course of development a number of spaces appear in the +mesoderm on each side of the axial line of the embryo. These spaces soon +unite to form two large cavities, one on each side. Taken together these +cavities constitute the _coelom_ or _body-cavity_, which becomes +subdivided in the adult mammal into the pleural, pericardial and +abdominal cavities. + +As these coelom cavities develop in the mesoderm the cells lining them +become distinctly epithelial. This mesodermic epithelium lining the +coelom is called the _mesothelium_. + +The development of the coelom space divides the mesoderm on each side +into an outer leaf, the _somatic_ or _parietal mesoderm_, and an inner +leaf, the _splanchnic_ or _visceral mesoderm_ (Figs. 18 and 19). The +former is closely applied to the ectoderm, forming with it the +_somatopleure_ or _body-wall_. The latter, in close contact with the +entoderm, forms with it the _splanchnopleure_ or wall of the alimentary +canal. In the dorsal median line both somatic and splanchnic mesoderm +become continuous with each other and with the axial mesoderm (Fig. 20). + +[Illustration: FIG. 19.--Transverse section of a seventeen and a half +day sheep embryo. (Bonnet.)] + +[Illustration: FIG. 20.--Curves of blastodermic layers and division of +mesoderm in amniote embryo. (Kollmann.)] + +The folds of the splanchnopleure, indenting the yolk-sac, form a gutter +directly connected with the yolk, the _primitive intestinal groove_ or +_furrow_, whose margins gradually approach each other (Fig. 20). In this +way the primitive alimentary canal becomes separated from the yolk. At +first this separation is ill-defined, and the channel of communication +between the primitive intestine and the yolk is wide (Figs. 13, 16, 17 +and 19). The folding of the splanchnopleure completes, at an early +period, the dorsal and lateral walls of the embryonic gut, but +ventrally, toward the yolk, the tube is incomplete and widely open. + +By union and coalescence of the splanchnopleural folds, proceeding from +the caudal and cephalic ends towards the center, this primitive wide +channel gradually becomes narrowed down, until the communication between +the yolk-sac and the intestine is reduced to a canal, the +_vitello-intestinal_ or _omphalo-mesenteric duct_. The intestinal gutter +is thus converted into a closed tube except at the point of implantation +of the vitelline duct during the persistence of this structure. In the +meanwhile the somatopleural folds forming the body-walls grow more and +more together from the sides, approaching the vitello-intestinal duct. +Finally touching each other they coalesce to form the ventral body wall, +in the same manner as the splanch[n]opleural folds met and united to +form the alimentary tube. + +At the same time the vitello-intestinal duct and the remnant of the +yolk-sac, to which it was attached ("umbilical vesicle"), normally +become obliterated and disappear. + +After the intestinal tube and the body cavity have thus become closed +the embryo straightens out and the alimentary canal appears as a nearly +straight cylindrical tube extending from the cephalic to the caudal end +of the embryo. This primitive alimentary tube at first terminates at its +cephalic extremity in a blind pouch, while at the caudal end in the +early stages the intestine is connected with the nerve-tube by a channel +called the _neuro-enteric canal_, forming in the earliest embryos a +communication between the ectoderm lining the bottom of the medullary +groove and the entoderm (Figs. 22 and 26). In man this stage is +encountered very early, in embryos of 2 mm. before the formation of +either heart or provertebrae. + +[Illustration: FIG. 22.--Caudal half of human blastoderm measuring 3 +mm., with open medullary groove. Dorsal view. x 30. (After Spee.)] + +[Illustration: FIG. 26.--Neuro-enteric canal in section of human embryo +of 2 mm. (After Spee.)] + +At the point where the canal develops the primitive groove presents a +thickened circumvallate spot, marking the beginning perforation of the +medullary plate from the ectoderm to the entoderm. The canal exists only +for a short period during the earliest stages of embryonal life. It +becomes rapidly closed, the neural and intestinal tubes henceforth +remaining permanently separated from each other. + +The embryonal caudal end of the primitive alimentary canal is not the +final adult termination of the tube. When the anal aperture is formed in +a manner to be presently detailed, the opening is situated cephalad of +the portion connected with the nerve-tube by the neuro-enteric canal. +Hence this terminal portion of the early embryonic alimentary canal is +called the "post-anal gut" (Fig. 21). + +[Illustration: FIG. 21.--Sagittal section of caudal extremity of cat +embryo of 6 mm. (Tourneux.)] + +The post-anal gut and the neuro-enteric canal are better developed in +the embryos of the lower than in those of the higher vertebrates. But in +all vertebrates of the present day both of these structures undergo +regressive changes and finally disappear altogether. They serve to +recall conditions which existed in bygone ages, and, while they have a +long and significant phylogenetic history, they have lost among living +vertebrates all physiological importance. + +After closure of the neuro-enteric canal and obliteration of the +post-anal gut the alimentary tube ends, during a short period, both +cephalad and caudad in a blind pouch. Very soon, however, the ectoderm +becomes invaginated at both extremities and finally perforates into the +lumen of the intestine, thus establishing the oral and anal +communications with the exterior. The anal ectodermal invagination +(proctodaeum) (Fig. 21), is smaller than the oral (stomadaeum) (Fig. 27), +but the intestinal tube forms an extensive pouch in the anal region +which descends to meet the ectodermal invagination of the proctodaeum. +The details of the embryonic processes leading to the final +establishment of the adult condition are of great interest on account of +the pathological importance of abnormal or arrested development in these +parts. Failure of the caudal intestinal pouch to establish a +communication with the anal invagination, or failure of development in +either anal invagination or intestinal pouch, leads to the condition +known as atresia ani or imperforate anus, of which there are several +varieties. + +[Illustration: FIG. 27.--Median section through head of embryo rabbit of +6 mm. (Mihulkovics.)] + +Before the anal opening forms the primitive caudal intestine receives +from above the stalk of the _allantois_, while the Wolffian duct, the +canal of the embryonic excretory apparatus, also opens into it. The +renal bud on the Wolffian duct in Fig. 28 indicates the beginning +development of the permanent kidney (metanephros), and the proximal +portion of the allantoic stalk is destined to form by a spindle-shaped +enlargement the future urinary bladder (Fig. 28). The caudal gut has as +yet no anal opening. Ventrad of the tail end of the embryo the ectoderm +presents at this time a depression (Fig. 21). The ectoderm lining the +bottom of this anal fossa or depression is separated by a little +mesoderm tissue from the entodermal lining of the blind pouch of the +caudal gut. Ectoderm and entoderm in this region with the intervening +mesodermal layer form the _cloacal membrane_ (Fig. 21). + +[Illustration: FIG. 28.--Reconstruction of caudal end of human embryo of +11.5 mm. (four and a half weeks), showing pelvic structures. x 40. +(After Keibel.)] + +=Development of Cloaca.=--The entodermal pouch or prolongation sent down +from the end-gut to meet the anal invagination enlarges and dilates to +form a short wide piece of the intestinal tube into which open on the +one hand the urinary and sexual ducts of the genito-urinary system, +while it receives on the other the termination of the _end-gut proper_ +(Figs. 28 and 29). + +[Illustration: FIG. 29.--Reconstruction of caudal end of human embryo of +14 mm. (five weeks). x 20 (After Keibel.)] + +This is the permanent condition of the terminal openings of the +alimentary and genito-urinary tracts in the lower vertebrates. It is +found in certain fishes, in all amphibia, reptiles and birds, and occurs +also in one order of mammals, the monotremes. In man and mammals +generally the anal orifice is separated from the genito-urinary opening, +lying dorsad of the same and provided with special sphincters. Only in +the monotremes do the anus and the genito-urinary tract open into a +common cloaca surrounded by a sphincter common to the anal and +genito-urinary openings (sphincter cloacae). In birds, reptiles, amphibia +and many fishes (especially the Plagiostomata) this cloacal formation is +the rule. In many fishes, especially the Teleosts, the anus and the +genito-urinary openings are separate, as in mammals, but their position +is reversed, the anus being ventral, while the genito-urinary opening is +placed dorsally. + +[Illustration: FIG. 23.--Genito-urinary tract and cloaca of _Iguana +tuberculata_, female. (Columbia University Museum, No. 1846.)] + +Fig. 23 shows the cloaca in a female specimen of _Iguana tuberculata_. +The ventral wall of the cloaca has been divided to the left of the +median line and turned over to the right, carrying with it the cloacal +opening of the bladder. The termination of the alimentary canal opens +into the cloaca from above. + +A transverse fold of the mucosa separates this upper compartment of the +cloaca (_coprodaeum_) from a lower space (_urodaeum_) which receives in +its dorsal wall the openings of the two oviducts and immediately above +them--upon two papillae--the openings of the ureters, while the ventral +wall contains the cloacal opening of the bladder. + +The right ovary has been removed--to show the abdominal opening of the +right oviduct--by dividing the mesovarian peritoneal fold. + +[Illustration: FIG. 24.--Genito-urinary tract and cloaca of the hen, +_Gallus bankiva_. (Columbia University Museum, No. 1208.)] + +Fig. 24--taken from a preparation of the hen--shows the typical +arrangement of the female genito-urinary tract and cloaca in the birds. + +The terminal portion of the alimentary canal, in entering the cloaca, +forms an expanded upper cloacal compartment for the accumulation of the +excreta, called the _coprodaeum_. + +It is separated by a prominent mucous fold from the central compartment, +or _urodaeum_ which receives the terminations of the two ureters and of +the single (left) oviduct. A second fold forms the distal limit of the +urodaeum and separates it from the lowest cloacal compartment, the +_proctodaeum_. + +[Illustration: FIG. 25.--Genito-urinary tract and cloaca of _Platypus +anatinus_, duck-billed platypus. (Columbia University Museum, No. +1802.)] + +Fig. 25 shows the male genito-urinary tract and the cloaca in the +monotreme, _Platypus anatinus_. The cloaca is a spacious sac formed by +the confluence of the rectum and the genito-urinary sinus. + +The penis, consisting of two large cavernous bodies, is contained in a +fibrous sac which arises from the junction of the genito-urinary sinus +and the cloaca, and is continued into the ventral wall of the cloaca +near its termination by an opening through which the penis can pass into +the cloaca and beyond the external cloacal aperture. + +The semen enters the penis at its root through a narrow opening situated +close to the junction of genito-urinary sinus and cloaca. + +For a short period, therefore, the human embryo and the embryos of the +higher mammalia present conditions which correspond to the permanent +structure of the parts in these lower vertebrates. In human embryos of +11.5 mm. cervico-coccygeal measure (32-33 days) (Fig. 28), the cloaca +appears as a short sac continuous dorsad with the intestine, ventrad +with the rudiment of the urinary bladder. The larger portion of the +caudal gut (postanal gut) has disappeared, having been reduced to a thin +epithelial strand which gradually becomes entirely absorbed. Only the +proximal portion of the end-gut is used for the development of the +cloaca, which, however, at first has no external opening (Fig. 28). + +The tail end of the embryo becomes more extended and between it and the +umbilical cord an interval appears in which the genital protuberance +develops. Behind this point the ventral cloacal wall is formed by the +cloacal membrane. + +A considerable interval also develops between the points of entrance +into the cloaca of the intestine proper and of the allantoic stalk +(urinary bladder). The growth of the mesoderm pushes the intestine +against the sacral vertebrae, while the stalk of the allantois with the +rudimentary urinary bladder is forced against the ventral abdominal +wall. These changes prepare the way for the first appearance of the +_genito-urinary sinus_. The neck of the embryonic bladder elongates and +receives the ducts of the urinary and genital glands (Fig. 29). In +embryos of 14 mm. cervico-coccygeal measure (36-37 days) (Figs. 29 and +30), the genito-urinary sinus perforates the cloacal membrane on the +ventral aspect of the genital protuberance, forming the _uro-genital +cleft_. The rectum remains closed for a few days longer. The perforation +is preceded by the formation of a transverse ectodermal reduplication, +producing a depression called the _transverse anal fissure_. This +depression increases in depth until a distinct anal invagination +results, known as the _proctodaeum_, which grows as a funnel-shaped fossa +toward the blind termination of the endgut. In embryos of 25 mm. +cervico-coccygeal measure (81/2-9 weeks) the intestine still ends in a +blind pouch. The anus is, therefore, independent of the end-gut in its +development. It is derived from the ectoderm and its production is +analogous to the formation of the oral cavity by means of the ectodermal +invagination called the _stomadaeum_. + +[Illustration: FIG. 30.--Human female foetus, 3.4 cm. long, +vertex-coccygeal measure. The external perineal folds separate the anal +invagination from the uro-genital opening. (Kollmann.)] + +Finally the cloaca is converted into a ventral tube from which part of +the urinary bladder, the urethra and genito-urinary sinus develop, and a +dorsal tube from which the _rectum_ is derived. This double disposition +of the cloaca is accomplished by gradual changes in the entoderm and +mesoderm. The entoderm proliferates until a partition is formed which +separates the two divisions of the cloacal tube from each other, and the +mesoderm likewise increases, surrounding the newly formed entodermal +tubes with tissue from which the muscles, connective tissue and blood +vessels of the parts are derived (Figs. 28 and 29). + +This partition, the _septum uro-rectale_, develops symmetrically on each +side, appearing first as paired folds on the right and left sides called +the _internal perineal folds_ (Figs. 28 and 29). When these folds have +reached the cloacal membrane they complete the separation of the cloaca +into two adjacent canals. Each of these canals is still closed caudad by +its respective portion of the cloacal membrane, now divided into an +_anal_ and _uro-genital_ segment. These two portions of the original +cloacal membrane become perforated separately, the uro-genital before +the anal. Hence the external opening of the uro-genital sinus is the +first to appear, to be followed by the anal perforation. The internal +perineal folds are supplemented by the formation of similar external +folds, ridges of mesoderm tissue which surround the anal orifice in the +form of a low wall and thus deepen the anal ectodermal invagination into +the fossa of the proctodaeum. + +These developmental stages in the formation of the end-gut are of +importance because they offer the explanation of the pathological +conditions which result from an arrest of development and from the +failure of either the uro-genital or anal opening to form in the usual +manner. These malformations must date back to an early stage, and +probably have their inception in disturbances occurring in the normal +development between the 15th and 23d day (embryos of 3-6 mm.). Perhaps +in some cases of atresia there may be a secondary obliteration of a +previously formed opening. In Fig. 31 the proctodaeum persists but the +perforation of the anal membrane into the end-gut has not occurred. The +ectoderm of the anal fossa and the intestinal entoderm remain separated +by a transverse mesodermal partition. Different degrees of this +malformation are observed. The layer separating the skin from the blind +end of the rectum may be so thin that the meconium contained in the +latter can be felt through it. On the other hand the rectum may +terminate high up in a blind pouch, which is separated from the skin by +a distance of several centimeters. + +[Illustration: FIG. 31.--Section of pelvis of human foetus, showing +atresia recti. (Esmarch.)] + +We may now briefly consider the genetic, histological and mechanical +conditions which the above-outlined course of development imposes on the +alimentary tract. + +The ectoderm forms the superficial covering of the embryo and in the +dorsal axial line develops the medullary groove which subsequently +becomes converted into the cerebro-spinal axis by closure of the +medullary plates and inclusion of the neural tube within the surrounding +mesoblast (Fig. 18). The entoderm forms the epithelial lining of the +interior of the alimentary canal and its appendages and derivatives +(Fig. 19). The mesoderm furnishes the skeletal, muscular and vascular +systems. At first single, like the two remaining layers of the +blastoderm, the mesoderm splits early on each side of the chorda +dorsalis into two layers, including between them spaces which after +coalescence form the _primitive pleuro-peritoneal_ or _body-cavity_ +(Fig. 20). One of these mesodermal layers bounding this space becomes +closely connected with the ectoderm, forming the _somatopleure_ or body +wall, while the other joins the entoderm to complete the wall of the +alimentary canal, forming the _splanchnopleure_. In the course of +further development the edges of these two layers approach each other +ventrally in the median line and finally fuse. + +The products of this fusion are two epithelial tubes, one included +within the other, with walls reinforced by tissue derived from the two +layers of the mesoderm. The internal or entodermal tube is of much +smaller diameter than the outer or ectodermal tube, but much longer. The +walls of the two tubes are placed in contact with each other by their +mesodermal elements dorsally in the axial line, but elsewhere are +separated from each other by the body-cavity (except in the region of +the ventral mesogastrium). + +The splanchnopleure is not so wide as the somatopleure. As it closes in +the ventral median line it includes the deepest or entodermal layer. It +now forms a tube whose walls are composed superficially of mesoderm +(splanchnopleure) while the lumen is lined by epithelium derived from +the entoderm. This tube is the _primitive enteric_ or _alimentary +canal_. The somatopleuric layers bounding the body cavity take a wider +sweep and after they have united ventrally in the median line they +embrace a much more extensive space, the _primitive body cavity_ or +_coelom_. The walls of this space are largely made up of the skeletal and +muscular elements developed from the mesoderm of the somatopleure, +covered superficially by the common ectodermal investment of the body. +It will be seen that the enteric tube thus becomes included within the +wider and more capacious coelom cavity. + +Both the somatic and the splanchnic leaf of the mesoderm consist at +first solely of a layer of flattened epithelial cells, the mesothelium. +But very early this tissue is increased to form a massive layer by +direct development from the mesothelium. The new mesodermal cells thus +produced constitute the _mesenchyma_, which includes the whole of the +mesoderm of the embryo except the mesothelial lining of the coelom. The +cells of the mesenchyma, connected with each other and with the +mesothelial cells by protoplasmic processes, are not as close together +as in an epithelium and do not form a continuous membrane. By migration +and multiplication a large mass of mesodermal tissue is produced which +fills the entire space between the mesothelium and the primary germ +layers. The mesenchymal tissue between the mesothelium and the ectoderm +forms the mass of the skeletal, muscular and vascular systems. The +mesenchymal tissue between the mesothelium and the entoderm forms an +important constituent of the alimentary canal and of its appendages. The +entoderm furnishes the internal epithelial lining of the tube upon which +the performance of the specific physiological function of the entire +apparatus depends. This epithelial tube is covered from without by the +splanchnic mesoderm. The mesodermal elements thus added to the enteric +entodermal tube consist of connective tissue and muscular fibers. The +latter, arranged in the form of circular and longitudinal layers, +control the contractility of the tube and regulate the propulsion of the +contents. The connective tissue of the splanchnic mesoderm appears as an +intermediate layer uniting the epithelial lining and the muscular walls. +Situated thus between the mucous and muscular coats of the intestine +this layer is known as the _submucosa_. It contains, imbedded in its +tissue, the glandular elements of the intestine derived from the +entodermal epithelium, and the blood vessels, lymphatics and nerves. The +second chief function of the splanchnic and somatic mesoderm is the +production of the serous membrane investing the body cavity and its +contents from the mesothelium lining the primitive coelom. This +mesothelial tissue, differentiated as a layer of flattened cells, lines +the interior of the body cavity and covers the superficial aspect of the +enteric tube. By subsequent partition of the common coelom the great +serous membranes of the adult, the pleurae, pericardium and peritoneum, +are developed from it. + +The entodermal enteric tube is, as already stated, closely attached at +an early period along its dorsal surface to the axial rod of mesoderm +containing the chorda dorsalis immediately ventrad of the neural canal. +In the earliest stages, just after the splanchnopleure and somatopleure +have closed to complete the alimentary tube and body cavity, the remnant +of these layers extends between the ventral abdominal wall and the +ventral surface of the intestine forming a partition which divides the +body into a right and left half. (Fig. 32, _A._) For the most part this +primitive connection between the ventral abdominal wall and the +intestinal tube is lost very early. The stomach, however, is always +connected by a ventral mesogastrium, from which the lesser omentum is +derived, to the ventral body wall. The disappearance of the ventral +mesentery caudad of this point establishes the condition indicated in +Fig. 32, _B._ The entodermal tube and the surrounding splanchnic +mesoderm forming the intestinal canal is attached along its dorsal +surface to the axial mesoderm of the dorsal mid-line. The primitive +mesothelial peritoneum is reflected along this line from the internal +surface of the body wall upon the ventral and lateral surfaces of the +intestine. The coelom of one side communicates ventrad of the intestine +with the coelom of the opposite side. Hence by the disappearance of the +ventral mesentery caudad of the stomach the paired body-cavities have +become fused into a single abdominal cavity--while cephalad the original +division into right and left halves is maintained by the portion of the +ventral mesentery which attaches the stomach to the ventral abdominal +wall. The mesodermal tissue which at this time attaches the alimentary +tube along its entire extent to the dorsal wall of the coelom carries the +primitive embryonic arterial vessel, the aorta. This vessel supplies a +series of small branches to the intestine, which reach the same by +passing ventrad imbedded in the mesoderm connecting the tube to the +dorsal body wall. + +[Illustration: FIG. 32.--Schematic diagrams, illustrating the vertebral +mesentery. _A._ earlier; _B._ later condition. (Minot.)] + +With the further development of the alimentary canal a gradual +elongation of this connecting band of mesoderm and of the contained +vessels is observed, the tube itself gradually receding from the +vertebral axis. The early broad attachment is replaced by a narrower +stalk into which the mesoderm is drawn out. With this narrowing in the +transverse and elongation in the sagittal direction the connecting +tissue assumes the character of a thin membrane with two free serous +surfaces, including the intestinal vessels imbedded between them. +Coincident with this elongation of the enteric attachment and its +narrowing in the transverse direction the primitive intestine becomes +more completely invested by the serous lining membrane of the coelom +cavity. In this stage we can speak of the double-layered membrane +attaching the tube to the dorsal body wall and carrying the intestinal +blood-vessels as the primitive dorsal mesentery. The intestinal canal +itself is invested by serous membrane except along a narrow strip of its +dorsal border where the mesentery is attached and where the vessels +reach the intestine. We can now distinguish the serous lining membrane +of the abdominal cavity, derived from the mesothelium of the splanchnic +and somatic mesoderm as the _peritoneum_. The membrane presents the +following topographical subdivisions: + +1. _Parietal Peritoneum_, lining the inner surface of the abdominal +walls. + +2. _Visceral Peritoneum_, investing the external surface of the +intestine and its derivatives. + +3. _Mesenteric Peritoneum_, connecting these two, carrying the +intestinal blood vessels and lymphatics and acting as a suspensory +support to the alimentary canal. + +The dorsal mesentery in fishes, amphibia and reptiles contains smooth +muscular fibers derived from the mesoderm. These bands of smooth muscle +fibers are also encountered, though less well developed, in the +mesentery of birds and mammals. The so-called "suspensory muscle of the +duodenum" belongs to this category. It consists of a few strands of +unstriped muscular and fibrous tissue which passes from the praeaortal +tissue around the origin of the superior mesenteric artery and coeliac +axis to the duodeno-jejunal angle. Fasciculi from this band may +penetrate into the root of the mesentery (Gegenbaur). + +Similar muscular fasciculi have been observed in the peritoneal folds of +the ileo-caecal junction (Luschka) and in the mesorectum--forming in the +latter situation the recto-coccygeal muscles of Treitz, and in the +female the recto-uterine muscles. + +In its earlier stages the primitive common mesentery forms a membrane +which carries the intestinal blood vessels between its two layers, +surrounds the embryonic alimentary canal and attaches the same to the +ventral aspect of the chorda dorsalis and aorta. This is the permanent +condition in many of the lower vertebrates in which the intestinal tube +is suspended by a simple dorsal mesentery, a condition which is repeated +by the embryos of man and the higher vertebrates. From this primitive +common mesentery are derived, by further development, displacement and +adhesion, all the other mesenteries, omenta and peritoneal folds of the +adult. The character and degree of these subsequent changes is +determined by the increase in length and change in position of the +intestine and the growth of large organs, like liver, spleen and +pancreas. Many portions of the intestinal canal, at first suspended by +the mesentery and freely movable within the abdominal cavity, become +later, by secondary adhesion, firmly connected with adjacent portions of +the tube or with the abdominal parietes. + +In certain of the lower vertebrates (fishes) large sections of the +intestine lie entirely free within the abdomen, their only connection +with the parietes being afforded by the blood vessels. This condition +depends upon _absorption_ of the original mesentery. A similar process, +though much more circumscribed, is observed in the omenta of many +mammals, which appear perforated at several points. + +=Derivatives of the Entodermal Intestinal Tube.=--The entodermal +epithelium is physiologically the characteristic element of the +alimentary canal. Besides lining the entire internal surface of the tube +it gives rise by budding and protrusion from the intestinal canal to a +series of organs which from the mode of their development must be +regarded as diverticular or derivatives of the alimentary canal (Figs. +33, 34, and 35). These organs, proceeding in order cephalo-caudad, are +the following: + + The salivary glands. + Thymus and thyroid. + The lungs. + Pancreas. + Liver. + +[Illustration: FIG. 33.--Schema of alimentary canal and accessory +organs, derived from same. (After Bonnet.)] + +[Illustration: FIG. 34.--Reconstruction of alimentary canal of human +embryo of 4.2 mm. x 24. (After His.)] + +[Illustration: FIG. 35.--Reconstruction of alimentary canal of human +embryo of 7 mm. (twenty-eight days). x 12. (After His.)] + +The epithelium of all these structures is derived from the primitive +entoderm of the intestinal tube, except the epithelium of the salivary +glands, which, being derived from the stomadaeal invagination, is +ectodermal in character. We have previously noted the general history +and appearance of the yolk-sac and its connection by means of the +vitello-intestinal duct with the intestine. In contradistinction to the +adult organs just noted the yolk-sac or umbilical vesicle is merely a +temporary embryonal appendage to the alimentary canal. It also differs +from them in the fact that it is not an extension or budding from the +completed intestinal tube, like the liver and pancreas, but indicates, +by the implantation of the duct (Fig. 21), the last point at which +closure of the intestinal canal takes place, when after obliteration of +the duct the separation of the intestine from the yolk-sac is completed. + +The segment of the primitive alimentary canal cephalad of the attachment +of the vitello-intestinal duct gives rise to the pharynx, oesophagus, +stomach, proximal portion of small intestine proper and its derivatives, +the liver and pancreas. + +The portion situated caudad of the duct produces the rest of the small +and all of the large intestine (Figs. 33 and 35). At times in man and +other mammals (cat) the vitello-intestinal duct does not become +absorbed, but persists and continues to develop as a part of the small +intestine, forming the blind pouch or appendage known as _Meckel's +diverticulum_ (Figs. 37 and 38). This diverticulum may vary in length +from 1.5 to 15 cm. It either projects freely into the abdominal cavity +as a pouch arising from the convex border of the small intestine +opposite to the mesenteric attachment, or else it reaches the abdominal +wall at the umbilicus and is attached to the same. In a few instances it +has not terminated in a blind pouch, but has remained open at the +umbilicus, in which case the aperture discharges intestinal contents. +Sometimes the process of obliteration which normally leads to the +absorption of the vitello-intestinal duct extends to the adjoining +segment of the small intestine, resulting in obliteration of the +intestinal lumen and consequent obstruction at this point. + +[Illustration: FIG. 36.--Reconstruction of alimentary canal of human +embryo of thirty-five days (13.8 mm.). x 8. (After His.)] + +[Illustration: FIG. 37.--Human adult ileum with Meckel's diverticulum. +Ileo-diverticular serous fold and persistent omphalo-mesenteric artery. +(Columbia University Museum, No. 1803.)] + +[Illustration: FIG. 38.--Human adult ileum, with Meckel's diverticulum. +(Columbia University Museum, No. 745.)] + +The intestinal opening of the diverticulum is situated at a varying +distance above the ileo-colic junction, ranging from 27.5 cm. to 290 +cm., with an average of 107 cm. + +While the obliteration and complete absorption of the duct is normal in +nearly all vertebrates, a remnant persists in some birds, in which a +short caecal pouch (_diverticulum caecum vitelli_) is found at about the +middle of the small intestine. A portion of the vitello-intestinal duct +thus persists throughout life in some wading and swimming birds. Figs. +39 and 40 show this condition in the small intestine of _Urinator lumme_ +and _imber_, the red-throated loon and the great northern diver. In +other birds, however, such as birds of prey, song birds, etc., the duct +is absorbed and disappears completely. + +[Illustration: FIG. 39.--Small intestine of the red-throated loon, +_Urinator lumme_, showing persistent caecal pouch, the remnant of the +vitelline duct. (Columbia University Museum, No. 997.)] + +[Illustration: FIG. 40.--Small intestine of great northern diver, +_Urinator imber_, with caecal pouch, the remnant of the vitelline duct. +(Columbia University Museum, No. 77, 1578.)] + +In order to complete the embryological history of the alimentary canal +it is necessary to take brief account of another structure derived from +it, namely the _allantois_. Its significance to the adult organism is +seen in connection with the genito-urinary tract, the urinary bladder +being formed by its persistent portion. In the embryo, however, it has +important nutritive and respiratory functions. In the embryos of the +higher vertebrates nutrition depends only in the earliest stages upon +the yolk-sac of the ovum, over which a vascular network extends. + +Very soon the caudal portion of the primitive intestine develops a +vascular sac-like outgrowth (Figs. 21 and 41). This pouch forms the +_allantois_. It is intimately connected with embryonal respiration, and +probably also forms a reservoir which receives the secretion of the +primitive kidney. This foreshadows the final destiny of the proximal +intra-abdominal portion of the allantoic sac which persists and is +converted into the urinary bladder of the adult. + +[Illustration: FIG. 41.--Diagram illustrating the later stages in the +formation of the mammalian foetal membranes. (Heisler, modified from +Roule.)] + +The allantois is present in Amphibia but is very small. In Amniota[1] it +is large and grows around the embryo. In those of the higher vertebrates +which are developed within an egg (reptiles, birds and monotremes) the +sac of the allantois comes to lie beneath the egg-shell and acts as a +respiratory organ. In the higher mammalia, developed within the uterus, +the allantois becomes attached by vascular villi to the uterine wall and +establishes a vascular connection between the foetal and maternal blood +vessels. In this way the _allantoic placenta_ is formed (Fig. 41). The +placenta, as just stated, is absent in the monotremes and is only +slightly developed in marsupials, in which animals the foetus develops to +maturity in the marsupial pouch after leaving the uterus. These animals +are therefore distinguished as _Aplacentalia_ from the remaining higher +mammals in which the allantoic placenta develops and which are hence +called the _Placentalia_. + +[1] In the embryos of reptiles, birds and mammals folds of the +somatopleure arise externally to the constricting furrows by means of +which the embryo is gradually separated from the yolk-sac, with the +resulting formation of the intestinal and abdominal walls. These folds, +situated at the head, tail and on the sides, grow upwards and finally +meet and unite to form a membranous sac called the _amnion_. Hence these +higher vertebrates (reptiles, birds and mammals) are called _Amniota_, +in contradistinction to fishes and amphibia who have no amnion and are +hence known as _Anamnia_. + +=Summary.=--To recapitulate, therefore, the intestinal tube gives origin +to two kinds of appendages or derivatives: + +1. Organs of the adult body, derived by budding from the alimentary +entodermal epithelium, in the form of pouch-like diverticula which +follow the glandular type of development and become secondarily +associated with mesodermal elements. These organs are again of two +kinds: + +(_a_) _Organs which retain their original connection with the lumen of +the digestive canal:_ + + The salivary glands,} + The liver, } Connected by their ducts with the digestive + The pancreas, } canal. + The lungs, } + +which open by means of the trachea and the laryngeal aperture into the +pharyngeal cavum. + +(_b_) _Organs which lose their primitive connection with the alimentary +canal._ + +Thymus and Thyroid Gland. + +2. Embryonic appendages of the alimentary tract. + +(_a_) The vitello-intestinal or omphalo-mesenteric duct and the yolk-sac +or umbilical vesicle. This structure does not form as an extension from +the intestinal tube after the same has been closed by coalescence of the +splanchnopleure in the ventral mid-line, but is the result of the +folding in of the layers of the embryonic germinal area, by means of +which the body-rudiment is constricted off from the yolk-sac. The +reduced channel of communication forms the vitello-intestinal duct. In +the vast majority of vertebrates this disappears completely by +absorption in the course of further development. It may persist in part +abnormally as Meckel's diverticulum. In a few birds its proximal portion +remains normally as a small blind pouch attached to the free border of +the small intestine. + +(_b_) The allantois. This is a hollow outgrowth from the embryonic +intestinal canal of the higher vertebrates, performing important +functions in connection with the early nutrition of the embryo. In the +course of subsequent development its proximal portion, situated within +the abdominal cavity, becomes converted into the urinary bladder. In +mammals it loses its original connection with the intestinal canal and +is assigned entirely to the genito-urinary tract. In some of the lower +vertebrates, amphibia and reptiles it retains its connection with the +ventral wall of the cloaca throughout life. (See Fig. 42, genito-urinary +tract of _Iguana tuberculata_.) + +[Illustration: FIG. 42.--Genito-urinary tract and cloaca of _Iguana +tuberculata_, female. (Columbia University Museum, No. 1846.)] + +After the intestinal canal has become separated from the yolk-sac it +forms at first a straight tube, running cephalo-caudad beneath the +chorda dorsalis. In most forms, however, the intestine grows much more +rapidly in length than the body-cavity of the embryo in which it is +contained. Hence the intestine is forced to form coils or convolutions. + +The entire alimentary canal, from the mouth to the anus, can be +separated into the following divisions and subdivisions: + +=I. Foregut=, including + + 1. The oral cavity. + 2. The pharynx. + 3. The oesophagus. + 4. The stomach. + +=II. Midgut=, closely associated at its beginning with the liver and +pancreas. + +It extends between the pyloric extremity of the stomach and the +beginning of the last segment, the endgut, frequently separated from +both by ring-like aggregations of the circular muscular fibers and +corresponding projections of the mucous membrane (pyloric and ileo-colic +valves). + +The midgut is usually the longest portion of the intestinal tube. + +=III. Endgut=, the last segment of the intestinal canal, courses through +the pelvic portion of the body cavity. From this short end-piece are +developed: (1) The colon, sigmoid flexure and rectum; (2) the cloaca +with the uro-genital sinus and the duct of the allantois. + + + + +PART I. + +ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY. + + +For the purpose of studying the adult human peritoneum it is in the +first place absolutely necessary to obtain a correct appreciation of the +disposition of the chief viscera within the abdominal cavity and of +their mutual relations. In the second place the visceral vascular supply +of the abdomen must be carefully considered in order to correctly +appreciate certain important relations of the peritoneal membrane. + +A review of the visceral contents of the abdomen shows that we have to +deal chiefly with the divisions of the alimentary tract below the +oesophagus and the structures directly derived from the same, as liver +and pancreas, or associated topographically with the alimentary canal, +as the spleen. Portions of the urinary and reproductive systems situated +within the abdominal and pelvic cavities will also require +consideration. + +The digestive apparatus as a whole presents, in the first place, a +segment designed to convey the food to the stomach, the +oesophagus--supplemented in mammalia by the special apparatus of the +mouth and pharynx, in which the food is mechanically prepared for +digestion by chewing and mixed with the secretion of the salivary +glands. + +The _digestive apparatus proper_, succeeding to the oesophagus, is +usually divisible into two sections differing in function and structure. + +1. The STOMACH, a short sac-like dilatation, in which chiefly +nitrogenous material is digested. + +2. The SMALL INTESTINE, a long and usually much convoluted narrow tube, +chiefly devoted to the digestion of starches, fats and sugars, and to +the absorption of the digested matters. + +In some of the lower vertebrates, as the _Cyclostomata_ (Fig. 43), +_Esox_, _Belone_, etc., among fishes (Fig. 48), _Necturus_ and _Proteus_ +among amphibians (Figs. 50 and 51), the separation of the digestive +portion of the alimentary tract into stomach and small intestine is not +clearly defined (vide infra, p. 43). + +[Illustration: FIG. 43.--Entire alimentary canal of the lamprey, +_Petromyzon marinus_, below the pericardium. (Columbia University +Museum, No. 1575.)] + +[Illustration: FIG. 48.--Alimentary canal of _Belone_, pickerel. +(Nuhn.)] + +[Illustration: FIG. 50.--_Necturus maculatus_, mud-puppy. Alimentary +canal and appendages. (Columbia University Museum, No. 1454.)] + +[Illustration: FIG. 51.--Alimentary canal of _Proteus anguineus_. +(Nuhn.)] + +A distinct digestive segment may even be entirely wanting, owing to its +failure to differentiate from the oesophagus on the one hand and from the +endgut on the other. In such forms the entire digestive canal appears as +a tube of uniform caliber extending from mouth to anus. It is necessary +to begin with these simple structural conditions in order to obtain a +clear conception of the disposition of the viscera in the adult human +abdomen. Such simple arrangement of the alimentary tract is found in the +embryo of man and of the higher vertebrates, and similar rudimentary +types are encountered, as the permanent condition, in some of the lower +forms. These latter are especially valuable for purposes of study, +because they afford an opportunity of examining directly, as macroscopic +objects, structural conditions which are found only as temporary +embryonal stages during the development of the higher mammalia (Fig. +43). + +In the early stages the alimentary tract of the mammalian embryo +consists of a straight tube of nearly uniform caliber (Fig. 44, _A_), +extending from the pharynx to the cloaca, along the median line in the +dorsal region of the body cavity, connected with the ventral aspect of +the axial mesoderm by a membranous fold forming the primitive common +dorsal mesentery. Subsequently differentiation of this simple tube into +successive segments takes place, marked by differences in shape and +caliber and in histological structure. + +[Illustration: FIG. 44.--Schematic diagram representing three stages in +the differentiation of the mammalian digestive tract: A. Early +undifferentiated stage, in which the entire canal appears as a tube of +uniform calibre. B. Spindle-shaped gastric dilatation. C. Typical +mammalian gastric dilatation.] + +[Illustration: FIG. 45.--Reconstruction of human embryo. 1, 2, 3, 4, +Gill-pouches. (After Fol.)] + +The first indication of the future stomach appears early, in human +embryos of from 5-6 days (Figs. 44, _B_, and 45; for later embryonal +stomach forms compare also Figs. 33, 35 and 36), as a small +spindle-shaped dilatation of a portion of the primitive entodermal tube, +placed in the median plane, dorsad of the embryonic outgrowth of the +liver, between it and the oesophagus. The appearance of this +dilatation marks the separation of the proximal cephalic part (pharynx +and oesophagus) from the distal caudal (intestinal) portion of the +primitive alimentary canal. + +Further growth of the stomach takes place chiefly along the dorsal +margin of the dilatation, rendering the same more convex. The ventral +border develops to a less degree and in the course of further and more +complete differentiation the dorsal margin of the future stomach assumes +even at this period the character of the greater curvature, while the +opposite ventral margin, the future lesser curvature, following the +dilatation of the tube dorsad, becomes in turn concave (Fig. 44, _C_). + +The early spindle-shaped dilatation has therefore assumed the general +shape of the adult organ. This differentiation of greater and lesser +curvature begins to appear in embryos of 5 mm. (Fig. 46) and is very +well marked in embryos of 12.5 mm., Fig. 36, of an embryo of five weeks, +indicates the adult form of the stomach clearly. + +[Illustration: FIG. 46.--Alimentary canal of human embryo of 5 mm. x 15. +(Reconstruction after His.)] + +It will, however, be noted that the oesophageal entrance is still at the +cephalic extremity of the rudimentary stomach, while the pyloric +transition to the intestine occupies the distal caudal point, under +cover of the liver, and turns with a slight bend dorsad and to the right +to pass into the duodenum. The future greater curvature is directed +dorsad and a little to the left toward the vertebral column, while the +concave lesser curvature is turned ventrad and a little to the right +toward the ventral abdominal wall. At this time there is but little +indication of the subsequent extension of the organ to the left of the +oesophageal entrance to form the great cul-de-sac or fundus of the adult +stomach. + +In this stage of its development the stomach therefore presents ventral +and dorsal borders, and right and left surfaces, while the continuity of +its lumen with the adjacent segments of the alimentary canal appears as +a proximal or cephalic oesophageal and a distal or caudal intestinal +opening. + + +COMPARATIVE ANATOMY OF FOREGUT AND STOMACH. + +A serial review of this portion of the alimentary tract in vertebrates +forms one of the most interesting and instructive chapters in +comparative anatomy. + +Not only is every embryonal stage in the development of the higher +mammalia represented permanently in the adult structure of some of the +lower types, but the far-reaching influence of function and of the +physiological demands on the structure of this portion of the digestive +tract is strikingly illustrated by the numerous and marked modifications +which are encountered. + +The foregut, strictly speaking, is in mammals separated from the oral +cavity by the musculo-membranous fold of the soft palate and uvula. In +all other vertebrates except the crocodile, the oral cavity and foregut +pass into each other without sharp demarcation (Fig. 47). In some of the +lower vertebrates the alimentary canal never advances beyond the +condition of a simple straight tube of nearly uniform caliber. There is +no gastric dilatation and hence no differentiation of a stomach properly +speaking. Such for example is the case in some teleost fishes, as the +pickerel (Fig. 48). In these forms we have to deal with the persistence +of the early embryonic pregastric stage of the higher types, before the +simple alimentary tube is differentiated by the appearance of the +distinct gastric dilatation. + +[Illustration: FIG. 47.--_Gallus canis_, dog-shark, male. Genito-urinary +tract and cloaca _in situ_. The foregut has been divided just caudad of +the communication with the oral cavity. (Columbia University Museum, No. +1694.)] + +In the _Cyclostomata_ (Fig. 43) the intestinal canal passes through the +body in a perfectly straight line and the three segments (mid-, fore- +and hindgut) are not clearly differentiated. + +In the _Ammocoetes_ the foregut begins behind the wide branchial basket, +dorsad of the heart, with a narrow entrance, which is succeeded by a +dilated segment. The entrance of the hepatic duct separates fore- and +midgut. + +In _Amphioxus_ the branchial pouch passes with a slight constriction +directly into the gut which extends through the body-cavity in a +straight line. + +The narrow segment is usually regarded as the "oesophagus." This is +followed by a slightly dilated segment, the "stomach," into which a +blind pouch enters. This caecal pouch is usually considered as a +_hepatic_ diverticulum (Fig. 49). + +[Illustration: FIG. 49.--_Amphioxus_, dissected from the ventral side. +The relatively enormous pharynx occupies more than half the length of +the body. The walls are separated by the gill-clefts, and the parallel +gill-bars abut at the midventral line on the _endostyle_. (Willey, after +Rathke.)] + +But even in these rudimentary forms the point where the liver develops +from the entodermal intestinal tube marks the separation of fore- and +midgut. The stomach, when it develops, is situated cephalad of the +entrance of the hepatic duct into the intestine. The section cephalad of +the duct opening may be very short, and the food digested further on in +the intestinal tube. Consequently a function which in these lower +vertebrates is assigned to the midgut becomes transferred in the higher +forms to a specialized segment of the foregut, situated cephalad of the +hepato-enteric duct. This segment is the + + +STOMACH. + +The distribution of the vagus nerve finds its explanation in this +derivation of the stomach. The primitive foregut is formed by the +passage between the branchial cavity and the midgut, and is within the +area supplied by the vagus. Hence when the stomach develops from the +foregut, as a specialized segment of the same, it is supplied by vagus +branches. The vertebrate stomach varies greatly in size and shape. + +The type-form is presented by a longitudinal spindle-shaped dilatation +of the foregut, which retains its foetal vertical position in the long +axis of the body. An example of this form, which is encountered among +fishes and amphibia, is presented by the alimentary tube of _Proteus +anguineus_ and _Necturus maculatus_ (Figs. 50 and 51). Since this +condition is common to all vertebrates in the earliest foetal period it +can be designated as the foetal or primitive stomach form. All others +appear as secondary derivatives from this typical early condition. + +The influences which bring about such derivations and modifications may +be enumerated as follows: + +1. The habitual amount of food required by the animal. + +2. The volume and digestible character of the food. + +3. The size and shape of the abdominal cavity in which the stomach is +contained. + +4. Structural modifications designed to increase the action of the +gastric juice on the food contained in the stomach. + +5. The assumption, on part of the stomach, of functions which are +usually relegated to other organs. + +Most of the individual stomach forms encountered among vertebrates owe +their production to several of these influences acting in conjunction. + +We may group the main types as follows: + +=1. Stomach Forms Depending on the Influence exerted by the Habitual +Amount of Food required by the Animal.=--The greater the activity of +tissue changes is, the greater will be the amount of food required and +the more pronounced will be the gastric dilatation of the alimentary +canal. Hence in the higher vertebrates generally the stomach appears as +a large and more sac-like dilatation than in lower forms, such as fishes +and amphibia and some reptilia, in which the stomach is usually smaller +and foetal in shape, forming a slight longitudinal dilatation situated in +the long axis of the body. An example is seen in the stomach of _Coluber +natrix_ (Fig. 52). Frequently this slight dilatation is scarcely +differentiated from the oesophagus at the cephalic and from the small +intestine at the caudal end. Many batrachians and perennibranchiates +possess this form among the amphibia. It is also encountered in the +pickerels, the _Cyprini_, and in _Labrus_ among fishes, and in some +saurians and ophidia among reptiles. It constitutes a slight advance in +development over the earliest stage represented, as we have seen, by the +nearly uniform and undifferentiated alimentary tube of amphioxus and the +cyclostomata. + +[Illustration: FIG. 52.--Alimentary canal of _Coluber natrix_. (Nuhn.)] + +This transition of the foetal form to the more advanced secondary types +of the stomach is marked by the development of two important structural +features: + +(_a_) The separation in the interior of the canal of the stomach from +the intestine by the appearance of a ring-shaped valve, the _pyloric +valve_. This is produced by an aggregation of the circular muscular +fibers of the intestine at this point, and causes a projection of the +mucous membrane into the lumen of the canal. It begins to appear in the +fishes (pickerel, sturgeon, etc.), is found in most amphibia and is +regularly present in the stomach of the higher vertebrates. (Figs. 54 +and 55.) A good example of the ring-shaped plate of the pylorus with +central circular opening produced by the aggregation of the circular +muscular fibers is afforded by the view of the interior of the +cormorant's stomach given in Fig. 69. The opposite or oesophageal +extremity of the stomach is less well differentiated from the afferent +tube of the oesophagus. + +[Illustration: FIG. 54.--Human adult. Pyloro-duodenal junction and +pyloric valve in section. (Columbia University Museum, No. 1842.)] + +[Illustration: FIG. 55.--Series of sections showing human pyloric valve +and gastro-duodenal junction: + +1. Stomach of foetus at term in section. + +2. Adult pyloric valve, gastric surface. + +3. Adult pyloric valve and gastro-duodenal junction in section. + +4. Foetal gastro-duodenal junction in section. Entrance of biliary and +pancreatic ducts on summit of papilla of duodenum. (Columbia University +Museum, No. 1851.)] + +There is no aggregation of muscular circular fibers in this situation +and no valve. Superficially the external longitudinal muscular fibers of +the oesophagus pass continuously and without demarcation into the +superficial gastric muscular layer. The separation between oesophagus and +stomach is, however, marked on the mucous surface by a well-defined line +along which the flat, smooth and glistening oesophageal tesselated +epithelium passes into the granular cuboidal epithelium of the gastric +mucous membrane. The oesophageo-gastric junction in the adult human +subject is shown in Fig. 53. + +[Illustration: FIG. 53.--Human adult. Mucous surface of +oesophageo-gastric junction. (Columbia University Museum, No. 1842.)] + +(_b_) The pyloric end of the stomach makes an angular bend, while the +rest of the organ remains in the original vertical position in the long +axis of the body. An example of this condition is presented by the +stomach of _Scincus ocellatus_ (Fig. 56; cf. also Fig. 202). + +[Illustration: FIG. 56.--Alimentary canal of _Scincus ocellatus_. +Pyloric extremity of the slightly marked gastric dilatation presents an +angular bend. (Nuhn.)] + +The purpose of both of these provisions is to retain the gastric +contents for a longer time within the stomach. Hence this form is +encountered especially in those fishes and amphibians in which the +nutritive demands require a more complete digestion of the food taken. +This is the case, for example, in _Gobius_ (Fig. 57), the plagiostomata +(Fig. 58), and many saurians. The same transitory stomach form is even +found in some mammals, as the seals. Fig. 59 shows the stomach in _Phoca +vitulina_, the harbor seal. With the further increase in the demand for +complete digestion of the food the entire stomach assumes a transverse +position to the long axis of the body. This may occur while the stomach +still retains its primitive tubular form, as in most chelonians (Fig. +60). In others the change in position occurs after the gastric +dilatation has assumed the sac-like form, as in many land-turtles, +crocodiles, some batrachians and all higher vertebrates (Figs. 61 and +62). This transverse position, at right angles to the long axis of the +body, forms the starting point for the derivation of all secondary types +of stomach. + +[Illustration: FIG. 57.--Alimentary canal of _Gobius niger_. (Nuhn.)] + +[Illustration: FIG. 58.--Alimentary canal of shark. (Nuhn.)] + +[Illustration: FIG. 59.--Stomach of _Phoca vitulina_, harbor seal. +(Columbia University Museum, No. 600.)] + +[Illustration: FIG. 60.--Stomach of _Pseudemys elegans_, pond turtle. +(Columbia University Museum, No. 1710.)] + +[Illustration: FIG. 61.--Stomach of _Chelydra serpentina_, snapping +turtle. (Columbia University Museum, No. 1852.)] + +[Illustration: FIG. 62.--Same in section.] + +=2. Stomach Forms Depending on the Influence Exerted by the Volume and +Digestible Character of the Foods.=--Vegetable substances usually have a +large volume in proportion to the amount of nutritive material which +they contain. Meat, on the other hand, contains considerable nutriment +in a comparatively small bulk. Hence carnivora (Fig. 63) usually have a +smaller stomach than herbivora (Fig. 64). + +[Illustration: FIG. 63.--Stomach of _Lutra vulgaris_, otter. (Nuhn.)] + +[Illustration: FIG. 64.--Stomach of _Equus caballus_, horse. (Nuhn.)] + +=3. Stomach Forms Influenced by Size and Shape of the Abdominal Cavity +in which they are Contained.=--In animals whose bodies are long and +slender, as in snakes (Fig. 52), most saurians (Fig. 56), many tailed +batrachians and perennibranchiates (Figs. 50 and 51), many teleosts +(Fig. 48), the stomach is likewise usually long and slender in shape, +unless special modifying conditions exist. When on the other hand the +body is broad and short, as in Lophius (Fig. 65), Pipa (Fig. 66), and +most higher vertebrates, the stomach is also broader and more sac-like. + +[Illustration: FIG. 65.--Stomach of _Lophius piscatorius_, angler. +(Nuhn.)] + +[Illustration: FIG. 66.--Stomach of _Pipa verucosa_. (Nuhn.)] + +=4. Stomach Forms Depending on Structural Modifications Designed to +Increase the Action of the Gastric Juice on the Food.=--This purpose is +accomplished: + +(_a_) By increasing the source of supply of the gastric juice. + +(_b_) By increasing the length of time during which the food remains in +the stomach. + +(_a_) The source of supply of the gastric juice is increased by adding +to the usual gastric glands of the stomach a special accessory glandular +compartment, either placed at the cardia, where the oesophagus enters, as +in _Myoxus_ or _Castor_ (Fig. 67) or attached to the body of the stomach +to the left of the cardia, as in the manatee (Fig. 68). The first +arrangement is similar to the universal position of the glandular +stomach of birds (Fig. 69). In birds, however, the glandular +proventriculus is the _only_ source of the gastric juice, while in the +above-mentioned mammalia (myoxus and beaver) the accessory glandular +stomach is merely an addition to the supply derived from the usual +gastric glands situated in the body of the organ. + +[Illustration: FIG. 67.--Stomach of _Castor fiber_, beaver. (Nuhn.)] + +[Illustration: FIG. 68.--Stomach of _Manatus americanus_, manatee. +(Nuhn.)] + +[Illustration: FIG. 69.--Stomach of _Phalacrocorax dilophus_, +double-crested cormorant; section. (Columbia University Museum, No. +67/1804.)] + +(_b_) The increase of the length of time during which the food remains +in the stomach subject to the action of the gastric juice can be +accomplished in one of several ways. + +1. The stomach, while it retains its general tubular form increases +considerably in length and assumes the shape and structure found in the +human large intestine. It is partially subdivided by folds projecting +into the interior and separating compartments resembling the colic cells +of the human large intestine. The time required for the passage of food +through the stomach is thus increased and the action of the gastric +juice is prolonged and rendered more intense. + +Such modifications of the structure of the stomach are encountered in +_Semnopithecus_ among the monkeys and in the kangaroo, among marsupials +(Figs. 70 and 71). + +[Illustration: FIG. 70.--Stomach of _Halmaturus derbyanus_, rock +kangaroo. (Columbia University Museum, No. 582.)] + +[Illustration: FIG. 71.--Stomach of _Semnopithecus entellus_, entellus +monkey. (Columbia University Museum, No. 62/1805.)] + +2. The same purpose is accomplished by the development of diverticula +from the stomach, in which the food is retained and acted on by the +gastric juice for longer periods. + +The herbivora, omnivora and such carnivora as live on animal food +difficult of digestion furnish examples of this type of stomach. The +same is also found in most teleosts. In the latter the caecal gastric +pouch lies in the long axis of the body, opposite the entrance of the +oesophagus. A marked example of this arrangement is seen in the stomach +of the eel, _Anguilla anguilla_ (Fig. 72). + +[Illustration: FIG. 72.--Alimentary canal of _Anguilla anguilla_, eel. +(Columbia University Museum, No. 1271.)] + +In other forms, and in the mammalia especially, the blind pouch is +developed from the portion of the stomach lying to the left of the +oesophageal entrance at the cardia, and is hence placed transversely to +the long axis of the body. + +This difference in the position of the cul-de-sac is explained by the +small transverse measure of the body in teleosts, while the greater +amount of available space in the abdominal cavity of mammalia permits +of the transverse position of the entire stomach and of the development +of the diverticulum from its left extremity. + +Most mammals have only a single pouch, whose size varies with the +digestibility of the food habitually taken. It is greater in herbivora +(Figs. 64 and 73) than in omnivora and carnivora (Figs. 74 and 75). In +some of the latter, as _Lutra_ (Fig. 63), the cul-de-sac is almost +wanting. + +[Illustration: FIG. 73.--Stomach of _Lepus cuniculus_, rabbit. (Nuhn.)] + +[Illustration: FIG. 74.--Stomach of _Nasua rufa_, coati. (Nuhn.)] + +[Illustration: FIG. 75.--Stomach of _Felis leo_, lion. (Nuhn.)] + +[Illustration: FIG. 76.--Stomach of _Erethizon dorsatus_, American +porcupine. (Columbia University Museum, No. 358.)] + +[Illustration: FIG. 77.--Stomach of _Cercopithecus cephus_, moustache +monkey. (Columbia University Museum, No. 158.)] + +In some forms, as the pig, the left extremity of the stomach carries a +caecal appendix with a spiral valve in the interior separating its lumen +from the general gastric cavity (Fig. 78). Others have two such caecal +appendices added to the left end of the stomach (Peccary, Fig. 79). +These caecal pouches may arise from the _body_ of the stomach, instead of +from the left extremity. An example of this condition is furnished by +the American manatee (Fig. 68). + +[Illustration: FIG. 78.--Stomach of _Sus scrofa_, pig. The fundus of the +stomach carries a caecal appendage separated in the interior by a spiral +fold of the mucous membrane from the gastric cavity.] + +[Illustration: FIG. 79.--Stomach of _Dicotyles torquatus_, peccary. The +fundus is a capacious pouch prolonged ventrally and dorsally into two +caecal appendages resembling the single appendage of the pig's stomach. +(Columbia University Museum, No. 1806.)] + +=5. Variations in the Form of the Stomach Depending upon the Assumption +by the Stomach of Special Functions, which are Usually Relegated to +other Organs.=--These functions are the following: + +(_a_) Storage of food in special receptacles or compartments for +subsequent use. + +(_b_) Mastication of the food is in some animals accomplished only +partly or not at all in the mouth, and is then performed in the stomach. +A portion of the stomach is thus converted into an apparatus for +mastication. + +(_c_) The provisions for these two accessory functions may be combined +in the same stomach. + +(_a_) Many of the higher vertebrates possess in connection with the +alimentary tract additional reservoirs for the storage of food until +used. Such reservoirs are found in mammals and birds connected with the +oral cavity, as cheek-pouches, or with the oesophagus, such as the crop +of the birds (Fig. 88). Fig. 80 shows the development of the +cheek-pouches in one of the primates, _Macacus nemestrinus_. + +[Illustration: FIG. 80.--_Macacus nemestrinus_, pig-tail macaque monkey; +cheek-pouches. (From a fresh dissection.)] + +In many mammals reservoirs of similar import are added +directly to the stomach and form an integral part of +the organ. Examples are furnished by the compound stomachs of many +rodents, ruminants, cetaceans and herbivorous edentates. The peculiar +appearance of these stomachs is explained if the additional reservoirs +are in imagination removed and the digestive stomach proper restored so +to speak to the type-form. The proximal or cardiac portion of the +stomach in many rodents is devoid of gastric glands and must be +interpreted as a storage chamber for food (Fig. 81). The same +significance attaches to the corresponding portion of the manatee's +stomach (Fig. 68). + +[Illustration: FIG. 81.--Stomach of _Cricetus vulgaris_, hamster. +(Nuhn.)] + +Similar contrivances are found in the ruminant stomach. The first +and second divisions (rumen and reticulum) are nothing but sac-like +gastric reservoirs or pouches, in which the food is collected, to be +subsequently returned to the mouth for mastication. When swallowed for +the second time the bolus is carried, by the closure of the so-called +oesophageal gutter, past the first and second stomach into the digestive +apparatus proper (the abomasum) (Figs. 82 and 83). Many ruminants +(_e. g._, _Moschus_) only have these three compartments. Most, however, +have four, the leaf stomach or psalterium being intercalated between +the retinaculum and the abomasum. The psalterium contains no digestive +glands. It may possibly serve for the absorption of the liquid portions +of the foods. + +[Illustration: FIG. 82.--Stomach of _Ovis aries_, sheep. (Columbia +University Museum, No. 1807.)] + +[Illustration: FIG. 83.--Scheme of ruminant compound stomach. (Nuhn.)] + +The rumen or first stomach of the camels and llamas is provided with +so-called "water-cells," for the storage of water. These cells are +diverticula lined by a continuation of the gastric mucous membrane. The +entrance into these compartments can be closed by a sphincter muscle +after they are filled with water (Fig. 84). + +[Illustration: FIG. 84.--Mucous membrane of stomach of _Camelus +dromedarius_, dromedary, showing water-cells. (Columbia University +Museum, No. 1123.)] + +[Illustration: FIG. 85.--Stomach of _Phocaena_, porpoise. (Nuhn.)] + +The three stomachs of the cetaceans are similar to those of the +ruminants (Fig. 85). The first is a crop-like reservoir for the +reception of the food when swallowed. The mucous membrane is entirely +devoid of digestive glands. In the dolphins the mucous membrane is +provided with a hard horny covering, which serves to break up the food +mechanically by trituration. The second stomach and the gut-like pyloric +prolongation constituting the third stomach contain gastric glands and +are hence digestive in function. + +(_b_) Stomach forms, in which a portion of the organ is converted into +an apparatus for mastication, are seen especially in birds, in which +animals, on account of the absence of teeth, mastication cannot be +performed in the mouth. + +The stomach of the bird is usually composed of two segments, one placed +vertically above the other. + +The first appears like an elongated dilatation of the oesophagus, forming +the _Proventriculus_ or glandular stomach. + +The second is larger, round in shape, with very strong and thick +muscular walls (Figs. 86 and 87). + +[Illustration: FIG. 86.--Stomach of _Urinator imber_, red-throated loon. +(Columbia University Museum, No. 1808.)] + +[Illustration: FIG. 87.--Scheme of stomach of granivorous bird. (Nuhn.)] + +The proventriculus furnishes the gastric juice exclusively. + +The second or muscular stomach, devoid of gastric glands, functions +merely as a masticating apparatus for the mechanical division of the +food. The thick muscular walls of this compartment may measure several +inches in diameter and carry on the opposed mucous surfaces lining the +cavity a hard horny plate with corrugated and roughened surface (Fig. +88). These hard plates are designed to crush the food between them, as +between two mill stones. The muscle stomach is best developed in +herbivorous birds, while both the muscular wall and the horny plate are +much weaker and thinner in carnivore wading and swimming birds (Fig. +89). + +[Illustration: FIG. 88.--OEsophagus and stomach of _Gallus bankiva_, hen. +(Columbia University Museum, No. 1809.)] + +[Illustration: FIG. 89.--Stomach of _Botaurus lentiginosus_, bittern. +(Columbia University Museum, No. 23/1810.)] + +In birds of prey, especially in the owls, the stomach walls are scarcely +more massive than in other animals, and the mucous membrane is soft and +devoid of a horny covering. The glandular and masticatory stomachs are +less sharply divided from each other in these forms, and the entire +organ conforms more to the general vertebrate type (Fig. 90). + +[Illustration: FIG. 90.--Stomach of owl sp. (Nuhn.)] + +In some birds (herons, storks, etc.) a small rounded third stomach, the +so-called pyloric stomach, is placed between the muscle stomach and the +pylorus (Fig. 91). It contains no gastric glands, and possibly may +function as an additional absorbing chamber. + +[Illustration: FIG. 91.--Stomach of _Ardea cinerea_, heron. (Nuhn.)] + +[Illustration: FIG. 92.--Stomach of crocodile. (Nuhn.)] + +Among reptiles the stomach of the crocodile resembles the organ in birds +(Fig. 92). It is flat and rounded in shape, the muscle wall carries a +tendinous plate, and there is a pyloric stomach. There is, however, no +glandular stomach or proventriculus, as in birds, and the mucous +membrane is not covered by a horny plate, but is soft and contains the +peptic glands. Figs. 93 and 94 show the stomach of _Alligator +mississippiensis_, in the ventral view and in section. + +[Illustration: FIG. 93.--Stomach of _Alligator mississippiensis_. +(Columbia University Museum, No. 1811.)] + +[Illustration: FIG. 94.--Same in section. Thin-walled cardiac segment +continues into cavity of pyloric ventriculus.] + +[Illustration: FIG. 95.--Stomach of _Bradypustridactylus_, three-toed +sloth. I. First stomach, devoid of gastric glands, corresponding to +rumen of ruminants. + +II. Second stomach, the homologue of the ruminant reticulum. + +III. Digestive stomach proper, provided with gastric glands connected by +a gutter with the oesophagus. + +IV. Muscular stomach, the walls formed by a thick muscular plate and +provided on the mucous surface with a dense corneous covering for +purposes of trituration.] + +(_c_) The combination of the two accessory functions just described in +the same stomach is found in the three-toed sloth (Fig. 95). + +There are here two large reservoirs, which correspond to the rumen and +retinaculum of the ruminants, and a digestive compartment containing +gastric glands, which corresponds to the ruminant abomasum, and is +connected by an oesophageal gutter directly with the oesophagus. At the +pyloric extremity the muscle wall is greatly increased and the mucous +membrane of this portion carries a thick horny covering, forming a +masticatory stomach greatly resembling the corresponding structure in +the bird. Its function is evidently to complete the mechanical division +of the food which has only been partly masticated in the mouth. + +The same significance is probably to be attached to the thickened +muscular walls which the pyloric segment of the stomach in _Tamandua +bivittata_, another edentate, presents (Fig. 96), in strong contrast +with the thinner walled cardiac segment and fundus. + +[Illustration: FIG. 96.--Stomach of _Tamandua bivittata_, collared +ant-eater, (Columbia University Museum, No. 68/1485.)] + + +INTESTINE. + +Continuing our consideration of the development of the alimentary canal +we find that changes from the simple primitive straight tube below the +stomach depend upon two factors: + +1. The increase in the length of the intestinal tube, which exceeds +relatively the increase in the length of the body cavity in which it is +contained. + +2. The differentiation into small and large intestine, the development +of the caecum and ileo-caecal junction, and the development of the +accessory digestive glands, liver and pancreas, by budding from the +proximal portion of the primitive entodermal intestinal tube. + +1. In embryos up to 5 mm. cervico-coccygeal measure (Fig. 97) the +intestinal tube follows the body curve without deviation. Subsequently +the elongation of the intestine causes a small bend, with the convexity +directed ventrad, to appear in the umbilical region. This bend gradually +increases until the gut forms a single long loop, beginning a short +distance below the pylorus and directed ventro-caudad. The apex of the +loop, to which the vitello-intestinal duct is attached (Fig. 98) (cf. p. +34) projects beyond the abdominal cavity into the hollow of the +umbilical cord, constituting the so-called "umbilical or embryonal +intestinal hernia." This entrance of the apex of the intestinal +umbilical loop into the umbilical cord begins in embryos of about 10 mm. +During the succeeding weeks--up to the tenth--the segment of the +intestine thus lodged within the hollow of the umbilical cord increases. +After this period the intestinal coils are gradually withdrawn within +the abdomen. The explanation of this temporary extrusion of the +intestine into the umbilical cord is probably to be found in the strain +produced by the yolk-sac which is attached by the vitello-intestinal +duct to the apex of the umbilical loop. As we have seen (p. 35) the site +of the original apex of the loop may still be indicated in the adult by +the persistence of a portion of the vitello-intestinal duct as a +"Meckel's diverticulum." + +[Illustration: FIG. 97.--Alimentary canal of human embryo of 5 mm. x 15. +(Reconstruction after His.)] + +[Illustration: FIG. 98.--Schema of human embryonic intestinal canal, +with intestinal umbilical loop, but before differentiation of the large +and small intestine.] + +In its simplest primitive condition the loop presents a proximal, +descending or efferent limb, an apex, and an ascending, returning or +afferent limb (Fig. 98). In the human embryo these segments of the loop +furnish the jejuno-ileum and portions of the large intestine, in a +manner to be subsequently detailed. + +This stage in the development of the higher vertebrate intestine is well +illustrated by the alimentary tract of the mud-puppy, _Necturus +maculatus_, shown in Fig. 99, which represents the entire situs viscerum +of an adult female animal. + +[Illustration: FIG. 99.--Viscera of _Necturus maculatus_, mud-puppy, _in +situ_. (Columbia University Museum, No. 1175.)] + +The stomach is tubular, not distinctly differentiated from the +oesophagus, placed vertically in the long axis of the body. The pyloric +end is marked by a constriction separating stomach from midgut and +immediately beyond this point the pancreas is applied to the intestine. +The rest of the intestinal canal forms a simple loop, the descending +limb presenting one or two primitive convolutions. There is no marked +differentiation between large and small intestine, the canal possessing +a nearly uniform caliber from pylorus to cloaca. + +2. The differentiation of the small from the large intestine, marked by +the appearance of the caecal bud or protrusion (Fig. 100), takes place in +the ascending segment of the umbilical loop a short distance from the +apex. In the human embryo the caecal bud appears in the 6th week as a +plainly marked protuberance, which grows very slowly in length and +circumference. It shows very early an unequal rate of development; the +terminal piece, not keeping pace in growth with the proximal portion, is +converted into the vermiform appendix, while the proximal segment +develops into the caecum proper. The increase in the length of the loop, +which begins to be marked in the 7th week, is not uniform. The apex is +the first portion to present the evidences of this growth. Subsequently +the descending limb grows in length very rapidly and is early thrown +into numerous coils of the future mobile portion of the small intestine +(jejuno-ileum). Even before the withdrawal of the apex of the loop +within the abdominal cavity a prominent coil of these convolutions is +found protruding in the umbilical region (Fig. 544). The ascending limb +of the loop from which a portion of the large intestine is developed, +grows comparatively slowly at this time. + +[Illustration: FIG. 100.--Schema of human embryonic intestinal canal +after differentiation of the large and small intestine.] + +The future portions of the human adult alimentary tract below the +stomach may be referred, in reference to their derivation, to this +primitive condition of the tube as follows: + +1. The segment of small intestine situated between the pylorus and the +beginning or point of departure of the proximal or descending limb of +the umbilical loop, develops into the _duodenum_. This portion of the +small intestine is indicated early in embryos of 2.15 mm. (Fig. 101), +by the origin of the hepatic duct from the intestinal tube. Somewhat +later, in embryos of 4.10-5 mm. length, (Fig. 102) it becomes +additionally marked by the origin of the pancreatic diverticulum. The +duodenum, at first straight, now begins to curve, forming a short +_duodenal loop_ or _bend_. In embryos of 6 weeks the duodenum forms a +simple loop placed transversely below the pyloric extremity of the +stomach (Figs. 103 and 104). + +[Illustration: FIG. 101.--Human embryo of 2.15 mm., twelve days old. +Seessel's sac is the cephalic blind termination of the embryonic +foregut before the communication with the ectodermal invagination of +the stomadaeum has been formed. (Reconstruction after His.)] + +[Illustration: FIG. 102.--Representation of alimentary canal and +appendages of human embryo of 4.1 mm.; isolated. x 15. (Kollmann, after +His.)] + +[Illustration: FIG. 103.--Alimentary canal and appendages of human +embryo of 12.5 mm. x 12. (Kollmann, after His.)] + +[Illustration: FIG. 104.--A. Schematic representation of alimentary +canal, with umbilical loop and mesenteric attachments in human embryo of +about six weeks. B and C, stages in the intestinal rotation.] + +2. The descending limb, the apex and a small part of the ascending limb +of the umbilical loop form the jejuno-ileum. + +3. The remainder of the ascending limb forms the caecum and appendix, the +ascending and transverse colon. + +4. The distal straight portion of the primitive tube forms the terminal +portion of the transverse colon (the splenic flexure), the descending +colon, sigmoid flexure and rectum. + +The primitive condition of the embryonal mammalian alimentary tract, +after differentiation of the large intestine is well illustrated by some +of the lower vertebrates in which development never proceeds beyond this +stage. Fig. 112 shows the entire alimentary canal of a teleost fish, the +conger eel (_Echelus conger_) isolated. + +[Illustration: FIG. 112.--Alimentary canal, isolated and in section, of +_Echelus conger_, the conger eel. (Columbia University Museum, No. +1812.)] + +The preparation forms a good illustration of the embryonal stage of the +higher vertebrates in which development has not proceeded beyond the +formation of the simple umbilical loop, about corresponding to the +schematic Fig. 98. The stomach is differentiated both by its caliber and +by the formation of a pyloric ring valve. + +The midgut forms a simple loop with a descending and ascending limb +closely bound together by mesenteric attachment. Different from the +course of development followed in the human embryo is the situation of +the ileo-colic junction. The same appears in the terminal straight +segment of the canal--corresponding to the human descending colon--while +in the human embryo the differentiation of small and large intestine +takes place in the course of the ascending limb of the loop. This +condition depends upon the relatively much shorter extent of the +teleost endgut compared with the human large intestine. Other examples +are afforded by the alimentary tract of some of the Amphibia and +Reptilia. Fig. 105 shows the alimentary canal of _Rana catesbiana_, the +common bull frog. The stomach, fairly well differentiated, is succeeded +by the small intestine of considerable length and uniform caliber. The +proximal portion of the small intestine is characterized as duodenum by +its connection with liver and pancreas. In the remaining portion of the +intestinal canal it is not difficult to recognize the elements of the +umbilical loop of the higher mammalian embryo. The larger mass of the +jejuno-ileal coils is developed from the descending limb of the loop; a +smaller number of convolutions belong to the returning or ascending +limb, which also includes the ileo-colic junction. The very short large +intestine of the frog passes straight down to enter the cloaca. Another +example, in which the early embryonal stages of the higher mammalia are +illustrated by the permanent structure of one of the lower vertebrates, +is given in Fig. 106, which shows the alimentary tract of a chelonian, +_Pseudemys elegans_, the pond turtle. The bilobed liver fits over the +well-differentiated stomach in the manner of a saddle. The stomach +itself, as in chelonians generally, has a markedly transverse position +and passes under cover of the right lobe of the liver into the duodenum. +The coils of small intestine form a prominent mass, which, however, when +unravelled as shown in the figure, permits us to recognize its identity +with the mammalian embryonic umbilical loop. The well-marked ileo-colic +junction is situated at the termination of the returning limb of the +loop, close to the beginning of the descending limb. This close +approximation of the duodenum and colon (duodeno-colic isthmus) forms +one of the most important factors in the further development of the +mammalian intestinal canal and will again be referred to below. + +[Illustration: FIG. 105.--_Rana catesbiana_, bull-frog. Alimentary canal +and appendages. (Columbia University Museum, No. 1454.)] + +[Illustration: FIG. 106.--_Pseudemys elegans_, pond turtle. Alimentary +canal. (Columbia University Museum, No. 1437.)] + +From the ileo-colic junction the large intestine of the turtle continues +caudad to the cloaca in a nearly straight line. The same primitive +condition of the intestinal canal may be observed in some members of +man's own class, the mammalia--as in certain edentates. Figs. 107 and +108 show the entire abdominal portion of the alimentary tract in +_Tamandua bivittata_, the little ant-eater of Brazil. The stomach is +turned cephalad and the great omentum elevated. The intestines are +turned over to the right side. + +[Illustration: FIG. 107.--Abdominal viscera of _Tamandua bivittata_, the +little ant-eater, seen from the left, with the intestines turned to the +right. (From a fresh dissection.)] + +[Illustration: FIG. 108.--The same view, from another specimen. Figures +107 and 108 should be studied and compared together, as each supplements +the other.] + +It will be observed that in spite of the numerous coils of the small +intestine the general arrangement of the alimentary canal corresponds to +the primitive scheme shown in Fig. 98. The entire intestinal canal is +attached by a continuous vertical mesentery to the dorsal median line of +the abdominal cavity ventrad of the vertebral column and aorta. The +growth in length of the small intestine has necessitated a corresponding +lengthening of the attached border of the mesentery--consequently the +membrane presents a pleated or crenated appearance. The caecum is well +developed, the ileo-caecal junction being situated within the returning +limb of the loop, a little distance from the apex. + +In Figs. 109 and 110, taken from the same specimens, the entire mass of +the small intestines has been turned to the left so as to exhibit the +right leaf of the common dorsal mesentery and the mesoduodenum, the +latter containing the head of the pancreas. It will be noted that the +mesentery, expanding beyond the duodeno-colic isthmus, is common to the +small and to the proximal portion of the large intestine, _i. e._, to +those segments of the alimentary canal which are developed from the two +limbs of the umbilical loop. Figs. 107-110 should be studied and +compared together, as each supplements the others. + +[Illustration: FIG. 109.--Abdominal viscera of _Tamandua bivittata_, the +little ant-eater, seen from the right, with the intestines turned to the +left. (From a fresh dissection.)] + +[Illustration: FIG. 110.--The same view, from another specimen.] + +It will be observed, in reference to the change from the primitive loop +to the subsequent increase in the length of the tube and the resulting +arrangement of the mesentery, that three successive stages are to be +considered, represented schematically in Fig. 111. In the earliest stage +(Fig. 111, I.) the two segments of the loop are of equal length, +parallel to one another, the distance between the beginning and +termination of the loop (1-2) being maintained throughout its +extent. Hence the mesentery is of equal width in all its parts within +the loop, only drawn out, _i. e._, away from the vertebral column, in +accordance with the length of the loop. In the next stage (Fig. 111, +II.) the increase in the length of the intestine is accompanied by a +corresponding widening of the mesentery. The points 1 and 2 are still +approximately the same distance apart as in the earlier stage, but the +increase in the length of the tube between these points forces the two +limbs of the loop to abandon their early parallel course, and to form +curved lines with the concavity turned toward the mesenteric attachment. +In this condition the mesentery consequently forms a widely expanded +membrane framed by the intestine and narrowing between the points 1 and +2 to a neck or isthmus which effects the transition between the expanded +segment surrounded by the intestine and the rest of the dorsal primitive +mesentery. Finally in the stage represented in Fig. 111, III., the +increase in the length of the small intestine has reached a point where +a single curve is no longer sufficient for the accommodation of the +growth. Consequently the tube now appears coiled and convoluted, and the +mesentery, as it is attached to the gut, of necessity follows all the +twists and appears fluted or pleated in its distal attached portion. + +[Illustration: FIG. 111.--Schematic representation of the development of +the mesentery of the umbilical loop.] + +If we now carefully examine the conditions presented by the intestine +and mesentery in a form like _Tamandua_ (Figs. 107 and 108) we will find +that they correspond to the developmental facts thus far considered. The +termination of the duodenum (1) and the bend in the colon (2) mark the +two points at which in the primitive schema (Fig. 111, I.) the umbilical +loop begins and terminates. The proximal of these two points (1) +corresponds to the termination of the duodenum, which segment extends +from here cephalad to the pyloric extremity of the stomach. The distal +point (2) is placed on the colon where the returning limb of the loop +resumes the original median vertical course of the large intestine. +These two points mark the neck of the loop, which we can describe as the +_duodeno-colic neck_ or _isthmus_. + +The same condition is well shown in the intestinal canal of the snapping +turtle (Fig. 113). The duodenum and colon approach each other very +closely at the isthmus and between these points the convolutions of the +intestine extend in a wide circle. We will find this approximation of +duodenum and colon a feature which persists throughout all the later +developmental stages of the higher vertebrates and has an important +bearing on the final arrangement of the intestinal canal in the human +adult. + +[Illustration: FIG. 113.--_Chelydra serpentina_, snapping turtle; +intestinal canal, pancreas, and spleen, isolated. (Columbia University +Museum, No. 1369)] + +=Further Changes in the Development of the Human Alimentary Canal. +Rotation of the Intestine. Formation of the Segments of the Colon. Final +Permanent Relations of the Segments of the Intestinal Tube.=--The next +important stage leading up to the final adult disposition of the +intestine in man and the higher mammals is the _rotation_ of the +portions developed from the two limbs of the primitive loop around an +oblique axis drawn from the duodeno-colic isthmus to the apex of the +loop. The portion of the large intestine, developed from the ascending +limb of the loop, moves in the third month to the middle line, coming +into contact with the ventral abdominal wall. From here the large +intestine passes, ventrad of the jejuno-ileal coils, toward the cephalic +end of the abdominal cavity and lies transversely along the greater +curvature of the stomach. The growing coils of the small intestine crowd +the colon more and more cephalad. In the fourth month the caecum turns to +the right, coming into contact with the caudal surface of the liver, +ventrad of the duodenum, and subsequently reaches the ventral surface of +the right kidney. As the result of this rotation the ileo-colic +junction, caecum and succeeding portion of the colon are carried from the +original position in the distal and left part of the abdomen cephalad +and to the right across the proximal (duodenal) portion of the small +intestine, while the coils of the jejuno-ileum, developed from the +descending limb and apex of the loop, are turned in the opposite +direction, caudad and to the left underneath the preceding (Figs. 114 +and 115). This change in the relative position of the parts of the +intestinal tract and the resulting altered bearing of the colon to the +duodenum will be best appreciated by considering in the first place the +effect of the change on the arrangement of the primitive mesentery and +the intestinal vessels, and secondly by repeating actually the rotation +in the intestinal tract of a mammal (cat) in which the adult arrangement +of the intestine and peritoneum permits us to perform the manipulations +and note the result. + +[Illustration: FIG. 114_A_.--Intestinal canal in stage of umbilical +loop--before rotation.] + +[Illustration: FIG. 114_B_.--First stage in rotation, colon crossing +duodenum.] + +[Illustration: FIG. 115_A_.--Second stage in rotation--rotation of small +intestine.] + +[Illustration: FIG. 115_B_.--Schema of intestinal canal after complete +rotation and descent of caecum.] + +=I. Effect of Rotation on the Disposition of the Primitive Mesentery and +on the Relative Position of Duodenum and Colon, and Consequent +Arrangement of the Intestinal Blood Vessels.=--It will be appreciated +that in Fig. 111, representing a profile view of the original +arrangement, or in Figs. 107 and 108, showing the intestinal canal of +_Tamandua_, the left layer of the primitive mesentery is turned toward +the observer. The membrane is seen to pass from the ventral aspect of +the vertebral column and aorta, through the narrow neck of the +duodeno-colic isthmus, to expand in the manner already indicated toward +its intestinal attachment. In the rotation of the intestine the twist +takes place at the duodeno-colic neck, carrying, as already stated, the +large intestine cephalad and to the right, while the jejuno-ileum is +turned in the opposite direction caudad and to the left. During this +rotation the duodeno-jejunal angle (Figs. 114, _B_ and 115, _A_) passes +to the left underneath the proximal segment of the colon, which now lies +ventrad and to the right of the duodenal portion of the small intestine. +The mesenteric peritoneum, occupying the bight of the umbilical loop, +will, after the rotation, in the left profile view shown in Fig. 104, +_A_ and _B_, turn its original right leaf toward the beholder, _i. e._, +toward the left, while the original left leaf is turned toward the +right. + +Observation of the difference in the position of the ileo-colic junction +will still further accentuate the change in the relative position of the +parts which has been effected by the rotation. In the primitive +condition shown in Fig. 104, _A_, the ileum enters the large intestine +from right to left, and the concavity of the caecal bud turns its +crescentic margin ventrad and to the right. + +After rotation is accomplished (Fig. 104, _B_ and _C_, and Fig. 115) the +ileo-colic entrance takes place in the opposite direction, from left to +right and the caecum turns its concave margin caudad and to the left. + +Figs. 116 and 117 show the intestinal tract of _Tamandua bivittata_ +arranged so as to correspond to the human embryonic condition after +rotation. The caecum has been brought up and to the right across the +proximal duodenal portion of the small intestine, while the jejuno-ileal +coils have been turned down and to the left. The rotation has been +accomplished by a twist at the duodeno-colic isthmus, and the original +right leaf of the mesentery has become the left and _vice versa_. +Comparison with Figs. 107 and 108, representing the condition before +rotation in the same animal, will indicate the changes which have been +accomplished by imitating the course of development followed in the +higher mammals. + +[Illustration: FIG. 116.--Abdominal viscera of _Tamandua bivittata_, +with the intestine rotated to correspond to the development in the human +subject. (From a fresh dissection.)] + +[Illustration: FIG. 117.--The same view as Fig. 116, from another +specimen.] + +Failure of rotation and arrest of development at the primitive stage, +with consequent persistent embryonic condition of the mesentery, occurs +occasionally in man. Such cases have been reported by W. J. Walsham, in +St. Barthol. Hosp. Rep., London, Vol. 16. The following four instances +of this condition, taken from the Columbia University museum, will +illustrate the disposition of the abdominal contents. + +Fig. 118 shows the arrangement of the abdominal viscera in an adult +female body. Beginning at the pyloric extremity of the stomach the +entire course of the duodenum can be overlooked and its continuation +into the jejuno-ileal division traced. The small intestines occupy the +ventral and right part of the cavity. The ileo-colic junction is placed +in the lower left-hand corner of the abdomen and the small intestine +enters the large from right to left, the ascending colon is situated to +the left of the median line and at its point of transition into the +segment representing the transverse colon is connected by several +adhesions with the ventral surface of the duodenum. The transverse +colon, folded into several coils bound together by adhesion, occupies +the upper left portion of the abdomen. + +[Illustration: FIG. 118.--Abdominal viscera of adult human female, in a +case of arrested rotation of the intestines. (Columbia University +Museum, Study Collection.)] + +[Illustration: FIG. 119.--The same preparation with the intestinal coils +displaced upward and to the left.] + +Fig. 119, taken from the same specimen, shows the entire mass of +intestines lifted up and turned to the left, exposing the background of +the abdominal cavity lined by parietal peritoneum. The duodenum is still +entirely free and non-adherent to the parietal peritoneum. The +continuity of the mesoduodenum with the jejuno-ileal mesentery is well +shown. The primitive right leaf of the mesentery is turned to the +observer. This layer after completed rotation would form the left layer +of the adult mesentery of the jejuno-ileum. + +Fig. 120 illustrates another instance of the same condition in the +adult. In this case the duodenum was coiled twice upon itself and +adherent to the prerenal parietal peritoneum. + +[Illustration: FIG. 120.--Abdominal viscera of adult human male; +non-rotation of intestine. (Columbia University Museum, Study +Collection.)] + +Fig. 121, presenting the same adhesion of the duodenum, illustrates very +perfectly the persistence of the narrow duodeno-colic isthmus in cases +of non-rotation, as well as the development of the different segments of +the adult tract from the limbs of the embryonal umbilical intestinal +loop. + +[Illustration: FIG. 121.--Abdominal viscera of adult human male; +non-rotation of intestine. (Columbia University Museum, Study +Collection.)] + +It will be observed that beyond the duodeno-colic isthmus the coils of +the jejuno-ileum have resulted from the increase in length of the +descending limb, the apex and the proximal part of the ascending or +recurrent limb, carrying the ileo-colic junction and caecum. The +remainder of the ascending limb, terminating in the embryonic condition +at the splenic flexure by passing into the descending colon, has in the +course of further development in this individual produced a straight +segment--the misplaced ascending colon--and a convoluted and bent +representative of the normal transverse colon. + +The same disposition of the large intestine may be noted in the other +preparations. + +Fig. 122 shows an instance of non-rotation observed in the human infant +at two years of age. + +[Illustration: FIG. 122.--Abdominal viscera of child, two years old; +non-rotation of intestine. (Columbia University Museum, Study +Collection.)] + +[Illustration: FIG. 123.--Human foetus at term; abdominal viscera, +hardened _in situ_; non-rotation of caecum. (Columbia University Museum, +No. 1813.)] + +Fig. 123, taken from a foetus at term, shows the result of failure to +completely rotate in the region of the caecum and ileo-colic junction. +The rest of the large intestine has rotated as usual and assumed the +normal position. The terminal ileum, however, passes behind the caecum +and enters the large intestine on its right side; the caecum is turned +upwards and to the right and the appendix lies ventrad of the beginning +of the ascending colon. In order to produce the normal arrangement, +shown in Fig. 124, taken from another foetus at term, it would be +necessary to turn the caecum and ileo-colic junction in Fig. 123 through +half a circle. The caecum would then turn upwards and to the left, the +ileum entering the large intestine from left to right, and the appendix +would be placed behind the caecum and ileo-colic junction. Figs. 125 and +126 show the normal and abnormal arrangement presented by these two +preparations diagrammatically. The instances in which in the adult the +ileo-colic entrance is placed on the right side of the large intestine +and in which the appendix is situated laterad of the ascending colon +unquestionably find their explanation in the failure of the intestine to +completely rotate at the ileo-colic junction. + +[Illustration: FIG. 124.--Human foetus at term; abdominal viscera, +hardened _in situ_; normal position of completely rotated caecum and +appendix. (Columbia University Museum, No. 1814.)] + +[Illustration: FIG. 125.--Just before final rotation of caecum and +terminal ileum. Concavity of caecum directed cephalad and to right. +Terminal ileum enters colon from right to left.] + +[Illustration: FIG. 126.--Rotation completed. Concavity of caecum turns +caudad and to left. Terminal ileum enters colon from left to right.] + +[Illustration: FIGS. 125, 126.--Schematic representation of final stages +in rotation of caecum and large intestine.] + +The resulting conditions are shown in Figs. 127 and 128, taken from +adult human subjects in which the final stage of rotation of the large +intestine has not taken place. + +[Illustration: FIG. 127.--Adult human subject with non-rotated caecum. +The terminal ileum turns caudad from right to left to enter right side +of colon.] + +[Illustration: FIG. 128.--Adult human subject with non-rotated caecum, +the ileum entering large intestine from the right and behind, and the +appendix placed to the right of the ascending colon. (From a fresh +dissection.)] + +In Fig. 127 the terminal ileum is sharply bent on itself and adherent to +the prerenal parietal peritoneum. It passes from right to left and +downwards to enter the right posterior circumference of the large +intestine. The caecum is turned cephalad and the appendix is in contact +with the right lobe of the liver. The caecum passes with a sharp bend +into the obliquely directed ascending colon. + +In Fig. 128 the ileum enters the colon from the right and below. The +apex of the caecum is turned cephalad and to the right and the appendix +extends beneath peritoneal adhesions along the lateral border of the +proximal segment of the colon. + +In the next place it is desirable to clearly understand the vascular +supply of the intestine before and after rotation and the final relation +of the superior mesenteric artery to the transverse portion of the +duodenum. + + +Development of Aortal Arterial System. + +The thoracic and abdominal aortae are at first double, the first aortic +arches continuing as so-called "primitive aortae" ventrad of the +vertebral column to the caudal end of the body. + +The cephalic portions of the two vessels unite in the chick on the third +day and from this point fusion into a single vessel proceeds slowly +caudad. + +In the rabbit the fusion of the primitive aortae begins on the ninth day +in the region of the lung-buds and progresses from here caudad until by +the sixteenth day a single aorta is formed (Fig. 129). + +[Illustration: FIG. 129.--Diagrams illustrating the arrangement of the +primitive heart and aortic arches. (After Heisler, modified from Allen +Thompson.)] + +That the entire descending aorta in man results from the fusion of two +vessels is shown by the rare cases in which the aorta is divided +throughout its entire length by a septum. + +The arteries of the allantois are originally the terminations of the +primitive aortae. After fusion of the primitive aortae to form the +abdominal aorta the allantoic arteries, now passing as the umbilical +arteries to the placenta, appear as the branches of bifurcation of the +abdominal aorta, in the same way as the common iliacs do in the adult. + +They furnish branches, which at first are very small, to the budding +posterior extremities and the pelvic viscera. In time these rudiments of +the future external and internal iliac arteries become larger, but as +the umbilical arteries continue to develop throughout the entire +intra-uterine period they appear even in the foetus at term as end +branches of the aorta, a condition which is only changed after birth by +the obliteration of the umbilical arteries and their conversion into the +lateral ligaments of the bladder, while the iliac vessels now appear as +the terminal aortic branches. The statement that the umbilical arteries +appear as the terminal branches of the embryonal aorta requires to be +modified in the following respect: + +When the allantois develops its arteries are in fact end-branches of the +two primitive aortae. After their fusion and after the formation of the +single aorta this vessel is continued beyond the umbilical arteries as +a small trunk, the caudal artery or rudiment of the adult sacralis +media. Consequently the umbilical arteries are really lateral branches +of a median vessel, viz., aorta abdominalis and arteria sacralis media. +But as the umbilical vessels are very large and the caudal aorta very +small, the former, even under these conditions, appear as the real +terminal branches of the abdominal aorta. + +The arteries supplying the yolk-sac and subsequently the intestinal +canal are the vitelline or omphalo-mesenteric. At first they are +branches derived from the two primitive aortae, and after the fusion of +these vessels they arise from the resulting single abdominal aorta. The +omphalo-mesenteric arteries are at first multiple and later are reduced +to two. When the primitive intestine loses its original close contact +with the vertebral column and the common dorsal mesentery develops, the +two omphalo-mesenteric arteries unite to form a single vessel, running +between the layers of the mesentery. After a short course this artery +divides again into two branches, passing one on each side, around the +intestinal tube, which has in the meanwhile become closed. Ventrad of +the intestine these branches reunite so that the gut is surrounded by a +vascular circle. The left half of this loop becomes obliterated and the +trunk of the omphalo-mesenteric artery now passes on the right side of +the intestine to the umbilicus. The peripheral segment of the +omphalo-mesenteric artery disappears with the cessation of the vitelline +circulation. The proximal portion, situated between the layers of the +mesentery, gives numerous anastomosing branches to the intestine and is +converted into the main trunk of the superior mesenteric artery. + +The derivation of the superior mesenteric as the fully developed +proximal segment of the embryonic omphalo-mesenteric artery passing to +the yolk-sac is responsible for the rare anomaly in the adult of a +branch of the superior mesenteric artery continuing beyond the intestine +to the umbilicus. I have encountered one instance of this persistence of +the intra-abdominal portion of the omphalo-mesenteric artery in a male +subject 54 years of age. A connective strand, containing a +small artery derived from the superior mesenteric vessels, extended +between the right layer of the mesentery, some distance from its +attached border, and the ventral abdominal wall at the umbilicus. The +vessel which was pervious throughout, was the size of one of the digital +arteries. + +Hyrtl has observed the same variation. An example of partial persistence +of the omphalo-mesenteric artery in the adult is well seen in the case +of Meckel's diverticulum shown in Fig. 37, where the arterial vessel +continued upon the diverticulum represents the embryonic +omphalo-mesenteric artery. + +The remaining intestinal arteries are at first more numerous and paired. +In man and most mammals they are early reduced in number, passing from +the abdominal aorta to the dorsal or attached border of the intestine, +between the two peritoneal layers of the primitive dorsal mesentery +(Fig. 104). The arterial blood supply of the intestinal canal then +presents three general divisions: + +1. Vessels pass from the proximal part of the abdominal aorta to the +stomach and pyloric portion of the duodenum. This set of vessels forms +the rudiment of the future coeliac axis. With the development of the +liver and pancreas by budding from the duodenum, and with the appearance +of the spleen in the mesoderm of the dorsal mesentery, branches +corresponding to these organs (hepatic and splenic arteries) are added +to the gastric and duodenal vessels and the adult arrangement of the +coeliac axis is thus obtained (Figs. 130, 131, 132 and 133). + +[Illustration: FIG. 130.--Diagrammatic representation of the arteries +proceeding to the alimentary canal and appendages prior to rotation of +intestine (stage of simple umbilical loop).] + +[Illustration: FIG. 131.--Diagrammatic representation of the arteries of +the alimentary canal in the first stage of intestinal rotation, showing +relation of superior mesenteric artery to the transverse portion of the +duodenum.] + +[Illustration: FIG. 132.--Arteries of alimentary canal in the later +stages of intestinal rotation.] + +[Illustration: FIG. 133.--Final arrangement of arteries of alimentary +canal after completed rotation of the intestines.] + +These vessels have an important bearing on the formation of the adult +peritoneal cavity in the retro-gastric space, and will be considered in +detail below with that portion of the subject. + +2. The next vessel in order derived from the aorta and supplying the +duodenum, pancreas, the small and a part of the large intestine is the +above-mentioned superior mesenteric artery, which arises from the aorta +a short distance caudad of the coeliac axis (Figs. 130, 131, 132 and +133). + +At the time when the intestine still presents the primitive arrangement +of the umbilical loop (Figs. 104 and 130) this vessel passes between +the layers of the dorsal mesentery through the narrow duodeno-colic neck +to reach the two limbs and the apex of the intestinal loop. In its +course it gives off successively branches to the gut from each side. +Those from the right side of the main vessel pass to the duodenum, +pancreas, jejunum and ileum. Those from the left side of the main vessel +accede in succession to the colic angle of the isthmus, the proximal +portion of the colon, the caecum and the ileo-colic junction. The +terminal portion of the superior mesenteric artery supplies the ileum +near the ileo-colic entrance. After rotation it will be found that the +turn has occurred at the point _X_ (Fig. 130), _i. e._, in that part of +the vessel which occupies the duodeno-colic isthmus. Hence it will be +found that the first branches derived from the right side of the +primitive superior mesenteric artery, supplying the duodenum and +pancreas (Art. pancreatico-duodenalis inferior) still arise after +rotation from the right side. They are succeeded, beyond the point _X_, +by the original highest _left_ branches passing to colon, caecum and +ileo-colic junction, while all the original right-sided vessels, except +the inferior pancreatico-duodenal, appear now as branches from the left +side of the main artery, supplying the coils of the jejuno-ileum. Hence +in the adult (Fig. 133) the succession of branches derived from the +right or concave side of the superior mesenteric artery is as follows: + + 1. Arteria pancreatico-duodenalis inferior. + 2. Arteria colica media. + 3. Arteria colica dextra. + 4. Arteria ileo-colica. + +On the other hand, the first branches from what has now become the left +or convex side of the vessel are the original lower right-hand vessels +to the small intestine developed from the descending limb of the loop. +Hence in the adult the left side of the superior mesenteric vessel gives +rise to the vasa intestini tenuis. + +3. The caudal intestinal arterial branch derived from the aorta is the +inferior mesenteric artery supplying parts of the transverse colon, the +descending colon, sigmoid flexure and rectum (Figs. 130, 131, 132, and +133). + +On the other hand in the cases of non-rotation of the intestine as above +described in Figs. 118-122, the embryonic type of the intestinal +arterial supply persists, as indicated schematically in Fig. 134. Not +only the pancreatico-duodenalis inferior, but all the remaining branches +to the small intestine are derived from the right side of the superior +mesenteric artery. The terminal branches of the main artery supply the +ileo-colic junction, while the arterial supply of the large intestine, +A. colica dextra and media, are given off from the left side of the +parent vessel. + +[Illustration: FIG. 134.--Schematic representation of intestinal +arterial supply from superior mesenteric artery in cases of arrested +rotation of the intestine.] + +II. =Demonstration of Intestinal Rotation in the Cat.=--The changes in +the relative position of the different intestinal segments and the final +disposition of the mesenteries and blood vessels can best be understood +by the direct examination of the abdominal contents in an animal whose +permanent adult arrangement corresponds to one of the early embryonal +human stages, and in which the necessary manipulations can readily be +carried out and their results noted. + +It is doubtful if the above detailed developmental stages in man can +ever be clearly comprehended unless the student will for himself examine +the conditions and perform the manipulations in one of the lower +mammals. + +The necessity of keeping the three dimensions of space in mind and the +fact that certain structures during and after rotation cover and obscure +each other, make diagrams and drawings unsatisfactory unless the actual +examination of the object itself is combined with their study. +Fortunately, among the common domestic animals of convenient size easily +obtained the cat answers every purpose of this study admirably. The +student is earnestly urged to pursue his study of the development and +adult arrangement of the human abdominal viscera and peritoneum in the +light which the anatomy of this animal can shed on the complicated and +obscure conditions encountered in the human subject. The plan of having +the opened abdominal cavity of the cat directly side by side with the +human subject, while the arrangement of the abdominal viscera and +peritoneum is considered, cannot be recommended too highly. + +=Directions.=--After killing the animal with chloroform the abdominal +cavity is to be freely opened by a cruciform incision and the skin flaps +turned well back and secured in this position. It is well to select a +male animal or an unimpregnated female, as the size of the pregnant +uterus in the later stages renders the examination of the abdominal +viscera and peritoneum more difficult. + +For purposes of careful study and comparison of the vascular relations +of the abdomen, it is highly desirable to inject the animal with +differently colored gelatine, starch or plaster of Paris mass. The +arterial injection can be made through the carotid artery, the systemic +venous injection through the femoral vein, and the portal circulation +can be filled after opening the abdomen, by injection through the +superior mesenteric or splenic veins. Animals prepared in this manner +are especially useful for the study of the upper portion of the +abdominal cavity and of the peritoneal relations of liver, stomach, +spleen, pancreas and duodenum. They may be kept for permanent reference +in a 5 per cent. solution of formaline or 50 per cent. alcohol. + +After opening the abdominal cavity turn the great omentum up over the +ventral surface of the thorax and secure it in this position, thus +exposing the underlying intestines completely (Fig. 135). Trace in the +first place the entire course of the intestinal tube from the pyloric +extremity of the stomach down. It will be noticed that the first portion +of the small intestine (duodenum) is freely movable, completely invested +by peritoneum and attached to the dorsal midline by a mesoduodenum +between the layers of which a portion of the pancreas is seen. + +[Illustration: FIG. 135.--Abdominal viscera of cat; great omentum +raised; intestines turned down and to left. (From a fresh dissection.)] + +Following the duodenum caudad it will be observed that the gut can be +traced directly continuous with the remaining coils of the small +intestine. The ileo-colic junction and the beginning of the large +intestine are marked by a short pointed caecum. The large intestine is +short, as it is in all carnivore mammals, and passes from the caecum +almost directly down into the pelvis. + +Take the caecum and the first portion of the large intestine and turn +them caudad and over to the left side as far as the peritoneal +connections will permit. + +Spread out the coils of the small intestine in the opposite direction, +_i. e._, over to the right side. + +The arrangement of the intestinal tract after these manipulations should +appear as shown in Figs. 136 and 137. + +[Illustration: FIG. 136.--Abdominal viscera of cat, hardened; omentum +removed to display derivation of intestines from umbilical loop and the +relation of the superior mesenteric artery and common dorsal mesentery +to the small and large intestines. (Columbia University Museum, No +728.)] + +[Illustration: FIG. 137.--Abdominal cavity of cat. (From a fresh +dissection.)] + +It will be seen that all the essential features described for the +corresponding stage in the human embryo (Fig. 104, _A_) exist here. The +proximal portion of the small intestine (duodenum) retains its freedom +and mobility, being attached to the ventral surface of the vertebral +column by the portion of the primitive mesentery which now constitutes +the mesoduodenum. The gut itself forms a bend with the convexity turned +to the right. + +Observe in the next place that the point (Fig. 136, _X_), where small +intestine and colon approach each other closely, marks the situation of +the foetal duodeno-colic isthmus. The small intestine at this point +corresponds to the future duodeno-jejunal angle as will be seen after +rotation has been accomplished. + +Recalling the development of the jejuno-ileum it will not be difficult +to recognize in the numerous coils of small intestine which succeed to +the duodeno-colic isthmus the results of the increase in length of the +descending or efferent limb of the human embryonal umbilical loop. +Tracing these coils it will be found that the terminal portions of the +ileum correspond to the apex and to the proximal part of the ascending +or recurrent limb of the primitive loop, while the remainder of this +limb furnishes the caecum and the next succeeding segment of the large +intestine. Following the tube up to this point the colic boundary of the +duodeno-colic isthmus will be reached; from here the short large +intestine of the carnivore descends straight into the pelvis, attached +to the ventral surface of the vertebral column by a mesocolon which +corresponds to the distal part of the original primitive dorsal +mesentery. + +Now with the parts still in this position examine carefully the +arrangement of the mesentery and of the intestinal blood vessels. +Starting with the duodenum it will be seen that the primitive sagittal +mesentery of this portion of the intestine has followed the gut in its +turn to the right, so that the original right layer of the sagittal +membrane is now directed dorsad and lies in contact with the parietal +peritoneum which invests the background of the abdominal cavity in the +right lumbar region below the liver and covers the ventral surface of +the right kidney. Beneath this parietal peritoneum the inferior vena +cava is seen, receiving the right renal vein and ascending to enter the +dorso-caudal aspect of the right lobe of the liver. If now we assume +that in the cat the opposed serous surfaces of the original right leaf +of the mesoduodenum, now directed dorsad, and of the parietal peritoneum +adhere to each other, and that the visceral peritoneum covering the +dorsal surface of the descending duodenum likewise becomes obliterated +by adhesion to the subjacent parietal peritoneum, we will obtain the +arrangement found in the adult human subject, in which the descending +duodenum is fixed by adhesion below the right lobe of the liver and +ventrad of the medial portion of right kidney, right renal vein and +inferior vena cava. During this process of anchoring the head of the +pancreas, which is found between the two layers of the free mesoduodenum +of the cat, would also become fixed to the abdominal background by +adhesion of the original right leaf of the mesoduodenum, investing what +has now become the dorsal surface of the pancreas, to the parietal +peritoneum. The original left layer of the primitive mesoduodenum would +then appear as _secondary_ parietal peritoneum covering what has now +become the ventral surface of the transversely disposed head of the +gland. The stages may be represented schematically in Figs. 138-140. + +[Illustration: FIGS. 138-140.--Diagrammatic representation of three +stages in the development of the mesoduodenum, duodenum, and pancreas +leading to the secondary "retroperitoneal" position of these viscera.] + +[Illustration: FIG. 138.--Free mesoduodenum in sagittal plane, including +head of pancreas between right and left layers.] + +[Illustration: FIG. 139.--Mesoduodenum folded to right; left leaf has +become ventral; right dorsal, directed toward primitive prerenal +parietal peritoneum.] + +[Illustration: FIG. 140.--Fixation of head of pancreas and duodenum +under cover of secondary parietal peritoneum by adhesion of apposed +surfaces of mesoduodenum and primitive parietal peritoneum.] + +Figs. 138 and 139 shows the arrangement in the cat where a free +duodenum and mesoduodenum exists, with the pancreas included between its +layers.[2] + +[2] The student should not be confused by the fact that a considerable +portion of the pancreatic gland in the cat will be found included +between the layers of the great omentum, extending over to the left side +of the abdomen. This circumstance will be found of importance in +studying the development of the dorsal mesogastrium and of the +structures connected with it. For the present attention should only be +given to the right extremity or head of the pancreas, situated close to +the duodenum and included between the layers of the mesoduodenum. + +It will be noticed that the duodenum in the cat can be carried over to +the median line (Fig. 138) exposing the entire ventral aspect of the +right kidney and the inferior vena cava beneath the primary lumbar +parietal peritoneum. This manipulation will also expose the dorsal +surface of the head of the pancreas, covered by what originally was the +right leaf of the mesoduodenum. + +Fig. 140 indicates the results of adhesion of the duodenum, pancreas and +mesoduodenum to the parietal peritoneum as it normally occurs in the +human subject. It will be seen that the primary parietal peritoneum can +be traced mesad over the ventral surface of the right kidney as far as +the point _X_, and that from here on to the median line the peritoneum +is _secondary_ parietal peritoneum, consisting of the visceral +peritoneal investment of the ventral surface of the duodenum and of the +original left leaf of the mesoduodenum, beneath which the ventral +surface of the pancreas is seen. Pancreas and duodenum occupy in the +adult secondarily a "retro-peritoneal" position, _i. e._, the peritoneum +now covering the ventral surface of these viscera appears as a +continuation of the parietal peritoneum, the transition between primary +and secondary parietal peritoneum occurring along the line marked _X_ in +Fig. 140. The opposed peritoneal surfaces indicated by the dotted lines +have become adherent and converted into loose connective tissue in which +the pancreas and duodenum lie imbedded. In the human embryo this process +of adhesion begins in the eighth week, starting at the duodeno-jejunal +flexure and ascending gradually toward the pylorus. At the end of the +fourth month the union is complete. + +Proceeding caudad it will next be observed that the peritoneum of the +mesentery occupies the narrow neck of the duodeno-colic isthmus, and +that large vessels (the superior mesenteric) pass between its two layers +at this point to supply the segments of the intestine forming the loop. +In conformity with the greatly increased length of the intestine it will +be found that the mesentery expands from the narrow pedicle at the neck +in a fan-shaped manner in order to develop a sufficiently long margin +for attachment to the intestine. The following points should be +carefully borne in mind in studying the mesentery with the intestines in +this position: + +1. The mesentery presents two free surfaces, right and left. With the +coils of the small intestine turned over to the right, the left leaf of +the mesentery is turned toward the observer. + +2. Inasmuch as the descending limb of the embryonic loop has developed +the greater part of the small intestine, while a portion of the large +intestine (caecum and colon up to the isthmus) is the result of +differentiation within the ascending or returning limb of the loop, it +will be at once apparent that the double peritoneal layer which extends +between the duodeno-colic isthmus and the attached border of the gut is +partly mesentery of the small intestine, partly mesocolon passing to the +large intestine (caecum and proximal colon). This condition may be +indicated schematically in Fig. 141. + +[Illustration: FIG. 141.--Schematic representation of mesentery of +umbilical loop, common to small intestine and proximal portion of large +intestine.] + +The curved line _A_ may be taken as an arbitrary division between the +portion of the membrane which on the right of the figure passes to the +small intestine, and the portion which proceeds to the left to be +attached to the large intestine. In other words the line will +schematically separate the true mesenteric from the mesocolic segment of +the primitive membrane. + +With the parts in their present position this line might be assumed to +indicate a strip along which the opposed serous surfaces of the parietal +peritoneum and the right leaf of the primitive mesentery became +adherent. In that case an actual division into a mesenteric and +mesocolic segment would have been effected. + +Ventrad and to the right of this line of adhesion we would trace that +portion of the primitive membrane which now passes to the coils of the +small intestine as the true mesentery, having an apparent origin in the +background of the abdomen to the dotted line of adhesion. In the same +manner the peritoneal layers passing to the left to reach the caecum and +beginning of the colon would appear as a free mesocolon with the same +line of apparent origin from the background of the abdomen. (cf. p. 80.) + +These considerations should be followed out in the dissection of the cat +in order to become familiar with the principle of _secondary lines of +origin_ for peritoneal layers. As we will see later this factor is of +importance in correctly estimating the value of the human adult +conditions. + +3. A brief consideration of the mechanical conditions and comparison +with the earlier stages will show why the peritoneal layers which occupy +the bight of the fully developed umbilical loop are especially prone to +develop secondary lines and areas of adhesion to other serous surfaces. +If we compare the dorsal mesentery in its primitive condition, before +the straight intestinal tube has become differentiated into the +subsequent segments, and before the umbilical loop has been formed (Fig. +142), with the later stages represented by the intestines of the cat as +now arranged (Figs. 143 and 144), it will be seen that the vertical line +of attachment to the ventral surface of the vertebral column, between +the points _a_ and _b_ corresponds in the advanced stages to the +interval _ab_ separating the two points of the duodeno-colic isthmus; +also that the entire mesenteric peritoneal surface beyond the isthmus is +the result of drawing out and lengthening the intestinal tract. +Consequently folding or overlapping of this extensive membrane affords +opportunities for adhesions between its own serous surfaces or between +it and the remaining visceral and parietal peritoneum of the abdomen. + +[Illustration: FIGS. 142-144.--Schematic representation of three stages +in the development of the mesentery of the umbilical intestinal loop.] + +[Illustration: FIG. 142.--Early stage before differentiation of +intestinal canal.] + +[Illustration: FIG. 143.--Stage of umbilical loop. Differentiation of +common dorsal mesentery of earlier stage into dorsal mesogastrium, +mesoduodenum, primitive mesentery of umbilical loop, and descending +mesocolon.] + +[Illustration: FIG. 144.--Final stage. With complete differentiation of +large and small intestine, the primitive mesentery of the umbilical loop +contains not only the mesentery of the future jejuno-ileum, but also the +mesocola and the ascending and transverse colon, developed from the +ascending or afferent limb of the umbilical loop.] + +Moreover, it will be appreciated that the entire extensive coil of +intestines extending between the two boundaries of the duodeno-colic +isthmus (_a_, _b_) is suspended from the back part of the abdomen by a +narrow pedicle and that consequently rotation will readily occur around +the axis drawn through the neck of the isthmus. + +Now proceed to illustrate on the cat the result of the rotation as it +occurs normally during the development of the primate intestinal tract. +Take the caecum and commencement of the colon and draw the same over to +the right across the duodeno-colic isthmus and the duodenum. Twist or +rotate the entire mass of small intestines around the isthmic pedicle, +so that the original left leaf of the mesentery will look to the right +and _vice versa_ (Fig. 145). The conditions thus established will be +found to correspond to the schemata shown in Figs. 114 and 115. The main +features of the intestinal tract in the rearranged position will be as +follows: + +1. The two points, _a_ and _b_, of the duodeno-colic isthmus (Fig. 145) +are still close together, but reversed in position, _b_ is in front and +to the right, _a_ behind and to the left, whereas before the rotation +_b_ was situated below and to the left, _a_ above and to the right (Fig. +135). + +[Illustration: FIG. 145.--Abdominal viscera of cat, with intestines +rotated to correspond to the stage in the development of the human canal +in which the caecum has reached the subhepatic position, but before the +establishment of the ascending colon. (From a fresh dissection.)] + +2. The direction of the ileo-colic entrance is reversed, the ileum now +entering the large intestine from below and the left upwards and to the +right, instead of from right to left. + +3. The descending duodenum is now situated dorsad to the colon. + +4. The original left leaf of the mesentery has become the right, and +_vice versa_. + +5. The superior mesenteric artery crosses over the transverse portion of +the duodenum, and with the exception of the inferior +pancreatico-duodenal artery the original right-sided branches now arise +from the left side of the vessel and _vice versa_. + +It is now time to compare the conditions established in the cat by the +manipulations just detailed with the arrangement of the adult human +intestinal tract and peritoneum below the level of the transverse colon +and mesocolon. + +I. The shortness of the large intestine in the cat will require careful +manipulation in order to produce a disposition in conformity with the +arrangement of this portion of the human intestinal tract. By +stretching the gut somewhat and pulling it well out of the pelvis +sufficient length will be obtained to establish an ascending, transverse +and descending colon. Move the caecum from the subhepatic position which +it occupies immediately after rotation (Fig. 145) down to the lower and +right-hand corner of the abdomen. Pull the distal portion of the large +intestine well out of the pelvis and obtain thus sufficient length to +establish an ascending, transverse and descending division each provided +with a free mesocolon (Fig. 146). In the formation of the three definite +main segments of the human large intestine, ascending, transverse and +descending colon, the following stages may be recognized: + +[Illustration: FIG. 146.--Abdominal viscera of cat, with the intestines +rotated to correspond to the adult human disposition, with ascending, +transverse, and descending segments of the colon. (From a fresh +dissection.)] + +1. Immediately after rotation the large intestine lies transversely +along the greater curvature of the stomach, with the caecum on the right +side in front of the duodenum and closely applied to the caudal surface +of the right lobe of the liver (Fig. 147). + +[Illustration: FIG. 147.--Human foetus, 6.6 cm., vertex-coccygeal +measure; liver removed. (Columbia University Museum, Study Collection.) +x 4.] + +PERSISTENCE OF SUBHEPATIC POSITION OF CAECUM IN ADULT.--The period at +which the caecum descends into the iliac fossa is liable to a +considerable range of variation. + +Treves found in two foetus, measuring respectively 41/2" and 51/2", the +caecum on a level with the caudal end of the right kidney, while in +several individuals at full term the caput coli was still placed +immediately below the liver, with no large intestine in the place of the +ascending colon. This condition is well illustrated in the foetus shown +in Fig. 124. + +The caecum may remain undescended throughout life. Treves, in an +examination of 100 bodies, found this condition in two subjects, both +females, one 41, the other 74 years of age. Both cases presented an +identical disposition. There was no large intestine in the place of the +ascending colon. The caecum was placed on the right side, immediately +underneath the liver, just to the right of the gall-bladder; it was +quite horizontal in position, continuing the long axis of the transverse +colon and included between the layers of the transverse mesocolon. From +the extremity of the caecum a horizontal fold was continued to the +abdominal parietes and upon it the edge of the liver rested. In one of +these instances the colon from the tip of the caecum to the splenic +flexure measured 38". The great omentum was attached only to the left +half of this portion. The descending colon was very long, measuring 15". + +In the other case the distance from the tip of the caecum to the splenic +flexure was 27", the great omentum commencing 5" from the former point. +The descending colon was of normal length. + +In both bodies the remaining viscera were normal. + +2. The caecum next descends ventrad of right kidney to the iliac fossa. +The future ascending colon is at this time placed very obliquely on +account of the large size of the foetal liver, and passes without a +marked angle into the transverse segment. Thus in Fig. 148, from a foetus +5" in length, the descending colon is vertical and the splenic flexure +well marked, forming the highest point of the colic arch. There is no +hepatic flexure, and no ascending and transverse colon, but instead of +these an oblique segment passing upwards and to the left between caecum +and splenic flexure. + +[Illustration: FIG. 148.--Abdominal viscera of human foetus of 12.5 cm., +vertex-coccygeal measure, hardened _in situ_; transverse and ascending +colon not yet differentiated. (Columbia University Museum, No. 1815.) +Natural size.] + +This disposition, due to the large size of the liver, is still marked at +times in the foetus at term, and occasionally even in children up to 2 or +3 years of age. + +3. The ascending colon is subsequently differentiated from the +transverse segment and the hepatic flexure formed consequent upon the +diminution of the relative size of the liver, which permits the foetal +oblique segment of the colon extending in the earlier stages between the +right iliac fossa and the spleen to become divided by a right-angled +(hepatic) bend or flexure into an ascending and a transverse segment +(Fig. 149). + +[Illustration: FIG. 149.--Abdominal viscera of human foetus at term, +hardened _in situ_; hepatic flexure formed and ascending and transverse +colon differentiated. (Columbia University Museum, No. 1816.)] + +4. The splenic flexure develops early and is well marked. It indicates +the point of transition of the original ascending limb of the umbilical +loop into the remaining vertical median segment of the large intestine, +from which the descending colon is formed. + +In the adult the ascending and descending portions of the colon are +vertical. The transverse colon is not quite horizontal since the +splenic flexure is higher and placed more dorsally than the hepatic +flexure. In the embryo the rapidly-growing coils of the small intestine +push the descending colon to the left and dorsad into close contact with +the dorsal abdominal wall. + +A small bend which appears about the middle of the third month in the +left iliac fossa indicates the rudiment of the future sigmoid flexure or +omega loop. + +The rest of the endgut follows the body wall in a well-marked curve, +whose termination lies within the concavity of the caudal portion of the +embryo (Fig. 150). From this terminal part the rectum develops after the +division of the cloaca and the union of the proctodaeum with the +entodermal intestinal pouch has taken place as detailed above. + +[Illustration: FIG. 150.--Caudal portion of human embryo of 5 mm., with +the end- and caudal gut at the highest stage of its development. x 25. +(Reconstruction after His.)] + +The early position of the colon produced by the large size of the foetal +liver, and before the descent of the caecum has occurred, is shown in +Fig. 124. In Fig. 123, where the liver has regained its normal +proportions with reference to the abdominal cavity and viscera, and the +caecum has descended into the right iliac fossa, the hepatic flexure is +well marked and the first segment of the colon has acquired the vertical +position on the right side, the single obliquely transverse segment of +Fig. 124, having become divided into an ascending and a transverse +colon. + +[Fig. 124. Early stage. Liver relatively large. Proximal portion of the +colon extends obliquely between the right lumbar region and the spleen. +The caecum has not yet descended. + +Fig. 123. Later stage. The caecum occupies the right iliac fossa. +Relative reduction in the size of the liver allows the colic segment to +be divided by the hepatic flexure into an ascending colon and a +transverse colon.] + +At times the transverse colon, whose normal average length in the adult +is 20", greatly exceeds this measurement and forms an arch which hangs +down or makes a well-marked V-shaped bend with the apex directed toward +the pubes. This is the normal arrangement of this portion of the large +intestine in many of the lower primates. Fig. 151 shows the abdominal +viscera of _Macacus rhesus_, hardened _in situ_, seen from the front +and the right side, with the omentum turned up over the stomach. The +transverse colon forms an extensive V-shaped bend, whose apex reaches to +the pubes, from which point the large intestine turns again cephalad and +dorsad to form the splenic flexure and then descends to the pelvis. + +[Illustration: FIG. 151.--Abdominal viscera of _Macacus rhesus_, rhesus +monkey, hardened in situ. (Columbia University Museum, No. 1817.)] + +The average length of the ascending colon in the adult, measured from +the tip of the caecum to the hepatic flexure, was found by Treves in his +series of 100 bodies to be 8", while the descending colon, from the +splenic flexure to the beginning of the sigmoid loop, measured 81/2". + +The descending colon may at times be much longer, up to 15", and become +convoluted. + +II. In the next place, in order to understand the arrangement of the +peritoneum in this lower larger compartment of the abdomen, disregard +for the present the peritoneal connections of the stomach, liver, +pancreas and spleen, and the folds of the great omentum entirely. This +latter membrane is adherent in the adult human subject by its dorsal +surface to the upper margin of the transverse colon, so that in turning +the omentum up over the ventral chest wall the transverse colon will be +carried with the omentum and the lower layer of the transverse mesocolon +will be put upon the stretch. This membrane forms in adult man by its +transverse attachment to the abdominal background the cephalic limit of +the larger lower compartment of the abdomen, which is framed laterally +by ascending and descending colon, continuous below with the pelvic +cavity and occupied chiefly by the freely movable coils of the +jejuno-ileum. + +Remember that the duodenum starting from the pyloric extremity of the +stomach first turns cephalad and dorsad in contact with the caudal +surface of the right lobe of the liver, forming the first portion or +hepatic angle of the duodenum; that in the next place the second or +descending portion of the duodenum passes down in front of the medial +part of the ventral surface of the right kidney and the inferior vena +cava, but _behind_ the right extremity (hepatic flexure) of what after +rotation and formation of the ascending colon appears as the transverse +colon; that consequently the descending duodenum is divided by its +intersection with the transverse colon into a cephalic supra-colic and a +caudal infra-colic segment. + +Also remember that the second angle of the duodenum (transition between +the descending and transverse portions) is consequently situated to the +right of the vertebral column below the level of the transverse colon +and the secondary attachment presently to be considered of the +transverse mesocolon to the background of the abdominal cavity. + +The third portion of the duodenum extends from this point more or less +transversely--depending upon the type--to the left, across the vertebral +column and aorta. This transverse portion, after the rotation of the +primitive loop at the duodeno-colic angle, is crossed in the direction +caudad and ventrad by the superior mesenteric vessels, which hence +divide this portion of the intestine into a right and left segment. + +The latter turns cephalad and ventrad on the left side of the vertebral +column (4th or ascending portion) to become continuous at the +duodeno-jejunal angle with the free or floating small intestine +(jejunum). + +If we imagine in the cat the duodenum anchored or fixed by adhesion of +the dorsal (originally right) leaf of the mesoduodenum and of its own +dorsal visceral peritoneum to the abdominal parietal peritoneum in the +manner above indicated (p. 70) as far as the duodeno-jejunal angle we +will have conditions established which correspond to those found in the +human adult abdominal cavity. + +III. It is next necessary to study carefully the disposition of the +primitive dorsal mesentery connected after rotation with the different +segments of the intestinal tube, ascending, transverse and descending +colon and free small intestine. + +In order to obtain in the cat a cephalic limit to the region now under +consideration which will correspond to the arrangement of the adult +human peritoneum, we will begin with the peritoneal membrane attached to +the portion of the colon which in the rearranged intestinal tract +represents the human transverse colon. This transverse segment of the +large intestine is now made to extend directly across the abdomen from +the liver to the spleen. The two layers composing the transverse +mesocolon are an upper or cephalic and a lower or caudal layer. + +Now it will be seen in the cat that the upper or cephalic layer of the +transverse mesocolon thus established is continuous on each side with +the dorsal (originally right) leaf of the ascending and with the dorsal +(originally left) leaf of the descending mesocolon, which peritoneal +layers are in direct opposition to the parietal lumbar and prerenal +peritoneum. On the other hand, the inferior or ventral layer of the +transverse mesocolon is continuous on each side of the median line with +the ventral (originally respectively left and right) leaves of the same +mesocola, while at the site of the duodeno-colic isthmus the two layers +of the transverse mesocolon are continuous as originally with the two +layers of the mesentery of the jejuno-ileum (Fig. 146). + +Now fix the transverse mesocolon firmly against the background of the +abdomen and place the ascending and descending colon as far as possible +over to the right and left side respectively. We will assume a line of +secondary adhesion between the transverse mesocolon and the parietal +peritoneum investing the dorsal abdominal wall. Along this line the +upper or cephalic surface of the transverse mesocolon would become +continuous with the dorsal parietal peritoneum, while the lower or +caudal layer would still be continuous with the left leaf of the +ascending and the right leaf of the descending mesocolon. We have +already seen that the duodenum and mesoduodenum become anchored in the +subhepatic region and that the visceral ventral peritoneum of the gut +and the original left leaf of the mesoduodenum appear then as secondary +parietal peritoneum. Hence a sagittal section through the right lumbar +region, right kidney and descending duodenum would, immediately after +rotation and establishment of the transverse mesocolon, show +the peritoneal arrangement indicated in Fig. 153. After adhesion of the +transverse mesocolon continuity would be established between its upper +or cephalic layer and the secondary parietal peritoneum investing the +supra-colic portion of the descending duodenum (Fig. 154) while its +caudal layer becomes continuous with the secondary parietal peritoneum +covering the infra-colic segment of the duodenum and the lower portion +of the ventral surface of the right kidney. + +[Illustration: FIGS. 152-154.--Schematic representation of peritoneum in +fixation of descending duodenum and formation of transverse colon and +mesocolon.] + +[Illustration: FIG. 152.--Sagittal section through right kidney and +descending duodenum before adhesion of latter to parietal peritoneum.] + +[Illustration: FIG. 153.--Adhesion of descending duodenum to primitive +parietal peritoneum. Colon and mesocolon after rotation of the +intestine, but before adhesion.] + +[Illustration: FIG. 154.--Adhesion of mesocolon to duodenum and +primitive parietal peritoneum, resulting in formation of root of +transverse mesocolon.] + +Reference to the schematic Figs. 152, 153 and 154, will show that the +adult duodenum becomes fixed to the posterior parietes of the abdomen by +adhesion of its visceral serous covering and of the dorsal layer of the +mesoduodenum to the primitive parietal peritoneum. The supra-colic +segment of the adult descending duodenum lies under cover of a single +peritoneal layer, derived from its own visceral investment and appearing +as secondary parietal peritoneum by continuity laterad along the line of +adhesion with the primitive parietal peritoneum covering the upper part +of ventral surface of right kidney, while mesad, the layer covering this +segment of the duodenum, is continued into the secondary parietal +peritoneum derived from the left or ventral leaf of the mesoduodenum and +covering the ventral surface of the pancreas (cf. Figs. 138-140). + +On the other hand, the infra-colic segment of the descending duodenum, +as well as the lower and mesal angle of the ventral surface of right +kidney, between ascending and transverse colon, is covered by a layer of +secondary parietal peritoneum derived from the ventral layer of the +ascending mesocolon and continuous with the caudal layer of the +transverse mesocolon. Beneath this secondary parietal peritoneum are two +obliterated layers, on the one hand the dorsal layer of the mesocolon, +on the other the visceral infra-colic duodenal serosa and the primitive +prerenal parietal peritoneum. + +In the further development of the adult human arrangement the changes +below the level of the transverse colon and mesocolon result in the +fixation of the ascending and descending colon to the background of the +right and left lumbar regions. The opposed serous surfaces of the +ascending and descending mesocola and of the dorsal parietal peritoneum +adhere and the process also usually involves the dorsal visceral +peritoneum of the ascending and descending colon, so that these portions +of the gut obtain a fixed position. + +Adhesion of the mesocolon to the dorsal body wall (parietal peritoneum) +does not occur at all points at the same time. Usually adhesion proceeds +from the midline laterad. The fixation of the ascending colon in the +human embryo begins about the fourth month. + +In the descending segment by the same time adhesion has usually +proceeded nearly up to the descending colon, but the intestine itself is +as yet free. In the fifth month the descending colon has usually become +fixed between the splenic flexure and the beginning of the sigmoidea. In +the latter region a free mesocolon usually persists throughout life. + +Differences in the rate of growth between the length of the body wall +and the length of the mesocolon may play an important part in the +production of peritoneal _fossae_, small pouches which in some regions of +the abdomen may assume considerable proportions. Such fossae are found +around the duodeno-jejuneal angle, the caecum and appendix, and the +sigmoid flexure. They will be considered more in detail with these +respective regions, especially in reference to their relation to +retro-peritoneal hernia. + +In a certain proportion of cases adhesion between the parietal +peritoneum and the ascending and descending mesocolon is incomplete or +entirely wanting, resulting in the formation of a more or less +completely free ascending and descending mesocolon. Treves, in an +examination of 100 bodies, obtained the following figures: + +In 52 subjects there was neither an ascending nor a descending +mesocolon, the intestine being fixed in the manner which is regarded as +normal. + +In 22 there was a descending, but no trace of an ascending mesocolon. + +In 14 a mesocolon was found in both the ascending and descending +segments of the large intestine. + +In 12 there was an ascending mesocolon, but no corresponding fold on the +left side. Hence from this series a mesocolon may be expected on the +left side in 36 per cent., on the right side in 26 per cent. + +Both development and comparative anatomy would lead us to expect that +the descending mesocolon would be found more frequently than the +ascending. + +In the lower animals the descending mesocolon is always an extensive and +conspicuous membrane. It is well developed in all monkeys and the +anthropoidea, as the remains of the primary vertical fold of the dorsal +mesentery, while the ascending mesocolon is a secondary production, +acquired during the development of the bowel by rotation. + +In most of the lower monkeys the ascending mesocolon is also largely or +entirely free. The descending mesocolon can always in these animals be +reflected to the median line (cf. Fig. 155). + +[Illustration: FIG. 155.--Abdominal viscera of _Macacus cynomolgus_, Kra +monkey, hardened _in situ_. (Columbia University Museum, No. 1801.)] + +The line of attachment in man of the descending mesocolon is usually +along the lateral border of the left kidney and vertical, while the line +of attachment of the ascending mesocolon is usually less vertical, +crossing the caudal end of the right kidney obliquely from right to left +and with an upward direction (Fig. 156). + +[Illustration: FIG. 156.--Schema of visceral and peritoneal relations of +ventral surface of right kidney.] + +In like manner when both the ascending and descending mesocola are +absent as free membranes the left or descending colon is adherent along +the lateral border of the kidney to the abdominal parietes, while the +ascending colon is fixed at the hepatic flexure a little obliquely +across the ventral surface of the caudal end of the corresponding gland +ascending toward the medial margin. + +Treves found in the cases of persistent ascending mesocolon in the adult +that the membrane varied in breadth from 1" to 2" while the persistent +fold on the left side varied between 2" and 3" in breadth. + +In the foetus, up to 5"-6" in length, the descending mesocolon is usually +an extensive fold. Its attachment is vertical, but nearer to the median +line than in the adult, usually along the medial border of the left +kidney. It is at times found attached along this line in the adult. + +An ascending mesocolon is rare even in the foetus. The caecum and +beginning of the ascending colon are completely invested by peritoneum, +but above the parts so invested the colon is usually adherent along an +oblique line to the ventral and medial aspect of the right kidney. + +In the foetus at full term, if the caecum is still undescended and in +contact with the liver, it is not uncommon to find the cephalic portion +of the descending colon provided with a mesocolon, while the caudal part +of the descending colon is fixed by adhesion to the ventral surface and +lateral border of the left kidney. This free membrane is then really a +part of the transverse mesocolon. Where the caecum descends to the iliac +fossa the portion of the foetal descending colon so invested is drawn +over to the right and incorporated in the transverse colon. + +Treves in two out of 100 bodies found the caecum in the right iliac +region, but both it and the whole of the ascending colon were entirely +free from any peritoneal connections with the dorsal parietes of the +abdomen. + +The gut from the tip of the caecum to the hepatic flexure was entirely +invested by peritoneum continuous with the mesentery. The ascending +colon was covered in the same manner and by the same fold as the small +intestine. The segment of large intestine thus free measured 8" in both +instances. + +The mesentery lacked the usual attachment to the dorsal abdominal wall +and its root was represented by the interval between the duodenum and +the transverse colon. The membrane had no other than its original +primary attachment, and small intestine and ascending colon formed +together a loop that practically represented the condition of the great +primary intestinal loop. (Compare p. 73.) + +The arrangement presented in these two subjects corresponds to that met +in many animals, such as the cat. + +A cross-section of the cat's abdomen arranged as above would show the +following disposition of the peritoneum, corresponding to the stage in +the human development preceding the fixation of the two vertical colic +segments (Fig. 157). It will be seen that the right and left mesocola +can be reflected to the median line where they become continuous ventrad +of the vertebral column and aorta with the mesentery of the small +intestine. The ventral surfaces of both kidneys are seen to be covered +by the primitive parietal peritoneum of the abdominal cavity. + +[Illustration: FIGS. 157, 158.--Schema showing peritoneal arrangement in +transection of infra-colic compartment of abdomen before and after +fixation of ascending and descending colon.] + +Fig. 158 shows the adult human arrangement of the same parts, after +fixation of the vertical colic segments by adhesion of the opposed +surfaces of their mesocola and the primitive parietal peritoneum. The +background of the abdomen is now seen to be covered by a layer of +secondary parietal peritoneum, _viz._, the original left leaf of the +ascending and right leaf of the descending mesocolon, continuous above +with the lower or caudal layer of the transverse mesocolon. + +This adhesion is so complete that the original condition is disregarded +in adult descriptive anatomy. The layer which has adhered to the +parietal peritoneum can no longer be recognized and the other has +assumed the role of parietal peritoneum. + +The connection of the transverse mesocolon with the dorsal lamella of +the great omentum will be considered below. + +The course of the vessels in the ascending and descending mesocola is +not altered by the secondary adhesions. These vessels are in the adult +situated behind the secondary parietal peritoneum derived from the +mesocola. + +The origin of the transverse mesocolon obtains by the fixation of the +hepatic and splenic flexures high up in the abdomen a transverse course, +and the transverse growth of the abdomen holds the membrane in this +position cephalad of the duodeno-jejunal flexure, so that on elevating +the transverse colon the mesocolon appears as separating the upper from +the lower abdominal compartment. This posterior line of attachment or +so-called "root of the transverse mesocolon," is nothing more than the +upper limit of the area of adhesion between the primitive parietal +peritoneum and the opposed surfaces of the ascending and descending +mesocola. Reference to the abdominal cavity of the cat after complete +rotation (Fig. 146) will show the original continuity of the three +mesocola very clearly. A secondary connection is established along the +lateral border of ascending and descending colon (Fig. 158), between the +primitive parietal peritoneum and the ventral visceral peritoneal +investment of the large intestine. Both of the vertical segments of the +colon now appear fixed. Their dorsal surfaces are uncovered by +peritoneum and can be reached in the lumbar region, laterad of the +kidney, without opening the peritoneal cavity (lumbar colotomy). + +The caudal portions of both kidneys are covered, beneath the secondary +parietal peritoneum, by a layer of loose connective tissue representing +the result of obliteration by adhesion of the first and second of the +original three layers of prerenal peritoneum, _viz._, the primitive +parietal (1) and the two layers of the mesocola (2 and 3). + +LINE OF ATTACHMENT OF THE MESENTERY OF THE JEJUNO-ILEUM.--Examination of +the caudal surface of the transverse mesocolon in the cat, with the +parts in the above outlined position, will show how and why in the adult +human abdomen the duodeno-jejunal angle appears to dip out from beneath +the transverse mesocolon, becoming gradually more and more free until +complete transition to the mobile jejunum is obtained. From this point, +situated to the left of the second lumbar vertebra, the dorsal +attachment of the adult human mesentery of the jejuno-ileum extends +somewhat obliquely caudad and to the right to terminate in the right +iliac fossa at the ileo-colic junction. + +Returning to the conditions presented by the cat's intestines to obtain +an explanation of this line of fixation we must recall the fact that in +the peritoneum included within the limits of the umbilical loop, after +differentiation of small and large intestine, but before rotation, we +have both the elements of the mesentery of the small intestines and of +the ascending and transverse mesocolon combined (Fig. 141). For it will +be seen that this membrane carries at this time vessels both to the +jejuno-ileum and to the segments of the large intestine (caecum, +ascending and transverse colon). This fact will be at once recognized if +the cat's intestines are arranged to correspond to the primitive +condition (Fig. 136) and the mesentery examined. + +After rotation and differentiation of the colic segments and after the +adhesion of the ascending and descending colon in man, the course of the +main trunk of the superior mesenteric artery passes, after crossing the +third portion of the duodenum, down and to the right to terminate near +the ileo-colic junction by anastomosis with its ileo-colic branch. The +adhesion of the right and left mesocola to the dorsal parietal +peritoneum proceeds mesad as far as this line, leaving free the +mesentery of the small intestines, which contains the vasa intestini +tenuis derived from the left side of the main vessel. The secondary line +of attachment of the mesentery to the abdominal background is therefore +along this line. To obtain a clear idea of these processes of +development in man assume that in the cat, after rotation and +establishment of the three divisions of the colon, the two vertical +(ascending and descending) mesocola become adherent to the dorsal +parietal peritoneum, leaving the mesentery of the small intestine free. + +Fig. 159 illustrates schematically the area of mesocolic adhesion in the +human subject after complete rotation, and the line of the mesentery of +jejuno-ileum. + +[Illustration: FIG. 159.--Schematic figure to show lines of mesocolic +adhesion, formation of root of transverse mesocolon and root of +mesentery of jejuno-ileum in human subject.] + +Fixation of the ascending and descending cola and of their mesocola +proceeds cephalad as far as the line _AB_, which thereby constitutes the +root of the free transverse mesocolon. + +The secondary parietal peritoneum derived from the ventral layer of the +ascending mesocolon covers the lower and inner portion of the ventral +surface of the right kidney, the infra-colic division of the descending +and the dextro-mesenteric segment of the transverse duodenum, while +along the root of the jejuno-ileal mesentery it becomes continuous with +the right layer of that membrane. The secondary parietal peritoneum +derived from the ventral layer of the descending colon covers the lower +part of the ventral surface of the left kidney and the +sinistro-mesenteric segment of the transverse duodenum and becomes +continuous along the mesenteric radix with the left layer of the +jejuno-ileal mesentery. + +Caudad the adhesion of the descending colon and mesocolon to the +parietal peritoneum proceeds only to the point _C_, following the dotted +line mesad and resulting in the formation of the free mesocolon of the +sigmoid flexure. + +=Resume of the Adult Arrangement of the Human Peritoneum in the Lower +Compartment of the Abdomen, Below the Level of the Transverse Colon and +Mesocolon.=--We should now consider the arrangement of the human +peritoneum in the adult below the dorsal attachment of the transverse +mesocolon in the light of the embryological and comparative anatomical +facts just stated. In doing this it will be advisable to study both the +actual conditions encountered and their significance in the sense of +determining the derivation of the peritoneal layers from the primitive +dorsal mesentery. Open the abdominal cavity in the usual manner by a +cruciform incision. + +Turn the great omentum up on the chest wall, exposing the underlying +intestines. This manipulation, as already stated, will cause the omentum +to carry the transverse colon with it, on account of the adhesion, in +the adult, of the gut to the dorsal layer of the omentum. Hence the +cephalic or upper layer of the transverse mesocolon will not be seen at +this stage because the omental adhesion just referred to prevents us +from passing between the greater curvature of the stomach and the +transverse colon without tearing peritoneal layers. It will, however, be +possible to trace on the right side the duodenum from the pylorus down +ventrad of the right kidney until the descending portion disappears +behind the hepatic flexure of the colon. With the omentum and transverse +mesocolon turned up, as stated, and the transverse mesocolon put upon +the stretch, it will be seen that the abdominal space now overlooked is +bounded cephalad by the lower layer of the transverse mesocolon and its +attachment to the dorsal abdominal wall. The lateral limits of the space +are given by the ascending and descending colon respectively. The +attachment of the mesentery of the small intestine to the oblique line +extending from the left of the vertebral column at about the level of +the second lumbar vertebra to the right iliac fossa subdivides the +entire space into a secondary right and left compartment. + +Begin by following the caudal layer of the transverse mesocolon dorsad +on the right side. In the angle between ascending and transverse colon +(hepatic flexure) pressure will locate the caudal portion of the ventral +surface of the right kidney. Remember that the peritoneum touched in +these procedures appears in the adult as parietal prerenal peritoneum, +but that in reality it is the left leaf of the originally free ascending +mesocolon, whereas the original right leaf of this membrane and the +primitive parietal peritoneum have, by adhesion of their serous +surfaces, been converted into the loose subserous connective tissue +covering the ventral aspect of the kidney beneath what now appears as +parietal peritoneum. + +Mesad of the resistance offered to the finger by the right kidney the +caudal (infra-colic) portion of the descending duodenum and the angle of +transition between it and the third or transverse portion will be found, +invested in the same way by secondary (mesocolic) parietal peritoneum. +It will be seen, especially if the duodenum is injected or inflated, +that the hepatic flexure of the colon lies ventrad of the vertical +descending second portion of the duodenum, so that one part of this +intestine is situated cephalad the other caudad of the colon. (Supra- +and infra-colic segments of descending duodenum.) + +Individual differences are observed in the area of colic attachment to +the duodenum. Usually the two intestines are in contact with each other +and adherent over a considerable surface. Exceptionally the transverse +mesocolon extends across to the right so as to include the hepatic +flexure. In this latter case the uncovered non-peritoneal surface of the +descending duodenum is small, represented by the interval between the +layers of the transverse mesocolon, and the hepatic flexure is then not +directly adherent to the gut. + +If we now trace the transverse duodenum from right to left we will +encounter the right layer of the root of the jejuno-ileal mesentery. The +caudal layer of the transverse mesocolon, the right leaf of the +mesentery and the secondary parietal peritoneum investing the ventral +surface of the transverse duodenum all meet at this point. Surround the +mesentery of the free small intestine with the fingers of one hand so +that the entire mass of intestinal coils can be swung alternately from +side to side. + +Turning them over to the left, as already stated, the proximal portion +of the transverse duodenum can be traced from right to left as far as +the root of the mesentery. Here the peritoneum investing the ventral +surface of the duodenum becomes continuous with the right leaf of the +mesentery. Now swing the whole mass of small intestines over to the +right, exposing the parietal peritoneum in the space to the left of the +vertebral column, between the attachment of the mesentery to the median +side, the root of transverse mesocolon cephalad and the descending colon +to the left. Remember that the same significance attaches to this +secondary parietal peritoneum as on the right side. It appears in the +adult as parietal peritoneum, but is in its derivation the original +right leaf of the descending mesocolon. Close to the root of the +mesentery the continuation from the right side of the transverse +duodenum will be seen, crossing the median line from right to left +ventrad of aorta and vertebral column and usually turning cephalad on +the left side of the lumbar vertebrae, as the fourth or ascending +duodenum, to reach the caudal surface of the transverse mesocolon near +its attachment, where the gut turns ventrad to form the duodeno-jejunal +angle and become continuous with the free small intestine. + +From the fact that the transverse duodenum is thus seen on each side of +the root of the mesentery it will be recalled that after rotation of the +primitive intestine the superior mesenteric artery crosses the +transverse portion of the duodenum to reach its distribution between +the leaves of the mesentery. Hence this portion of the small intestine +consists of a dextro- and sinistro-mesenteric segment. This intersection +of mesentery and duodenum marks the site of the primitive duodeno-colic +isthmus through which the superior mesenteric artery passed to reach its +distribution to the gut composing the embryonic umbilical loop. + +To the left of the ascending duodenum a portion of the caudal surface of +the pancreas will be seen, covered by the continuation of the caudal +leaf of the transverse mesocolon into the parietal peritoneum. The +consideration of this relation of peritoneum and pancreas will +profitably be deferred until we have studied the developmental changes +in the region of the dorsal mesogastrium and great omentum. + +In the angle between termination of the transverse colon and proximal +part of descending colon (splenic flexure) the caudal part of the +ventral surface of the left kidney will be felt. The disposition of the +peritoneum and its significance is the same as on the right side. +Inasmuch as we have already seen that the secondary parietal peritoneum +covering the dorsal abdominal wall on each side of the small intestine's +mesenteric attachment is derived from the primitive ascending and +descending mesocolon, it will be readily understood why the blood +vessels supplying the ascending and descending colon (arteria +ileo-colica, a. colica dextra, a. colica sinistra) are placed _behind_ +the parietal peritoneum, while the colica media, supplying the +transverse colon, runs between the layers of the transverse mesocolon. +Originally the same condition obtained for the two vertical colic +segments, but with the anchoring of these portions of the large +intestine and the adhesion of their mesocola to the parietal peritoneum +the blood vessels which formerly ran between the two layers of the +membrane, as long as it remained free, now appear as retroperitoneal +vessels placed beneath the parietal peritoneum derived secondarily from +the mesocola. + +This fact must be borne in mind in studying the arrangement of certain +folds and fossae of the parietal peritoneum which are now to be +considered. + +=Duodenal Fossae. Fossa of Treitz and Retro-peritoneal Hernia.=--The +peritoneal cavity of the cat can be used to great advantage in order to +obtain a clear idea of the formation of these folds and fossae, whose +relation to the so-called "retro-peritoneal hernia" has led to an +exaggerated elaboration of minute detail and a somewhat puzzling +terminology in human descriptive anatomy. + +=Directions for Examining the Folds and the Formation of the +Duodeno-jejunal Fossa in the Cat.=--Turn the omentum and the coils of +the small intestine cephalad out of the abdomen until they rest upon the +ventral thoracic wall. Press the large intestine over to the left side, +putting the mesocolon on the stretch until the parts are arranged as +shown in Fig. 160. The loop of the duodenum with the head portion of the +pancreas will be seen caudad of the liver and ventrad of the right +kidney. A well-marked peritoneal fold, somewhat sickle-shaped, with the +concavity of the free edge directed caudad and to the right, will be +seen extending from the convex border of the duodenum, directly opposite +the mesenteric or attached margin, to the right leaf of the mesocolon. +This fold indicates the beginning adhesion of the duodenum to the +mesocolic peritoneum, the first step toward the subsequent complete +fixation of the gut as it is found in man. + +[Illustration: FIG. 160.--Abdominal cavity of cat, with intestines +everted and elevated to show duodenal fold. (From a fresh dissection.)] + +[Illustration: FIG. 161.--Abdominal viscera of _Nasua rufa_, brown +coaiti. (From a fresh dissection.)] + +Fig. 161 shows the abdominal cavity of Nasua rufa, the brown +Coati-mundi, a South American arctoid carnivore, with the intestines +everted and turned to the left side. In this animal the large intestine +is very short, there is no caecum, the ileo-colic junction is only marked +on the surface by a pyloric-like constriction of the tube and in the +interior by the projection of a ring-valve (Fig. 408). + +The duodenal fold is very well developed, passing between the convex +surface of the duodenal loop and the adjacent right leaf of the short +mesocolon. + +In Primates, in which complete rotation of the intestine, on the plan of +the human development, takes place, still further and more extensive +agglutination of the serous surface of the duodenum to the peritoneum of +the mesocolon occurs. Fig. 162 shows the condition in _Hapale +vulgaris_, one of the marmosets. The ascending and descending mesocola +and the mesoduodenum of this animal are still free, but the surface of +the duodenum has become fastened to the opposed mesocolon. With fixation +of the hepatic flexure and adhesion of the ascending colon, such as +occurs in man, the duodenum is carried dorsad against the ventral +surface of the right kidney, and now anchoring of the duodenum, by +obliteration of the mesoduodenum and adhesion to the prerenal parietal +peritoneum, takes place as already detailed above. To return now to the +formation of the duodeno-jejunal fossa by means of this fold, as +illustrated in the cat. Perform the manipulations already described in +rotation of the intestine. The appearance of the parts then will be as +shown in Fig. 163. The large intestine is drawn over so as to represent +the human ascending and transverse colon in one segment, the descending +colon in the other, and the mesocolon appears correspondingly as +transverse and descending. In other words the cat's intestines as +arranged in the figure would represent the stage in the human +development in which caecum and beginning of large intestine are still +subhepatic in position ventrad of the right kidney, before +differentiation of ascending and transverse colon by descent of caecum +into right iliac fossa. + +[Illustration: FIG. 162.--Abdominal viscera of _Hapale vulgaris_, the +marmoset. (Columbia University Museum, No. 1818.)] + +[Illustration: FIG. 163.--Abdominal viscera of cat; intestines rotated +and turned to the right to show duodenal fold. (From a fresh +dissection.)] + +In the human subject, as we have seen, the transverse mesocolon obtains +a secondary attachment to the background of the abdominal cavity, its +caudal surface remaining free. + +The descending mesocolon turns its original right leaf ventrad, its left +leaf dorsad, and the latter adheres to the primitive parietal peritoneum +covering the left lumbar region and ventral surface of left kidney. This +area of adhesion extends up to and usually involves the dorsal surface +of the descending colon, anchoring the same in the left lumbar region, +down to the point where the sigmoid flexure begins and where the +original mesocolon again appears free. + +In the cat, therefore, with the intestines arranged to correspond to the +course of the human large intestine after rotation has been +accomplished, the lines representing the peritoneal human adhesions +should be fixed, as shown in the schema, Fig. 159: AB, line of secondary +attachment after rotation resulting in the formation of the "root" of a +free transverse mesocolon. BC, line of limit of secondary adhesion to +the original parietal peritoneum involving the entire left (now dorsal) +layer of the descending mesocolon and the dorsal surface of the +descending colon, resulting in the fixation of the latter part of the +large intestine. + +This establishes, as already stated, a secondary parietal peritoneal +surface in the left lumbar region derived from the original right leaf +of the descending mesocolon. Inasmuch as the inferior mesenteric vessels +originally passed to the descending colon between the layers of the +mesocolon they will now apparently be placed beneath the (secondary) +parietal peritoneum of the left lumbar region. + +If now the duodenal fold in the cat be examined after rotation of the +intestine it will be found presenting the original relations (Figs. 160 +and 163), viz., passing from the convex margin of that portion of the +duodenal loop which would correspond to the human fourth or ascending +portion, to the original right layer of the mesocolon, which in man +becomes secondarily converted into the parietal peritoneum of the left +lumbar region. Hence the connections of the fold are as follows: + +_On the right_: ventral surface of the ascending duodenum. + +_On the left_: right layer of mesocolon (secondary lumbar parietal +peritoneum in the adult human subject). + +_Cephalad_ it abuts against the caudal layer of the transverse mesocolon +along the line which would correspond to the root of the mesocolon in +the adult human subject. + +The concave _caudal_ edge is free and bounds the entrance into a fossa, +the "superior duodenal fossa" of anthropotomy. This fossa opens caudad +and extends cephalad to the root of the transverse mesocolon. The +ventral and left wall of the fossa is formed by the fold in question, +its background by the mesocolon (right leaf); to the right the left +circumference of the ascending duodenum enters into the formation of +the fossa, and its fundus is formed by the confluence of the fold and of +the caudal layer of the transverse mesocolon. The inferior mesenteric +vessels are found near the left margin of the entrance into the fossa. + +Fig. 164 shows the appearance of the fold in _Nasua rufa_ after rotation +of the intestine. The short course of the large intestine in this +animal, and the consequent reduction of the mesocolon, brings the fold +much below the level which it occupies in the cat. + +[Illustration: FIG. 164.--Abdominal viscera of _Nasua rufa_, the brown +coaiti, showing the position of the duodenal fold after rotation of the +intestine. (From a fresh dissection.)] + +If we now look for the corresponding structures in man we will find +certain modifications depending chiefly upon still closer adhesion +between duodenum and the mesocolon which is destined to become the left +parietal peritoneum after anchoring of the descending colon. We have +already encountered an example of such closer connection in the marmoset +shown in Fig. 162. + +In all cases the "superior duodenal" fold, corresponding to the fold +just encountered in the cat, is the original condition, and the duodenal +fossa consequently opens caudad. In many instances this will be the only +fold and fossa encountered in the adult human subject. In other +instances more extensive duodeno-mesocolic adhesions result in the +addition of an "inferior fold," bounding a fossa the entrance into which +is directed cephalad toward the transverse mesocolon. Such a condition +is seen in Fig. 165 taken from a foetus at term. The duodenal fossa in +this case is bounded by an "upper" and "lower" duodenal fold continuous +with each other on the left side, but separated on the right at their +attachment to the duodenum. It will be seen that the inferior mesenteric +vein runs in the left margin of the fold, following along the left +border of the entrance into the fossa. A segment of the colica sinistra +artery may occupy the same position. This position of the vein, or +artery, or of both vessels, is not the cause leading to the formation of +the duodenal fossa, but is more or less accidental and variable. In many +cases the vessels run at some distance from the folds bounding the +fossa. + +[Illustration: FIG. 165.--Abdominal viscera of human foetus at term, +arranged to show duodenal folds and fossa. The jejuno-ileum, ascending +and transverse colon have been removed. (Columbia University Museum, No. +1819.)] + +In some subjects the "inferior" fold is the only one found, and the only +duodenal fossa then encountered looks cephalad. This condition, when +associated with the course of the inferior mesenteric vessels in the +free edge of the fold, constituted the classical "fossa +duodeno-jejunalis" of Treitz, and is described as "Treitz's fossa." + +Fig. 166 shows the condition in which only a small inferior fold +attaches itself to the termination of the transverse duodenum. There is +practically an entire absence of duodenal or duodeno-jejunal folds and +fossae. The inferior mesenteric vessels course under cover of the +mesocolic secondary parietal peritoneum, but do not produce a fold. + +[Illustration: FIG. 166.--Abdominal viscera of human foetus at term. +(Columbia University Museum, No. 1820.)] + +[Illustration: FIG. 167.--Abdominal viscera of adult human subject, +showing duodenal folds and fossa. (From a fresh dissection.)] + +Fig. 167, from an adult human subject, illustrates the further +development of the fossa from the foetal conditions shown in Fig. 165. +The well-marked duodenal fossa is bounded by a superior and inferior +duodenal fold, uniting laterally in a crescentic margin containing a +segment of the inferior mesenteric vein and colica sinistra artery. The +lower division of the peritoneal recess thus produced corresponds to the +typical (vascular) "fossa of Treitz." Mesally the projection of the +fourth portion of the duodenum bounds the fossa. + +In Fig. 168, also taken from an adult human subject, an extensive +duodenal recess is bounded in the same way by a superior and inferior +duodenal fold. In the interior of the fossa a third duodenal +reduplication of the peritoneum ("intermediate duodenal fold") is seen, +as is also the trunk of the inferior mesenteric vein, while the main +trunk of the colica sinistra artery courses laterally behind the +secondary mesocolic parietal peritoneum near the margin of the +descending colon. + +[Illustration: FIG. 168.--Abdominal viscera of adult human subject, +showing duodenal folds and fossa. (From a fresh dissection.)] + +It will be seen that the freedom of the ascending or fourth portion of +the duodenum depends largely upon the disposition and extent of these +folds. Inasmuch as they are the product of varying degrees of adhesion +of this segment of the intestine they are subject to great individual +variations and have given rise to an unnecessary and complicated +classification of the duodenal folds and fossae. The close relation +maintained between the duodeno-jejunal angle and the caudal layer of the +transverse mesocolon near its root at times leads to the production of +a peritoneal fold connecting this membrane with the duodeno-jejunal +knuckle of intestine (duodeno-jejunal or mesocolic fold) and may result +in the formation of a duodeno-jejunal or mesocolic fossa of the +peritoneum. An instance of this fold is seen in Fig. 168. + +The importance of the duodenal fossae, and of similar peritoneal recesses +in other parts of the abdominal cavity, is founded on the fact that by +gradual enlargement they may lodge the greater part of the movable small +intestine in their interior, leading to the formation of intra- or +retro-peritoneal herniae.[3] + +[3] For full details of the anatomical and pathological conditions +involved consult B. G. A. Moynihan "On Retro-peritoneal Hernia"--London, +1899. + +=Fossa Intersigmoidea.=--A second peritoneal pocket or fossa is +encountered in the region of the sigmoid flexure and its mesocolon. The +formation of this fossa is closely associated with the adult disposition +of the sigmoid mesocolon as part of the original primitive vertical +dorsal mesentery. In the typical arrangement of the parts the sigmoid or +omega loop of the large intestine has a free mesocolon. The adhesion of +the descending mesocolon to the parietal peritoneum usually ceases along +a line drawn horizontally from the lateral margin of the left psoas at a +level with the crest of the ilium to the medial side of the iliac +vessels. This line, along which the mesocolon ceases to be adherent to +the parietal peritoneum, joins the attachment of the distal portion of +the sigmoid mesocolon, which partially retains its primitive vertical +origin to the dorsal midline, at a right angle. This angle is the site +of the _intersigmoid fossa_, the entrance into which is seen usually as +a round opening of variable size on elevating the sigmoid flexure and +putting its mesocolon on the stretch. Fig. 159 shows the area of +adhesion between the primitive descending mesocolon and the parietal +peritoneum (from C mesad) which results in the formation of a free +mesocolon for the sigmoid flexure. Frequently in the angle formed by the +horizontal and vertical line of attachment of the sigmoid mesocolon a +non-adherent strip of the primitive mesocolon roofs in a more or less +extensive intersigmoid fossa, whose fundus is directed upwards and +inwards. + +=Caecum, Appendix and Ileo-colic Junction.=--Several peritoneal fossae and +folds are found in the ileo-colic region in connection with the caecum, +appendix and termination of the ileum. The practical importance of this +portion of the intestinal tract and the great morphological interest +which attaches to the same make it worth while to consider its anatomy +in a separate chapter. + + + + +PART II. + +ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT OF THE ABDOMEN. + + +We have already seen that the transverse colon and mesocolon effect a +general division of the adult human abdominal cavity into a cephalic +supra-colic compartment, situated between the diaphragm and the level of +the transverse colon and mesocolon, comprising in general the +hypochondriac and epigastric regions, and a larger caudal infra-colic +space which includes the entire rest of the abdominal cavity and is +continued caudad into the pelvic cavity. The arrangement of the +peritoneum and viscera in this latter space has just been considered. +The fact will be recalled that the second or descending portion of the +duodenum, passing dorsad of the hepatic colic flexure, forms so to speak +the visceral connection between the portions of the alimentary tube +situated in the supra-colic compartment and those situated in the +infra-colic space. The fixation of this segment of the duodenum and its +consequent secondary retroperitoneal position in the adult human subject +masks this continuity of the alimentary canal to a certain extent so +that it requires more than a superficial examination in order to trace +correctly the course of the duodenum from the pylorus to the +duodeno-jejunal angle, dorsad of the colon, root of transverse mesocolon +and mesentery, and under cover of the secondary parietal peritoneum. + +We have now to turn our attention to the viscera contained in the +cephalic or supra-colic compartment of the abdomen and to consider the +disposition of the serous membrane investing them and connecting them +with each other and with the abdominal parietes. + +The visceral contents of the supra-colic compartment comprise the +liver, pancreas, spleen, stomach and the proximal portion of the +duodenum, including the hepatic angle and the supra-colic part of the +descending duodenum. Less directly the cephalic portions of the right +and left kidney and the corresponding suprarenal capsules belong to this +visceral group. + +In this region of the abdomen we meet with the most extensive +modifications of the primitive dorsal peritoneal membrane, producing +conditions which, considered without reference to development and +comparative anatomy, are complex and difficult of comprehension. These +changes lead to the formation of the so-called "lesser sac," a term +which in some respects is unfortunate as it implies a more complete +degree of separation from the general peritoneal cavity or "greater sac" +than actually exists. + +In order to clearly understand the adult arrangement of the peritoneum +in this region it is advisable to consider the subject in two distinct +subdivisions, dealing successively with the two cardinal facts which +contribute to effect the change from the simple primitive to the +complicated adult condition. + +These two main elements are: + +1. Developmental changes in the position of the stomach, alterations in +the disposition of the proximal part of the primitive dorsal mesentery +attached to the stomach, and the development of pancreas and spleen in +connection with this membrane. + +2. The development of the liver and the successive stages in the +production of the final adult vascular and serous relations of this +organ. + +=1. Stomach and Dorsal Mesogastrium.=--We have already considered the +early stages in the differentiation of the stomach from the primitive +intestinal tube of uniform caliber (p. 40). It will be recalled that the +stomach at a certain period, while it already presents the main +structural features familiar in the adult organ, occupies a vertical +position in the abdominal cavity, turning its concave margin (lesser +curvature) ventrad, while the convex dorsal border (greater curvature) +is directed toward the vertebral column, being attached to the same by +the layers of the proximal part of the primitive dorsal mesentery. At +this time the stomach presents right and left surfaces, and the +oesophageal entrance is at the highest or cephalic point of the organ, +while the pyloric transition to the small intestine occupies the distal +caudal extremity. + +The primitive dorsal mesentery, as already stated, passes as a thin +double-layered membrane between the ventral surface of the vertebral +column and the dorsal border of the stomach, which, as we will presently +see, becomes during the later stages of development the caudal (lower) +margin or greater curvature. + +It will be seen that the embryonic differentiation of the intestinal +tract into successive segments justifies the application of a +terminology based on this differentiation to the corresponding portions +of the primitive common dorsal mesentery. + +Thus the proximal portion extending between the vertebral column and the +dorsal border or greater curvature of the stomach becomes the +_mesogastrium_; we differentiate this portion still further as the +"_dorsal mesogastrium_" to distinguish it from a "_ventral +mesogastrium_" which we will presently encounter in considering the +development of the liver and the connected peritoneum. + +In the same way the section of the primitive common dorsal mesentery +attached to the duodenal loop becomes the _mesoduodenum_, that connected +with the mobile part of the small intestine (jejuno-ileum) the +_mesentery_ proper, while the portion passing to the colon forms the +_mesocolon_, to be subsequently still further subdivided, after the +different segments of the large intestine have become mapped out, as the +_ascending_, _transverse_ and _descending mesocolon_, the +_mesosigmoidea_ and the _mesorectum_. + +In tracing the development of the adult human peritoneum it is well to +consider certain stages, which we will find illustrated by the permanent +conditions presented by some of the lower vertebrates: + +These stages comprise: + +(_a_) Changes in the position of the stomach. + +(_b_) Changes in the direction and extent of the dorsal mesogastrium. + +(_c_) Development of the pancreas and spleen in connection with the +mesogastrium. + + +A. Changes in the Position of the Stomach. + +The primitive position of the organ above outlined (p. 41) is changed +during the course of further development by a twofold rotation. + +1. The primitive vertical position, in which the oesophageal entrance +occupies the highest cephalic extremity, while the pyloric opening is at +the opposite caudal end, is exchanged for one directed more +transversely, approximating the two gastric orifices to the same +horizontal level. In human embryos of 13.9 mm. the fundus has already +descended, the pylorus moving cephalad and to the right, while the +cardia becomes shifted more to the left. At the same time the greater +growth and prominence of the convex border or greater curvature becomes +marked in comparison with the relatively short extent of the opposite +margin or lesser curvature. + +2. Coincident with this change in position is a rotation around the +vertical axis, by means of which the original left side of the stomach +is turned ventrad, becoming the ventral or "anterior" surface, while the +original right surface of the organ now looks dorsad toward the +vertebral column, becoming the dorsal or "posterior" surface of human +anatomy. The oesophageal or cephalic end is placed to the left of the +median line, while the caudal or pyloric end is situated on the right +side (Figs. 169 and 170). + +[Illustration: FIGS. 169, 170.--Two front views of the entodermal canal. +(Minot, after His.)] + +[Illustration: FIG. 169.--Embryo Sch. 1 of His.] + +[Illustration: FIG. 170.--Embryo Sch. 2 of His.] + +The original ventral border, now the "lesser curvature" or "upper +border," looks cephalad and to the right, toward the caudal surface of +the liver, while the original dorsal border, as the "greater curvature" +or "lower border" is directed in the main caudad and to the left. + +The prominence of this border is still further increased by the greater +development of the stomach to the left of the oesophageal entrance +resulting in the formation of the "fundus" or "great cul-de-sac." + +This rotation of the stomach explains the asymmetrical position of the +vagus nerve in the adult, the left side of the embryonic stomach, +innervated by the left vagus, becoming the "anterior" surface of adult +descriptive anatomy and _vice versa_. + +It will be readily appreciated that a comparatively flat organ like the +stomach, will, as long as it occupies a sagittal position, with right +and left surfaces, help to divide the upper part of the abdominal cavity +to a certain extent into a right and left half, even if the peritoneal +connections of the organ are left out of consideration. As soon, +however, as the above-described changes in position take place and the +surfaces of the stomach are directed ventrad and dorsad, the relative +arrangement and extent of this right and left abdominal space becomes +altered by the different disposition of the septum, _i. e._, the +stomach. The original right side of the organ is now directed dorsad, +and the rotation of the organ has created a space between this dorsal or +"posterior" surface of the stomach and the background of the abdominal +cavity, which is the inception of the "lesser peritoneal cavity" or +retrogastric space. We will find that this space becomes well defined +and circumscribed by the peritoneal connections of the stomach, but we +will realize, even at this stage, that the _dorsal_ surface of the +stomach will form a part of the general _ventral_ wall of the lesser +peritoneal space. + +On the other hand, the partial division of the abdomen into a right and +left half, effected by the stomach in its primitive sagittal position, +disappears after rotation of the organ. We now pass uninterruptedly from +left to right across the ventral (original _left_) surface of the +stomach. + + +B. Changes in the Direction and Extent of the Dorsal Mesogastrium. + +The effects of the altered position of the stomach on the disposition of +the abdominal space have just been considered in relation to the organ +itself, without reference to its natural connections with the parietes +and with adjacent viscera. Their true significance and their influence +on the adult anatomical arrangement of the abdomen is, however, only +appreciated when the changes in the arrangement of the peritoneal +membrane which they involve, are taken into account. + +The dorsal mesogastrium changes more than any other portion of the +peritoneum in the course of development. It not only becomes displaced +and altered in direction by the rotation of the stomach, but in addition +it grows so extensively that it finally hangs down like an apron over +the entire mass of small intestines, forming the great omentum. + +If we begin with the primitive disposition of the sagittal stomach and +dorsal mesogastrium shown in Fig. 171 it will be observed that both +structures together actually divide the dorsal portion of the abdominal +cavity into symmetrical right and left halves (Fig. 172). + +[Illustration: FIG. 171.--Schematic representation of dorsal +mesogastrium before rotation of stomach.] + +[Illustration: FIG. 172.--Semi-diagrammatic representation of +mesogastrium in human embryo of the sixth week. (Kollmann.)] + +After rotation of the stomach (Fig. 173) the mesogastrium loses its +original sagittal direction. It follows the altered position of the +original dorsal border of the stomach, which has now become the caudal +margin or "greater curvature," by turning caudad and to the left, being +at the same time considerably elongated. This occurs during the second +month. Hence the dorsal mesogastrium, after leaving the vertebral +column, turns ventrad and to the left to reach its gastric attachment +along the greater curvature. This is the first indication of the +formation of the great omental or epiploic bursa. + +[Illustration: FIGS. 173-175.--Schema of dorsal mesogastrium after +rotation of stomach.] + +[Illustration: FIG. 173.--Early stage.] + +[Illustration: FIG. 174.--Later stage, extension of mesogastrium beyond +stomach to left, with fundus of blind retrogastric pouch thus created at +X.] + +[Illustration: FIG. 175.--After adhesion over area of dotted line +between dorsal mesogastrium and primitive parietal peritoneum. Secondary +line of transition from dorsal mesogastrium to parietal peritoneum at +X.] + +The stomach is here considered as developing in situ and as influencing +by its growth and change of position the arrangement and direction of +the peritoneal layers with which it is connected. As a matter of fact it +is well to note that the stomach at first lies above the primitive +diaphragm or septum transversum, migrating, however, at an early period +into the subhepatic abdominal position. This migration produces a +corresponding increase in the length of the oesophagus (Fig. 34) and the +stomach, in consequence of this change in position, acquires its ventral +and dorsal mesogastrium. For the purpose of explaining the adult +peritoneal relations of the organ it is, however, more +convenient to regard the stomach as an abdominal organ from the +beginning and to deal with the subsequent changes in position from this +standpoint. The inaccuracy is slight and renders the comprehension of +the succeeding stages easier. + +It will be noticed (Fig. 173) that the rudimentary retro-gastric space +or "lesser peritoneal sac" is bounded ventrally by the dorsal (the +primitive _right_) surface of the stomach, while its dorsal boundary is +furnished by the ventral (originally _right_) layer of the dorsal +mesogastrium. + +In the primitive condition, therefore, dorsal mesogastrium and stomach +form together a straight line sagittal in direction and placed in the +median plane of the body. As the result of the developmental changes +above outlined this straight line becomes bent at the point where the +mesogastrium reaches the stomach (Fig. 173, x). The two component +elements of the line (stomach and mesogastrium) hinge on each other +here, and the angle which they form opens to the right. + +The changes which are to be observed in the later stages depend +principally upon a peculiar feature characteristic of the development of +the dorsal mesogastrium. This feature consists in the extreme redundancy +of the membrane which grows out of proportion to the requirements of its +visceral connections, and to a certain extent becomes independent of the +direct mechanical purpose of carrying blood vessels to the viscera. +Hence in a transverse section at this period (Figs. 174 and 175) the +mesogastrium no longer passes in a direct line between its points of +attachment, viz. the greater curvature of the stomach and the vertebral +column, but extends beyond the stomach to the left. We will appreciate +the significance of this extensive growth of the mesogastrium especially +in considering the development of the spleen and pancreas. For the +present it will suffice to note (Figs. 174 and 175) that the growth has +carried the mesogastrium well to the left of the stomach, consequently +the retrogastric space is now bounded toward the left by the bend which +the original right leaf of the primitive sagittal mesogastrium takes in +order to reach its gastric attachment. The retrogastric space therefore +terminates toward the left in a blind pocket formed by this +reduplication of the mesogastrium. + +One more factor is to be taken into consideration, namely the tendency, +already noted, of peritoneal surfaces to become adherent to each other. +Such adhesion involves the apposed surfaces of the mesogastrium and of +the primitive parietal peritoneum to the left of the vertebral column. +The dorsal (original _left_) layer of the mesogastrium adheres to the +parietal peritoneum covering the left side of the abdominal background +and the cephalic portion of the ventral surface of the left kidney up to +the end of the blind pouch which forms the extreme left limit of the +retrogastric space. Hence, after this process of adhesion is completed, +the dorsal wall of the retrogastric space is lined by secondary parietal +peritoneum covering the left kidney (original right leaf of primitive +mesogastrium) (Fig. 175). We obtain (Fig. 175 at x) an apparent +continuity of the parietal peritoneum with that portion of the +mesogastrium which, derived from the original left layer of the +membrane, appears now to extend, as the ventral one of two layers, +between the stomach and the abdominal parietes near the lateral border +of the left kidney. (Primitive gastro-splenic omentum.) + +It should be remembered that the disposition of the peritoneum just +indicated is modified by the development of the pancreas and spleen, +both of which organs are intimately associated with the mesogastrium. +The foregoing statements and diagrams are therefore merely given for the +purpose of affording a general view of the extent, growth and changes of +the dorsal mesogastrium before proceeding to consider the development of +the pancreas and spleen in and from the membrane itself. + +[Illustration: FIG. 176.--Schematic ventral view of stomach, duodenum, +and dorsal mesogastrium, after rotation of stomach and extension of +omental bursa caudad beyond greater curvature of stomach. The ventral +mesogastrium is detached along the lesser curvature.] + +[Illustration: FIG. 177.--Semi-diagrammatic representation of peritoneal +membrane in human embryo. (After Kollmann.)] + +In the view directly from in front the redundancy of the peritoneum +forming the mesogastrium is shown in Figs. 176 and 177. Just as the +membrane extends further to the left than required by its visceral +connection with the stomach, so the downward growth exceeds the demand +made by the rotation of the attached border (greater curvature) caudad +and to the left. The mesogastrium, forming, as it now does, the great +omentum, enlarges in descending toward the transverse colon (Fig. 177). +The bag thus formed can be distended with air in a foetus of from 8 to 9 +cm. vertex-coccygeal measure, as shown in the figure. Consequently in +sagittal section the membrane is seen to extend caudad beyond the level +of the greater curvature, and must turn on itself and pass again +cephalad in order to reach the stomach (Fig. 178). By reason of this +excessive growth the limits of the primitive retrogastric space are +enlarged, not only toward the left, but more especially in the caudal +direction. The bend made by the mesogastrium in returning to the stomach +forms the blind extremity of a pouch which continues the retrogastric +space caudad beyond the stomach, and whose dorsal and ventral walls are +formed by the reduplicated mesogastrium. This pocket or pouch +constitutes the _omental_ or _epiploic bursa_ of the lesser peritoneal +cavity, for the great omentum is the direct product of this redundant +growth of the mesogastrium caudad. It will be observed that the great +omentum is made up of four peritoneal layers, the folding of the +double-layered mesogastrium naturally producing this result. The first +or ventral and the fourth or dorsal layer are derived from the original +left layer of the primitive sagittal mesogastrium; the intermediate +second and third layers, separated from each other at this stage by the +cavity of the omental bursa, are products of the primitive right leaf of +the mesogastrium. Since the entire retrogastric space with its +extensions becomes the "lesser cavity" of the human adult peritoneum, it +will be seen that its serous membrane is derived from the original right +leaf of the mesogastrium (second and third omental layers). After the +above-described adhesion of the mesogastrium to the parietal peritoneum +overlying the ventral surface of the left kidney, the membrane would be +traced in sagittal section (Fig. 179) from the dorsal surface of the +stomach caudad, lining the interior of the omental bursa (second layer) +to the turn or blind end of the pouch; thence cephalad as the third +omental layer, forming the dorsal wall of the epiploic bursa, to invest, +as secondary parietal peritoneum, the cephalic segment of the ventral +surface of the left kidney. + +[Illustration: FIG. 178.--Schematic sagittal section through stomach and +dorsal mesogastrium, after rotation and formation of omental bursa.] + +[Illustration: FIG. 179.--Schematic sagittal section through stomach and +dorsal mesogastrium after adhesion to prerenal parietal peritoneum.] + + +C. Development of Spleen and Pancreas in the Dorsal Mesogastrium and +Changes in the Disposition of the Great Omentum. + +In order to obtain a correct conception of the adult human conditions it +is finally necessary to consider the development of the spleen and +pancreas in their connection with the dorsal mesogastrium and to note +the changes which are produced by adhesion of portions of the great +omentum to adjacent serous surfaces. It will be advisable to discuss +these subjects at first separately, and to subsequently combine all the +facts in an attempt to gain a correct impression of their share in +determining the disposition of the adult human peritoneum. + +=1. Development of Spleen.=--The spleen develops from the mesoderm +between the layers of the dorsal mesogastrium, near its point of +accession to the greater curvature, in the region of the subsequent +fundus. It has therefore, like the stomach, originally free peritoneal +surfaces. After rotation of the stomach the organ lies between the two +layers of the membrane at the extreme left end of the retrogastric space +(Fig. 180). + +[Illustration: FIG. 180.--Schematic transverse section of the abdomen, +showing early stage of development of spleen from extreme left end of +dorsal mesogastric pouch.] + +=Vascular Connections.=--The splenic artery accedes to the mesal surface +of the spleen from the vessel which originally passed directly to the +dorsal border (subsequent greater curvature) of the stomach, between the +layers of the mesogastrium. + +With the further growth of the spleen the segment of this vessel +situated between its origin from the coeliac axis and the hilum of the +spleen becomes relatively larger, forming the adult splenic artery, +while the continuation of the original vessel to the greater curvature +of the stomach appears now as a branch of the splenic artery, viz., the +arteria gastro-epiploica sinistra. + +Through the development of the spleen the dorsal mesogastrium has been +subdivided into a proximal longer vertebro-splenic, and a distal shorter +gastro-splenic segment. The former, as we have seen, loses its identity +as a free membrane in the human adult, by fusing with the parietal +peritoneum investing the ventral surface of the left kidney. Hence, +after this adhesion has taken place, the splenic artery courses from the +coeliac axis to the spleen behind peritoneum which functions as part of +the general parietal membrane, but which is derived from the original +right leaf of the proximal vertebro-splenic segment of the primitive +mesogastrium (Fig. 181). On the other hand the distal segment of this +membrane, beyond the spleen, remains free, carrying, as the +gastro-splenic omentum, the left gastro-epiploic artery between its +layers from the splenic artery to the greater curvature of the stomach. + +[Illustration: FIG. 181.--Schematic transverse section of the abdomen, +showing later stage of development of spleen and arrangement of +peritoneum after adhesion of dorsal layer of mesogastrium and primitive +prerenal parietal peritoneum.] + +The lateral limit of the area of adhesion between mesogastrium and +parietal peritoneum is situated along the lateral border of the left +kidney. Hence, in the final condition of the parts, the main splenic +vessels at the hilum are situated between two peritoneal layers of which +the ventral (Fig. 181) appears as the parietal peritoneum forming the +dorsal wall of the retro-gastric space, while the dorsal layer (Fig. +181) forms a reflection from the mesal surface of the spleen, along the +dorsal margin of the hilum, to the adjacent lateral border of the left +kidney (lieno-renal ligament) and to the diaphragm. At this point of +adhesion subsequently firmer strands of connective tissue develop in the +serous reduplication forming the _ligamentum phrenico-lienale_ of +systematic anatomy. This process of adhesion takes place during the +second half of intra-uterine life. A connection with the colon, produced +by adhesion of the mesogastrium to the splenic flexure of the large +intestine, forms the adult _lig. colico-lienale_, while a similar +adhesion between great omentum, transverse mesocolon and phrenic +parietal peritoneum just caudad of the spleen, gives rise to the +_colico-phrenic_ or _costo-colic "supporting" ligament_ of the spleen. + +On the other hand, the ventral one of the two layers constituting the +gastro-splenic omentum and including between them the left +gastro-epiploic artery, is formed by the distal part of the primitive +left layer of the mesogastrium, while the dorsal layer of the same fold +is the portion of the primitive right layer beyond the spleen, which +has not been converted into secondary parietal peritoneum, but forms now +part of the ventral wall of the lesser peritoneal sac between the spleen +and the stomach (Fig. 181) (lig. gastro-lienale). Since, therefore, the +gastro-splenic omentum is a specialized part of the fully-developed +dorsal mesogastrium, and since we have seen that the great omentum is +formed directly by the excessive growth of this membrane caudad, it is +not difficult to understand why in the adult human subject the ventral +layer of the gastro-splenic omentum is directly continuous with the +ventral layer of the great omentum along the greater curvature of the +stomach to which both are attached. The dorsal layer of the +gastro-splenic omentum would, in the same way, be continuous with the +second layer of the great omentum, lining the ventral wall of the +omental bursa, if it were not for the fact that in the adult adhesions +usually obliterate the cavity of the bursa. + +Fig. 182 shows the stomach, left kidney, spleen and splenic flexure of +the colon hardened in situ and removed from the body of a two-year-old +child. The great omentum has been divided along the line of adherence to +the transverse colon. + +[Illustration: FIG. 182.--Part of the abdominal viscera of child, two +years old, hardened _in situ_ and removed from body. The great omentum +has been detached along the line of the transverse colon. (Columbia +University, Study Collection.)] + +[Illustration: FIG. 183.--The same preparation with the spleen removed, +showing lines of peritoneal reflection on mesial surface of the organ.] + +In Fig. 183 the spleen has been removed from the preparation by division +of its peritoneal and vascular connections, and is shown in its mesal +aspect (gastric and renal surfaces, intermediate margin and hilum). It +will be seen that the peritoneal reflections are arranged in the form of +two concentric elliptical lines. The two ventral lines form the +gastro-splenic omentum and correspond to the reflection of the +peritoneum from spleen to left end of stomach carrying the gastric +branches derived from the splenic artery. The third line from before +backwards results from the division of the secondary parietal peritoneum +of the lesser sac, covering splenic artery, and ventral surface of +pancreas and derived from the dorsal mesogastrium; while the most dorsal +fourth line represents the divided reflection of the peritoneum from the +renal surface of spleen to lateral border of left kidney and diaphragm +(lig. lieno-renale). + +Between the second and third lines of peritoneal reflection appears the +portion of the mesal surface of the spleen in contact with and invested +by the extreme left end of the lesser peritoneal sac. + +Fig. 184, taken from an adult human subject with the viscera hardened in +situ, shows the left or splenic extension of the lesser peritoneal +cavity. + +[Illustration: FIG. 184.--Upper abdominal viscera of adult human +subject, hardened _in situ_, with liver and colon removed and stomach +turned up. (Columbia University, Study Collection.)] + +=2. Development of the Pancreas.=--The pancreatic gland is derived from +the hypoblast of the enteric tube. The secreting epithelium and that +lining the ducts of the adult gland is formed by budding and +proliferation of the intestinal epithelium. The gland develops primarily +from two outgrowths which are at first separate and distinct from each +other. + +1. The proximal and dorsal bud grows directly from the hypoblast lining +the duodenum immediately beyond the pyloric junction. + +In embryos of 8 mm. (four weeks) (Fig. 185) it appears as a small +spherical outgrowth connected by a slightly narrower stalk with the +epithelial intestinal tube. + +[Illustration: FIG. 185.--Pancreatic and hepatic buds of human embryo of +four weeks. (Kollmann.)] + +2. The distal and ventral outgrowth is separated from the preceding and +is from the beginning closely connected with the similar embryonic +outgrowth from the enteric tube which is to form the liver. This portion +of the pancreas is, strictly speaking, derived primarily from the +epithelium of the primitive hepatic duct and not directly from the +duodenum. This primary arrangement of the gland, being formed of two +main collections of budding hypoblastic cells, corresponds to the adult +system of the pancreatic excretory ducts. The proximal or dorsal +outgrowth furnishes that portion of the head of the gland whose +excretory system terminates in the _secondary pancreatic duct_ or _duct +of Santorini_, while the distal (ventral) outgrowth includes within its +area the termination of the principal pancreatic duct or _canal of +Wirsung_, which is closely connected with the end of the common +bile-duct at the intestinal opening common to both (Figs. 186-187). The +method of union of the two pancreatic outgrowths and their respective +share in building up the adult gland explains the usual adult +arrangement of the excretory system and its variations. + +[Illustration: FIG. 186.--Pancreatic buds of human embryo of five weeks. +(Kollmann, after Hamburger.)] + +[Illustration: FIG. 187.--Pancreatic buds of human embryo of six weeks. +(Kollmann, after Hamburger.)] + +In the embryo of five weeks (Fig. 186) the two portions have grown in +length. The dorsal or proximal outgrowth, developing between the layers +of the mesoduodenum, is at this time the larger of the two, composed of +a number of glandular vesicles clustered around the stalk represented by +the parent duct. + +The distal or ventral pancreatic growth, connected with the liver duct, +is as yet small and presents only a few vesicular appendages. The duct +of this portion empties in common with the hepatic duct into the +duodenum. + +In embryos of the sixth to seventh week (Fig. 187), the two glandular +outgrowths have become connected with each other at a point which +corresponds exactly to the divergence of the duct of Santorini from the +main pancreatic duct (canal of Wirsung) in the adult gland (Fig. 188). + +[Illustration: FIG. 188.--Human adult. Corrosion of pancreatic and +common bile-ducts: ventral view. (Columbia University Museum, No. +1712.)] + +The secondary pancreatic duct (of Santorini) of the adult corresponds to +that section of the proximal or larger embryonic outgrowth situated +between the intestine and the point where the two glandular diverticula +fuse with each other. Hence the canal of Wirsung in the adult is a +compound product. It includes the duct system developed, in connection +with the bile duct, in the head of the gland, forming the intestinal +termination of the main duct. Its distal body portion on the other hand +is derived from the duct system of the originally larger proximal +outgrowth, including the entire peripheral portion which has become +secondarily added to the duct of the ventral outgrowth to form together +with it the canal of Wirsung. On the other hand the proximal portion of +the duct system of this originally larger part becomes secondarily +differentiated as the duct of Santorini. + +Fig. 188 shows the normal adult arrangement of the pancreatic and +biliary ducts in a corrosion preparation of the canal. + +The duct of Santorini in this case opened by a separate orifice into the +duodenum above the common opening of the biliary and pancreatic ducts +(cf. p. 113). + +=Explanation of Adult Arrangement of Human Pancreatic Ducts and Their +Variations Dependent Upon the Embryonic Development.=--The smaller +distal embryonic outgrowth is, as we have seen, from its inception in +close connection with the duodenal end of the common bile-duct (Fig. +185). + +The proximal outgrowth, situated nearer to pylorus and derived directly +from the duodenal epithelium, is the larger and forms the greater part +of the bulk of the adult pancreas (Figs. 186, 187). + +If, notwithstanding this primitive arrangement, the distal duct (canal +of Wirsung) appears as the main pancreatic duct in the adult, while the +proximal (duct of Santorini) is secondary, this depends upon a union of +the products of the two outgrowths in such a manner that the greater +part of the duct system of the proximal and larger portion is +transferred to the distal duct to form the adult canal of Wirsung, while +the smaller segment of the proximal duct, between its opening into the +duodenum and the point of fusion of the two outgrowths, forms the adult +secondary duct of Santorini. This duct opens usually into the duodenum +upon a small papilla situated about 2.5 cm. above the common duodenal +termination of the bile-duct and canal of Wirsung (papilla Vateri) (Fig. +193). The duct of Santorini usually tapers toward the duodenal opening +from its point of departure from the main duct, its caliber gradually +diminishing in the direction indicated, so that it is smaller at the +duodenal opening than at the point of confluence with the main duct +(Fig. 189). Hence the secretion from the proximal head portion of the +pancreas, conveyed by this duct and its tributaries, passes usually into +the main pancreatic duct and not directly into the intestine through the +duodenal opening of the duct of Santorini. The latter is, however, thus +enabled to vicariously take upon itself the conduct of the pancreatic +secretion in cases of obstruction or obliteration of the main duct +(calculi, ulcers, cicatrices, etc.). In these cases of obstruction of +the main duct the duct of Santorini enlarges and performs its functions. + +[Illustration: FIGS. 189-192.--Series of schemata showing normal and +variant adult types of biliary and pancreatic ducts.] + +[Illustration: FIG. 189.--Usual human adult type.] + +[Illustration: FIG. 190.--Persistence of early embryonal type.] + +[Illustration: FIG. 191.--Duct of Santorini has no duodenal orifice.] + +[Illustration: FIG. 192.--Duct of Santorini forms the only pancreatic +duct. Separate duodenal openings of biliary and pancreatic ducts, +resulting from failure of development of distal embryonal pancreatic +bud.] + +[Illustration: FIG. 193.--Mucous surface of human duodenum, showing +entrance of biliary and pancreatic ducts and diverticulum Vateri. +(Columbia University Museum, No. 1842.)] + +Occasionally, without obstruction of the main duct, the duodenal opening +of the duct of Santorini is large, and the flow of secretion evidently +the reverse of the usual, _i. e._, directly into the intestine. + +In other cases, also without pathological conditions, the proximal duct +is the larger of the two and serves as the principal channel of +pancreatic secretion, the canal of Wirsung being small. This is +evidently a persistence and further development of the early embryonic +relative condition of the two outgrowths above described (Fig. 190). On +the other hand the duct of Santorini may not open at all into the +duodenum, terminating in small branches which drain the proximal part of +the head of the gland (Fig. 191). + +Schirmer has examined the arrangement of the pancreatic ducts in 105 +specimens. In 56 of these the duct of Santorini passed from the main +duct into the duodenum, opening upon a papilla situated 2.5 cm. above +the common opening of the bile duct and canal of Wirsung. + +In 19 the duct of Santorini was well developed but did not open into the +duodenum. + +In but 4 cases the duct of Santorini formed the only pancreatic duct, +the lower opening being occupied by the bile duct alone (Fig. 192). We +may assume in these cases failure of development of the distal outgrowth +connected with the primitive hepatic bud, leaving only the proximal +duodenal outgrowth to form the entire adult gland. + +Figs. 188 and 189 show the normal arrangement of the duodenal openings +of the biliary and pancreatic ducts. + +Figs. 190 to 192 show schematically the variations in the relative +development and the adult arrangement of the pancreatic ducts. + +=Diverticulum and Papilla Vateri.=--From what has been said regarding +the embryonic union of the distal pancreatic outgrowth with the hepatic +bud it will be easy to recognize the corresponding features in the +arrangement of the adult duodenal termination of the common bile-duct +and canal of Wirsung. The dilated interior of the duodenal papilla +(diverticulum Vateri) corresponds to the embryonic segment between the +intestinal opening of the primitive liver duct and the point when this +duct gives off the distal larger pancreatic outbud (Figs. 186, 187, 188, +193 and 194). + +[Illustration: FIG. 194.--Adult human subject. Mucous membrane of +pyloro-duodenal junction and of duodenum. (Columbia University Museum, +No. 1840.)] + +The union of the pancreatic and biliary ducts to form the recess of the +diverticulum Vateri, which then opens by a single common orifice into +the duodenum, is better marked in some of the lower vertebrates than in +man. + +[Illustration: FIG. 195.--Duodenum, with entrance of pancreatic and +biliary ducts and well-developed diverticulum Vateri in the cassowary, +_Casuarius casuarius_. (Columbia University Museum, No. 1821.)] + +Fig. 195 shows the proximal portion of the duodenum of the cassowary +(_Casuarius casuarius_) with the biliary and pancreatic ducts and the +diverticulum at their confluence in section. + +The development of these two main digestive glands as diverticula from +the intestinal canal also explains the direct continuity of the mucous +membrane of their ducts with that lining the duodenum, a fact which is +of considerable importance in the pathological extension of mucous +inflammations from the intestine to the duct system of the glands. + +=Development of the Pancreas in Lower Vertebrates.=--In the embryo of +the _sheep_ two pancreatic buds are found, but the duct of the dorsal +(proximal) outgrowth (duct of Santorini) subsequently fuses entirely +with the main duct. + +In the _cat_ there are likewise two pancreatic outgrowths. + +In the _chick_ three pancreatic buds are visible about the fourth day. + +_Amphibia_ likewise present three embryonic pancreas buds. + +The ventral (distal) outgrowth is double, the two portions proceeding +symmetrically from each side of the hepatic duct. The single dorsal +outgrowth is derived directly from the duodenal epithelium. Later on all +these outgrowths fuse to form the single adult gland. + +_Fish_ also possess several (up to four) embryonic pancreatic +outgrowths. + +Recently in human embryos of 4.9 mm. cervico-coccygeal measure three +pancreatic outgrowths have been observed, all entirely distinct from +each other, one dorsal, budding from the epithelium of the primitive +duodenum and two ventral, proceeding from the grooved gutter which +represents the primitive ductus choledochus at this period. In embryos +of from 6 to 10 mm. the two ventral outgrowths have already fused, hence +only two buds, a single ventral and a dorsal, are now encountered.[4] + +[4] Iankelowitz, Arch. f. Mikr. Anat., Bd. 46, 1895. + +These observations place the development of the human pancreas in line +with the triple pancreatic outgrowths, two ventral and one dorsal +characteristic of the majority of the lower vertebrates, which have been +hitherto carefully examined. The ventral or distal bud is probably +double in the majority of vertebrates. The two segments fuse, however, +so early that the derivation of the pancreas from a double outgrowth, as +described above for the human embryo, practically obtains. In forms in +which the adult gland presents a number of separate openings into the +duodenum (cf. p. 118), the development would probably show multiple +embryonic outgrowths from the intestinal hypoblast. + +In any case the dorsal pancreatic bud appears to have developed in the +vertebrate series before the ventral outgrowth and to be hence +phylogenetically the older structure. + + +COMPARATIVE ANATOMY OF THE PANCREAS. + +With the exception of _Amphioxus_ and probably also of the +_Cyclostomata_, the gland appears to be present in all vertebrates, +varying, however, much in size, shape and relation to the intestinal +tube. Usually it appears as an elongated, flattened, more or less +distinctly lobulated organ, in close apposition to the duodenum between +the layers of the mesoduodenum. In all forms in which the gland is found +it is connected with the post-gastric intestine and marks the beginning +of the midgut. In structure the gland is usually acinous, resembling the +salivary glands. It is well developed in the selachians, forming a +triangular body connected with the beginning of the midgut (Fig. 202). +In some instances the gland elements do not extend beyond the intestine +itself, but remain imbedded in the wall of the midgut, as in +_Protopterus_. In certain adult teleosts the pancreas is surrounded by +the liver (Fig. 196), in others it does not appear as a compact gland +but is distributed in the form of finely scattered lobules throughout +the mesentery between the two layers of this membrane. On account of +this concealed position of the gland it was formerly believed that the +adult teleosts did not possess a pancreas. The pyloric caeca (cf. p. 119) +found in these forms were consequently considered to be homologous with +the pancreas of the higher vertebrates. + +[Illustration: FIG. 196.--A portion of alimentary canal of _Pleuronectes +maculatus_, the flounder, with pancreas attached to biliary duct and +concealed in the substance of the liver, which has been removed. +(Columbia University Museum, No. 1491.)] + +In _Myxinoids_ a peculiar lobulated glandular organ is found imbedded in +the peritoneal coat of the intestine near the entrance of the bile-duct, +into which its lobules open separately. This organ possibly corresponds +to the higher vertebrate pancreas. + +An organ which may represent a dorsal pancreas is also developed in +_Ammocoetes_ (larva of _Petromyzon_), but its exact homology is still +doubtful. It is possible that a true pancreas has not yet developed in +the cyclostomata. In _Amphioxus_ no trace of a pancreas is found. In all +other vertebrates the gland is present. In certain amphibians, as the +frog, the single pancreatic duct opens into the common bile duct (Fig. +197). + +[Illustration: FIG. 197.--Pancreas and biliary ducts of _Rana +esculenta_, frog. (Wiedersheim, after Parker; both from Ecker.)] + +In lacertilians and in some chelonians a lateral offshoot of the +pancreas is directed transversely and is adherent to the spleen. Fig. +113 shows the gland in _Chelydra serpentaria_. While the gland usually +has a single duct, yet two ducts are found in a number of animals (many +mammals, birds, chelonians and crocodiles). At times three ducts are +encountered, as in the chicken and pigeon. + +The arrangement of the pancreatic duct system among mammalia presents +the following variations: + +1. Mammals with _one_ pancreatic duct, either connected with the +bile-duct or entering the intestine independently: + +Monkeys, most rodents (except the beaver), marsupials, carnivora (except +dog and hyena), many ungulates (pig, peccary, hyrax, etc.), most +ruminating artiodactyla. + +(_a_) The pancreatic duct joins the common bile-duct before entering the +duodenum in the monkeys, marsupials, carnivora, in the sheep, goat and +camel. + +The point of entrance of the combined duct into the intestine varies. In +some forms it is near the pylorus, in others at some distance from the +same. The common opening is situated 11/2" to 2" beyond the pylorus in +carnivora, and one foot behind the same point in the goat and sheep. + +(_b_) The pancreatic duct does not join the bile-duct, but empties +separately into the intestine, in most rodents and in the calf and pig. + +In the calf the pancreatic duct opens into the duodenum 15' beyond the +bile-duct and 3' beyond the pylorus. + +In the pig the pancreatic opening is 5"-7" beyond that of the bile-duct +and 6"-8" behind the pylorus. + +2. Mammals with _two_ pancreatic ducts, of which one usually joins the +bile-duct: perissodactyla (except the ass according to Meckel), +elephant, beaver, several carnivora, dog, hyena, and according to +Bernard the cat. In the perissodactyla the proximal of the two +pancreatic ducts empties, either combined with the bile-duct, or +separate from it, but very close to it, 3"-4" behind the pylorus. The +second distal duct is smaller and opens several inches further down. + +[Illustration: FIG. 198.--_Necturus maculatus_, mud puppy. Dissection of +intestinal canal, liver, pancreas, and spleen, with blood-vessels +injected. (Columbia University Museum, No. 1863.)] + +[Illustration: FIG. 199.--Pancreas and pancreatic ducts of rabbit. +(Nuhn.)] + +In most rodents the pancreatic entrance is placed at some distance from +the pylorus. Fig. 199 shows the arrangement of the parts in the rabbit, +in which animal the main distal pancreatic duct empties at a distance of +13"-14" from the pylorus into the end of the duodenum, which intestine +forms a very long loop, while the biliary duct, receiving the smaller +proximal pancreatic duct, opens near the pylorus. + +In the _beaver_ the smaller proximal duct joins the bile-duct or even +enters the duodenum anterior to the bile-duct, nearer the pylorus, +while the distal larger pancreatic duct opens into the intestine +16"-18" behind the biliary duct. Of the two ducts found in the dog +(Fig. 200) the smaller proximal either joins the bile-duct or opens +into the intestine close to it, 1" to 11/2" beyond the pylorus. The +larger distal duct opens into the duodenum 1" to 11/2" behind the biliary +duct. Fig. 201 shows the dog's stomach and proximal portion of the +duodenum in section. The proximal smaller pancreatic duct here joins +the biliary duct, and opens with it by a single orifice into the +duodenum. The distal larger pancreatic duct opens independently into +the intestine further caudad. + +[Illustration: FIG. 200.--Abdominal viscera of dog, showing arrangement +of pancreatic ducts. (Nuhn.)] + +[Illustration: FIG. 201.--Section of dog's stomach, and proximal portion +of duodenum, with entrance of biliary and pancreatic ducts. (Columbia +University Museum, No. 1822.)] + +The parts in _Hyaena_ present a similar arrangement. + +Bernard always found _two_ pancreatic ducts in the _cat_, one large +principal duct and a second smaller accessory duct. Of these, the one +situated nearest to the pylorus always united with the bile-duct. The +pancreatic duct thus joining the bile-duct was sometimes the main duct, +sometimes the accessory smaller duct. + +Since the main function of the pancreatic juice is the conversion of +starch into sugar, the gland appears better developed in general in +herbivora than in carnivora, without, however, disappearing in the +latter. In fact it is of considerable size in the carnivora, because the +secretion also acts on the albuminous food substances and, though to a +lesser degree, on the fats. + + +PYLORIC CAECA OR APPENDICES. + +In the _Cyclostomata_ and _Selachians_ the intestinal canal is in the +main free from caecal appendages, while a large portion of the tube is +provided with a special fold of the mucous membrane which projects into +the lumen of the gut (spiral valve). Fig. 43 shows the straight +intestinal tract with the spiral valve of the longer distal segment in a +cyclostome, _Petromyzon marinus_ or lamprey. In Figs. 202 and 203 the +selachian (shark) intestine is represented in two examples, while the +similar spiral valve in a Dipnoean or lung fish, _Ceratodus_, is seen in +Fig. 204. + +[Illustration: FIG. 202.--Alimentary tract with spleen and pancreas of +_Squalus acanthias_, the dog-fish. (Columbia University Museum, No. +1405.)] + +[Illustration: FIG. 203.--Alimentary canal of _Galeus canis_, dog-shark, +in section, showing spiral intestinal valve. (Columbia University +Museum, No. 1429.)] + +[Illustration: FIG. 204.--Alimentary canal with spiral valve of +_Ceratodus forsteri_, the Australian lung-fish (Barramunda). (Columbia +University Museum, No. 1645.)] + +On the other hand in the Ganoids and in many Teleosts longer or shorter +finger-shaped diverticula of the midgut are found immediately beyond the +pylorus in the region of the bile-duct. + +These pouches or diverticula of the intestine form the so-called pyloric +caeca or appendices of these fish. They vary very much in length, +diameter and number in different forms. + +Thus but a single diverticulum appears in _Polypterus_ and _Ammodytes_ +(Fig. 205). _Rhombus maximus_ and _Echelus conger_ (Figs. 112 and 206) +have two, and the same number appear in _Lophius piscatorius_ (Fig. +207). Perca has three and the _Pleuronectidae_ have three to five. + +[Illustration: FIG. 205.--Alimentary canal of _Polypterus bichir_. +(Columbia University Museum, No. 1823.)] + +[Illustration: FIG. 206.--Alimentary tract of _Echelus conger_, Conger +eel. Stomach, mid- and end-gut, liver, and spleen. (Columbia University +Museum, No. 1430.)] + +[Illustration: FIG. 207.--Stomach, duodenum, and pyloric caeca of +_Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)] + +Fig. 208 shows the stomach and the beginning of the midgut with four +pyloric caeca in _Pleuronectes maculatus_, and Fig. 209 the same parts of +this animal in section. + +[Illustration: FIG. 208.--_Pleuronectes maculatus_, window-pane. Stomach +and mid-gut with pyloric caeca and hepatic duct. (Columbia University +Museum, No. 1432.)] + +[Illustration: FIG. 209.--_Pleuronectes maculatus_, window-pane. Stomach +and mid-gut with pyloric caeca, in section. (Columbia University Museum, +No. 1433.)] + +[Illustration: FIG. 210.--_Paralichthys dentatus_, summer flounder. +Stomach and mid-gut with pyloric caeca and liver. (Columbia University +Museum, No. 1431.)] + +Fig. 210 shows the stomach and midgut of _Paralichthys dentatus_, the +summer flounder, with three well-developed conical pyloric caeca. On the +other hand in some forms the number of pyloric appendices is enormously +increased, while their caliber diminishes. Thus 191 caecal appendages are +found surrounding the beginning of the midgut in _Scomber scomber_. A +well-marked example of prolific development of the pyloric appendages is +furnished by the common cod, _Gadus callarias_ (Fig. 211). The +appendices are in the natural condition bound together by connective +tissue and blood vessels, so as to form a compact organ, resembling a +gland (Fig. 211, A), and a similar arrangement is found in _Thynnus +vulgaris_ and _alalonga_, _Pelamys_ and _Accipenser_ (Fig. 212). + +[Illustration: FIG. 211.--Pyloric caeca of _Gadus callarias_, codfish. +(Columbia University Museum, No. 1825.) _A._ Bound together by +connective tissue and blood-vessels. + +_B._ Dissected to show confluence of caeca to form a smaller number of +terminal tubes of larger calibre entering the intestine.] + +[Illustration: FIG. 212.--Alimentary canal of _Accipenser sturio_, +sturgeon. Numerous pyloric caeca are bound together to form a gland-like +organ. (Columbia University Museum, Nos. 1826, 1827, and 1828.) + +In the smaller upper figure on the left the stomach, mid-gut, and +pyloric caeca are seen in section, showing the lumen of the latter and +their openings into the mid-gut. + +The lower left-hand figure shows the mid- and end-gut in section, the +latter provided with a spiral mucous valve.] + +In some Teleosts (Siluroidea, Labroidea, Cyprinodontia, Plectognathi and +Leptobranchiates) the appendices are entirely wanting. If there are not +more than 8-10 appendices they usually surround the gut and empty into +the same in a circle. In other cases they are arranged in a single line, +or in a double row, opposite to each other (Fig. 213). Each appendix may +open into the intestine independently, this especially where the number +is limited and the individual pouches large (cf. Figs. 206-210), or +several may unite to form a common duct. + +[Illustration: FIG. 213.--_Melanogrammus aeglifinus_, haddock. Stomach, +mid-gut, and pyloric caeca; spleen. (Columbia University Museum, No. +1598.)] + +Fig. 211, _B_, shows the appendices in _Gadus callarias_, the cod, freed +by dissection from the investing connective and vascular tissue. It will +be noticed that a considerable number of the tubes unite to form ducts +of larger caliber which open into the intestine, as seen in the section +shown in Fig. 214. + +[Illustration: FIG. 214.--Stomach and mid-gut of _Gadus callarias_, +codfish, in section, showing intestinal openings of pyloric caeca. +(Columbia University Museum, No. 1830.)] + +The pyloric appendices apparently have the same _significance_ as the +spiral intestinal fold of the Selachians, Cyclostomes and +Dipnoeans, _i. e._, the production of an increase in the area of +the digestive and absorbing surfaces of the intestinal mucous membrane. +Hence, as stated, the appendices and the spiral fold are found to vary +in inverse ratio to each other. Thus, for example, _Polypterus_ (Fig. +205) still has a fairly well developed spiral fold and only a single +pyloric appendix, while _Lepidosteus_, with but slightly developed +spiral fold, has numerous appendices. It was formerly held that the +pyloric caeca and the pancreas were mutually incompatible structures, and +that where one is found the other will be wanting. + +Hence the appendices were regarded as homologous with the pancreas of +the higher forms. Recent observations have shown that this view is not +strictly and entirely correct, while at the same time it merits +consideration in several respects. + +It is true that the pancreas in certain teleosts is now known to be +present although concealed from observation in the liver or scattered in +the form of small lobules between the layers of the mesentery (cf. p. +117), and that in a number of fish, such as _Salmo salar_, _Clupea +harengus_, _Accipenser sturio_, both the appendices and the pancreas are +encountered. Consequently these structures are not identical or even +completely homologous, since they occur side by side in the same form. + +On the other hand Krukenberg has demonstrated that the appendices +pyloricae may function physiologically as a pancreas by yielding a +secretion which corresponds to the pancreatic juice in its digestive +action. In the majority of forms, however, they apparently merely +increase the intestinal absorbing surface, secreting only mucus. + +These structures are nevertheless very interesting and instructive since +they furnish a perfect gross morphological illustration of the embryonal +stages just considered in connection with the development of the +mammalian pancreas. In the adult ganoid or teleost these blind +diverticula or pouches, varying greatly in shape, number and size, +protrude from the intestine immediately beyond the pylorus, usually in +close connection with the duodenal entrance of the bile-duct. Two or +more of these pouches may unite to form a common duct or canal opening +into the intestine. + +These forms, therefore, offer direct and valuable morphological +illustration of the manner in which the pancreas of the higher +vertebrates develops, _i. e._, as a set of hollow outgrowths or +diverticula from the hypoblast of the primitive enteric tube. We can +establish a consecutive series, beginning with forms in which only one +or two diverticula are found, and extending to types in which the number +of the little cylindrical pouches reaches nearly two hundred and in +which they are bound together by connective tissue and blood vessels so +as to closely resemble the structure of a glandular pancreas. This is +one of the most striking instances in which the minute embryological +stages of the higher types are directly illustrated by the permanent +adult conditions found in the lower vertebrates. [The same statement, as +we will see, holds good in reference to the development of the _liver_.] + + +RELATION OF THE PANCREAS TO THE PERITONEUM. + +The gland becomes very intimately connected with the serous layers of +the primitive dorsal mesentery. In order to clearly comprehend the adult +serous relations it is necessary to make a distinction between two +divisions or portions of the gland, based upon the altered relations of +the primitive dorsal mesentery which result from the differentiation of +the primitive simple intestinal tube into stomach and duodenum. + +1. The primary outgrowth of the pancreatic tubules from the duodenum, +_i. e._, the part which is to form the "head" of the adult gland, is +situated between the two layers of that division of the primitive dorsal +mesentery which forms, after differentiation of stomach and small +intestine, the _mesoduodenum_. Coincident with the rotation of the +stomach, as we have seen, the duodenum and mesoduodenum exchange their +original sagittal position in the median plane of the body for one to +the right of the median line, balancing, so to speak, the extension of +the stomach to the left (Fig. 218). + +The original right layer of the mesoduodenum and the right surface of +the duodenum now look dorsad and rest in contact with the parietal +peritoneum investing the right abdominal background and the ventral +surface of the right kidney and inferior vena cava. We have already seen +that the descending portion of the duodenum in man becomes anchored in +this position by adhesion of these apposed peritoneal surfaces. This +fixation includes, of course, the structures situated between the layers +of the mesoduodenum, _i. e._, the head of the pancreas. Consequently, +after rotation and adhesion, this portion of the gland turns one surface +ventrad, invested by secondary parietal peritoneum, originally the left +leaf of the free mesoduodenum, while the original right surface of the +gland has become the dorsal and has lost its mesoduodenal investment by +adhesion to the primary parietal peritoneum. + +2. In order to understand the way in which the body and tail of the +pancreas obtain their final peritoneal relations it is necessary to +consider the development of the dorsal mesogastrium to form the omental +bag. If we regard the primitive dorsal mesentery in the profile view +from the left side (Fig. 215) it will be seen that, as already stated, +the mesoduodenum is the first part of the membrane to be invaded by the +pancreatic outgrowth from the intestine. Cephalad of the mesoduodenum +the primitive dorsal mesogastrium (Fig. 215) is seen to protrude to the +left and caudad to form, as already explained, the cavity of the omental +bursa of the retrogastric space ("lesser peritoneal sac"). The further +growth of the pancreas carries the developing gland from the district of +the mesoduodenum into that portion of the dorsal mesogastrium which now +forms the dorsal wall of the omental bursa (Fig. 216). + +[Illustration: FIG. 215.--Cephalic segment of primitive mesentery in +schematic profile view.] + +[Illustration: FIG. 216.--Schematic profile view of primitive +mesenteries with formation of omental bursa and developing spleen and +pancreas.] + +[Illustration: FIG. 217.--_Sos scrofa foet._, foetal pig. Portions of +thoracic and abdominal viscera hardened in situ. (Columbia University +Museum, No. 1449.)] + +This double relation of the pancreas to the mesoduodenum and to the +mesogastrium forming the omental bursa is well seen in foetal pigs +between two and three inches in length (Fig. 217). + +The head portion of the pancreas is seen developing between the layers +of the mesoduodenum, while the body and tail of the gland, extending to +the left, grows between the two dorsal layers of the omentum bursa +towards the spleen, which organ is found connected with the left and +dorsal extremity of the omental sac derived from the dorsal +mesogastrium. + +Before the growth of the great omentum is pronounced the continuity of +the mesoduodenum and dorsal mesogastrium can be readily appreciated +(Fig. 218). But after the redundant growth of the membrane has carried +the great omentum further caudad, the stomach and the two omental layers +attached to the greater curvature lie in front of the structures +included between the two dorsal layers and conceal them from view (Fig. +177). + +[Illustration: FIG. 218.--Schematic view of primitive mesentery after +intestinal rotation and incipient formation of omental bursa from dorsal +mesogastrium.] + +In sagittal sections to the left of the median line (Figs. 221 and 222) +the pancreas now appears included between the layers of the great +omentum near their point of departure from the vertebral column. (This +point is of course identical with the prevertebral attachment of the +primitive dorsal mesogastrium from which the omentum is developed.) + +[Illustration: FIGS. 221, 222.--Schematic sagittal sections through +stomach, pancreas, great omentum, and left kidney.] + +[Illustration: FIG. 221.--Before adhesion between dorsal and +mesogastrium and parietal peritoneum.] + +[Illustration: FIG. 222.--After adhesion.] + +The foregoing considerations will, therefore, lead to the conclusion +that the pancreas presents, in regard to its peritoneal relations, two +distinct segments: + +1. The portion adjacent to duodenum (head and neck of the gland) is +developed between the layers of the mesoduodenum. + +2. The distal portion of the gland, comprising the body and tail, +develops between the layers of the great omentum (dorsal segment), +derived from the primitive dorsal mesogastrium. + +The transections of the dorsal mesogastrium shown in Figs. 180 and 181 +will now have to be amplified by the introduction of the body of the +pancreas between the two layers of the vertebro-splenic segment, in +addition to the splenic artery (Figs. 219 and 220). + +[Illustration: FIGS. 219, 220.--Schematic transection of dorsal +mesogastrium, pancreas, spleen, and stomach.] + +[Illustration: FIG. 219.--Before adhesion to primitive parietal +peritoneum (arrow indicates the direction in which the adhesion takes +place).] + +[Illustration: FIG. 220.--After adhesion and formation of secondary line +of transition between mesogastrium and parietal peritoneum (lieno-renal +ligament).] + +Hence the following facts will be understood: + +1. In the adult the splenic artery supplies a series of small branches +to the pancreas as it courses along the cephalic border of the gland on +its way to the spleen. + +2. After the above-described adhesion of the original left leaf of the +dorsal mesogastrium (vertebro-splenic segment) to the parietal +peritoneum (Fig. 220), the dorsal surface of the body of the pancreas +loses its peritoneal investment and becomes attached by connective +tissue to the ventral surface of the left kidney. + +3. The ventral surface of the body of the pancreas is in the adult lined +by peritoneum of the "lesser sac"; in other words the organ has +practically assumed a "retro-peritoneal" position, its ventral +peritoneal covering appearing now as the dorsal parietal peritoneum of +the retro-gastric space. + +4. When completely developed the extreme end (tail) of the pancreas +extends to the left, following the splenic artery, until it touches the +mesal aspect of the spleen at the hilus. + +5. If we, therefore, leave out of consideration for the moment the +transverse colon and duodenum, which will be taken up presently, and +confine ourselves to the arrangement of the stomach, pancreas and great +omentum, a sagittal section to the left of the median line would result +as shown in Fig. 222, after the adult condition of adhesion has been +established. + +The same process of fixation, which resulted in the anchoring of +duodenum and head of pancreas, extends to the body of the gland and the +investing omentum. The peritoneum lining the original left, now the +dorsal surface of the gland, fuses with the primitive parietal +peritoneum covering the diaphragm and the left kidney. The main body of +the pancreas in the adult appears prismatic, giving a triangular +sagittal section. The dorsal surface is adherent to the ventral surface +of the left kidney; the ventral surface is covered by the secondary +parietal peritoneum (original right layer of mesogastrium) which lines +the dorsal wall of the retrogastric space and omental bursa (lesser +peritoneal sac). The great omentum now appears to take its dorsal point +of departure along the sharp margin which separates this ventral surface +of the pancreas from a third narrower surface directed caudad. This +surface, under the conditions which we are at present examining, would +be lined by the peritoneum continued onto it from the dorsal layer of +the great omentum. This peritoneum merges along the dorsal margin of +this caudal surface of the pancreas with the general parietal peritoneum +covering the left lumbar region and the caudal part of ventral surface +of the left kidney. We have, therefore, along this line a secondary +transition from visceral to parietal peritoneum, obtained by the +obliteration of the original visceral peritoneum investing the dorsal +surface of the pancreas before adhesion to the parietal peritoneum. + +The pancreas assumes, therefore, in the adult a secondary +retro-peritoneal position, covered on its ventral surface by peritoneum +of the "lesser sac," while the caudal surface is lined by part of the +general peritoneal membrane of the "greater sac." The dorsal surface, +denuded of serous covering by obliteration, is adherent to the crura of +the diaphragm, the aorta and the ventral surface of the left kidney. + +It is now proper to compare the conclusions just derived from the study +of the development of the human dorsal mesogastrium and connected +structures (spleen and pancreas) with the conditions presented by the +corresponding parts in one of the lower mammalia, which illustrate some +of the human embryonal stages. Here again the abdominal cavity of the +cat forms an instructive object of study. + +The purpose of the following comparison should be twofold: + +I. The mesogastrium, spleen and pancreas in the cat will clearly +illustrate the process of human development above outlined. + +II. The abdominal viscera of the cat, if properly arranged, will enable +us to complete the consideration of this region by including the very +important relations which the transverse colon and third portion of the +duodenum bear in man to the great omentum and pancreas. + + +I. SPLEEN, PANCREAS AND GREAT OMENTUM OF CAT. + +After opening the abdominal cavity it will be seen that the great +omentum can be lifted up, exposing the subjacent coils of the small and +large intestine, to which it adheres at no point. In other words the +entire dorsal surface of that part of the original mesogastrium which +forms the great omentum is free. It will be remembered that this is not +the case in the adult human subject, because here the dorsal surface of +the great omentum adheres to the transverse colon. Consequently in man +only that portion of the dorsal surface of the omentum can be seen which +extends between the transverse colon and the caudal free edge of the +membrane. + +It will be noted that on the left side the spleen is connected by its +mesal surface to the omentum and through it with the stomach +(gastro-splenic omentum). In other words the cat illustrates the human +embryonal stage in which the spleen has appeared between the layers of +the dorsal mesogastrium at the extreme left or blind end of the +retrogastric pouch formed by the rotation of the stomach and elongation +of the mesogastric membrane, but _before_ the adhesion has taken place +between the original _left_ (now _dorsal_) layer of the vertebro-splenic +segment of the mesogastrium and the primitive parietal peritoneum +apposed to it (Fig. 219). Consequently the dorsal wall of the "lesser" +sac in the cat is still composed of the two layers of the free +vertebro-splenic segment of the mesogastrium, the primitive right (now +ventral) layer not having been converted, as is the case in man, into +secondary parietal peritoneum by adhesion of the original left (now +dorsal) layer to the primitive prerenal parietal peritoneum. + +If we now examine the relation of the pancreas to the peritoneum we can +establish the following facts: + +1. The portion of the gland adjacent to the duodenum, corresponding to +the "head" of the human organ, is included between the two layers of +the mesoduodenum. This membrane is free, so that the dorsal surface of +this portion of the pancreas is seen to be invested by the dorsal layer +of the mesoduodenum (Fig. 223). The duodenum and the mesoduodenum, the +latter containing the head of the pancreas between its layers, can be +turned toward the median line, so as to expose the entire ventral +surface of the post-cava and right kidney. To illustrate the arrangement +which is found in the adult human subject the descending duodenum and +pancreas should be allowed to fall over to the right so as to cover the +vena cava and the mesal part of the ventral surface of right kidney. The +adult human condition will now be produced if we assume that the +structures are fixed in this position by the obliteration of the apposed +serous surfaces, viz., the parietal peritoneum over kidney and vena cava +on the one hand and the right layer of the mesoduodenum and the dorsal +visceral peritoneum of the duodenum on the other. + +[Illustration: FIG. 223.--Abdominal viscera of cat, hardened and removed +from body, showing relation of pancreas to mesoduodenum and dorsal +mesogastrium, respectively. (Columbia University Museum, No. 728.)] + +2. In following out the pancreas of the cat in its entire extent, +proceeding to the left of the pylorus, it will be seen that the body of +the gland has extended between the two dorsal layers of the great +omentum (primitive dorsal mesogastrium) over to the spleen (Fig. 223). +Consequently the arrangement in the cat corresponds to the stage in the +human development shown in Fig. 219 and Fig. 221 in which adhesion of +the dorsal surface of the pancreas to the parietal peritoneum has not +yet taken place. + +It will be quite easy to reconstruct from the facts as demonstrated by +the arrangement of the parts in the cat, the stage in the development of +the lesser peritoneal sac in which the dorsal wall of the space is still +formed by the proximal portion of the free dorsal mesogastrium (great +omentum) and the structures included between its two layers. + +It must then become apparent that the entire serous surface which in the +adult human subject we regard as "parietal peritoneum of the lesser sac" +lining the dorsal wall of the retrogastric space is derived from what +originally was the right layer of the primitive sagittal dorsal +mesogastrium. + + +II. RELATION OF GREAT OMENTUM TO TRANSVERSE COLON, TRANSVERSE MESOCOLON +AND THIRD PART OF DUODENUM. + +The second purpose to be accomplished by the study of the cat's +abdominal cavity at this stage is the correct appreciation of the adult +human conditions which are produced by areas of adhesion between the +transverse colon, transverse mesocolon and third part of the duodenum on +the one hand, and the dorsal mesogastrium, as great omentum, with the +structures contained between its layers, on the other. + +Perform the manipulations of the large and small intestine in the cat +(see p. 67) which are required in order that the tract may be arranged +so that it will correspond in general to the topographical conditions +presented by the adult human subject. Locate the transverse colon and +mesocolon and the third portion of the duodenum produced by these +manipulations in imitation of the corresponding human structures. Then +proceed to plot the different parts out successively as they would +appear in a sagittal section (Fig. 224). + +[Illustration: FIG. 224.--Schematic sagittal section of abdominal +viscera of cat, after the intestines have been rotated to correspond to +the adult human disposition, to show lines of peritoneal reflection +before adhesion.] + +The following facts are to be noted and indicated on the plan of the +section: + +1. The great omentum is free, hanging down from the greater curvature of +the stomach over the coils of intestine. Turning the omentum up it will +be observed that the body of the pancreas is included between the two +dorsal layers of the membrane. + +2. The omentum, containing the pancreas, can be lifted up, exposing the +next succeeding structure, viz., the transverse colon and mesocolon. In +the cat the large intestine has been brought over, by the manipulations +above indicated, into a transverse position so as to represent the human +transverse colon and its mesocolon. It is therefore necessary to +remember that in this mammal the fixation of the transverse mesocolon in +the position indicated, by adhesion of ascending and descending mesocola +to the parietal peritoneum of the abdominal background, has not yet +occurred. Consequently the membrane must be held in the transverse +position in order to represent the human arrangement. + +It will of course be observed that both surfaces of the transverse +mesocolon established in this way are free, not adherent to either +omentum or pancreas on the one hand, nor to the transverse duodenum on +the other. + +3. The third or transverse portion of the duodenum is seen to be +attached by the distal part of the mesoduodenum, both of the serous +surfaces of the membrane being free. The duodenum having been brought +from right to left transversely across vertebral column and aorta, +underneath the superior mesenteric artery, the mesoduodenum, in the +segment corresponding to the transverse duodenum, exchanges its original +sagittal position for one in a horizontal plane, with cephalic +(primitive left) and caudal (primitive right) surfaces. + +Now compare the above arrangement of the intestines and peritoneum in +the cat at once with the conditions presented in the adult human +subject, reserving certain intermediate stages, as exhibited by some of +the lower monkeys, for subsequent study. + +[Illustration: FIG. 225.--The same figure indicating the areas of +adhesion and peritoneal obliteration (shaded) which produce the +arrangement of the adult human peritoneum. + +1. Area of adhesion between opposed surfaces of great omentum and +transverse mesocolon and colon. + +2. Area of adhesion between parietal peritoneum, duodenum, and caudal +layer of transverse mesocolon. + +3. Adhesion of opposed walls of omental bursa leading to obliteration of +distal portion of pouch and producing "gastro-colic" ligament of adult +human subject.] + +The examination of a similar sagittal section representing schematically +the adult human arrangement of the parts (Fig. 225) will reveal the +following points of difference as compared with the cat: + +1. The peritoneum covering the dorsal surface of the pancreas, derived +from the primitive dorsal mesogastrium, has become adherent to the +parietal peritoneum, as previously described. + +2. The cephalic surfaces of the transverse colon and mesocolon fuse with +the corresponding area of the dorsal (4th) layer of the great omentum +(dorsal mesogastrium). + +In the human foetus in the 4th month the connection is still so slight +that the omentum can readily be separated from the transverse colon and +mesocolon. + +Further dorsad the cephalic layer of the transverse mesocolon adheres to +the serous investment of the caudal surface of the pancreas, derived, as +we have seen, from the same dorsal layer of the great omentum. + +3. The duodenum and mesoduodenum are fixed by adhesion on the one hand +to the parietal peritoneum, on the other to the caudal layer of the +transverse mesocolon near the root of that membrane. + +4. The cavity of the omental bursa is usually obliterated in the adult +caudad of the level of the transverse colon, by adhesion of the apposed +surfaces of the two intermediate omental layers. + +We have therefore three general areas of secondary peritoneal adhesion +to deal with (Fig. 225), viz.: + + 1. Dorsal layer of primitive } { Parietal peritoneum, cephalic + mesogastrium (great } { layer of transverse + omentum) including the } { mesocolon and cephalic surface + serous investment of the } to { of transverse colon. + dorsal and caudal surfaces } { + of the pancreas (Fig. 225, } { + 1). } { + + 2. Transverse duodenum } { Parietal peritoneum and + and mesoduodenum (Fig. } to { caudal layer of transverse + 225, 2). } { mesocolon. + + 3. Between the apposed serous surfaces of the intermediate omental + layers (Fig. 225, 3). + +[Illustration: FIG. 226.--Schematic sagittal section of adult human +peritoneum]. + +These areas of adhesion result naturally in the production of secondary +lines of peritoneal transition as follows: + +1. Figs. 225, 1; 226, 1, from the omentum, dorsal layer, to the caudal +surface of transverse colon, caudal layer of transverse mesocolon and +caudal surface of the pancreas. + +2. Figs. 225, 2; 226, 2, from the caudal layer of the transverse +mesocolon across the transverse portion of the duodenum to the parietal +peritoneum and mesentery of the jejuno-ileum. + +3. Figs. 225, 3; 226, 3, between the intermediate omental layers, +forming the secondary caudal limit of the lesser sac. + +These changes consequently result in the rearrangement of the adult +human peritoneum in accordance with the following schema (Fig. 226): + +We trace the peritoneum as the ventral or superficial layer of the great +omentum from the greater curvature of the stomach caudad around the +distal free edge of the omentum and cephalad, as the dorsal layer, to +the ventral border of the transverse colon. Here apparently this layer +is continued across the caudal surface of the large intestine and beyond +as the caudal layer of the transverse mesocolon. While this condition +obtains practically in the adult it is to be remembered that the +adhesion (at 1 in Fig. 225) prevents us from lifting the omentum away +from the colon, and that consequently the apparent continuity of the +dorsal layer of the great omentum with the caudal layer of the +transverse mesocolon is the result of this peritoneal fusion. + +Near the dorsal attachment or "root" of the transverse mesocolon the +caudal layer of the membrane becomes continuous with the parietal +peritoneum investing the transverse portion of the duodenum on its +ventral aspect, which peritoneum in turn passes into the free mesentery +of the jejuno-ileum (Fig. 225, 2). Comparison with the previous figures +will show that we are dealing here with another area of secondary +peritoneal fusion. + +If we now open the "lesser peritoneal cavity" by dividing the two layers +of the omentum attached to the greater curvature of the stomach (Figs. +225 and 226 in direction of arrow) we will apparently reach the upper or +cephalic surface of the transverse mesocolon. This layer can be followed +dorsad to the sharp border which separates the ventral and caudal +surfaces of the pancreatic body and the membrane can be traced thence +over the ventral surface of the gland to the diaphragm. (The connections +with the liver and stomach shown schematically in the diagram (Fig. 225) +are to be considered in detail subsequently.) + +In the adult the peritoneal surface just described appears as the +cephalic layer of the transverse mesocolon and its continuation dorsad. +From the facts previously considered it will be at once apparent that we +are really dealing here with a part of the third layer of the primitive +omentum. We do not see the original cephalic layer of the transverse +mesocolon. This membrane has become fused with the fourth omental layer, +and its free serous surface obliterated in the stretch between the +vertebral column and the transverse colon. Hence the human adult +transverse mesocolon is apparently composed of _two_ layers; the +cephalic of these layers appears as peritoneum of the "lesser sac," in +conformity with its derivation from the original third omental layer +lining the interior of the omental bursa. The caudal layer, on the other +hand, is a part of the general or "greater" peritoneal membrane. The +entire adult transverse mesocolon, hence, comprises _four_ peritoneal +layers, of which only two remain as permanently free serous surfaces. +These differ in their derivation, the cephalic layer being a part of the +primitive dorsal mesogastrium (third omental layer), while the caudal +layer is part of the primitive mesocolon. Between these two layers of +the adult transverse mesocolon are included the two obliterated +embryonic membranes, _viz._, the fourth omental layer and the original +dorsal layer of the transverse mesocolon. + +Caudad the two layers of the adult transverse mesocolon surround the +transverse colon and are continuous along the ventral margin of the +intestine with the layers of the great omentum. Toward the vertebral +column these layers again diverge. The cephalic layer, lining the +"lesser peritoneal cavity" invests the ventral surface of the pancreas. +The caudal layer continues over the caudal surface of the body of the +gland and transverse portion of the duodenum into the parietal +peritoneum and the free mesentery of the jejuno-ileum. Consequently the +returning layers of the great omentum are said to surround the +transverse colon and unite along the dorsal border of the intestine to +form the transverse mesocolon, which membrane is continued dorsad toward +the vertebral column as two layers. At the "root" of the transverse +mesocolon these layers are then described as diverging, the cephalic +passing up to line the ventral surface of the pancreas, while the caudal +continues over the caudal surface of the pancreas and third portion of +the duodenum into the parietal peritoneum and mesentery. + +Wherever in this discussion of the transverse mesocolon the transition +between the caudal layer of the membrane and the "parietal" peritoneum +is referred to it is necessary to remember that this "parietal" +peritoneum is the _secondary_ investment of the abdominal background, +formed by the surface of the ascending and descending mesocolon which +remains free after the opposite surface and the vertical segments of the +large intestine have been anchored by adhesion to the _primary_ parietal +peritoneum (cf. p. 81, Fig. 158). + +A summary at this point of the course of the dorsal mesogastrium, in +forming the great omentum and its subsequent connections, would show us +that the membrane first enlarges and descends towards the transverse +colon (Fig. 177). The omental bag is formed by the descending or +superficial segment (starting from the greater curvature of the +stomach), turned toward the observer in the figure, and by the ascending +or deep layer which is attached above to the dorsal abdominal wall, in +front of the vertebral column and aorta along the original line of +origin of the dorsal mesogastrium. Gradually growing and descending +further, the deep segment becomes attached to the transverse colon. It +also becomes connected, especially on the left side, with the +diaphragmatic peritoneum (phrenicocolic lig.), so that its original +starting point is no longer distinct. Finally the development of the +spleen and pancreas between the layers of the dorsal segment and their +subsequent connections obscure the original conditions. + +Fig. 297 shows the primitive condition at a time when the connection +with the transverse colon and mesocolon has not yet taken place. + +The omental bag or bursa epiploica develops in the region of the dorsal +mesogastrium and the viscera included between its layers, by changes in +the position and extent of the membrane which finally result in placing +a part of the right half of the primitive coelom cavity behind the +stomach. Up to the sixth week the line of origin of the dorsal +mesogastrium is from the mid-dorsal line of the abdomen. It deviates +from this origin to the left because the great curvature of the stomach +to which it is attached turns in this direction. On this account, and +because of the rapid growth of this portion of the mesogastrium, a bag +or space is formed behind the stomach. The entrance into this space is +situated to the right of the lesser curvature, behind the peritoneal +layers connecting the same with the liver (lesser or gastro-hepatic +omentum and hepato-duodenal ligament). The ventral wall of this space is +formed by the dorsal surface of the stomach itself, the dorsal wall by +the mesogastrium, turning to the left and presenting its original right +surface, now directed ventrad. The caudal limit of the retro-gastric +space is given by the turn of the mesogastrium to reach its attachment +along the greater curvature of the stomach (rudiment of great omentum). + +The stomach, in contributing to produce these changes, passes from the +vertical to the oblique and finally into the transverse position. The +pylorus, formerly directed caudad, passes up and to the right. The +fundus develops and the original left side of the stomach becomes the +ventral, the right side the dorsal. The original dorsal border, now the +greater curvature, moving caudad, carries the attached dorsal +mesogastrium with it into its new position. The mesogastrium now pouches +to form the great omentum and rapidly enlarges. At first hardly +projecting beyond the greater curvature, it increases in length until it +forms a four-layered apron which hangs down as a loose sac over the +transverse colon and the coils of the small intestine (Fig. 177). In the +foetus of six months the cavity of the omental bag extends caudad as far +as the lower edge of the omentum. Later adhesions between the peritoneal +surfaces lining the interior of the bursa limit this extension. + +The omental bursa is therefore formed by a ventral lamella, consisting +of two peritoneal layers, which hangs down from the greater curvature of +the stomach and passes around the caudal free edge of the omentum into +the double-layered dorsal lamella, which ascends, over the transverse +colon, to the original starting point of the dorsal mesogastrium along +the front of the vertebral column and aorta. Hence the "great omentum" +is originally composed of four layers of peritoneum. + +The dorsal double lamella becomes adherent over a considerable area to +the parietal peritoneum of the dorsal abdominal wall. In this way the +organs developed between the two layers of the lamella obtain their +final fixed position. The pancreas becomes anchored and appears in the +adult as a "retro-peritoneal" structure, while the spleen is attached by +the "phrenico-lienal ligament" to the diaphragm. + +In addition the dorsal omental lamella adheres in the fourth month to +the cephalic layer of the transverse mesocolon and to the transverse +colon. + +Important illustrations of some of the intermediate stages in the human +development of this portion of the peritoneal tract are afforded by the +permanent adult conditions found in the abdominal cavity of some of the +lower primates, notably certain of the cynomorphous monkeys. + +[Illustration: FIG. 227.--Abdominal cavity of _Macacus rhesus_, Rhesus +monkey, with the small intestine removed. (Columbia University Museum, +No. 63/1831.)] + +Fig. 227 shows the abdominal cavity and disposition of the peritoneum in +a macaque monkey (_Macacus rhesus_, male) in the ventral view, with the +coils of small intestines removed and the omentum lifted up and +reflected upon the ventral body wall. The following important points of +difference from the arrangement in the _cat_ on the one hand, and in +_man_ on the other, are to be noted: + +1. The large intestine presents the typical primate course, with an +ascending, transverse and descending colon. The ileo-caecal junction is +situated in the right iliac fossa. + +2. The ascending and descending mesocola are still _free_, not having +become adherent to the parietal peritoneum along the dorsal abdominal +wall. Hence the caudal portions of the ventral surfaces of the two +kidneys are still covered by the _primitive parietal peritoneum_. + +3. The great omentum is not yet adherent to the transverse colon and +mesocolon except for a short distance on the extreme right. At this +point the dorsal layer of the omentum has begun to contract adhesions to +the hepatic flexure of the colon and ascending colon, but the rest of +the transverse colon is free. Differing from the human arrangement is a +line of adhesion, uniformly present in these monkeys, between the dorsal +surface of the omentum along its right edge and the ventral +surface and right border of the _caecum_ and _ascending colon_, parts +which normally are not adherent to the omentum in man. + +4. Hence in tracing the omentum to the left of the limited adhesion to +the hepatic flexure and ascending colon, _i. e._, nearly throughout the +entire extent of the transverse colon, we find the membrane passing +freely without adhesion over the cephalic surface of the transverse +mesocolon, which preserves its original free condition, independent of +the omentum. This arrangement is shown in the schematic sagittal section +in Fig. 230. + +5. Tracing the omentum dorsad beyond the transverse colon and mesocolon +the pancreas is reached. Here we encounter the first extensive area of +omental or mesogastric adhesion. The omental peritoneum continues over +the ventral and caudal surfaces of the gland, investing the same, but +the dorsal surface has lost its serous covering and is anchored to the +ventral surface of the left kidney. Hence a sagittal section would show +the arrangement of the monkey's omentum as indicated in the schematic +Figs. 229 and 230. Making now a general comparison of the peritoneal +membrane of this animal with that of man, and of both with the preceding +common embryonal condition, we can draw the following conclusions, +indicated schematically in the five figures 228-232. + +[Illustration: FIGS. 228-232.--Schematic sagittal sections of dorsal +mesogastrium and omental bursa, in man, monkey, and cat.] + +[Illustration: FIG. 228.--Common embryonal condition, as illustrated by +cat, after rotation and formation of omental bursa.] + +[Illustration: FIG. 229.--Area of adhesion between dorsal mesogastrium +and primitive parietal peritoneum in _Macacus_, producing condition +shown in Fig. 230.] + +[Illustration: FIG. 230.--Arrangement of great omentum as found in +_Macacus rhesus_, shown without reference to areas of peritoneal +obliteration.] + +[Illustration: FIG. 231.--Corresponding section of human adult +peritoneum showing, along dotted lines, area of peritoneal adhesion.] + +[Illustration: FIG. 232.--Section showing human adult peritoneum without +reference to area of adhesion.] + +1. The dorsal layer of the monkey's omentum in its proximal segment +behaves in the same way as in man, _i. e._, it becomes adherent to the +primitive parietal peritoneum down as far as the caudal margin of the +dorsal surface of the pancreas included between the primitive +mesogastric layers forming by their further growth the omental apron. + +Therefore we find, as in the human subject, + +(_a_) The pancreas adherent to the ventral surface of the left kidney. + +(_b_) A portion of the ventral surface of the kidney, cephalad of the +pancreas, and the dorsal wall of the retrogastric (lesser peritoneal) +space lined by secondary parietal peritoneum derived from the third +layer of the omentum (original right layer of dorsal mesogastrium). + +2. The monkey differs from adult man in the behavior of the dorsal +omental layer in relation to the cephalic surface of the transverse +mesocolon. The adhesion, which in the human subject fuses this layer +with the transverse colon and mesocolon, does not occur in the monkey. + +Hence we have in this animal the following conditions: + +(_a_) The omentum is non-adherent to the transverse colon and transverse +mesocolon. + +(_b_) The caudal surface of the pancreas is lined by its original +mesogastric peritoneum. + +(_c_) The transverse mesocolon is formed by the original two layers of +the primitive dorsal mesentery; hence its cephalic layer is not +"peritoneum of the lesser sac" as is the case in man. + +(_d_) The caudal part of the ventral surface of the left kidney below +the pancreas, is covered by the original parietal peritoneum. + +(_e_) Only one point or line of _secondary peritoneal transition_ +exists, where the dorsal layer of the omentum in the adult becomes +continuous with the parietal peritoneum covering the caudal surface of +the pancreas and the ventral surface of the left kidney. + +_Note_: In the schematic sections shown in Figs. 228 to 232 the +transverse colon is represented as far removed from the ventral surface +of the left kidney, in order to make the peritoneal lines of the +mesocolon more clear. Actually a sagittal section which would divide the +kidney would cut the transverse colon at its extreme left end, where it +turns close to the ventral surface of the left kidney and then follows +its lateral border to form the splenic flexure (Fig. 235). The caudal +part of the ventral surface of the left kidney in the adult human +subject is covered by the peritoneum which, as secondary parietal +peritoneum, is derived from the upper part of the right leaf (later +ventral leaf) of the descending mesocolon. Hence it should be remembered +that these diagrams present _combinations_ of sections. A section which +will show the full development of the transverse mesocolon is mesad of +the kidney; while a section through the kidney would be too far laterad +to show the transverse mesocolon. + +[Illustration: FIGS. 233-235.--Series of schematic sagittal sections +through left kidney and adrenal, pancreas, and transverse colon, to show +development of adult peritoneal relations.] + +[Illustration: FIG. 233.--Embryonic condition, as illustrated by cat, +after rotation of intestine. Pancreas free between dorsal layers of +great omentum. Transverse colon and mesocolon free. Kidney behind +primitive parietal peritoneum.] + +[Illustration: FIG. 234.--Area of adhesion between: 1. Primitive +parietal peritoneum. 2. Mesogastrium forming great omentum. 3. Colon and +mesocolon.] + +[Illustration: FIG. 235.--Adult human arrangement, shown without +reference to obliterated areas.] + +Figs. 233, 234 and 235 show sagittal sections through the left kidney +with the adult arrangement of the peritoneum and colon and the embryonic +and adhesion stages leading to the same. + +It will be observed that in all the schematic sections of the early +embryonic stages the two layers of the transverse mesocolon are shown +without dorsal attachment, as turning with the formation of a fold (Fig. +228 at x) into two layers descending ventrad of the parietal peritoneum. +This is because the dorsal attachment of the mesocolon is at this stage +still in the median line and would hence not be encountered by a +sagittal section through the kidney, and because the two layers of the +transverse mesocolon, immediately after rotation of the large intestine, +are still directly continuous with the two layers of the descending +mesocolon. That is to say, the cephalic layer of the transverse +mesocolon is continuous with the dorsal (originally the left) layer of +the descending mesocolon, and the caudal layer of the transverse +mesocolon with the ventral (originally the right) layer of the +descending mesocolon, which is, in the human subject, to assume +subsequently the character of parietal peritoneum after the dorsal layer +and the primitive parietal peritoneum have become obliterated by +adhesion (Fig. 235). + +Fig. 236 shows this continuity of the descending and transverse +mesocolon as a permanent adult condition in the macaque. The fold of +transition between the two is seen at x in Fig. 228. It will be noticed +that the ventral surface of the left kidney, caudad of the adherent +pancreas, is covered by the primitive parietal peritoneum, corresponding +to section in Fig. 230. + + +RELATIONS OF SPLEEN AND OMENTUM IN _MACACUS RHESUS_. + +The spleen in this animal has not contracted any extensive adhesions to +the parietal peritoneum (the phrenico-lienal lig. of anthropotomy is not +developed). It can be turned mesad so as to expose the lateral border +and an adjacent segment of the ventral surface of the left kidney, as +well as the dorsal surface of the tail of the pancreas at its tip, still +covered by mesogastric peritoneum. Hence in the monkey the adhesion of +the original vertebro-splenic segment of the mesogastrium, including the +pancreas, to the primitive parietal peritoneum is less complete than in +man. + +[Illustration: FIG. 236.--Abdominal viscera of _Macacus cynomolgus_, Kra +monkey. (Columbia University Museum, No. 1801.)] + + +MEDIAN ATTACHMENT OF DESCENDING MESOCOLON AND ITS RELATION TO THE +MESOCOLON OF THE SIGMOID FLEXURE IN THE _MACAQUE_. + +Fig. 236 shows the abdominal viscera, hardened in situ, of _Macacus +cynomolgus_, the Kra monkey, in the ventral view and from the left side. + +The great omentum is lifted up, the pancreas is adherent to the ventral +surface of the left kidney, the caudal portion of which is covered by +the primary parietal peritoneum, which can be exposed by turning the +still free descending mesocolon mesad. The mesocolon retains its +primitive attachment to the median line ventrad of the large +prevertebral blood vessels. It is readily seen that adhesion between the +left leaf of this free descending mesocolon and the parietal peritoneum +down to the level of the iliac crest would produce the conditions found +in the human adult, with an attached descending colon and a free sigmoid +flexure; also that limited adhesion of the mesocolon of the sigmoid +flexure to the parietal peritoneum would produce, as previously +explained (cf. p. 97), the intersigmoid peritoneal fossa. + +=2. Ventral Mesogastrium and Liver.=--The peritoneal reflections from +the stomach to the liver, and the arrangement of the membrane in +connection with the latter organ, remain for consideration. + +Certain complicated adult conditions, encountered in this part of the +abdominal cavity, make it desirable to arrange the subject for purposes +of study under the following subdivisions: + +I. The development of the liver and of its vascular system, and the +significance of the adult circulation of the liver and of the foetal +remnants connected with the organ. + +II. The anatomy of the ventral mesogastrium and the changes produced in +the arrangement of the membrane by the development of the liver. + +=I. A. Development of the Liver.=--The liver, like the pancreas, is +developed from the duodenum as an outgrowth from the hypoblast lining +the enteric tube. As we have previously noted, the first outgrowth of +the hepatic diverticulum is closely associated with the distal +pancreatic outbud; in fact the latter arises as a derivative from the +hepatic duct rather than as a distinct outbud from the intestinal tube. +(This close association of the hepatic duct with the pancreas is well +seen in the arrangement of the concealed pancreas of some teleosts (cf. +p. 117, Fig. 196).) + +In point of time the liver is the first accessory structure to develop +by budding from the primitive alimentary canal, the pancreas and lung +following. + +[Illustration: FIG. 237.--Longitudinal section of an embryo of +_Petromyzon planeri_, four days old. (Minot, after Kupffer.)] + +In the primitive type of development, as seen in _Petromyzon_ and in the +Amphibia, the liver appears very early, as a diverticulum of the +embryonic intestinal tube, near its cephalic extremity, projecting on +the ventral aspect down into the mass of yolk-cells (Fig. 237). The +short stretch of the primitive alimentary canal cephalad of the hepatic +diverticulum corresponds to the foregut. With the development of the +heart the primitive foregut becomes divided into pharynx and +post-pharyngeal segment (oesophagus and stomach). The hepatic +diverticulum then lies immediately dorsad of the caudal or venous +extremity of the heart. Hence it is probable that the liver is an older +organ in the ancestral history of the vertebrates than the pharynx or +even the heart. The liver diverticulum lies in close connection with the +omphalo-mesenteric veins which return the blood from the yolk-sac to the +heart. In the course of further development, as will be seen below, the +liver comes into very intimate relations with the venous circulation. + +In human embryos of 3.2 mm. the primitive hepatic duct appears as a wide +hollow pouch composed of hypoblast cells, growing between the two layers +of the ventral mesogastrium, which membrane, extending between the +ventral border of the primitive stomach and the ventral abdominal wall, +will be subsequently considered in detail. The liver, in developing +between the layers of the ventral mesogastrium, approaches very early +the _septum transversum_ or rudimentary diaphragm and becomes connected +with the same. A mass of mesodermal cells, derived from the mesogastrium +and from the primitive mesodermal intestinal wall surrounding the +hypoblastic lining of the tube, covers the caecal termination of the +primitive hepatic duct, forming the so-called embryonic _hepatic ridge_. +This mesodermal tissue accompanies the duct in its further growth and +branching, forming the connective tissue envelope, known in the adult as +the capsule of Glison. The primitive hepatic duct is directed cephalad +in the mesogastrium between the vitelline duct and the stomach (Fig. +101). + +In embryos measuring 4.25 mm. the duct is 0.24 mm. long. Later (in +embryos of 8 mm.) the primitive single duct divides into two secondary +branches, indicating, even at an early stage, the adult arrangement of +the duct, as formed by the union of the right and left hepatic ducts +(Fig. 185). + +The gall-bladder in embryos of this size (8 mm.) is a well-defined caecal +diverticulum, branching caudad from the main hepatic duct. + +The vesicular mucous surface is thus derived from the enteric hypoblast +in the same way as the epithelial lining of the bile-ducts and +capillaries. The external muscular and fibrous coats of the gall-bladder +are developed from the mesoderm of the mesogastrium. + +It is to be noted that at an early stage the gall-bladder is derived +from the main duct close to the intestine, the latter duct being very +short. Later on the common duct grows in length, making the liver more +and more a gross anatomical organ distinct from the intestine. The +cystic duct develops as the result of a similar increase in length of +the cystic diverticulum. The two principal secondary branches of the +hepatic duct give origin to sprouts or buds. These are derivatives of +the hypoblastic cells of the larger ducts and may from the beginning be +hollow, possessing a lumen continuous with that of the parent duct +(Selachians, Amphibians). In warm-blooded animals these sprouts are at +first solid, forming the s. c. _hepatic cylinders_, and only +subsequently become hollowed out with the further development of the +biliary duct system of the liver. The rapid growth of the organ leads to +a great increase in the number of the hepatic cylinders. They spread out +on all sides, finally coalescing with adjacent buds so as to form an +interlacing network whose meshes are filled by blood vessels. After the +hepatic cylinders have become canalized they preserve the same +arrangement, hence the resulting biliary capillaries of the adult form +an anastomosing network. Amphioxus and the amphibians have a single +hepatic outgrowth (Fig. 49). + +In the Selachians the liver arises as a ventral outgrowth at the hinder +end of the foregut immediately in front of the vitelline duct, thus +bringing the liver from the beginning into close proximity with the +vitelline veins entering the heart. Almost as soon as formed the +outgrowth develops two lateral diverticula, opening into a median canal. +The two diverticula are the rudimentary lobes of the liver and the +median canal uniting them is the rudiment of the common bile-duct and +gall-bladder. + +In the Teleosts the liver arises quite late (in the trout about the 25th +day) as a solid outgrowth from the intestinal canal close to the heart. +In the Amniota the liver arises in the same position as in the Anamnia, +but, at least in birds and mammals, shows its bifurcation almost, if not +quite, from the start. The two forks embrace between them the +omphalo-mesenteric or vitelline veins just before they empty into the +sinus venosus of the heart. + +In the chick the liver appears between the 56th and 60th hour, the right +fork being always of greater length but less diameter than the left. The +hepatic outbud in the rabbit appears during the 10th day, and during the +11th day begins to send out branches. + +In man, as above stated, the bud appears well marked in embryos of 3 +mm. + +[Certain adult variations make it appear possible that there are two +human embryonic hepatic buds, a cranial and a caudal, as is the case in +birds.] + +=I. B. Comparative Anatomy of the Liver.=--The liver, phylogenetically a +very old organ, occurs in all vertebrates, for the caecal diverticulum of +the intestine of amphioxus (Fig. 49) has probably the significance of a +hepatic outbud. + +The primitive form of the liver is symmetrically bilobed, a type which +is seen well in the chelonian organ (Fig. 238). + +[Illustration: FIG. 238.--_Pseudemys elegans_, pond turtle. Alimentary +canal. (Columbia University Museum, No. 1437.)] + +In size the liver is subject to great variations. It is usually larger +in animals whose food contains much fat. Hence carnivora in general have +a larger liver than herbivorous animals. + +Its shape also varies considerably, depending on the form of the body +cavity and on the amount and disposition of the available space. Hence +in the snakes the organ appears long drawn out, flattened, almost +ribbon-like (Fig. 239), while the relatively very large coronal diameter +of the body cavity in the turtles permits the liver to expand +transversely (Fig. 238). + +[Illustration: FIG. 239.--Stomach, mid-gut, pancreas, and liver of _Boa +constrictor_, boa. (Columbia University Museum, No. 1832.)] + +[Illustration: FIG. 240.--Liver of _Macacus cynomolgus_, Kra monkey. +(Columbia University Museum, No. 28/1833.)] + +[Illustration: FIG. 241.--Liver of _Pleuronectes maculatus_, flounder. +(Columbia University Museum, No. 1679.)] + +In general, when the liver is large and the available space for its +reception limited, it is usually split into several (two to seven) +lobes, which permit, by mutual displacement, the accommodation of the +organ to varying space-conditions of the body cavity (Fig. 240). Under +the opposite circumstances, on the other hand, even the primitive +bilobed character may disappear and the liver is then unlobed (Fig. +241). + +The presence or absence of a gall-bladder depends apparently largely on +the character of the food and on the habitual type of digestion. In many +vertebrates digestion is carried on nearly continuously, without marked +interruption, especially in many ungulates, ruminants and rodents. In +such animals the gall-bladder is absent. It is also absent in several +birds (most Parrots, Doves, Ostrich, Rhea americana, the Cuculidae, +Rhamphastos, etc.). This variability emphasizes the morphological fact +that the biliary bladder is only a modified portion of the hepatic duct +system, as shown by the development above outlined. + +A great variety is observed in the arrangement of the biliary ducts, +through which, at the period of intestinal digestion, bile passes from +the liver and gall-bladder into the intestine, while in the intervals of +digestion the secretion is only carried from the liver to the bladder. +The following main types of the biliary duct system may be recognized: + +1. The hepatic duct joins the cystic to form the common bile-duct, +entering the duodenum by passing obliquely through the intestinal wall +(Fig. 242). This form is encountered in man and in most mammals. It is +also found in some birds (_Buceros_), many amphibians, and in some fish +(_Lophius_). Instead of one hepatic duct two may join the cystic duct +separately to form the common bile duct (_Phoca litorea_), or the number +of hepatic ducts may be further increased. The separate hepatic ducts +then unite successively with the cystic duct. This occurs in many +mammals (as _Tarsius_, _Galeopithecus_, monotremes) and in some fishes +(_Xiphias_, _Trigla_, _Accipenser_) (Fig. 243). + +[Illustration: FIG. 242.--Schema of hepatic and cystic ducts. (Nuhn.)] + +[Illustration: FIG. 243.--Schema of hepatic and cystic ducts. (Nuhn.)] + +2. Of two hepatic ducts only one helps to form with the cystic duct the +common duct, while the other leads from the liver transversely into the +bladder, especially into the neck, forming the hepatico-cystic duct +(Fig. 244). This arrangement is found in several mammals (calf, sheep, +dog). + +[Illustration: FIG. 244.--Schema of hepatic and cystic ducts. (Nuhn.)] + +[Illustration: FIG. 245.--Schema of hepatic and cystic ducts. (Nuhn.)] + +3. No common bile-duct is formed. The hepatic and cystic ducts each +empty separately into the intestine (hepato-enteric and cysto-enteric +ducts), while a hepato-cystic duct carries the bile directly from the +liver to the gall-bladder (Fig. 245). + +_Lutra vulgaris_ among mammalia, the majority of the birds and several +reptilia present this type. + +When the gall-bladder is absent a single large hepato-enteric duct is +found, or instead a number of smaller ducts which enter the intestine +successively. + +=I. C. Development of Vascular System of Liver.=--In order to comprehend +the peritoneal relations of the adult liver it is absolutely necessary +to have a clear understanding of the development of the vascular system +in connection with the gland. + +For our purpose, in the first place, a serial consideration of the +successive stages, illustrated by schematic diagrams, will prove most +practicable. These diagrams represent the structures in the dorsal view, +_i. e._, in the position which they would occupy in the adult liver with +the gland resting on its upper or convex surface and with the ventral +sharp margin turned toward the beholder (see Fig. 259). + +The development of the venous system, especially in connection with the +liver, presents a somewhat complicated series of successive conditions. +After having become familiar with the principal typical embryonal +stages, as shown in the following diagrams, the student is strongly +recommended to cement this knowledge by the comparative examination of +the venous system. The permanent veins of the lower vertebrates, while +in many cases not strictly homologous to those of the higher forms, yet +are excellent objects for study, since they serve to illustrate +temporary stages in the development of the mammalian venous system, and +to that extent are of aid in comprehending one of the most difficult and +important chapters in human anatomy. At the conclusion of the +diagrammatic consideration of the mammalian development a number of +comparative facts will be put together for this purpose. + +=1. Early Stage.=--In the earlier developmental stages in mammalian +embryos the primitive dorsal aorta extends caudad along the ventral +aspect of the vertebral axis, giving off paired vitelline or +omphalo-mesenteric arteries to the yolk-sac and allantoic arteries to +the embryonic urinary bladder or allantois (Figs. 246 and 247). + +[Illustration: FIG. 246.--Diagram of embryonic vascular system, without +the portal circulation. (Parker, after Wiedersheim.) The dorsal aorta is +formed by the junction of the right and left aortic roots arising from +the confluence of the branchial arterial arches.] + +The blood is returned from the vascular area of the yolk-sac by two +vitelline or omphalo-mesenteric veins, which unite near the heart to +form a common trunk, continued as the _sinus venosus_ into the caudal or +auricular extremity (venous end) of the primitive tubular heart (Figs. +246, 247 and 248). + +[Illustration: FIG. 247.--Diagram of the circulation of the yolk-sac at +the end of the third day of incubation in the chick. (After Balfour.) +The median portion of the first aortic arch has disappeared; but its +proximal end forms the external, its distal the internal carotid +arteries. The whole blastoderm has been removed from the egg and is +viewed from below. Hence the left appears on the right, and _vice +versa_. + +Arteries in black. + +Veins in outline.] + +[Illustration: FIG. 248.--Schema of vitelline veins.] + +=2. Development of Allantois. Stage of Placental Circulation.=--The +placental circulation, replacing the temporary vitelline circulation of +the earliest stages, is inaugurated by the appearance of two umbilical +veins, which pass cephalad, imbedded in the tissue of the ventral +mesogastrium, to empty into the sinus venosus near the vitelline veins +(Fig. 249). The umbilical veins return the oxygenated blood from the +placenta to the embryo. At first the right umbilical vein is the larger +of the two. + +[Illustration: FIG. 249.--Schema of umbilical veins, early stage.] + +The sinus venosus at this time also receives two large veins, +transversely directed, called the ducts of Cuvier, which are formed near +the heart by the union of the anterior cardinal (primitive jugular) and +posterior cardinal veins, draining respectively the head end of the +embryo, and the body walls and Wolffian bodies. + +The vitelline veins are placed on each side of the primitive small +intestine, and become connected with each other by a broad anastomotic +branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the +vitelline veins send out branches which break up into a wide-meshed +capillary network in the mesodermic tissue enveloping the hepatic +cylinders. Hence at this period the circulation in the vitelline veins +is made up of three districts: + +(_a_) Distal segment of veins, coursing along duodenum, and joined by a +transverse anastomosis, before reaching the liver bud (subintestinal +veins). + +(_b_) Middle segment, from which capillary vessels are derived, +ramifying upon and between the developing hepatic cylinders. + +(_c_) Proximal segment, formed by the continuation of the proximal part +of the vitelline veins into the sinus venosus of the heart. + +[Illustration: FIG. 250.--Schema of primitive portal circulation.] + +=3. Formation of Portal Circulation. A.=--With the further development +of the liver the direct connection of the distal segment of the +vitelline veins with the sinus venosus becomes lost, the intermediate +segment being entirely broken up into an intrahepatic network (Fig. +250). Hence all the blood brought to the liver by the vitelline veins +(venae hepaticae advehentes) passes through the hepatic capillary +circulation, before it is carried by the proximal segment of the +vitelline veins (venae hepaticae revehentes) into the sinus venosus. The +amount of this blood increases with new connections which the vitelline +veins make with the venous radicles developing in the intestinal tract +and its appendages. In proportion as, with the development of the +placenta and reduction of the yolk-sac, the original significance of the +vitelline veins as nutritive and respiratory vessels disappears, this +secondary connection of the vitelline veins with the veins of the +alimentary tract becomes more and more important, until finally the +original vitelline veins, now properly called omphalo-mesenteric veins, +return the blood from the intestinal tube, pancreas and spleen to the +liver. + +The venae hepaticae advehentes, becoming connected in this way with the +developing intestine, pancreas and spleen, form the rudiments of the +future portal system, while the venae hepaticae revehentes are prototypes +of the hepatic veins of the adult circulation. + +=B. Development of the Portal Vein.=--The distal subintestinal segments +of the vitelline veins are early united by a transverse anastomotic +branch. The section of the veins above this anastomosis is seen already +in Fig. 250 to have assumed an annular shape, while the veins below the +primary anastomosis are approaching each other to form a second +ring-like junction. + +In Fig. 251 the subintestinal segments of the two vitelline veins are +seen to have communicated with each other by transverse anastomotic +branches around the duodenum, two of these branches being situated +ventrad and one dorsad of the intestinal tube. These branches, and the +portions of the primitive vitelline veins between their points of +derivation, form two vascular loops or rings, encircling the primitive +duodenum (Fig. 251). + +[Illustration: FIG. 251.--Schema of further development of portal +circulation and connection of same with umbilical veins in early +stages.] + +The distal portions of the vitelline veins, before reaching the caudal +annular duodenal anastomosis, next fuse into a single longitudinal +vessel which also receives the veins from the stomach, intestine, +spleen, and pancreas, and forms the beginning of the portal vein. + +By atrophy of the right half of the lower, and of the left half of the +upper duodenal venous ring (Figs. 252 and 253), the proximal portion of +the portal vein is formed as a single vessel, taking a spiral course +around the duodenum (Fig. 256). Hence in the adult the portal vein and +its principal branch (the superior mesenteric vein) crosses over the +ventral surface of the duodenum (third portion), turns along the mesal +side of the second portion, and then continues to the liver along the +dorsal aspect of the first portion (Fig. 254). _Note_--In comparing Fig. +254 with the schematic figures it should be noted that the same presents +the parts in the _ventral_ view, while the schemata offer the _dorsal_ +aspect. + +[Illustration: FIG. 252.--Second stage in development of circulation +through portal and umbilical veins. The proximal segment of the main +portal vein is formed by the persistence of the left half of the distal +and right half of the proximal periduodenal vascular ring of the +omphalo-mesenteric veins. The distal segment of the main portal vein is +the product of the fusion of the omphalo-mesenteric veins, and becomes +connected with the veins of the intestinal canal, pancreas, and spleen. +The proximal terminal segment of both umbilical veins becomes included +in the system of the venae hepaticae revehentes.] + +[Illustration: FIG. 253.--Third stage in development of portal and +umbilical veins during the placental period.] + +[Illustration: FIG. 254.--Corrosion preparation showing course of portal +vein and tributaries in relation to duodenum. (Columbia University +Museum, No. 1857.)] + +[Illustration: FIG. 255.--Human embryo of 10 mm. cervico-coccygeal +measure. Heart and ventral body-wall removed to show sinus venosus and +entering veins. (Kollmann, after His.)] + +[Illustration: FIG. 256.--Final stage of development of portal and +umbilical veins in the placental period.] + +=4. Changes Leading to the Final Arrangement of the Umbilical Veins.=--A +very important rearrangement of the umbilical veins takes place. These +veins originally course in the lateral abdominal wall, close to the fold +of the amnion (Fig. 255), and then turn cephalad of the developing liver +along the septum transversum to empty into the sinus venosus at each end +(Figs. 249 and 250). The right umbilical vein is at first the larger. + +This symmetrical arrangement, and the direct connection of the umbilical +veins with the sinus venosus, now becomes lost by the occurrence of the +following changes: + +1. At first (Fig. 249) all the blood carried to the liver by the +omphalo-mesenteric veins passes through the hepatic capillary network +before being conducted by the venae revehentes to the sinus venosus. Very +early, however, a new intrahepatic channel develops, the ductus venosus +(Figs. 250-253), which passes obliquely between the entrance of the left +omphalo-mesenteric vein into the capillary system (l. v. advehens) and +the termination of the right omphalo-mesenteric vein (r. vena revehens) +in the sinus venosus. + +In human embryos of 4 mm. the ductus venosus can already be +distinguished, and in embryos of 5 mm. the vessel has assumed +considerable proportions. + +2. A communication is next established on both sides between the +capillary hepatic network in the portion of the liver nearest to the +abdominal wall and the umbilical veins as they ascend imbedded in the +abdominal wall (Fig. 251). + +This connection is usually from the start larger on the left side and +connects with the left omphalo-mesenteric vein just at the point where +the same is about to be continued into the ductus venosus. This +connection becomes rapidly larger, so that the ductus venosus, which at +first appeared merely as an anastomotic channel between the left +omphalo-mesenteric vein and the terminal portion of the right +omphalo-mesenteric vein, now forms the main continuation of the left +umbilical vein. This vessel grows very rapidly up to its connection with +the ductus venosus and soon exceeds the right umbilical vein in size +(Fig. 252). Beyond the ductus venosus on the other hand the proximal +segment of the left umbilical vein diminishes in size, and loses its +independent character by incorporation in the hepatic circulation. Only +its terminal portion, emptying into the sinus venosus, is preserved. +This is surrounded by the growing masses of hepatic cylinders and is +converted into a vena revehens. + +The connection of the right umbilical vein with the liver vessels is at +first symmetrical to that on the left side, but less strongly developed. +The effect of this connection is to reduce in the same way the proximal +segment of the right umbilical vein and to convert its termination into +a vena revehens. With the great development of the left vein, however, +the vein on the right side gradually diminishes and finally loses its +connection with the intrahepatic circulation altogether. The right +umbilical vein is now reduced to a vessel of the ventral abdominal wall, +which carries blood in the reverse of the original direction, _i. e._, +from the abdominal wall caudad _into_ the left umbilical vein (Figs. 253 +and 255). + +The connection thus established between the umbilical vein and the +portal circulation results in the formation of a single large (the +original left) umbilical vein which, throughout the remainder of foetal +life, returns all of the placental blood (Fig. 253). + +The newly developed hepatic portion of the left umbilical vein becomes, +however, not only connected with the ductus venosus, but also with the +right part of the upper venous ring, derived from the right +omphalo-mesenteric vein (Fig. 253). This connection forms the left +portal vein of the adult, and enlarges rapidly. + +The terminations of the ductus venosus and of the venae hepaticae +revehentes undergo a number of secondary changes in relative position. +The left hepatic vein loses its direct connection with the sinus +venosus, and now opens into the termination of the ductus venosus, into +which the right hepatic vein also empties. This common vessel (v. +hepatica communis) subsequently forms the proximal segment of the +postcava when this vessel develops (Fig. 256). + +The blood, therefore, returned to the liver by the left umbilical vein +divides at the transverse fissure into three streams. Two of these pass +through the connection with the portal vein and through branches +developed from the hepatic part of the umbilical vein into the capillary +system of the right and left lobe. The third continues through the +ductus venosus to the common hepatic vein and sinus venosus (Fig. 256). +The ductus venosus thus becomes the chief vessel returning arterialized +placental blood to the heart. When the postcava develops fully the +hepatic segment of this vessel also joins the terminal part of the +ductus venosus (Fig. 256) and gradually replaces the same as the main +returning venous channel, the proximal part of the ductus venosus being +incorporated in the vena cava (Fig. 257). The postcava then receives the +right hepatic veins separately, while the left hepatic veins and ductus +venosus open together into the main vein. This condition obtains up to +the time of birth and the consequent interruption of the placental +circulation. + +[Illustration: FIG. 257.--Schema of relation of postcava to hepatic +veins and ductus venosus.] + +While at first the ductus venosus communicates throughout its entire +length with the meshwork of the hepatic capillary system, a separation +into two segments, _i. e._, ductus venosus proper and intrahepatic +segment of umbilical vein, is established after the free communication +with the left umbilical vein takes place. This condition is exhibited in +Fig. 258, which represents the corroded venous system of the foetal +liver, and in Fig. 259, showing an injected liver in the foetus at term. + +[Illustration: FIG. 258.--Corrosion preparation of venous system of +human liver in foetus at term. (Columbia University Museum, No. 1834.)] + +[Illustration: FIG. 259.--Injected and hardened human liver from foetus +at term. (Columbia University Museum, No. 1853.)] + +It will be observed that the umbilical vein on entering the liver gives +off a large branch to the left lobe, and a smaller branch on the right +side to the quadrate lobe, which act as the main venae advehentes of +these portions of the liver. Arrived at the transverse fissure the +umbilical vein divides into three branches, at right angles to each +other. The left branch enters the left lobe, the right branch becomes +directly continuous with the left main division of the portal vein, +while the central branch, continuing the direction of the umbilical +vein, passes dorsad, as the ductus venosus proper, to join the left +hepatic vein close to its entrance into the postcava. + +=5. Changes Consequent upon the Establishment of Pulmonary +Respiration.=--After birth the umbilical vein and its continuation, the +ductus venosus, become obliterated, the former constituting the round +ligament of the liver, the latter the ligament of the ductus venosus, +both structures imbedded in corresponding portions of the sagittal +fissure on the caudal and dorsal surfaces of the adult liver (Figs. 284 +and 286). The lateral branches of the umbilical vein, however, in its +course from the ventral margin of the liver to the transverse fissure +(Fig. 258), remain pervious and are transferred to the portal +circulation. + +[Illustration: FIG. 260.--Diagram of intrahepatic foetal venous +circulation.] + +[Illustration: FIG. 261.--Diagram illustrating the changes in the +intrahepatic venous circulation resulting from the cessation of the +placental circulation at birth.] + +It will be noticed, in reference to the _direction_ of the blood +current, that at birth a sudden reversal takes place in the right +terminal branch of the umbilical vein at the transverse fissure (Figs. +260 and 261). Before birth the blood current of the umbilical vein +divides into three streams, right, left and central. The latter enters +the ductus venosus. The left enters the liver directly, the right +traverses, from left to right, the segment between the termination of +the umbilical and the bifurcation of the portal vein. This segment in +the adult carries blood from right to left, as left branch of the portal +vein. In the foetus, however, the blood traverses this segment from left +to right, in passing from the umbilical to the right branch of the +portal vein. The blood entering the liver through the portal vein passes +chiefly into the right division of that vessel (Fig. 260). + +After birth all the venous blood entering the liver passes +through the portal vein. In the right division the direction of the +current is the same as in the foetus. + +On the left side, however, the current is now from right to left, from +the bifurcation of the portal into the channels of the left lobe +formerly connected with the umbilical vein (Fig. 261). + +Hence the direction of the current in this segment is reversed at birth. + + +SUMMARY OF HEPATIC CIRCULATION. + +The foregoing consideration of the development shows us that the hepatic +circulation presents successively three main stages: + +=1. Omphalo-mesenteric or Vitelline Stage=, which results in the laying +down of the primary capillary circulation of the liver and in the +establishment of its connection with the developing veins of the +alimentary tract (primitive portal channels). + +=2. Umbilical or Placental Stage=, in which the greater part of the +blood circulating through the liver is oxygenated blood returned from +the placenta by the umbilical vein, accounting for the rapid growth and +relatively large size of the organ during foetal life. + +The placental blood uses the preformed capillary channels of the +vitelline or primitive portal system in the liver, and the same rapidly +extend and enlarge with the accelerated growth of the gland. During this +stage venous blood is also returned from the alimentary tract to the +liver by the portal vein, produced by fusion of the distal segments of +the primitive vitelline veins and their secondary connection with the +mesenteric, splenic and pancreatic veins (omphalo-mesenteric development +of primitive vitelline veins). + +=3. Adult or Portal Stage.=--With the interruption of the placental +circulation the portal vein assumes again its original position as the +only vein carrying blood to the liver. With the establishment of +intestinal digestion and absorption this vessel grows rapidly in size. + + +COMPARATIVE ANATOMY OF THE HEPATIC VENOUS CIRCULATION. + +For the purpose of fixing the main facts in connection with the +development of the higher mammalian hepatic circulation, and in order to +obtain a demonstration of the cycle through which the different veins +pass, the student is recommended to examine, preferably by personal +dissection, a limited series of lower vertebrates which can be readily +procured and easily injected. The following series has been selected, +but it will be understood that other forms can be substituted, according +to the local conditions which govern the supply of the material. + +1. _Fish._ _A Selachian_, the common skate (_Raja + ocellata_) or dog-fish (_Acanthias vulgaris_). + +2. _Amphibian._ + (_a_) Urodele. _Necturus maculatus._ + (_b_) Anura. The common _frog_. + +3. _Reptile._ + +Preferably, on account of the ease of injection, one of the larger +lizards, as _Iguana tuberculata_. + +The turtles, although somewhat more difficult objects to prepare, can be +substituted. + +4. _Bird._ The common fowl. + +5. Human foetus at term. + +=1. Fish.=--The venous system can be injected by tying a canula in the +lateral vein, and injecting both cephalad and caudad, or by injecting +cephalad through the caudal vein. The injection of the systemic veins +can also be made caudad through one of the ducts of Cuvier, combined +with an injection cephalad of the caudal vein. + +[Illustration: FIG. 262.--Diagram of the veins of a selachian. +(Wiedersheim, after Parker.) + +The lateral vein arises from a venous network surrounding the cloaca, +receiving one or more cutaneous veins of the tail, veins of the +body-wall, and veins of the pelvic fins. + +The caudal vein divides at the posterior end of the kidney into the two +renal-portal veins, from which the advehent veins of the renal-portal +system are derived. The revehent renal-portal veins join to form the +posterior cardinal veins, which, after dilating enormously to form the +cardinal sinuses, join with the anterior jugular, subclavian, and +lateral veins to form the ducts of Cuvier. The latter receive the +inferior jugular veins, from the deep parts of the head and neck and the +terminations of the hepatic portal system (hepatic sinus). + +The hepatic portal vein is formed by the veins of the oesophagus, +stomach, and intestines. After traversing the capillary vessels of the +liver, the revehent hepatic veins unite to form an extensive hepatic +sinus before entering the heart.] + +The following main facts are to be noted in the venous system of the +Selachian (Fig. 262): + +=1. There are Two Portal Systems.= (_a_) _Renal Portal System._--The +caudal vein divides near the vent into two branches which course along +the lateral border of the kidneys, sending _afferent_ or _advehent_ +veins into the organ. The blood traverses the renal capillaries and is +gathered together by the _efferent_ or _revehent_ veins, which empty +into median paired vessels, the posterior cardinals. + +(_b_) _Hepatic Portal System._--The veins of the digestive tract and +appendages unite to form a hepatic portal vein. The blood after +traversing the capillary system of the liver is collected by hepatic +veins, which form a dilated hepatic sinus emptying into the sinus +venosus of the heart. + +2. The middle segment of the intestine, presenting a spiral valve in the +interior, gives rise to a vein emptying into the portal vein which +corresponds to the subintestinal vitelline vein of the mammalian embryo +(Fig. 202). + +3. The posterior cardinal veins, also greatly dilated and forming the +posterior cardinal sinus, join, near the heart, the veins returning +blood from the head, the anterior cardinal or jugular, to form a +transversely directed trunk, the duct of Cuvier, which empties into the +sinus venosus at the auricular extremity of the heart. Into the duct of +Cuvier empties on each side a _lateral vein_ returning the blood from +the body walls. This vein can be considered, for our present purpose, as +representing in general the abdominal vein of amphibians and reptiles, +and the umbilical vein of the mammalian embryo. + +The adult selachian venous system is therefore to be considered as +illustrating the following conditions above encountered in our study of +the embryology of the mammalian venous system. + +1. The heart illustrates excellently the stage in the mammalian +development, in which auricular and ventricular segments have +differentiated, but before the division of the cavities into a pulmonary +and systemic portion by the development of the auricular and ventricular +septa and the division of the arterial trunk into pulmonary artery and +aorta. + +The sinus venosus still exists, as an ante-chamber to the auricular +cavity proper, receiving on each side the ducts of Cuvier, which +represent the fusion product of the systemic veins, anterior and +posterior cardinal. + +2. The hepatic portal circulation corresponds to the mammalian stage in +which the vitelline veins have become omphalo-mesenteric by joining the +intestinal veins. + +The spiral vein remains as a portion of the original vitelline vein +corresponding to the subintestinal segment of the mammalian embryo (cf. +Figs. 248 and 249). + +The selachian portal vein represents the united vitelline veins, into +which the veins of the digestive tract open. + +In the liver we find a simple system of venae advehentes, derived from +the branching of the portal vein, a hepatic capillary network, and venae +revehentes, the proximal remnants of the original vitelline veins which +carry the liver blood to the sinus venosus. The condition of the hepatic +circulation corresponds therefore to the stage shown in Fig. 250 of the +mammalian development. There is as yet no association of the hepatic +venous system with the representative of the umbilical vein (the lateral +vein of the selachian). + +3. The lateral veins, which we can, as stated, regard for purposes of +illustration, without prejudging their genetic significance, as +representing the mammalian embryonic umbilical veins, still present the +condition corresponding to the early mammalian embryonal stage shown in +Fig. 250. They are veins of the body walls, emptying cephalad of the +liver, directly into the ducts of Cuvier, and through them into the +sinus venosus of the heart. + +Fig. 262 shows the arrangement of the venous system in a typical +selachian diagrammatically. + +=2. Amphibian.= (_a_) =Urodele.=--The following points are to be noted +in comparison with the preceding form: + +1. The two ducts of Cuvier entering into the sinus venosus are formed by +the anterior cardinal and subclavian veins, which latter, having +appeared with the full development of an anterior extremity, receives +the posterior cardinal veins, representing the mammalian azygos system. + +2. The renal portal circulation persists. The caudal vein is, however, +no longer the only afferent vein of this system. With the full +development of a posterior extremity an iliac vein returns the blood +from the same and gives a large branch (afferent to the portal renal +system), while the trunk continues cephalad as an anterior abdominal +vein, corresponding to the lateral selachian vein, emptying in the +hepatic portal vein. + +3. The efferent veins of the renal portal system no longer unite to form +the posterior cardinal, as in the Selachian, but empty into a new median +vessel, the inferior vena cava, or postcava, which has replaced the +distal segments of the posterior cardinal veins. + +The postcava now carries the blood from the kidneys directly to the +heart. The original posterior cardinal veins still persist in their +proximal segments, as smaller trunks connecting the distal part of the +postcava with the ducts of Cuvier through the subclavian veins. The +ducts of Cuvier represent the precavae (venae cavae superiores) of mammalia +and the postcardinals the mammalian azygos veins. + +4. The hepatic portal system differs in two respects from the Selachian +type. + +(_a_) The blood returned to the liver from the digestive tract by the +portal vein becomes mixed before entering the gland with the blood +returned from the posterior extremities and abdominal walls by the +abdominal vein. + +This vein, paired below and continuous with the lateral of the two +branches into which the iliac vein divides, becomes united into a single +trunk above and empties into the portal vein. + +The abdominal vein represents the lateral vein of the Selachian and +corresponds to the umbilical vein of the higher vertebrates. + +(_b_) The venae hepaticae revehentes do not empty directly into the sinus +venosus, but into the proximal portion of the postcava. + +Hence the adult urodele venous system illustrates, in reference to the +mammalian development, these stages: + +1. The umbilical (abdominal) vein has lost its direct connection with +the sinus venosus. The proximal segment, cephalad of the liver, has +disappeared, and its blood now passes directly into the hepatic +circulation by its union with the portal vein. + +(Cf. stage schema Figs. 251 and 252.) + +2. The postcaval vein has made its appearance, largely replacing the +posterior cardinal veins, whose proximal segments became converted into +secondary vessels (azygos) uniting the system of the postcava with that +of the duct of Cuvier (mammalian praecava), while their distal segments +are transformed into the distal portion of the postcava. + +The postcava, therefore, is made up of two districts: + +(_a_) The proximal portion is a new vessel, developed in connection with +the hepatic venous system. + +(_b_) The distal portion is derived from the distal segments of the +original posterior cardinal veins. + +The termination of the hepatic veins in the postcava corresponds to the +stage shown in schema Fig. 256. + +Fig. 263 gives a schematic representation of the arrangement of the +venous system in a typical urodele amphibian (_Salamandra maculosa_). + +[Illustration: FIG. 263.--Diagram of the veins of urodele amphibian +(_Salamandra maculosa_). (Wiedersheim.) + +The caudal vein bifurcates at the posterior extremity of the kidneys to +form the afferent trunks of the renal-portal system along the lateral +border of the kidneys, from which the advehent veins of the renal-portal +system are derived. The iliac or femoral vein divides into an anterior +and a posterior branch, the latter opening into the afferent +renal-portal vein, while the former, uniting with the one of the +opposite side, forms the abdominal vein, and receives vessels from the +bladder, cloaca, and end-gut. The revehent veins of the renal-portal +system, emerging upon the ventral surface of the kidneys, empty into a +single median vessel, the distal or renal section of the postcava or +vena cava inferior. Proceeding cephalad, the proximal or hepatic section +of this vessel, after traversing the liver and receiving the revehent +hepatic veins of the hepatic portal system, empties into the sinus +venosus of the heart. Previous to entering the liver the postcava gives +off the two posterior cardinal or azygos veins, which continue cephalad, +receiving tributary segmental veins from the body-walls and reach the +sinus venosus by joining the subclavian veins. These latter uniting with +the anterior cardinal (jugular) veins form the ducts of Cuvier (precaval +veins). + +The abdominal vein continues cephalad in the ventral mesogastrium to the +liver, giving off a number of smaller branches, which enter the hepatic +portal circulation by penetrating the ventral surface of the liver +between the layers of the ventral mesogastrium, while the main +continuation of the vessel joins the hepatic portal vein at its point of +entrance into the liver. + +The hepatic portal vein is formed by tributaries returning the blood +from the digestive tract (intestinal canal, spleen, pancreas). The +blood, after traversing the hepatic portal circulation, is conducted by +the hepatic revehent veins to the proximal section of the postcava. A +number of secondary or accessory portal veins pass from the anterior +portion of the intestinal canal (oesophagus, stomach) directly to the +liver.] + +In Fig. 264 the dissected venous system of _Necturus maculatus_, the mud +puppy, is shown in an injected preparation. + +[Illustration: FIG. 264.--Dissection of veins of _Necturus maculatus_, +mud-puppy. (Columbia University Museum, No. 1835.) + +The postcava has been divided at the cephalic end of the liver just +before entering the sinus venosus, and the postcardinals have been cut +prior to their junction with the subclavian veins. + +The stomach has been turned caudad. The abdominal vein has been divided +after the common trunk has been formed by branches from the iliac veins. +The latter are seen entering the afferent renal-portal vein, derived +from the bifurcation of the caudal vein, along the lateral border of the +kidneys. + +The junction of the main trunk of the abdominal vein with the hepatic +portal vein takes place close to the liver under cover of the pancreas. +A series of accessory portal veins continuous with the abdominal vein +enter the ventral surface of the liver between the layers of the ventral +mesogastrium. The inter-renal segment of the postcava receives the +revehent renal-portal veins. The iliac vein enters the advehent +renal-portal veins derived from the caudal vein.] + +[Illustration: FIG. 265.--Venous system of _Rana esculenta_, frog. +(Ecker.)] + +(_b_) =Anure.=--The venous system of _Rana esculenta_ is shown in Fig. +265. Comparison with venous system of _urodele_: + +1. The abdominal vein, corresponding to the mammalian umbilical vein, +has assumed a greater importance in reference to the hepatic +circulation. It is a large trunk, continuous below with the pelvic vein, +terminating above in two branches, which enter the liver as afferent +veins, being joined just prior to the division by the hepatic portal +vein. + +2. A small cardiac vein, coming from the heart, empties into the angle +of bifurcation of the abdominal vein. + +3. The postcava is well developed, formed by large efferent renal veins. +It entirely replaces the posterior cardinal veins which are absent in +the adult animal. + +4. A right and left praecaval vein is formed by the union of two jugular +trunks with the vein of the anterior extremity and a large +musculo-cutaneous vein. + +Comparison with the mammalian development: the venous system of this +amphibian can be used to illustrate the mammalian embryonal stage shown +in schema Fig. 252, in which the abdominal or umbilical vein has become +the most important vessel in the afferent hepatic venous system. + +The communication existing by means of the cardiac vein between the +heart and the hepatic afferent system may suggest, but _purely for +illustrative purposes_, the direct connection of the umbilical vein with +the heart by the ductus venosus in the mammalian embryo (cf. schema +Figs. 250-256). + +=3. Reptile.=--In _Iguana_ the renal portal system is well developed. +The caudal vein, returning the blood from the tail and the cavernous +tissue of the genital organs, continues for a short distance upon the +fused caudal end of the two kidneys (Fig. 269) and then divides into two +afferent renal veins which ascend on the ventral surface of the glands, +giving branches to the renal capillary system. About the middle of the +kidney each afferent vein is joined by a large transverse branch from +the abdominal vein (Fig. 266). + +[Illustration: FIG. 266.--Systemic veins of _Iguana tuberculata_. The +alimentary canal and appendages, together with the hepatic portal vein +and the intrahepatic segment of the postcava, have been removed. The +liver occupies the space between the divided ends of the postcava. The +vertebral vein represents the rudimentary proximal segment of the +postcardinal vein corresponding to the mammalian azygos vein. (Columbia +University Museum, No. 1320.)] + +[Illustration: FIG. 267.--Veins of _Iguana tuberculata_. Connection of +systemic veins with sinus venosus of heart. The rudimentary system of +the vertebral (azygos) veins and their proximal connection with the +subclavian vein are shown. (Columbia University Museum, No. 1859.)] + +[Illustration: FIG. 268.--Corrosion preparation of venous system of +liver in _Iguana tuberculata_. The hepatic portal system and its +connection with the abdominal vein, as well as the relation to the +postcava, are shown. The preparation supplements Fig. 266, showing the +parts which have been removed in the latter. (Columbia University +Museum, No. 1860.)] + +[Illustration: FIG. 269.--_Iguana tuberculata_, male. Genito-urinary +tract, dorsal view, with renal-portal, postcardinal, and postcaval +veins. (Columbia University Museum, No. 1862.)] + +The renal efferent system begins by a number of inter-renal anastomoses +which unite along the mesal border of the right kidney into a large +ascending trunk, while the corresponding vessel of the left side, +starting from the same anastomosis, is considerably smaller (Figs. 266 +and 269). Each of these vessels also receives blood from the testis, +epididymis, vas deferens and adrenal body in the male, and from the +ovary and oviduct in the female. They represent, in fact, the distal +functional part of the right and left embryonic postcardinal vein. Just +caudad of the left testis the vein of the left side crosses obliquely +ventrad of the aorta and joins the right vessel to form the trunk of the +postcava, which enters, immediately beyond the cephalic pole of the +right testis, the prolonged caval lobe of the liver (Figs. 266 and 269). +Ascending in the substance of this gland and receiving the afferent +hepatic veins (Fig. 268), the vena cava emerges from the cephalic +surface of the liver greatly enlarged and proceeds to the right +auricle. + +The abdominal vein divides below into two branches which pass caudad on +each side of the bladder, receiving tributaries from the same, to the +lateral border of the kidneys (Figs. 266 and 269). Here the vessel is +connected by the transverse branch above described with the afferent +renal portal system derived from the caudal vein. At the same point it +receives the sciatic vein, the principal venous vessel of the posterior +extremity. Above, the main abdominal vein, resulting from the union of +the two branches referred to, ascends on the dorsal surface of the +ventral abdominal wall, receiving a few twigs from the ventral +mesogastrium within whose free caudal edge the vessel runs. Just before +reaching the liver the abdominal vein turns dorsad on the caudal surface +of the gland and joins the hepatic portal vein (Figs. 268 and 275). +Several accessory veins, two or three in number, belonging to the system +of the abdominal vein, pass above this point from the ventral body wall +between the layers of the ventral mesogastrium, to enter the liver +separately on its convex ventral surface, above the fusion of the main +abdominal vein with the portal vein. These additional branches on +entering the liver join the portal system, forming a set of ventral +accessory portal veins. + +The hepatic portal vein derives its principal tributaries from the +splenic, gastric, pancreatic and intestinal veins. One or two additional +branches (accessory vertebral portal veins), as above stated, connect +the system of the segmental and vertebral veins with the portal +circulation, entering the liver separately. In like manner one or two +gastric veins (accessory gastric portal veins) enter the dorsal aspect +of the liver separately, passing from the stomach to the gland between +the layers of the gastro-hepatic omentum (Fig. 275). + +Compared with the development of the mammalian type, the venous system +of Iguana serves to illustrate the stage in the history of the umbilical +vein (represented by the abdominal vein of the reptile) in which the +connection of the vessel with the portal vein has been formed and +transmits the greater part of the blood returned by the +umbilical vein to the liver, while the proximal segment above this +point, originally continued into the sinus venosus, has begun to +disappear, being, however, still represented by the vessels which, as +accessory ventral portal veins, pass in the ventral mesogastrium, from +the body wall to the liver. + +It will be noted that all the hepatic portal blood, whether conducted by +the main portal and abdominal vein, or by the accessory portal branches, +traverses the capillary circulation of the liver before entering the +postcava. + +The vertebral and segmental venous system, representing the azygos veins +of the mammalia, is very rudimentary (Figs. 266 and 267). The distal +portions of the postcardinal veins form the efferent renal branches and +the ascending trunks of the postcava. + +The next segment of the vertebral veins appears as a trunk on the right +side which enters the portal circulation. A second vein higher up is +connected with both the gastric portal system and with the longitudinal +chain of the vertebral veins. Finally a proximal venous branch on each +side of the vertebral column, representing the upper portion of the +postcardinal veins, receives the proximal segmental veins and empties +into the subclavian vein (Fig. 267). + +[Illustration: FIG. 270.--Veins of pigeon, _Columba livia_. (Modified +from Parker and Haswell.) The renal-portal vein of the right side is +supposed to be dissected to show its passage through the right kidney.] + +=4. Bird.=--The characteristic change in the venous system of the bird, +as compared with that of the amphibian and reptile, is found in the +nearly complete abolition of the renal portal system. The caudal vein +bifurcates, sending on each side a large trunk, which receives the +pelvic (int. iliac) veins, to the kidney (renal afferent portal vein), +but only a few small branches enter the substance of the gland (Fig. +270, afferent renal V). The main vessel continues cephalad through the +kidney and, after receiving the vein from the posterior extremity +(femoral), unites as common iliac vein with the vessel of the opposite +side to form the postcava. This vessel traverses the liver, receiving +the hepatic afferent veins of the portal system. The portal vein is +formed by tributaries from the intestinal canal, pancreas and spleen, +and is also joined by a large coccygeo-mesenteric vein, which is given +off at the point of bifurcation of the caudal vein and receives +tributaries from the lower part of the alimentary canal. The abdominal +vein of amphibians and reptiles is represented probably by the +epigastric vein, which returns the blood from the omental mass of fat to +the hepatic veins. + +Compared with the mammal on the one hand, and with the lower types on +the other, the venous circulation of the bird illustrates the following +points: + +1. Extensive reduction of the renal portal system and direct formation +of postcava by the iliac veins, foreshadowing the condition found in the +mammal. + +2. Complete separation of the portal and systemic venous circulation in +the adult. Disappearance of the ventral abdominal vein as a vessel of +the body wall. + +=5. Human Foetus at Term.=--The student is recommended to examine, by +dissection and injection, the venous system of a foetus at term, noting +the following facts: + +1. Course of umbilical vein in ventral abdominal wall and along free +edge of falciform ligament to liver (Fig. 241), corresponding to the +position of the amphibian and reptilian abdominal vein (Figs. 264 and +275). + +2. Connection of umbilical vein in liver: + (_a_) With portal system (Figs. 258 and 271). + ({~GREEK SMALL LETTER ALPHA~}) With portal vein. + ({~GREEK SMALL LETTER BETA~}) With portal system of left and quadrate lobes by + branches derived directly from umbilical vein while + situated in the umbilical fissure (Fig. 258). + (_b_) With hepatic veins and postcava by the ductus venosus + (Figs. 258 and 271). + +[Illustration: FIG. 271.--Human foetus at term. Corrosion preparation of +heart and vascular system. (Columbia University Museum, No. 1858.)] + +3. Connection of the postcaval and precaval systems by the azygos veins +representing the proximal segments of the embryonic postcardinal veins +(Fig. 272). + +[Illustration: FIG. 272.--Human foetus at term. Postcava and azygos +veins. (Columbia University Museum, No. 1861.)] + +If possible the dissection of an injected foetus should be combined with +the examination of corrosion preparation of the foetal circulation and +especially of the venous system of the foetal liver (Figs. 258 and 271). + +3. The remnants of foetal structures in the adult liver (round ligament +and ligament of the ductus venosus) should be compared with the +structures from which they are derived in the foetus at term (umbilical +vein and ductus venosus). + + +II. THE VENTRAL MESOGASTRIUM. + +This membrane has been heretofore mentioned on several occasions. It now +remains for us to carefully consider its arrangement in detail, both as +regards the peritoneal relations of the liver and in reference to its +influence on the abdominal space as a whole. We can best accomplish this +purpose by considering the membrane in the first place in a purely +schematic manner. In contradistinction to the primitive common dorsal +mesentery, which extends the entire length of the alimentary tube, the +ventral mesentery, or properly the ventral mesogastrium, is confined to +the stomach and proximal portion of the duodenum. We can represent the +membrane as extending between the ventral abdominal wall and the ventral +border (later the lesser curvature) of the stomach and of the hepatic +angle of the duodenum. Cephalad it is connected with the embryonic +septum transversum (future diaphragm). Caudad its two layers pass into +each other in a free concave edge, including between them the umbilical +vein (free edge of falciform ligament of adult). Consequently a +schematic profile or lateral view of the membrane and its attachments in +the earlier stages would appear as represented in Fig. 273, while the +arrangement in transection would be as shown in Fig. 274. It will be +observed that the separation of the cephalic portion of the abdominal +cavity into symmetrical right and left halves, previously indicated in +discussing the primitive stomach and the dorsal mesogastrium, is +actually completed by the ventral mesogastrium. This complete separation +of the lateral halves of the coelom cavity ceases at the point where the +ventral mesogastrium terminates in the free concave edge carrying the +umbilical vein. Hence caudad of this falciform edge the two halves of +the cavity communicate freely with each other ventrad of the intestine +and dorsal mesentery. + +[Illustration: FIG. 273.--Schematic profile view of ventral mesogastrium +with developing liver.] + +[Illustration: FIG. 274.--Schematic transection of abdomen in region of +ventral mesogastrium.] + +This difference in the extent of the mesogastria is perhaps best +understood by reference to their relation to the first portion of the +duodenum. We have seen that the duodenum in the early stages is attached +dorsally by a portion of the common dorsal mesentery, which, after +differentiation of the intestinal tract, immediately follows the dorsal +mesogastrium proper, forming the mesoduodenum (Fig. 172). The proximal +portion of the duodenum (hepatic angle) is still included within the +fold of the ventral mesogastrium which membrane terminates immediately +beyond this point in the free edge surrounding the umbilical vein +(subsequent round ligament) (Fig. 172). The remainder of the duodenum is +devoid of any ventral attachment, being only connected to the dorsal +body wall by the mesoduodenum (Fig. 197). + +Subsequently, after the fourth month, while the right surface of the +mesoduodenum and descending duodenum adhere to the parietal peritoneum, +the peritoneal investment of the first portion or hepatic angle remains +free. This peritoneal covering of the proximal duodenal segment is +situated at the point where the caudal end of the ventral mesogastrium, +after surrounding the first portion of the duodenum, becomes continuous +with the dorsal mesentery forming the mesoduodenum. Obliteration of the +latter membrane by adhesion to the parietal peritoneum leaves the first +portion of the duodenum invested on both surfaces by the _lesser +omentum_, derived from the ventral mesogastrium. The ventral surface of +the gut is covered by the ventral layer, the dorsal surface by the +dorsal layer of the lesser omentum. These two layers become continuous +around the right free edge of the lesser omentum (hepato-duodenal +ligament) forming the ventral boundary of the foramen of Winslow (cf. +infra, p. 177). + +Returning to the schematic consideration of the ventral mesogastrium +above outlined (Figs. 273 and 274) we have to note the first important +change in the arrangement depending upon the development of the liver. +This organ, growing, as we have seen, from the duodenum, extends between +the two layers of the ventral mesogastrium, receiving a serous +investment from the same. At an early period the liver, developing thus +between the mesogastric layers, reaches the septum transversum and +becomes closely connected with it, laying the foundation for the +subsequent extensive attachment of the gland to the diaphragm. + +Extending caudad the liver grows beyond the caudal free edge of the +ventral mesogastrium on each side, carrying the serosa with it. +Consequently the ventral margin of the liver becomes indented at this +point; the umbilical vein and subsequently its fibrous remnant, the +round ligament, are imbedded in a notch and fissure (umbilical notch and +fissure) continued from the ventral margin dorsad along the caudal +surface of the liver (Fig. 259). + +This growth of the liver has now effected a division of the primitive +ventral mesogastrium into two segments: + +1. Ventral portion, between diaphragm and liver, forms the broad +falciform or suspensory ligament of the liver. + +2. The dorsal portion, between liver and stomach, forms the lesser or +gastro-hepatic omentum. + +The caudal free edge of the ventral mesogastrium extends between the +umbilicus and the caudal surface of the liver, carrying the umbilical +vein between its layers. The growth of the liver serves to bury this +free edge and the contained vein in a fissure on the caudal surface of +the liver. The same obtains in the case of the ductus venosus continued +from the umbilical vein (umbilical fissure and fissure of ductus venosus +of adult liver). Consequently the original continuity of the broad +ligament and lesser omentum, as parts of the primitive ventral +mesogastrium, is not readily seen in the adult. + +The broad ligament extends across the convex cephalic surface of the +liver uniting it to the ventral abdominal wall and diaphragm, while its +free falciform edge apparently stops at the umbilical notch in the +ventral border of the organ. Actually, however, the obliterated vein is +surrounded in the bottom of the fissure, by a peritoneal fold which +effects the junction between broad ligament and lesser omentum. + +We will see later in what way the permanent adult arrangement of the +lesser omentum is brought about. For the present we can state, on the +hand of the schematic Fig. 273, that the free caudal edge of the +falciform ligament containing the umbilical vein, and the free edge of +the gastro-hepatic omentum form together originally the caudal free edge +of the ventral mesogastrium, which membrane becomes separated, by the +growth of the liver, into suspensory or broad ligament and lesser or +gastro-hepatic omentum. + +[Illustration: FIG. 275.--Abdominal viscera of _Iguana tuberculata_. +(Columbia University Museum, No. 1313.)] + +This primitive disposition of the ventral mesogastrium and the viscera +connected with the same, is well shown in some of the lower vertebrates +in whom the development never proceeds beyond the early mammalian +stages. Fig. 275 shows in profile view from the right side the situs +viscerum and peritoneum in _Iguana tuberculata_.[5] The two dorsal +aortic roots are seen to unite to form the main aorta, which descends +between the layers of the dorsal mesentery, sending branches to the +dorsal margin of oesophagus and stomach. From the opposite border of the +stomach the ventral mesogastrium is derived. Its dorsal segment +(gastro-hepatic omentum) connects liver and stomach, carrying between +its layers the portal vessels, hepatic artery and biliary duct. The +ventral segment of the membrane, forming the suspensory or broad +ligament, extends between abdominal wall and ventral surface of the +liver. Caudad, the lesser omentum and the suspensory ligament are seen +to have a common concave falciform edge. + +[5] _Iguana tuberculata_, one of the large lizards native of South +America. This animal forms an excellent object for the comparative study +of the visceral and vascular anatomy of the abdomen. It possesses a +well-differentiated intestinal tract, several coils of small intestine, +a well-marked caecum and large intestine. The examination of this or a +similar reptilian form is to be highly recommended. Iguana is easily +obtained in any of our large cities, as a considerable number of these +animals are annually imported from Mexico and the South American states. + +The ventral abdominal vein ascends between the layers of the suspensory +ligament and near the liver becomes connected by a large branch with +the portal vein. A few smaller branches are seen passing from the +abdominal wall beyond this point. In this reptile, therefore, the +permanent vascular arrangement corresponds to an early human embryonic +stage. + +The reptilian ventral abdominal vein is the homologue of the umbilical +vein of the placentalia. The large branch passing to the portal vein +represents the connection established in the human embryo between the +umbilical and portal veins. The small branches, continuing cephalad +between the mesogastric layers, represent the temporary proximal +remnants which in the human embryo the umbilical veins form in +connection with abdominal walls. The permanent adult arrangement of this +part of the vascular system in this animal corresponds therefore to one +of the stages of development in the human embryo, as previously +indicated (cf. p. 149; Figs. 251 and 252). + + +PERITONEAL RELATIONS OF LIVER. + +It is well to begin the study of the peritoneal connections of the liver +with the consideration of the embryonic stage shown in Fig. 273 +schematically. + +If we imagine this embryonic liver detached from its connections in such +a manner as to leave the divided peritoneal layers of the ventral +mesogastrium as long as possible, and if we regard the preparation from +behind, the appearance of the parts could be represented in Fig. 276.[6] + +[Illustration: FIG. 276.--Schematic view of embryonic liver detached +from its connections, seen from behind, with lines of peritoneal +reflection.] + +[6] I am indebted to Dr. J. A. Blake, former Assistant Demonstrator of +Anatomy at Columbia University, for the valuable suggestion which led to +the preparation of Figs. 276, 277 and 278 together with the correlated +text. + +It will of course be seen that the area of direct adhesion to the +diaphragm, extending transversely, would separate the lesser omentum +from the suspensory ligament. + +As is seen in the transection (Fig. 274), the right and left layers of +the suspensory ligament, at its attachment to the liver, turn into the +visceral peritoneum investing the organ on its ventral and cephalic +surfaces. Continuing around the borders of the liver this visceral +peritoneum then invests in like manner the dorsal or caudal surface +directed toward the stomach, until, at the region of the future portal +or transverse fissure, this visceral peritoneum becomes in turn +continuous with the two layers of the lesser or gastro-hepatic omentum. +Consequently in the embryonic detached liver the lines of peritoneal +reflection would be nearly cruciform, the vertical limb of the cross +being formed on the cephalic surface by the two layers of the suspensory +ligament, while on the caudal surface it is formed by the layers of the +lesser omentum. The horizontal arm of the cross is formed by the upper +and lower limits of the area of diaphragmatic attachment, along which +the parietal diaphragmatic peritoneum turns into the visceral hepatic +investment (forming the two layers of the primitive coronary ligament). +In the liver shown thus schematically from behind we would overlook the +dorsal and adjoining portions of the cephalic and caudal surfaces of the +adult human liver. + +The primitive biliary duct, portal vein and hepatic artery reach the +liver between the layers of the lesser omentum. The venae revehentes +(hepatic veins) reach the sinus venosus at the attachment of the liver +to the septum transversum (primitive diaphragm). + +The first important change, resulting in a rearrangement of these +peritoneal layers, is produced by the connection of the umbilical with +the rudimentary portal vein. + +This junction occupies a relatively wide area on the caudal surface of +the liver, and the layers of the lesser omentum are separated somewhat +at this point to accommodate the enlarging vascular structures between +them. More especially is this the case with the right leaf of the +primitive gastro-hepatic omentum. A species of lateral diverticulum is +formed by this leaf so as to include the umbilical vein at its junction +with the portal. The membrane in the region of this diverticulum turns +its surfaces dorsad and ventrad, and its free edge toward the right +(Fig. 277). With the gradual increase in the size of the vessels, and +with the transverse position which the rotation of the stomach imparts +to the opposite border of the lesser omentum attached to the lesser +curvature, this transversely disposed portion gradually exceeds in +length and size the part of the original omentum enclosing the umbilical +vein. This vessel and the investing peritoneum become lodged in a +sagittal depression on the caudal surface of the liver (rudimentary +umbilical fissure), while the transverse portion, developed as +indicated, surrounds the structures connected with the liver at the +future transverse or portal fissure. + +[Illustration: FIG. 277.--Schematic view of embryonic liver, showing +influence of vascular connections on the arrangement of the lines of +peritoneal reflection.] + +[Illustration: FIG. 278.--Later stages, showing development of +transverse fissure, Spigelian and caudate lobes.] + +Schematically this rearrangement of the hepatic peritoneal lines of +reflection can be shown in Fig. 278. + +It will be observed that in this way a small part of the caudal surface +of the right lobe has become partially marked off from the remainder as +a rudimentary Spigelian lobe, bounded ventrally by the transverse +fissure and lesser omentum attached to the same; to the left by the two +layers of the lesser omentum containing the ductus venosus; while the +limit cephalad is afforded by the reflection of peritoneum from liver to +diaphragm, forming part of caudal layer of right coronary ligament. To +the right this rudimentary Spigelian surface is directly continuous with +the rest of the dorsal and caudal surface of the right lobe (Fig. 277). +Finally a definite right limit is given to the Spigelian lobe by the +increasing size of the postcava and its closer connection with the +liver. This vessel now assumes the position of the main venous trunk +entering the heart from below. + +This inclusion of the vena cava in the fissure or fossa of that name on +the dorsal surface of the liver affords, so to speak, the vertical +measure of the non-peritoneal area of the liver attached directly to the +diaphragm. As the vein develops the interval between the two layers of +the right coronary ligament increases, producing the well-known large +non-peritoneal area on the dorsal surface of the adult liver, which is +directly attached to the diaphragm. + +Immediately to the left of the vena cava, however, the original +condition persists. The area of direct diaphragmatic attachment is +narrow and consequently the two layers of the coronary ligament are +close together at this point.[7] + +[7] It should be remembered that in the final adult arrangement of the +abdominal viscera the liver shifts relatively backwards, so that the +diaphragmatic attachment, originally directed cephalad, now looks dorsad +and forms part of the dorsal or "posterior" surface of the adult organ. +The original ventral surface looks cephalad, as well as ventrad, forming +the convex surface which in the adult rests in contact with the +abdominal wall and diaphragmatic vault, while the surface originally +directed dorsad toward the stomach finally in large part has an +inclination caudad forming the "inferior" surface of human anatomy. + +In this way a species of recess (Spigelian recess or hepatic antrum of +lesser sac) is formed. A portion of the dorsal liver surface lying just +to the left of the vena cava, between it and the ductus venosus, remains +invested by peritoneum which is reflected from the boundaries of this +space to the diaphragm. This forms the Spigelian lobe (Fig. 278). + +The lobe is bounded to the right by the postcava, to the left by the +reflection of the lesser omentum to the stomach along the fissure for +the ductus venosus; cephalad the boundary is formed by the reflection of +the caudal layer of the coronary ligament to the diaphragm. + +The caudal boundary is afforded by the transverse position which the +lesser omentum has assumed in the region of the transverse or portal +fissure. + +It will be seen that the original continuity of the Spigelian lobe with +the caudal surface of the right lobe is maintained by the narrow bridge +of liver tissue connecting the caudal right angle of the rectangular +Spigelian lobe with the right lobe. This narrow isthmus, situated +between vena cava dorsad and the free right edge of lesser omentum +ventrad, forms the so-called _caudate lobe_. + +[Illustration: FIG. 279.--Liver of human foetus at eighth month. View of +caudal and dorsal surfaces. (Columbia University Museum, No. 1854.)] + +Fig. 279 shows a human foetal liver at the end of the eighth month in the +view from below and behind. The original continuity of the layers of the +lesser omentum, attached along the fissure for the ductus venosus, with +the fold of the falciform ligament occupying the umbilical fissure can +still be made out for a short distance beyond the left extremity of the +transverse fissure. The section of the lesser omentum which occupies the +transverse fissure and, including the portal vein, hepatic artery and +duct between its layers, terminates in the free right margin, is +evidently derived by a lateral extension from the right layer of the +primitive sagittal lesser omentum, whose original direction is preserved +along the fissure of the ductus venosus. + +In Fig. 280 the lines of peritoneal reflection on the cephalic, dorsal +and caudal surfaces of a human foetal liver at term are shown. + +[Illustration: FIG. 280.--Human foetal liver at term, showing lines of +peritoneal reflection on cephalic, dorsal, and caudal surfaces. +(Columbia University Museum, No. 1855.)] + +We can now proceed to trace the reflection of the peritoneum from the +liver to adjacent structures. + +Begin with the caudal layer of the coronary ligament on the extreme +right, where fusion with the corresponding cephalic layer produces the +right triangular ligament. The caudal layer of the coronary ligament +proceeds from right to left along the caudal margin of the +non-peritoneal dorsal diaphragmatic surface of right lobe, being +reflected along this line from the liver to the adjacent portions of the +diaphragm and ventral surface of right kidney and suprarenal capsule +(hepato-renal ligament). A small cephalic part of ventral surface of +right suprarenal capsule lies above this line of reflection, is hence +non-peritoneal and firmly connected with the liver just to the left of +entrance of vena cava into the caval fissure. Continuing, the caudal +layer of the coronary ligament crosses the ventral surface of the vena +cava and turns, immediately to the left of the vein, at a right angle, +ascending to form the left boundary of the Spigelian recess, being +reflected along this line from the left margin of the caval fissure to +the pillars of the diaphragm. Arrived at the opening of the central +tendon permitting passage of vena cava into pericardium, and at the +level of the entrance of the left hepatic vein into the cava, the +peritoneum turns again at a right angle and runs from right to left, +forming the cephalic limit of the Spigelian recess. Turning caudad along +the fissure for the ductus venosus, as right leaf of that portion of the +lesser omentum which is attached to this fissure and has preserved its +sagittal position, the peritoneal line of reflection reaches the left +extremity of the portal or transverse fissure. It now turns to the +right following the fissure as the dorsal layer of the transverse +segment of the lesser omentum, and becomes continuous, with the +formation of a free right edge, with the ventral layer of the same +membrane, passing from right to left, the two layers including between +them the structures entering and leaving the liver at the transverse +fissure (portal vein, hepatic artery, duct). Arriving at the left +extremity of the transverse fissure the ventral layer of the transverse +segment of the lesser omentum--as we practically trace it in the adult +as a free membrane--turns directly into the left leaf of the sagittal +segment attached along the fissure for the ductus venosus, and becomes +continuous along the dorsal border of the left lobe with the caudal +layer of the left coronary ligament. This direct continuity, as just +stated, exists practically in the adult. From the development of the +membrane, however, it will be seen that the ventral layer of the +transverse lesser omentum, at the left extremity of the portal fissure, +becomes continuous with the right layer of the primitive mesogastrium +enclosing the umbilical vein. After surrounding this vein it is +continued into the left leaf of the same membrane, which in turn passes +into the left layer of the portion attached along the fissure for the +ductus venosus. + +This original connection can at times be traced very clearly in young +specimens (Fig. 279), and occasionally is also still evident in the +adult liver. + +Usually, however, the round ligament of the adult and its investing +peritoneum is buried so deeply in the umbilical fissure, or even bridged +over in part by liver tissue, that the connection is not evident. The +ventral layer of the transverse omentum then appears directly continuous +with the left layer of the sagittal omentum attached along the fissure +for the ductus venosus. + +We can sum up the facts just considered as follows: + +1. The rotation of the stomach from the sagittal into the transverse +position, and the development of the umbilical and portal veins, +rearrange the original sagittal plane of the lesser omentum, dividing it +into two districts: + +(_a_) Cephalic portion, remaining in the original sagittal plane, +follows the fissure for the ductus venosus. With the incorporation of +the Spigelian lobe in the adult dorsal or "posterior" surface of the +liver, this segment of the omentum assumes a vertical direction, forming +the left boundary of the Spigelian recess, being reflected from the +fissure for the ductus venosus to the abdominal portion of the oesophagus +and the part of the lesser curvature of stomach adjacent to the cardia. + +(_b_) Distal caudal portion of the lesser omentum is twisted laterally +and turned to the right by the change in the position of the stomach and +the development of the structures connected with the liver at the +transverse fissure. It is reflected from this fissure to the distal part +of the lesser curvature and to the first portion of the duodenum. This +transverse segment of the lesser omentum is a secondary derivative from +the right leaf of the primitive membrane, produced by the enlarged area +for entrance of umbilical and portal veins at the transverse fissure. It +lies ventrad of caudal border of Spigelian lobe. + +2. The distal segment of the original omentum containing the umbilical +vein (round ligament), continues imbedded in the umbilical fissure, to +the ventral margin of the liver, where it joins the layers of the +suspensory ligament passing over the cephalic surface. + +3. The adult lesser omentum at the transverse fissure may be regarded as +a diverticulum of the right leaf of the primitive embryonal sagittal +omentum. + +With the reduction of the umbilical vein after birth to form the round +ligament this structure becomes deeply buried in the umbilical fissure. +The ventral and dorsal layers of the lesser omentum at the transverse +fissure thus become continuous with respectively the left and right +layers of the second segment of the omentum which ascends vertically +along the fissure for the ductus venosus. + +4. The cephalic layer of the coronary ligament (Fig. 280) remains +practically in the embryonic condition. The adult convex cephalic +surface of the liver is traversed in the sagittal direction by the +suspensory ligament which connects it with the abdominal surface of the +diaphragm, and thus effects the division into right and left lobes on +the convex surface. Arrived at the dorsal border of this surface +(junction of "superior" and "posterior" surfaces) the right and left +leaves of the falciform ligament turn at right angles into the cephalic +layer of the right and left coronary ligament, which at each extremity +meet the right and left caudal layers to form the triangular ligaments. +It will thus be seen that the apparent irregularity in the relative +arrangement of the s. c. "upper" and "lower" layers of the coronary +ligaments, produced by the Spigelian recess, is only a difference in the +interval between the two layers, caused by the vertical extent of the +non-peritoneal direct diaphragmatic attachment of the right lobe to the +right of the vena cava. + +=Comparative Anatomy of Spigelian Lobe and Vena Cava in the Cat.=--The +lines of peritoneal reflection in the _cat's_ liver and the arrangement +of the Spigelian lobe and recess are seen in Fig. 281, taken from a +preparation hardened in situ. + +[Illustration: FIG. 281.--Liver of cat, hardened _in situ_. (Columbia +University Museum, No. 1836.)] + +Compared with the human liver it will be noted that the area of +diaphragmatic adhesion is much less developed. The dorsal surface of the +right lobe to the right of the postcava is peritoneal, there being no +extension laterad of the right coronary and triangular ligaments. The +postcava enters the liver in a special prolongation of the liver +substance (caval lobe). + +The boundaries of the Spigelian recess and the lines of attachment of +the gastro-hepatic omentum correspond to the human arrangement. + + +RELATION OF THE HEPATIC PERITONEUM TO THE "LESSER SAC." + +_Foramen of Winslow._--We have previously seen that the rotation of the +stomach and the further growth of the dorsal mesogastrium lead, in the +first instance, to the formation of the "lesser peritoneal cavity." This +cavity is in fact primarily the retrogastric space created by the +transverse position of the stomach, augmented by the cavity of the +omental bursa developed from the dorsal mesogastrium. + +We have now to consider the additional boundaries of this space +contributed by the peritoneal connection of the lesser curvature with +the liver. + +The lesser omentum follows, of course, along its gastric attachment to +the lesser curvature the general direction of the stomach, passing from +the cardia transversely downwards and to the right. We distinguish the +two layers of the adult membrane as ventral and dorsal, which meet in +the free right edge and include between them the main structures +entering and leaving the liver at the transverse fissure, viz.: the +portal vein, hepatic artery and bile-duct. + +The lesser omentum therefore prolongs the plane of the stomach cephalad +towards the liver and thus forms the continuation of the ventral +boundary of the lesser peritoneal sac. We can now consider the line of +its hepatic attachment in the light of the facts previously adduced, and +combine the same with the line of gastric attachment to the lesser +curvature. Fig. 282 shows the foetal liver and stomach in their relative +position in the dorsal view, and Fig. 283 gives the lines of the +peritoneal reflections. The vertical segment of the omentum, occupying +the fissure for the ductus venosus, passes to the cardiac part of the +lesser curvature, its ventral layer covering the ventral and left side +of the oesophagus, while its dorsal layer passes to the dorsal and right +side of the oesophagus at its entrance into the stomach. The transverse +segment of the omentum, attached on the liver to the portal or +transverse fissure, accedes to the pyloric part of the lesser curvature. +Of course the ventral and dorsal layers of the omentum are continuous +with the serous visceral investment of the ventral and dorsal surfaces +of the stomach. + +[Illustration: FIG. 282.--Dorsal view of human liver and stomach in +foetus at term, showing lines of hepatic and gastric attachment of lesser +omentum.] + +[Illustration: FIG. 283.--Schema of lines of reflection of peritoneum on +dorsal surface of liver and in the formation of the gastro-hepatic +omentum. _A B_, transverse section of lesser omentum attached to +transverse fissure of liver and to pyloric section of lesser curvature +(_A' B'_); _B C_, vertical section of lesser omentum passing between +fissure of ductus venosus and cardiac section of lesser curvature of +stomach (_B' C'_); _C D_, line of reflection of peritoneum from cephalic +border of Spigelian lobe to diaphragm; _D E_, line of reflection of +peritoneum from right border of Spigelian lobe to left margin of +postcava and diaphragm.] + +[Illustration: FIG. 284.--Portion of abdominal viscera of adult human +subject, hardened _in situ_. (Columbia University, Study Collection.) +The segment of stomach between cardiac and pyloric orifices has been +removed, dividing the lesser omentum to this extent, but leaving the +right extremity of the membrane (lig. hepato-duodenale) intact. Behind +this portion the arrow passes through the foramen of Winslow.] + +[Illustration: FIG. 285.--Liver and stomach of _Macacus pileatus_. +(Columbia University, Study Collection.)] + +Fig. 284 shows this right-angled course of the lesser omentum at the +hepatic line of attachment in a preparation of the abdominal viscera +hardened in situ, with the segment of the stomach between the cardiac +and pyloric orifices removed. The arrow is passed behind the right free +edge of the lesser omentum. This portion of the membrane is still +intact, not having been disturbed by the removal of the body of the +stomach, and includes between its layers the structures connected with +the liver at the transverse fissure (duct, hepatic artery and portal +vein). The lesser omentum is seen to be attached to the liver along the +transverse fissure (Fig. 284, _A_) and along the fissure for the ductus +venosus (Fig. 284, _B_), constituting the transverse and vertical +segments above referred to, which pass into each other at the angle of +junction between the transverse fissure (left end) and the fissure for +the ductus venosus (Fig. 284, _C_). The caudal and left border of the +Spigelian lobe is exposed by the division of the omentum, and the extent +of the Spigelian or hepatic recess of the lesser peritoneal sac is +shown. Fig. 285 shows the liver, stomach and lesser omentum of a Macaque +monkey hardened in situ, and demonstrates still more conclusively that +the uniform curve of the omentum along the lesser curvature of the +stomach becomes a broken line at the hepatic attachment, the angle being +placed at the left end of the transverse fissure at the point where the +same encounters the fissure for the ductus venosus. + +[Illustration: FIG. 286.--Abdominal viscera of adult human subject, +hardened _in situ_; with liver lifted up after incision of the +gastro-hepatic omentum. (Columbia University Museum, No. 1845.)] + +In Fig. 286 finally the hardened abdominal viscera of an adult human +subject are shown in the ventral view with the lesser omentum incised. +The cut through the lesser omentum exposes the hepatic recess of the +lesser peritoneal cavity immediately to the left of the foramen of +Winslow. Toward the right free margin of the omentum the divided portal +vein, hepatic artery and duct are seen between the layers of the omentum +imbedded in the pancreas and coursing behind the first portion of the +duodenum on their way to the transverse fissure. + +To the left of these structures the omental tuberosity of the pancreas +projects above the level of the lesser curvature under cover of the +secondary parietal peritoneum forming the dorsal wall of the lesser sac, +while the lower edge of the Spigelian lobe appears in the upper angle of +the incision. + +If we remember that the liver is itself welded to the diaphragm between +the layers of the coronary ligament (Fig. 280), it will become +apparent that the serous surface of the Spigelian lobe forms part of the +ventral wall of a peritoneal recess situated behind the lesser omentum, +between this membrane and the diaphragm. Access to this recess, without +the division of peritoneal layers, can only be obtained by passing from +right to left, along the caudate lobe, between the vena cava behind, +covered by parietal peritoneum, and the free right edge of the lesser +omentum in front. (In the reverse direction of the arrow shown in Fig. +284.) This hepatic or Spigelian recess of the lesser peritoneal cavity +has categorically the following boundaries (Figs. 282 and 283): + +Dorsal: Parietal peritoneum, reflected along the line CD, from the +caudal layer of the coronary ligament to the diaphragm. + +Ventral: Visceral peritoneum investing the Spigelian lobe and the +gastro-hepatic omentum. + +Right: Reflection of peritoneum along the line DE (caval fissure) to +become the parietal peritoneum covering the diaphragm. + +Left: Right layer of lesser omentum, reflected along the fissure for the +ductus venosus (CB) to the cardiac portion of the lesser curvature, +continuous with the dorsal layer of the lesser omentum reflected from +the transverse fissure to the pyloric segment of the lesser curvature +(AB). + +We will presently see that certain relations of the vessels connected +with the liver at the transverse fissure and of the duodenum prevent the +finger, when passed from right to left behind the free right edge of the +lesser omentum and along the caudate lobe of the liver, from proceeding +downward at this point. A narrow channel of communication is thus formed +between the Spigelian recess and rest of the lesser sac on the one hand, +and the general greater peritoneal cavity on the other. This channel is +the so-called foramen of Winslow. + +Having once passed this narrow space the finger will be in the Spigelian +recess and can palpate its boundaries. Further progress cephalad and to +the right is barred by the diaphragmatic adhesions of the liver just +detailed. But in the direction downward behind the lesser omentum and +along the dorsal surface of the stomach, as well as to the left toward +the spleen the excursion is limited only by the length of the examining +finger. + +After opening the abdominal cavity of the human adult, elevating the +liver and depressing the stomach, the hepatic attachment of the lesser +omentum can be traced as already described. It will then be observed +that the gastric attachment of the membrane lies in one plane following +the lesser curvature while the hepatic attachment forms a broken line, +with the angle situated at the left extremity of the transverse fissure. +The vertical segment of the hepatic attachment, occupying the fissure +for the ductus venosus, turns at this angle into the transverse segment +which follows the transverse fissure to its right extremity where the +two layers pass into each other around the right free omental margin +(hepato-duodenal ligament). Consequently we overlook, in an abdominal +cavity thus exposed, the entire caudal surface of the liver, including +the caudal surfaces of right, left, and quadrate lobes. The junction of +right and caudate lobes can be seen between vena cava and right edge of +the omentum, or rather, it can be felt at this point. But the Spigelian +lobe, turning its surface dorsad against the parietal peritoneum +covering the diaphragm, forms part of the "posterior" liver surface and +is not visible, although--as just stated, it can be palpated by passing +the finger through the foramen of Winslow. The Spigelian lobe cannot be +overlooked in its entire extent until the liver is removed from the body +and regarded from behind. The caudal edge (continuation of its right +angle into the caudate lobe and papillary tubercle) can be seen by +tearing through the layers of the lesser omentum and lifting the liver +up forcibly (Fig. 286). + +=Caudal Boundary of Foramen of Winslow.=--We have above referred to the +fact that the finger introduced through the foramen of Winslow meets in +this canal with resistance if an attempt is made to pass downwards. +After passing this constricting point the free excursion into the +Spigelian recess and behind the omentum and stomach and toward the +spleen can be performed. + +In considering the elements which produce this narrowing of the +communication between the two peritoneal sacs at the foramen of Winslow +we have to deal with two factors, one primary and constant, the other +secondary and inconstant. + +1. The first of these is afforded by the arrangement of the arterial +vessel supplying the liver. The hepatic artery is a branch of the coeliac +axis, furnishing arterial blood to the liver tissues and supplying, in +addition, branches to the stomach, duodenum and pancreas. + +This vessel is, of course, placed primarily, like all other arterial +branches supplying the alimentary tract, between the layers of the +primitive dorsal mesentery. Originally the vessel supplies the distal +(pyloric) portion of the stomach along its dorsal attached border +(subsequently the greater curvature) corresponding to the adult +gastro-epiploica dextra of the hepatic (gastro-duodenalis). + +It likewise gives branches to the adjacent pyloric portion of the +duodenum and the pancreas, as that gland develops from the intestine, +corresponding to the adult superior pancreatico-duodenal branch, and to +the ventral border (lesser curvature) of stomach, corresponding to the +adult pyloric branch of the hepatic. + +With the development of the liver from the duodenum arterial branches +derived from this primitive gastro-duodenal vessel pass to the sprouting +hepatic cylinders by continuing around the duodenum, beneath its serous +investment, to reach the interval between the two layers of the ventral +mesogastrium, in which the liver develops, near the free margin of this +membrane. + +After the rotation, which turns the right side of the stomach, duodenum +and mesoduodenum dorsad, the branch which passes over the dorsal surface +of the duodenum to reach the liver becomes more favorably situated and +develops into the main hepatic artery which reaches the liver at the +transverse fissure between the folds of the lesser omentum. The original +right side of the duodenum, now turned dorsad, adheres to the parietal +peritoneum. The hepatic artery which reached the liver by passing over +this surface of the duodenum, beneath its visceral serous covering, +becomes imbedded in connective tissue by the adhesion of the visceral +duodenal and the primitive parietal peritoneum. Hence in the adult the +hepatic artery courses imbedded in the connective tissue which binds the +duodenum to the abdominal background to reach the interval between the +two omental layers which carry it to the transverse fissure. + +The hepatic artery, therefore, derived from one of the primitive +intestinal branches (gastro-duodenal) is, notwithstanding its hidden +position in the adult, originally situated between the layers of the +free primitive dorsal mesogastrium. + +It now becomes necessary to regard the development of the great omentum +from the primitive dorsal mesogastrium in relation to this course of the +hepatic artery. We have seen that the great omentum and the cavity of +the omental bursa is produced by the extension of the dorsal +mesogastrium to the left and caudad, subsequent to the rotation of the +stomach. The splenic artery and the left gastro-epiploic branch pass +from the coeliac axis to the left between the layers of the mesogastrium, +as previously seen (Figs. 291 and 292). + +The hepatic artery, however, is so to speak placed on the border line +between the portion of the primitive mesentery which, as dorsal +mesogastrium, is to turn to the left and caudad to form the great +omentum, and the portion which, as mesoduodenum, turns to the right and +passes to the duodenal loop (Fig. 287). + +[Illustration: FIG. 287.--Primitive dorsal and ventral mesogastrium with +course of hepatic artery.] + +[Illustration: FIG. 288.--The liver divides the ventral mesogastrium +into a dorsal segment, the gastro-hepatic or lesser omentum, and a +ventral segment, the suspensory or falciform ligament of the liver.] + +[Illustration: FIG. 289.--Stages in the development of the dorsal +mesogastrium (omental bursa) and mesoduodenum to show relation of +hepatic artery to these two segments of the primitive common dorsal +mesentery.] + +In the further course of development the dorsal mesogastrium grows more +and more, forming the omental bag, while the mesoduodenum on the other +hand becomes anchored early and obliterated as a free membrane by +adhesion of its original right layer to the primitive parietal +peritoneum. The hepatic artery runs on the line dividing these two +different mesenteric segments. We can imagine, so to speak, that the +redundant growth of the omentum to the left and caudad, takes place over +the hepatic artery as a resistant support (Figs. 288 and 289). Cephalad +of the hepatic artery is the developing omentum, caudad of the vessel +the mesoduodenum. The artery follows the cephalic limit of the +mesoduodenum and becomes, as stated, adherent to the abdominal +background in the segment between its origin from the coeliac axis and +the point where, after having crossed the dorsal surface of the +duodenum, it enters the right edge of the lesser omentum on its way to +the liver. + +=Pancreatico-gastric Folds.=--If we open the lesser peritoneal cavity by +dividing the gastro-hepatic omentum and look into the background of the +retro-omental space, we will see a fold of the secondary lining parietal +peritoneum (derived from the mesogastrium), which can be traced from the +cephalic border of the pancreas to the pyloric extremity of the stomach. +This fold carries the hepatic artery to the lesser omentum behind the +first portion of the duodenum, and is called the right or main +pancreatico-gastric fold. A similar fold, further to the left, carries +in a like manner the coronary artery of the stomach to the cardiac end +of the lesser curvature. This fold forms the left or secondary +pancreatico-gastric fold. Between the two folds the caudal margin of the +Spigelian lobe projects into the lesser cavity. + +The appearance of the two pancreatico-gastric folds in the adult human +subject is well seen in Fig. 284. + +[Illustration: FIG. 290.--Abdominal viscera of _Nasua rufa_, brown +coaiti, with stomach turned up and great omentum divided. (From a fresh +dissection.)] + +Fig. 290 shows the abdominal cavity of _Nasua rufa_, with great omentum +divided to bring into view the vessels passing from coeliac axis to liver +and stomach and elevating the retrogastric parietal peritoneum to +produce the pancreatico-gastric folds. + +(The course of the hepatic artery from coeliac axis to liver in the +dorsal view in the cat is seen in Fig. 223.) + +[Illustration: FIG. 291.--Schematic transection through foramen of +Winslow before adhesion of dorsal mesogastrium and mesoduodenum to +parietal peritoneum.] + +[Illustration: FIG. 292.--The same section after the adult conditions +have been established by adhesion.] + +Figs. 291 and 292 represent schematically cross-sections directly +through the foramen of Winslow, showing the method by means of which the +hepatic artery reaches the upper border of the duodenum and the effect +of the adhesion of duodenum and mesoduodenum upon the disposition of the +vessel. + +The coronary artery, like the splenic, is at first situated between the +layers of the dorsal mesogastrium (vertebro-splenic segment). Like the +splenic the coronary artery becomes anchored to the abdominal background +and placed secondarily behind the parietal peritoneum of the lesser sac +by the adhesion of this mesogastric segment to the primitive parietal +peritoneum. To reach the lesser curvature at the cardia and to run +thence from left to right along the lesser curvature between the layers +of the gastro-hepatic omentum, the vessel raises the investing parietal +peritoneum (originally the right leaf of the dorsal mesogastrium) into a +crescentic fold, extending between its origin from the coeliac axis at +cephalic margin of pancreas and the beginning of the lesser curvature of +the stomach. Hence this fold is called the left pancreatico-gastric +fold. (Seen well in Fig. 284.) + +In the next place it must be borne in mind that the relation of the +primitive hepatic artery to the vascular supply of the stomach, pancreas +and duodenum produces a permanent shortening of the primitive mesentery +at this point. This result is indicated in the schematic figures 287, +288 and 289. + +In the original condition the dorsal mesentery, passing to a practically +straight intestinal tube, is of uniform sagittal measure (Fig. 287). + +As development proceeds, and as the liver grows from the duodenum, the +hepatic artery develops from the primitive pyloric vessel as above +indicated. This vessel, assuming greater importance with the rapid +growth of the liver, is not lengthened out as happens with the remaining +purely intestinal branches which follow the increase in the length of +the intestinal canal. The hepatic artery, therefore, will mark the point +where the original short sagittal extent of the primitive mesentery will +tend to be preserved. Cephalad of this point the dorsal mesogastrium +grows out into the great omentum (Figs. 288 and 289); caudad of the same +point the membrane, in following the development of the intestine, +becomes drawn out into the permanent mesentery and mesocolon. + +The hepatic artery, in addition, marks the cephalic limit of the +adhesion which anchors the duodenum and mesoduodenum to the parietal +peritoneum. Consequently in the adult the vessel courses in as direct a +manner as possible, taking the shortest course from the coeliac axis to +the liver, passing dorsad of the duodenum and giving what now appear as +secondary branches to supply the intestine, the stomach and pancreas +(pyloric and gastro-duodenal arteries (pancreatico-duod. superior and +gastro-epiploica dextra)). + +Even if no fixation of the duodenum and mesoduodenum takes place this +course of the hepatic artery will produce a constricted passage between +the liver (caudate lobe) cephalad, abdominal parietes and aorta dorsad, +lesser omentum and pyloric duodenum ventrad, and hepatic artery caudad. +This passage leading from the general peritoneal cavity into the +retrogastric space is the _primitive foramen of Winslow_. This condition +is well represented in the abdominal cavity of some of the lower +mammalia, in which duodenum and mesoduodenum remain permanently free. + +Fig. 293 shows a view of the abdominal cavity from the right side in a +specimen of the ant-eater, _Tamandua bivittata_. + +[Illustration: FIG. 293.--Abdominal viscera of _Tamandua bivittata_, the +little ant-eater, with the intestines turned downward and to the left. +(From a fresh dissection.)] + +The right kidney is seen in the background, covered by the parietal +peritoneum. The duodenum and mesoduodenum are free and can be turned +toward the median line. The opening of the foramen of Winslow leading +into the retrogastric space is seen between the liver cephalad, kidney +and vena cava dorsad, lesser omentum and pyloric extremity of the +stomach ventrad, and a fold of peritoneum carrying the hepatic artery +caudad. Exactly similar conditions prevail in the cat and in many other +mammals. + +It will be seen in all these instances that neither portal vein nor +bile-ducts limit the foramen caudad. These structures can be lifted up +and turned toward the median line with the free duodenum and +mesoduodenum. But the hepatic artery must pass to the liver from the +_retroperitoneal coeliac axis_. In doing this the vessel traverses the +cephalic border of the pancreas, and the pyloric extremity of the +stomach and duodenum, to reach the lesser omentum which conveys it to +the liver. + +Consequently there must always be a narrow peritoneal neck between the +liver cephalad, aorta dorsad, hepatic artery caudad, and pyloric +extremity of stomach and duodenum together with the lesser omentum +ventrad. It should be remembered that the vessel which extends after the +development of the liver into the lesser omentum as the _hepatic_ +artery, was originally destined for the supply of these latter +structures. In the adult these primary embryonic terminal branches to +the intestine appear as secondary branches derived from the hepatic as +the main vessel. Their origin, however, serves to keep the beginning of +the small intestine in comparatively close connection with the hepatic +artery which courses over the dorsal surface of the duodenum to reach +the liver. The narrow space thus left between aorta, hepatic artery, +duodenum, lesser omentum and liver forms the framework of the foramen of +Winslow and appears always as a confined and narrow channel. This +relation is shown in the accompanying schematic Figs. 294 and 295 which +represent a sagittal section through the foramen. This primitive foramen +is thus bounded cephalad by the liver (caudate lobe, connecting +Spigelian and right lobes), ventrad by the first portion of the duodenum +and the lesser omentum, with hepatic artery behind the intestine and +between the omental layers; dorsad by the abdominal background and large +retroperitoneal vessels, and caudad by the coeliac axis and beginning of +the hepatic artery. + +[Illustration: FIG. 294.--Schematic sagittal section through foramen of +Winslow before fixation of pancreas by adhesion of mesoduodenum.] + +[Illustration: FIG. 295.--The same section after adhesion of +mesoduodenum and pancreas. The pancreas appears secondarily +retroperitoneal, after adhesion of apposed surfaces of mesoduodenum and +primitive parietal peritoneum over dotted area, producing fixation of +dorsal surface of pancreas.] + +2. In the forms which possess in the adult an adherent duodenum and +mesoduodenum, as in man, the foramen of Winslow obtains a secondary +caudal limit by the agglutination of the descending duodenum and the +parietal prerenal peritoneum. This is the secondary and inconstant +factor referred to above in the caudal boundary of the foramen. The +result of this anchoring of duodenum and mesoduodenum is to bring the +margin of the foramen further to the right and to bury the hepatic +artery still further from view. Thus in the adult human subject the +structures bounding the foramen at the margin of the entrance into the +narrow channel would be above caudate lobe of liver, behind postcava, +below duodenum adherent to ventral surface of right kidney, in front +first portion of duodenum and lesser omentum. The hepatic artery will be +felt on introducing the finger through the foramen in its original +position, but it will be seen that the actual boundaries of the foramen +have been moved so to speak a little further to the right by the +duodenal adhesion. + +[Illustration: FIG. 296.--Dissection of adult liver, pancreas, spleen, +and duodenum, with vessels, to show structures concerned in the +formation of the foramen of Winslow. (Columbia University, Study +Collection.)] + +Fig. 296 shows a complete dissection of the adult human viscera and +vessels concerned in the formation of the foramen, hardened in situ. + +The stomach is removed, dividing of course the coronary artery and vein +and the left gastro-epiploic artery. The portal vein, hepatic artery and +bile-duct are seen entering and leaving the liver at the transverse +fissure. Behind them and to the right the vena cava enters the liver. +The hepatic artery distributes its pancreatico-duodenal branches to the +duodenum and pancreas. The left angle of the Spigelian lobe and the +fissure for the ductus venosus appear to the left of the portal vein and +hepatic artery. The right angle of the Spigelian lobe and its +continuation into the right lobe by means of the caudate lobe is hidden +by the structures occupying the transverse fissure. We would enter the +beginning of the foramen of Winslow by passing between the vena cava +behind, the structures in the transverse fissure (portal vein, hepatic +artery and duct) in front, caudate lobe of liver above and duodenum +below, the latter in the undisturbed condition of the parts adherent to +the right kidney. Continuing to the left the finger would pass between +aorta behind, coeliac axis and hepatic artery below and in front, and +liver above. These structures bound the permanent and primary narrow +channel of communication between the retrogastric or lesser peritoneal +space and the general peritoneal cavity, which exists even if a free +duodenum and mesoduodenum allow us to lift the intestine away from vena +cava and right kidney. + +The main facts pertaining to the structure of the lesser peritoneal sac +and its connection with the greater peritoneal cavity by means of the +foramen of Winslow may be summed up as follows: + +The mesogastrium as a whole, expanding originally in the sagittal plane +in a fan-shaped manner between the vertebral column and the ventral +abdominal wall, from the level of the umbilicus to the septum +transversum (diaphragm), divides the cephalic part of the abdominal +cavity into a symmetrical right and left half. + +Figs. 172 and 273 represent the membrane as seen in a profile view from +the left side. We distinguish the segment dorsad of the stomach as the +dorsal mesogastrium, directly continuous with the remaining segments of +the common primitive dorsal mesentery, while the portion ventrad of the +stomach forms the ventral mesogastrium in which the liver develops. The +segment of the ventral mesogastrium between liver and stomach becomes +the lesser or gastro-hepatic omentum, while that between liver and +ventral abdominal wall forms the falciform or suspensory ligament. + +A transection, showing the dorsal and ventral mesogastrium at the level +of the fundus of the stomach, is given in Fig. 298. The mesogastria are +here seen to be short, while in the schematic Figs. 291 and 292 the +membrane is, for the sake of distinctness, represented as being of +considerable extent. + +[Illustration: FIG. 297.--Schematic sagittal section of the ventral and +dorsal mesogastria and epiploic bursa in a human embryo of eight weeks. +(Modified from Kollmann.)] + +[Illustration: FIG. 298.--Transection of human embryo of 3 cm., +vertex-coccygeal measure. (Kollmann.)] + +The ventral mesogastrium surrounding the liver and stomach extends +caudad to include the first portion of the duodenum. Beyond this point +it terminates in a thickened free edge which includes the umbilical +vein. This vein extends from the umbilicus to the transverse fissure of +the liver (Fig. 297), lying within the umbilical fissure on the caudal +surface of the gland. + +At the point where the vein enters the liver the thickened margin of the +ventral mesogastrium is continued, as ligamentum hepato-duodenale, to +the upper part of the duodenum and forms the ventral boundary of the +foramen of Winslow. Between the layers of the mesogastrium which meet in +this margin are situated the portal vein, biliary duct and hepatic +artery, together with the nerves and lymphatics of the liver. + +The mesogastrium originally divided the abdominal cavity between +umbilicus and diaphragm into symmetrical right and left halves of equal +size and extent. This early symmetrical arrangement becomes disturbed +about the seventh week by the rotation of the stomach and the resulting +altered course of the mesogastrium, which render the two original equal +halves of the abdominal cavity unequal and asymmetrical. The original +right half becomes placed behind the stomach and is converted into a +blind sac with its opening directed to the right. + +The communication of the general abdominal cavity with the retrogastric +space by means of this channel is still wide in the embryo, but +gradually becomes narrowed in the course of further development to form +the foramen of Winslow. This opening is situated between the +hepato-duodenal ligament and the parietal peritoneum covering the vena +cava. It is constricted from below by the curve of the hepatic artery as +this vessel passes from the coeliac axis to reach the liver at the +transverse fissure between the layers of the lesser omentum. + +The earlier developmental stages of the higher mammalian embryos are in +general well illustrated by the permanent adult conditions found in some +of the lower vertebrates, in which development does not proceed beyond +the primitive condition. + +In reptiles, birds and mammals the epiploic bursa is generally formed, +while in amphibia the dorsal mesogastrium is very short and connects the +stomach directly to the dorsal midline of the abdominal cavity without +forming the sac-like extension of the great omentum. + +The dorsal mesogastrium with the stomach, and the ventral mesogastrium +including the liver between its layers, divides in these animals the +cephalic part of the body cavity into two halves, corresponding to the +earlier embryonic stages in man and in the higher mammalia. + +The foramen of Winslow of the higher forms appears in the lower +vertebrates as the wide-open space leading from below into the right +half of the coelom cavity. The dorsal mesogastrium remains short, not +forming the pouch-like extension of the great omentum. The stomach +retains more or less its primitive vertical position without rotation or +elevation of the pyloric extremity, and the intestinal canal is simple, +short and comparatively straight. + + + + +PART III. + +LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION AND CAECUM. + + +In considering the anatomy of the human caecum and vermiform appendix +many structural conditions are encountered which can only be correctly +appreciated in the light of the physiology of the digestive tract. The +alimentary canal as a whole affords one of the most striking examples of +the adaptation of structure to function. The constant renewal of the +tissues of the body by the absorption of nutritive material, the +necessary concomitant egestion of undigestible remnants, the variety in +the quantity and character of the food habitually taken, all serve to +explain why the alimentary canal responds structurally in individual +forms so completely to the physiological demands made upon it. This will +become especially evident if we extend our observations to include, in +addition to man, a review of the corresponding structures in +representative types of the lower vertebrates. Moreover the human caecum +and appendix are in part rudimentary structures, representing a portion +of the alimentary tract which, in accordance with altered conditions of +food supply and nutrition, has lost its original functional significance +to the organism and which consequently exhibits the wide range of +structural variation which characterizes the majority of rudimentary and +vestigial organs. + +The vermiform process of man and the higher primates is thus one of +several indications given in the structure of the alimentary canal (the +character of the dentition is another example) which suggests that at +one phylogenetic period the forms composing the order or their immediate +ancestors were largely or entirely herbivorous, and hence possessed a +more extensively developed caecal apparatus than their omnivorous +descendants of to-day. In approaching, therefore, the study of the +human caecum and appendix we will at once meet with conditions which call +for the simultaneous physiological and morphological consideration of +the adjacent small and large intestine. + +Again many of the structural peculiarities which characterize the human +caecal apparatus can only be correctly valued by comparison with the +corresponding parts in the lower vertebrates. Our inquiry will, +therefore, most profitably include the following subdivisions of the +subject: + + +I. General review of the functional and structural characters of the +vertebrate large and small intestine. + +II. Systematic consideration of the ileo-colic junction and the +connected structures in the vertebrate series. + +III. Phylogeny of the types of vertebrate ileo-colic junction and caecum, +and their probable lines of evolution. + +IV. Detailed morphology of the human caecum and vermiform appendix. + + +I. GENERAL REVIEW OF THE MORPHOLOGY AND PHYSIOLOGY OF THE VERTEBRATE +INTESTINE. + +We have seen that the intestinal tube of all vertebrates is the product +of two of the embryonal blastodermic layers, the entoderm and mesoderm. +The former furnishes the characteristic and cardinal elements of the +digestive tract, viz., the secretory and absorbing epithelium of the +mucous membrane and of the accessory digestive glands, the liver and +pancreas. + +From the mesoderm, on the other hand, are derived the muscular and +connective tissue coats which surround the epithelial tube and +contribute to the thickness of the intestinal wall, as well as the blood +vessels and lymphatics. The alimentary canal separates from the yolk-sac +of the embryo by the development of cranial, caudal and lateral folds, +and at an early period communicates with the neural canal by the +primitive postanal gut (cf. p. 23). This connection subsequently becomes +lost. The oral and anal openings, by means of which the alimentary canal +communicates with the exterior, are formed _secondarily_ by entodermal +invaginations which finally break through into the lumen of the canal +(cf. p. 24). + +At an early embryonic stage the alimentary canal appears therefore as a +straight cylindrical tube running cephalo-caudad in the long axis of the +body-cavity, and suspended by the primitive mesentery from the ventral +aspect of the chorda dorsalis. + +In Amphioxus, the cyclostomata, certain teleosts, dipnoeans and lower +amphibians the canal remains permanently in this condition (cf. Fig. +310). + +In the remaining vertebrates the uniform non-differentiated tube of the +embryo develops further and appears more or less distinctly divided into +a proximal segment, the _foregut_, a central segment, the _midgut_, and +a distal segment, the _hindgut_, or _endgut_. This differentiation of +the tube into successive segments is closely connected with the +character and quantity of the food habitually taken and with the method +and rapidity of its elaboration in the process of digestion, absorption +and excretion. In general the _foregut_ is formed by the segment which +succeeds to the oral cavity, and includes the _pharynx_, _oesophagus_ and +_stomach_. The _midgut_ is composed of a longer or shorter narrower tube +of nearly uniform caliber, the _small intestine_, which follows the +gastric dilatation. Even in forms in which the stomach is not distinctly +differentiated (cf. p. 40) the connection of the biliary duct with the +intestinal canal serves to separate the fore- and midgut. The _hindgut_ +or _large intestine_ is usually separated from the preceding segment by +an external circular constriction, with a corresponding annular valve or +fold of the mucous membrane in the interior. + +The beginning of the large intestine is marked in many forms by the +development of an accessory pouch or diverticulum, the _caecum_. The +hindgut extends from its junction with the midgut to the cloacal or anal +opening. + + +1. Midgut or Small Intestine. + +The small intestine is the segment of the alimentary canal in which +digestion of the non-nitrogenous food substances takes place, and which +affords the necessary area of mucous surface for the absorption of all +digested matters. Consequently the character and habitual quantity of +the food here elaborated exerts a very marked influence on the _length_ +of the small intestine, _i. e._, on the extent of the digestive and +absorbing surface represented by its mucous membrane. + +The relative length of the small intestine in any individual form will +vary with both the quantity and volume of the food and with the rapidity +of the metabolic processes. Animals, in which digestion is rapid and the +usual food small in bulk and concentrated in its nutrient qualities, +have a relatively short intestine, while the canal is longer in forms +subsisting on food which is bulky and which demands considerable time +for its elaboration. Hence we find the relatively shortest intestine in +carnivora, the longest in herbivora, while the canal in omnivora +occupies an intermediate position in regard to its relative length. + +The rapidity of tissue-metabolism also exerts a marked influence on the +length and development of this portion of the alimentary canal. + +In the warm-blooded animals (mammals and birds) the tissue-changes are +constant and rapid and call for a large amount of nutrition within a +given period, while the metabolic processes in the cold-blooded +vertebrates (reptiles, amphibia and fishes) are slow, these animals +being able to go without food for long periods. Consequently in the +former class the small intestine is relatively much longer than in the +latter. Thus in certain birds and herbivorous mammals the small +intestine exceeds the total length of the body many times. This +influence of the quantity and quality of the food on the length of the +intestinal canal is seen, for example, very well during the course of +development in the frog. + +The increase in the length of the intestine, and the consequent varying +degrees of coiling and convolution, are therefore secondary +acquired characters, depending for their development upon the habitual +kind and volume of the food. Additional provisions for increasing the +efficiency of the digestive apparatus are encountered throughout the +whole of the intestinal canal. In many forms the digestive secretory and +absorbing area is augmented by the development of folds, valves, +diverticula, villi and papillae from the mucous surface of the intestine. +Certain valves and folds, moreover, both control the direction in which +the contents of the canal move and retain the same for a longer period +in the intestinal segment in which they develop. Such folds appear +especially well developed in the intestine of certain cyclostomes, +selachians and dipnoeans (cf. Figs. 203 and 204). In these forms the +alimentary canal is usually short and straight, and the fold which has a +typical spiral course and projects far into the lumen of the gut, +evidently makes up to a very large extent for the shortness of the +intestine, serving the threefold purpose of + +(_a_) Increasing the digestive and absorbing surface; + +(_b_) Prolonging the period of retention of the food-substances in the +intestine, and thus increasing the time available for elaboration and +absorption. + +(_c_) Regulating the direction in which the intestinal contents move. + +We will see presently that a similar spiral mucous fold is also +encountered in some of the higher vertebrates, especially in the large +intestine. Examples are found in the well-developed spiral valve in the +caeca of the ostrich (Fig. 341), the similar fold in the large intestine +of many rodents (Figs. 387 and 388) and in the crescentic plicae of the +primate large intestine (Figs. 471, 472 and 473). + +To the same physiological category belong the _digestive diverticula_ of +the intestinal canal, such as the pyloric appendices of the midgut found +in many teleosts and ganoids (cf. p. 119) and the varieties of caeca or +blind diverticula of the hindgut encountered throughout the vertebrate +series. They all function as reservoirs which increase the available +digestive and absorbing surface and which in addition are especially +adapted to retain substances difficult of digestion until the processes +of elaboration have been completed. + +=Divisions of the Small Intestine.=--In the higher forms the segment of +the small intestine which succeeds to the pylorus is distinguished as +the _duodenum_. Into it empty the ducts of the liver and pancreas. In +some animals a pear-shaped enlargement is found, corresponding to the +_duodenal antrum_ of the human intestine, as the dilated proximal +portion of the duodenum immediately beyond the pylorus is called. +Examples of this condition are furnished by the cetaceans, several +rodents, the llama and dromedary and the koala (Phascolarctos). + +In the birds and in many mammals (_e. g._, dog, Fig. 200, and many +rodents, as the rabbit) the duodenum is drawn out into a long loop +surrounding the pancreas. + +=Structure of the Small Intestine.= =1. Secretory Apparatus.=--The +glands whose ducts empty into the small intestine and which furnish the +digestive secretions, may be divided as follows: + +(_a_) Glands situated in the substance of the intestinal walls. + +Two kinds are distinguished: + +1. Brunner's glands, small acinous glands confined to the first part of +the duodenum. + +2. Glands of Lieberkuehn, small caecal pits distributed not only over the +entire small intestine, but also found in the mucous membrane of the +large intestine. + +These structures furnish the intestinal juice, whose chief function is +the conversion of starches into sugar, while aiding in carnivorous +animals also the digestion of proteid substances. The glands are hence +best developed in herbivora, while in carnivora they are present in +diminished numbers since they assist in the digestion of proteid +substances. + +The size and number of these glands also depends on the amount of food +digested within a given period. When a considerable quantity of +digestive fluid is required, in order to obtain the nutritive value of +the food for the organism rapidly, the glandular apparatus of the +intestine will be well developed. Hence mammalia, in whom these +conditions exist, possess both the glands of Brunner and of Lieberkuehn. +In birds the latter structures are still found, but the former are +absent, while amphibia and fishes are devoid of both kinds. In these +lower vertebrates the typical intestinal glandular apparatus of the +higher forms is to a certain extent replaced by small pits and +depressions of the mucous membrane bounded by reticular folds. + +(_b_) _Glands situated outside the intestinal tube, into whose lumen +their ducts empty._ + +The liver and pancreas fall under this head. The liver functions in the +digestion of the fatty substances of the food, while the secretion of +the pancreas converts the starches into sugars, and aids in the +digestion of albumenoid substances and to a lesser degree in that of the +fats. + +=2. Absorbing Apparatus of Small Intestine.=--The mucous membrane of the +intestine is provided with villi, containing lymphatics, by whose agency +the digested matters are absorbed. These structures are developed in +individual forms in direct proportion to the ease and rapidity with +which the food is habitually absorbed. + +The more rapid and complete the digestion is the greater will be the +amount of digested nutritive material at any given time in the +intestine, and the greater will be the development of the absorbing +structures. Hence the villi of the small intestine are especially large +and prominent in the carnivora, while they are small and insignificant +in herbivora and omnivora. Intestinal villi are found in nearly all +mammals and in many birds. Fig. 300 shows the villi of the intestinal +mucous membrane in a carnivore mammal (_Ursus maritimus_, polar bear) +and Fig. 301 the same structures in the cassowary (_Casuarius +casuarius_) in which bird they are very well developed. The villi are +not confined to the two highest vertebrate classes, but are encountered +also in the mucous membrane of the midgut in certain reptiles, notably +the ophidia. + +[Illustration: FIG. 299.--Schematic sagittal section of abdomen to +illustrate the intestinal branches of the abdominal aorta. The gastric +and hepatic arteries are shown for the sake of convenience as arising +together from the coeliac axis (_B_), hence the left and right +gastro-pancreatic folds carrying these vessels appear fused at their +beginning, separating the hepatic recess of the lesser peritoneal sac +(_A_) from the cavity of the omental bursa (_C_).] + +[Illustration: FIG. 300.--Small intestine, of polar bear, _Ursus +maritimus_. Mucous surface. (Columbia University Museum, No. 782.)] + +[Illustration: FIG. 301.--Duodenum, with entrance of pancreatic and +biliary ducts and well-developed diverticulum Vateri in the cassowary, +_Casuarius casuarius_ (Columbia University Museum, No. 1821.)] + +[Illustration: FIG. 302.--Mucous membrane of midgut of _Boa +constrictor_. (Columbia University Museum, No. 1837.)] + +Fig. 302 shows the intestine mucous membrane of the boa constrictor with +well-developed and prominent villous projections. + +Some birds, such as the snipes, herons and crows, have in place of the +intestinal villi projecting folds of the mucosa, often arranged in a +reticular manner. This type is prevalent in amphibia and fish (Fig. 112, +distal segment of midgut). Collections of lymphoid tissue in the mucous +membrane of the small intestine, either aggregated to form Peyer's +patches (Fig. 309) or as solitary follicles, are only found in the two +highest vertebrate classes, birds and mammals. In the former they appear +scattered over the surface of the mucous membrane, in the latter they +may be arranged in aggregations or regular rows. They are not secreting +structures, but their exact function in absorption is not known. This +lymphoid or adenoid tissue in certain forms is especially well developed +at the ileo-colic junction, forming the _lymphatic sac_ of some rodents, +as lepus (cf. Fig. 386). It is not confined to the small intestine, but +is found in the large intestine as well. At times it appears especially +well developed in the terminal portion of the caecal pouch (appendix), as +in _Lepus_ (Fig. 388). + +The _valvulae conniventes_ or _valves of Kerkring_ of the human small +intestine serve to very greatly increase the secreting and absorbing +mucous surface. They are not found in this complete development in any +other mammals, although a very few forms present a transverse +reduplication of the intestinal mucosa and the circular layer of +muscular fibers. An example of this is found in the intestinal mucous +membrane of a species of antelope, shown in Fig. 303. + +[Illustration: FIG. 303.--Mucous surface of small intestine of a species +of African antelope, _Cervicapra arundinacea_. (Columbia University +Museum, No. 1843.)] + +The complete development of the valvulae conniventes in man is possibly +also associated with a mechanical function in connection with the +upright posture. In some mammalia, as in certain rodents and the +porpoise (Fig. 304), the mucous membrane of the terminal part of the +small intestine is thrown into _longitudinal folds_. + +[Illustration: FIG. 304.--Mucous surface of small intestine of _Phocaena +communis_, porpoise. (Columbia University Museum, No. 1057.)] + +[Illustration: FIG. 305.--Mucous membrane of mid-gut of _Lophius +piscatorius_, the angler, 18 cm. caudad of pylorus. (Columbia University +Museum, No. 1838.)] + +The mucosa of the midgut in the lower vertebrates may be smooth, or +thrown into longitudinal folds, or the longitudinal folds may become +connected by oblique and transverse secondary folds, resulting finally +in a more or less complicated reticulated pattern of crypts. A very good +example of the type-form from which the more complicated conditions are +derived is seen in Fig. 305, showing the mucous membrane of the midgut +in _Lophius piscatorius_, the angler. The specimen is taken 18 cm. from +the pylorus and shows a ground plan of longitudinal plicae connected by +short oblique cross folds. + +[Illustration: FIGS. 306, 307.--Intestinal mucous membrane of +logger-head turtle, _Thalassochelys caretta_. (Columbia University +Museum, No. 1839.)] + +[Illustration: FIG. 306.--Mid-gut.] + +[Illustration: FIG. 307.--End-gut.] + +Fig. 306, showing the midgut mucosa of the loggerhead turtle +(_Thalassochelys caretta_), exhibits the same arrangement further +developed, resulting in a fine reticulated pattern, while in the endgut +of the same animal the primitive longitudinal folding is resumed (Fig. +307). + +The number and size of the human valvulae conniventes vary in different +parts of the small intestine (Fig. 309). They are not usually found in +the beginning of the duodenum (Fig. 308), but commence in the second or +descending portion. + +[Illustration: FIG. 308.--Adult human subject. Mucous membrane of +pyloro-duodenal junction and of duodenum. (Columbia University Museum, +No. 1840.)] + +[Illustration: FIG. 309.--Adult human subject. Mucous membrane of small +intestine, showing arrangement of valvulae conniventes in successive +portions of jejunum and ileum. (Columbia University Museum, No. 1841.)] + +They become very large and closely packed immediately beyond the common +entrance of the biliary and pancreatic ducts and continue to be well +developed and numerous throughout the rest of the duodenum and upper +half of the jejunum (Figs. 308 and 309). From here on they become +smaller, more irregular and less closely packed, and finally in the +terminal two feet of the ileum disappear almost entirely (Fig. 309). +This varying development of the valvulae is the chief reason why a given +segment of the ileum weighs less than a corresponding length of the +jejunum. This reduction in the fold-formation of the intestinal mucosa +toward the terminal portion of the midgut is seen even in the lower +vertebrates. Thus in Fig. 112, showing the entire intestinal tract of +the conger eel, _Echelus conger_, in section, the plicae of the mucous +membrane in the proximal segment of the midgut, at and immediately +beyond the entrance of the biliary duct, are prominent and numerous. +This redundancy continues but slightly reduced in the descending limb of +the intestinal loop, while in the ascending limb and up to the +ileo-colic junction the folds are reduced to a fine reticulated +meshwork. Beyond the ileo-colic valve plate, in the short endgut, the +mucosa again presents numerous pointed reduplications. + + +II. ENDGUT OR LARGE INTESTINE. + +In this segment of the intestinal canal the undigested remnants of the +food are collected and evacuated from time to time. + +In addition, the mucous membrane of the large intestine absorbs all +_digested_ material which is passed from the small intestine. While +digestion of food-substances will not be _inaugurated_ in the large +intestine, material already in the process of digestion and mixed with +the intestinal juices of the preceding segment, will be further +elaborated in this portion of the canal and the nutritive products +absorbed. This is especially the case in herbivora and omnivora, whose +food is bulky, containing a large amount of refuse material, and is +hence only slowly digested. On the other hand the food of the carnivora +is easily and rapidly digested and absorbed. After passing through the +small intestine hardly any substances remain which are capable of +digestion and absorption. Hence the large intestine of herbivora and +omnivora is uniformly longer in proportion to the small intestine than +it is in carnivorous animals. In the former this segment of the canal +functions as an accessory digestive apparatus and hence, as we will see, +often develops accessory structural modifications, such as a large caecum +and spiral colon, while in the latter it acts almost solely as a canal +for the evacuation of the indigestible remnants. + +Again, the large intestine is better developed in the higher animals, in +mammalia and to a lesser degree in birds, in whom the functional demands +for nutrition are active and require that a relatively large amount of +food should pass through the digestive tract in a given time. On the +other hand in the lower cold-blooded vertebrates the metabolism is less +active, less food is taken and it is not necessary to secure all the +nutrient material contained in the same for the organism. The great +differences observed in the vertebrate series in regard to length, width +and structure of the large intestine depend upon these physiological +conditions. The divisions of the human large intestine into caecum, +ascending, transverse and descending colon, sigmoid flexure and rectum +are found only in the primates, and here not uniformly. + +In the lower vertebrate classes the endgut is very short, corresponding +only to the pelvic segment of the Mammalia (rectum), a colon proper +being absent in these forms (cf. Fig. 112, _Echelus conger_). The human +large intestine exhibits a very characteristic structure. Throughout the +greater part of the colon the longitudinal muscular layer is mainly +disposed in the form of three bands or taenia (ligamenta coli). The canal +itself is longer than these bands, thus producing a folding of the walls +in the form of three rows of pouches (cellulae coli), in the intervals +between the bands. The pouches of each row are separated from each other +externally by constrictions, internally by projecting crescentic folds +(plicae coli) (Figs. 471, 472 and 474). + +This arrangement of the large intestine is also found in the monkeys +(Fig. 473) and in certain Rodents (Fig. 474). + +In other mammals the large intestine is smooth and cylindrical and the +longitudinal layer of muscular fibers uniform (Fig. 475). + +In general the vertebrate large intestine is _wider_ than the small, +usually in the proportion of 5:1 or 6:1. + +In some ruminant Herbivora, however, the great length of the colon leads +to a reduction of the caliber in certain segments so that the large +intestine does not exceed the width of the small, or even falls below +the same. + +The _length_ of the large intestine, as in man, is usually much less +than that of the small intestine. As already stated this disproportion +is more marked in Carnivora than in Herbivora. + +The ratio in length of the large to the small intestine is very low in +the Seals (1:14), and in several Edentates, as _Myrmecophaga_, +_Tamandua_ and _Bradypus_ (1:9-11). + +In the carnivorous mammals it ranges 1:5-7. + +In some of the ruminant Herbivora, as the cow and sheep, it is 1:4, +while in the deer, horse, certain Rodents (as _Lepus_ and _Cricetus_) it +reaches as high as 1:2 or 1:3. + +The large intestine is usually relatively short in birds, reptiles, +amphibia and fish. + +In the Cassowary the length of the large to the small intestine is 1:6. + +In some of the birds of prey (eagle) the proportion falls as low as 1:68 +or 70. + +Exceptions to the general rule are furnished by some of the herbivorous +Cetaceans and by the Dugong (_Halicore_) in whom the large intestine is +twice as long as the small. Again in the Ostrich the large intestine in +one example measured 40', while the length of the small intestine was +only 22'. This unusual development of the large intestine indicates the +necessity of retaining the food, which is bulky and difficult of +digestion, until the elaboration is completed. The same significance +belongs to the enormously developed caeca of these birds (cf. p. 204). + +The separation of the small and large intestine is marked externally by +the _caecum_, when present, and internally by the _valve of the colon_. +The details of the vertebrate ileo-colic junction will be considered in +the following pages. + + +II. SERIAL REVIEW OF THE ILEO-COLIC JUNCTION AND CONNECTED STRUCTURES IN +VERTEBRATES. + +I. FISHES. + +In the Cyclostomata there is no differentiation between the mid- and +hindgut. Fig. 310 shows the entire alimentary canal of _Petromyzon +marinus_, the lamprey, caudad of the pericardium. + +[Illustration: FIG. 310.--_Petromyzon marinus_, lamprey. Entire +alimentary canal below pericardium. (Columbia University Museum, No. +1575.)] + +In some fishes the midgut is differentiated from the hindgut by an +external circular constriction, corresponding to an annular projecting +fold of the mucosa in the interior which resembles the pyloro-duodenal +valve. There is no caecum, and the short hindgut empties into the +cephalic and ventral aspect of the cloaca. Fig. 311 shows the entire +intestinal tract of a Teleost fish, _Echelus conger_, the conger eel. +The midgut, provided at the beginning with a short globular pyloric +appendix (cf. p. 119), constitutes the longest individual segment of the +canal. The hindgut, separated from the preceding by aconstriction, is +very short and of large caliber. Fig. 312 shows the broad annular valve +with central circular opening which separates mid- and hindgut in the +interior, and Fig. 313 the ileo-colic junction in section in the same +animal. + +[Illustration: FIG. 311.--_Echelus conger_, Conger eel. Alimentary +canal, stomach, mid- and hindgut, liver, and spleen. (Columbia +University Museum, No. 1430.)] + +[Illustration: FIG. 312.--_Echelus conger_, Conger eel. Ileo-colic +junction, opened. (Columbia University Museum, No. 1434.)] + +[Illustration: FIG. 313.--_Echelus conger_, Conger eel. Section of +mid- and end-gut, with ileo-colic junction, hardened. (Columbia +University Museum, No. 1349.)] + +A similar type of ileo-colic junction is seen in other Teleosts, as in +_Gadus callarias_, the cod (Fig. 314), _Pleuronectes maculatus_, the +flounder (Fig. 315), and in some Ganoids, as _Accipenser sturio_, the +sturgeon (Fig. 212). In some Selachians an appendicular diverticulum, +the so-called "rectal" or "digitiform gland," is found connected with +the terminal segment of the gut near the entrance of the same into the +cloaca (Fig. 316). + +[Illustration: FIG. 314.--_Gadus callarias_, cod-fish. Ileo-colic +junction. Intestine on each side opened, with probe passed through +constricted opening of ileo-colic valve. (Columbia University Museum, +No. 1260.)] + +[Illustration: FIG. 315.--_Pleuronectes maculatus_, flounder. +Ileo-colon, opened to show ileo-colic valve. (Columbia University +Museum, No. 1493.)] + +[Illustration: FIG. 316.--_Galeus canis_, dog-shark, male. +Genito-urinary tract and cloaca _in situ_. (Columbia University Museum, +No. 1694.)] + +[Illustration: FIG. 317.--_Accipenser sturio_, sturgeon. Alimentary +canal. (Columbia University Museum, Nos. 1826, 1827, and 1828.)] + + +II. AMPHIBIA. + +The alimentary canal is simple and usually comparatively short. There is +no caecal pouch. Differentiation of mid- and endgut is usually marked +externally by a constriction and by the increased caliber of the +terminal intestinal segment. + +[Illustration: FIG. 318.--_Rana catesbiana_, bull-frog. Alimentary canal +and appendages. (Columbia University Museum, No. 1454.)] + +[Illustration: FIG. 319.--_Necturus maculatus_, mud-puppy. Alimentary +canal and appendages. (Columbia University Museum, No. 1582.)] + +[Illustration: FIG. 320.--_Cryptobranchus alleghaniensis_, hellbender. +Ileo-colic junction. (Columbia University Museum, No. 1711.)] + +Fig. 318 shows the alimentary canal of the bull-frog, _Rana catesbiana_, +Fig. 319 that of a Urodele Amphibian, _Necturus maculatus_, and Fig. 320 +the ileo-colic junction isolated in _Cryptobranchus alleghaniensis_, the +hellbender. + + +III. REPTILIA. + +In reptiles a well-marked differentiation of small and large intestine +is the rule. + +Four types of ileo-colic junction are encountered in this class: + +1. The transition from small to large intestine is marked by the greatly +increased caliber of the latter and by an annular valve in the interior. +An example of this type is furnished by _Alligator mississippiensis_ +(Fig. 321) and a similar form is encountered in some lizards, as +_Heloderma suspectum_, the Gila monster (Fig. 322). + +[Illustration: FIG. 321.--_Alligator mississippiensis_, alligator. +Ileo-colon; dried preparation. (Columbia University Museum, No. 179.)] + +[Illustration: FIG. 322.--_Heloderma suspectum_, Gila monster. (Columbia +University Museum, No. 69/1536.)] + +2. The large intestine immediately beyond the ileo-colic junction +protrudes along the convex border to form a rudimentary lateral caecum. +This type is found in many Chelonians, _e. g._, in _Pseudemys elegans_, +the pond turtle (Figs. 323 and 324) and _Chelydra serpentaria_, the +snapping turtle (Fig. 325). + +[Illustration: FIG. 323.--_Pseudemys elegans_, pond-turtle. (Columbia +University Museum, No. 1069.)] + +[Illustration: FIG. 324.--_Pseudemys elegans_, pond-turtle. Ileo-colic +junction, opened. (Columbia University Museum, No. 1524.)] + +[Illustration: FIG. 325.--_Chelydra serpentaria_, snapping turtle. +Intestinal canal, pancreas, and spleen. (Columbia University Museum, No. +1369.)] + +3. The ileo-colic junction is provided with a well-developed sacculated +caecal pouch derived from the proximal segment of the colon and divided +in the interior by folds into several secondary compartments. + +[Illustration: FIG. 326.--_Iguana tuberculata_, iguana. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +243.)] + +This type is found in some of the phytophagous lizards, as _Iguana +tuberculata_ (Figs. 326 and 327). The small intestine of this animal is +of considerable length and of uniform caliber from the pylorus to the +ileo-colic junction. The caecum is a large sacculated pouch developed +chiefly along the convex border of the large intestine opposite to the +mesenteric attachment. + +[Illustration: FIG. 327.--_Iguana tuberculata_, iguana. Mid-gut, +ileo-colic junction, caecum, and end-gut; dried preparation. (Columbia +University Museum, No. 178.)] + +[Illustration: FIG. 328.--_Iguana tuberculata_, iguana. Ileo-colic +junction and caecum in section. (Columbia University Museum, No. 1321.)] + +[Illustration: FIG. 329.--Drawing taken from same preparation (No. 1321) +to elucidate more clearly internal structure of caecal pouch.] + +[Illustration: FIG. 330.--_Cyclura teres_, smooth-backed cyclura. +Ileo-colic junction and caecum in section. (Columbia University Museum, +No. 1523.)] + +The examination of the interior of this pouch reveals a complicated +structure (Figs. 328 and 329). Fig. 330 shows the same structures in a +closely allied form, _Cyclura teres_. The entrance of the small +intestine is guarded by an annular sphincter valve, whose central +circular opening leads into a proximal compartment of the caecum. This +compartment is in turn separated from the remainder of the caecal pouch +by a second circular valvular fold with central opening. Beyond this +valve the interior of the pouch carries a number of crescentic mucous +folds, corresponding to the external constrictions between the caecal +sacculations. The entire pouch gradually diminishes in caliber and +finally passes with a sharp angular bend into the terminal portion of +the endgut. At this point the lumen of the canal is slightly diminished +by a sphincter-like thickening of the muscularis, producing an annular +projection of the mucous membrane. The entire caecal pouch appears as a +specialized segment of the large intestine interposed between the +termination of the midgut and the terminal portion of the endgut, which +latter is characterized by uniform caliber and increased thickness of +the muscular walls. + +The highly developed and complicated structure of the caecal apparatus in +_Iguana_ and allied forms exemplifies very clearly the influence of +_vegetable_ food on the development of this segment of the alimentary +tract when compared with the simple type of ileo-colic transition found +in _carnivorous_ lizards, as _Heloderma_ (Fig. 322). _Iguana_ subsists +on leaves, fruits and other vegetable matter and the caecal pouch is +invariably found filled with the firmer and less digestible portions of +this food. These are undoubtedly retained in the pouch by the series of +valves and folds until digestion and absorption of all available +nutritive material forwarded from the small intestine is completed. On +the other hand _Heloderma_ lives almost entirely on bird eggs, a +concentrated and easily digested food. Consequently the ileo-colic +junction in this lizard is exceedingly simple and rudimentary, marked +merely by a slight external constriction, with an annular valve in the +interior, and an increase in the caliber of the short hindgut, +resembling the form found in many teleost fishes. + +4. Finally in some Ophidians a typical lateral caecal pouch of +considerable dimensions is found connected with the endgut immediately +beyond the ileo-colic junction. + +An example of this reptilian type, closely resembling the corresponding +structure in many Mammalia, is presented by _Eunectes marinus_, the +anaconda, shown in Figs. 331 and 332. + +[Illustration: FIG. 331.--_Eunectes marinus_, anaconda. Mid- and +end-gut, with ileo-colic junction and caecum. (Columbia University +Museum, No. 72/1535.)] + +[Illustration: FIG. 332.--_Eunectes marinus_, anaconda. Mid- and +end-gut, with ileo-colic junction and caecum laid open. (Columbia +University Museum, No. 1709.)] + + +IV. ILEO-COLIC JUNCTION IN BIRDS. + +In the birds the length of the intestine is subject to great variations. +The canal is short in species subsisting on fruits and insects, long in +those feeding on seeds, plants and fish. The large intestine, +immediately beyond the ileo-colic junction, is provided typically with +two symmetrical lateral caeca which extend in some forms for a +considerable distance cephalad on each side of the small intestine to +which they are bound by peritoneal connections. + +[Illustration: FIG. 333.--_Buteo harloni_, black hawk. Ileo-colic +junction and caeca. (Columbia University Museum, No. 1502.)] + +[Illustration: FIG. 334.--_Phalacrocorax dilophus_, double-crested +cormorant. Ileo-colic junction and caeca. (Columbia University Museum, +No. 67/1534.)] + +[Illustration: FIG. 335.--_Gallus bankiva_, hen. Ileo-colic junction and +caeca. (Columbia University Museum, No. 1486.)] + +[Illustration: FIG. 336.--_Chen hyperborea_, Canada snow-goose, +Ileo-colic junction and caeca. (Columbia University Museum, No. +47/1448.)] + +As a rule carnivorous birds have short and rudimentary pouches (Figs. +333 and 334), whereas they are long in herbivorous forms (Figs. 335 and +336). Some carnivorous birds, as _Corvus_, _Strix_, etc., have fairly +long caeca (Fig. 337). In the passerine birds living on seeds and +insects, the caeca are of considerable length as they are also in some of +the piscivorous divers (Figs. 338 and 339). They are long in the Ratitae, +and in the Lamellirostra, who feed chiefly on plants (Fig. 340). + +[Illustration: FIG. 337.--_Bubo virginianus_, great horned owl. +Ileo-colic junction and caeca. (Columbia University Museum, No. 672.)] + +[Illustration: FIG. 338.--_Urinator lumme_, red-throated loon. +Ileo-colic junction and caeca. (Columbia University Museum, No. 1001.)] + +[Illustration: FIG. 339.--_Merganser serrator_, red-breasted merganser. +Ileo-colic junction and caeca. (Columbia University Museum, No. 1798.)] + +[Illustration: FIG. 340.--_Casuarius casuarius_, cassowary. (Columbia +University Museum, No. 1799.)] + +[Illustration: FIG. 341.--_Struthio africanus_, African ostrich. +Ileo-colic junction and caeca. (Columbia University Museum, No. +48/1573.)] + +The enormously elongated caeca of the African ostrich contain a spiral +fold of the mucous membrane in the interior (Fig. 341). + +In place of the usual double avian caecum a single pouch is found in a +few forms, namely in the Herons (Fig. 342). + +[Illustration: FIG. 342.--_Ardea virescens_, green heron. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1132 a.)] + +[Illustration: FIG. 343.--_Urinator lumme_, red-throated loon. Small +intestine with caecal pouch; the remnant of the vitello-intestinal duct. +(Columbia University Museum, No. 997.)] + +[Illustration: FIG. 344.--_Urinator imber_, great northern diver. Small +intestine with caecal pouch; the remnant of the vitello-intestinal duct. +(Columbia University Museum, No. 78/1573)] + +In some birds the small intestine is also provided with a caecal pouch, +the remnant of the vitello-intestinal duct corresponding in its +significance to the occasional mammalian diverticulum of Meckel (Figs. +343 and 344). (cf. p. 35.) + + +V. ILEO-COLIC JUNCTION, CAECUM AND VERMIFORM APPENDIX IN THE MAMMALIA. + +I. Subclass: Ornithodelphia. + +I. Order: Monotremata. + +In many particulars the anatomical structure of these animals reveals a +close relationship to the Sauropsida. They represent the mammalian class +in its lowest stage of evolution. + +The ileo-colic junction in all the existing forms is direct, without +angular bend at the entrance of the small into the large intestine. The +caecum is a long narrow pouch, slightly dilated at the extremity, derived +from the beginning of the colon and extending backward along the free +margin of the small intestine. The caecum resembles in its general shape +and structure the pouches seen in many birds, except that it is +unilateral, while the birds normally have two symmetrical caeca. The +caecum of _Ornithorhynchus anatinus_, the platypus or duck bill, is shown +in Figs. 345 and 346, and that of _Echidna hystrix_, the spiny +ant-eater, in Fig. 347. These two animals represent the two genera into +which the order is divided. + +[Illustration: FIG. 345.--_Ornithorhynchus anatinus_, duck mole. +Ileo-colon and caecum. (Columbia University Museum, No. 1499.)] + +[Illustration: FIG. 346.--_Ornithorhynchus anatinus_, duck mole. +Ileo-colon and caecum. (Columbia University Museum, No. 1500.)] + +[Illustration: FIG. 347.--_Echidna hystrix_, spiny ant-eater. Ileo-colon +and caecum. (Columbia University Museum, No. 1501.)] + + +II. Subclass: Didelphia. + +II. Order: Marsupialia. + +The Didelphia are represented by numerous species, which are united by +certain common anatomical characters of the reproductive organs and +dentition to form the order of the Marsupialia. The individual species +included within this order differ widely in abit, food, mode of +locomotion, etc., and consequently exhibit great diversity in the +structure of the skeletal and muscular systems and of the alimentary +canal. With the exception of the Opossums inhabiting the new world, the +families composing the order are confined to the Australian continent +and the adjacent islands. In respect to the alimentary tract in general +and the ileo-colic junction in particular, we are evidently dealing with +a group of animals which, while they retain the common characters above +indicated as uniting them in the marsupial order, yet have in the +structure of their digestive canal adapted themselves to widely +divergent conditions of food supply and environment. Consequently within +the confines of this single and largely isolated order, we encounter +nearly all the types of caecum and ileo-colic junction which are found +among the remaining mammalia. The group in its individual +representatives has passed, so to speak, through the different stages of +development and evolution which, on a very much larger scale, are +exhibited by the remaining mammalian orders. + +We can, independently of the systematic zoological classification, +arrange the forms composing the order under the following types: + +1. _Forms with large well-developed simple caeca, of uniform caliber, +with rounded globular termination._ + +This type is encountered among the herbivorous Marsupials, such as the +opossums, kangaroos and wallabys. Fig. 348 shows the structures in +_Didelphis virginiana_, the common opossum, Fig. 349 in a small species +of opossum from Trinidad, and Fig. 350 the same parts in _Halmaturus +derbyanus_, the rock wallaby. + +[Illustration: FIG. 348.--_Didelphis virginiana_, opossum. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1533.)] + +[Illustration: FIG. 349.--_Didelphis sp._? opossum. Ileo-colic junction +and caecum. (Columbia University, Study Collection.)] + +[Illustration: FIG. 350.--_Halmaturus derbyanus_, rock kangaroo. +Ileo-colic junction and caecum. (Columbia University Museum, No. 727.)] + +2. _Forms with enormously developed sacculated caeca, coiled spirally, +with or without additional convolutions of the proximal colon; the +terminal portion of the caecal pouch diminishes in caliber to form a +pointed appendage._ + +This type of caecum characterizes the _Phalangeridae_ or Phalangers and +the _Phasolarctidae_. + +[Illustration: FIG. 351.--_Trichosurus vulpinus_, vulpine phalanger. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1800.)] + +[Illustration: FIG. 352.--_Phascolarctos cinereus_, koala. Ileo-colic +junction and caecum. (Drawn from preparation.) (Columbia University +Museum, No. 1528.)] + +Examples are shown in Figs. 351 and 352, representing the structures in +_Trichosurus vulpinus_, the vulpine phalanger, and _Phascolarctos +cinereus_, the koala. + +3. _Forms with simple caeca of moderate size._ + +The _Peramelidae_ or bandicoots. + +Fig. 353 shows the ileo-colic junction, caecum and proximal segment of +the colon in _Perameles nasuta_, the bandicoot. + +[Illustration: FIG. 353.--_Perameles nasuta_, bandicoot. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1481.)] + +4. _Forms with sacculated short caeca, whose terminal portion is reduced +to constitute a typical vermiform appendix._ + +The caecum of the _Phascolomyidae_ or wombats, resembles, in its general +structure and in the presence of a typical vermiform appendix, very +closely the corresponding parts of the alimentary canal in man and the +anthropoid apes. Fig. 354 shows these structures in _Phascolomys +wombat_, the common wombat. + +[Illustration: FIG. 354.--_Phascolomys wombat_, wombat. Ileo-caecum and +appendix. (Columbia University Museum, No. 1508.)] + +5. _Forms with simple direct ileo-colic junction without caecum._ + +In the purely carnivorous Marsupials, comprising the family of the +_Dasyuridae_, the reduction of the caecal apparatus, foreshadowed by the +appearance of the distal rudimentary segment as a vermiform appendix in +the wombats, has been carried to the complete elimination of the pouch. +The ileo-colic junction in these animals is simple, marked externally by +a circular constriction and internally by an annular valve. The colon +forms a very short terminal segment of the alimentary canal. The parts +are shown in Fig. 355 in a typical representative of the family, +_Dasyurus viverinus_, the Tasmanian devil. + +[Illustration: FIG. 355.--_Dasyurus viverinus_, dasyurus, Tasmanian +devil. Intestinal canal. Ileo-colic junction. (Columbia University +Museum, No. 1463.)] + +The structural modifications encountered in the digestive tract of these +carnivorous Marsupials can properly be regarded as the result of their +habitual diet, and we will meet with analogous and identical examples of +caecal reduction in the typical Carnivores among the placental mammals +(cf. p. 212). + + +III. Subclass: Monodelphia. + +III. Order: Edentata. + +In all probability the Sloths, Ant-eaters and Armadillos composing this +order represent a highly specialized remnant of an ancient group now +largely extinct. In respect to the ileo-colic junction the Edentates may +be arranged in two groups which offer, within the limited number of +existing species, a very complete transitional series. + + +I. SYMMETRICAL TYPE OF ILEO-COLIC JUNCTION. + +1. _Differentiation in caliber, with direct funnel-like transition of +small into large intestine. No caecum._ + +Beyond the ileo-colic junction the caliber of the large intestine +increases gradually. The terminal ileum is thus implanted into the apex +of a funnel formed by the proximal segment of the colon. + +Examples of this type are furnished by _Myrmecophaga jubata_, the great +ant-eater (Fig. 356), and by _Choloepus didactylus_, the two-toed sloth +(Fig. 357). + +[Illustration: FIG. 356.--_Myrmecophaga jubata_, great ant-eater. +Ileo-colic junction. (Columbia University Museum, No. 1519.)] + +[Illustration: FIG. 357.--_Choloepus didactylus_, two-toed sloth. +Ileo-colic junction. (Columbia University Museum, No. 714.)] + +2. _Abrupt demarcation of small and large intestine, with median +transition of ileum._ + +The caliber of the intestine enlarges rapidly immediately beyond the +ileo-colic junction. This form is derived from the preceding by the +substitution of the abrupt ileo-colic transition for the gradual +funnel-shaped development of the large intestine. + +The type is illustrated by _Tatusia novemcincta_, the nine-banded +armadillo (Fig. 358), and is also found in two other armadillos, +_Tolypeutes_ and _Xenurus_. + +[Illustration: FIG. 358.--_Tatusia novemcincta_, nine-banded armadillo. +Ileo-colic junction. (Columbia University Museum, No. 176.)] + +3. _The colon on each side of the ileo-colic junction is prolonged +backward along the small intestine, forming two symmetrical lateral +globular colic caeca._ + +This type, which is to be regarded as a further development of the +preceding form, is also found in the armadillos. Fig. 359 represents the +structures in _Dasypus sexcinctus_, the six-banded armadillo, and a +similar arrangement of the parts exists in _Chlamydophorus_, another +species of armadillo. + +[Illustration: FIG. 359.--_Dasypus sexcinctus_, six-banded armadillo, +Ileo-colic junction and caeca. (Columbia University Museum, No. 1478.)] + +4. _The caecal pouches are more completely differentiated, communicating +with the colon by a constricted neck._ + +This results in an arrangement which recalls the structure of many avian +caeca (cf. Fig. 337) and is seen in the double caecal pouches of +_Cyclothurus didactylus_, the little ant-eater (Fig. 360). + +[Illustration: FIG. 360.--_Cyclothurus didactylus_, little ant-eater. +Ileo-colic junction and caeca. (Columbia University Museum, No. 1512.)] + + +II. ASYMMETRICAL TYPE OF ILEO-COLIC JUNCTION. + +The second general group of the Edentates is characterized by the +gradual development of a single lateral asymmetrical caecum, in place of +the median symmetrical ileo-colic transition found in the forms just +considered. The species composing this group thus form a link leading up +to the right-angled accession of ileum to large intestine and the +lateral caecum characteristic of most other mammalia. + +[Illustration: FIG. 361.--_Manis longicauda_, long-tailed pangolin. +Ileo-colic junction; dry preparation. (Columbia University Museum, No. +95.)] + +[Illustration: FIG. 362.--_Manis longicauda_, long-tailed pangolin. +Ileo-colic junction. (Columbia University Museum, No. 328.)] + +1. This type may be considered as being inaugurated by the form of +ileo-colic junction found in the _Manidae_ or _Pangolins_, as illustrated +by Figs. 361 and 362, taken from the long-tailed pangolin, _Manis +longicauda_. There is no caecum and only a slight differentiation in +caliber between the small and large intestine. The gut in all the forms +examined shows a very characteristic bend at the ileo-colic junction, +being twisted into a figure of 8 and held in place by mesenteric folds. + +[Illustration: FIG. 363.--_Arctopithecus marmoratus_, three-toed sloth. +Ileo-colic junction. (Columbia University Museum, No. 1479.)] + +2. The second stage, illustrated by _Arctopithecus (Bradypus) +marmoratus_, the three-toed sloth (Fig. 363), reveals a distinct +increase in the caliber and convexity of the large intestine opposite +the mesenteric border immediately beyond the ileo-colic junction. + +[Illustration: FIG. 364.--_Tamandua bivittata_, Tamandua ant-eater. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1590.)] + +[Illustration: FIG. 365.--_Tamandua bivittata_, Tamandua ant-eater. +Ventral view of abdominal viscera from the left side. (Study Collection, +Columbia University.)] + +3. This leads in the third stage, represented by _Tamandua bivittata_, +the Tamandua ant-eater (Figs. 364 and 365), to the development of a +distinct lateral caecal pouch. I have had no opportunity of examining the +structures in _Orycteropus_, but from the published descriptions[8] the +large caecum of this animal would form the final link in this series. + +[8] Flower and Lyddecker, "Mammals, Living and Extinct," p. 209. + + +IV. Order: Sirenia. + +Of the two living representatives of this remarkable mammalian order the +dugong (_Halicore_) is described as possessing a single caecum, while the +caecal pouch of _Manatus americanus_, the manatee, is symmetrically bifid +at the extremity (Fig. 366). + +[Illustration: FIG. 366.--_Manatus americanus_, American manatee. +Ileo-colic junction and bifid caecum. (Columbia University Museum, No. +673.)] + +V. Order: Cetacea. + +In the majority of the whales the ileo-colic junction is simple without +caecum, as in _Physeter_, _Delphinus_, _Monodon_ and _Phocaena_ (Fig. +367). + +[Illustration: FIG. 367.--_Phocaena communis_, porpoise. Ileo-colic +junction. (Columbia University Museum, No. 1007.)] + +A few forms have a small caecal pouch. + + +VI. Order: Ungulata. + +The intestinal canal, in conformity with the herbivorous habit of the +group, is uniformly provided with a large caecum, and in many forms the +proximal segment of the colon immediately beyond the ileo-colic junction +is more or less extensively coiled in a spiral manner. This arrangement +is, without doubt, to be regarded as being functionally accessory to the +caecal apparatus, in the sense of increasing very much the area of the +secreting and absorbing surface and of prolonging the period during +which food-substances, which are slow and difficult of elaboration, are +retained in this segment of the alimentary canal. + + +1. SUBORDER: ARTIODACTYLA. + +A. NON-RUMINANTIA.--In the _Suidae_ the caecum is large and the spiral +colon well developed (Fig. 368). + +[Illustration: FIG. 368.--_Sus scrofa foet_, foetal pig. Ileo-caecum and +spiral colon _in situ_. (Columbia University Museum, No. 1111.)] + +In the peccaries (_Dicotyles_) the terminal portion of the caecal pouch +is reduced, constituting a centrally implanted appendage. + +[Illustration: FIG. 369.--_Dicotyles torquatus_, collared peccary. +Ileo-colic junction, caecum, and spiral colon. (Columbia University +Museum, No. 60/1462)] + +[Illustration: FIG. 370.--_Dicotyles torquatus_, collared peccary. +Ileo-colic junction and caecum, isolated. (Columbia University Museum, +No. 1464.)] + +Fig. 369 shows the ileo-colic junction and spiral colon in _Dicotyles +torquatus_, the collared peccary, and Fig. 370 the caecum and appendix of +the same animal detached from the spiral colon. In the hippopotamus, on +the other hand, the caecum is said to be absent. If this is the case the +animal forms an isolated exception among the Ungulates. + +B. RUMINANTIA.--The caecum is very large and the spiral coil of the colon +extensive. + +Fig. 371 shows the caecum of _Capra aegagrus_, the Bezoar goat, detached +from the adjacent intestine, and illustrates the type of the ruminant +pouch, of considerable length and caliber, without terminal reduction. +The same parts in a preparation of _Boselaphus tragocamelus_, the +nilghai, are shown in Fig. 372. + +[Illustration: FIG. 371.--_Capra aegagrus_, Bezoar goat. Ileo-colic +junction and caecum, isolated; dried preparation. (Columbia University +Museum, No. 194.)] + +[Illustration: FIG. 372.--_Boselaphus tragocamelus_, nilghai. Ileo-colic +junction and caecum, isolated. (Columbia University, Study Collection.)] + +Fig. 373 shows the caecum and ileo-colic junction, together with the +spiral coil of the colon, in _Bos indicus_, the zebu, and Fig. 374 the +same structures with a typical example of the spiral colon from _Cervus +sika_, the Japanese deer; Fig. 375 is taken from a preparation of the +parts in a foetal sheep, while Fig. 376 shows the spiral colon isolated +in _Oryx leucoryx_, the oryx. + +[Illustration: FIG. 373.--_Bos indicus_, zebu. Ileo-colic junction, +caecum, and spiral colon. (Columbia University Museum, No. 676.)] + +[Illustration: FIG. 374.--_Cervus sika_, Japanese deer. Ileo-colic +junction, caecum, and spiral colon. (Columbia University, Study +Collection.)] + +[Illustration: FIG. 375.--_Oris aries foet_, foetal sheep. Ileo-colic +junction, caecum, and spiral colon. (Columbia University Museum, No. +1379.)] + +[Illustration: FIG. 376.--_Oryx leucoryx_, oryx. Spiral colon, isolated. +(Columbia University, Study Collection.)] + + +2. SUBORDER: PERISSODACTYLA. + +In the horse and the rhinoceros the caecum is very large and of uniform +caliber. + +[Illustration: FIG. 377.--_Tapirus americanus_, American tapir. +Ileo-colic junction, caecum, and colon. (Columbia University Museum, No. +624.)] + +In the American tapir (Fig. 377) the large caecum tapers at its +extremity, to form a species of rudimentary appendix, resembling +somewhat the corresponding structure in _Dicotyles_ (cf. Figs. 369 and +370). The proximal segment of the colon is bent on itself in the form of +an extensive loop with closely adherent limbs, illustrating an early +stage in the development of the ruminant spiral colon (cf. p. 233). + + +3. SUBORDER: HYRACOIDEA. + +This suborder is formed by the single family of the _Hyracidae_. In +addition to their other isolated and puzzling structural peculiarities +the members of this small group present a most unusual arrangement of +the intestinal canal, which is unique among living mammalia. In addition +to a large sacculated caecal pouch, situated in the usual position at the +beginning of the colon, the large intestine is provided further on with +two supplementary elongated pointed conical pouches (Fig. 378). + +[Illustration: FIG. 378.--_Hyrax syriacus_, hyrax or coney. Intestinal +canal, with ileo-colic junction, proximal ileo-colic caecum, and distal +paired colic caeca. (Columbia University, Study Collection.)] + +This unique arrangement, which is not found in any other known +vertebrate, may possibly be led back to a type-form encountered in +certain saurians (see p. 234). + + +4. SUBORDER: PROBOSCIDEA. + +The caecum of the elephant is a very large sacculated pouch with rounded +termination, illustrated in Fig. 379, taken from the Asiatic elephant. + +[Illustration: FIG. 379.--_Elephas indicus_, Asiatic elephant. +Ileo-colic junction and caecum. (Columbia University Museum, No. 995.)] + + +VII. Order: Rodentia. + +With the exception of a single group, the dormice (_Myoxus_) (Fig. 380), +the rodents possess a well-developed caecal apparatus. + +[Illustration: FIG. 380.--_Myoxus avellanarius_, common dormouse. +Alimentary canal. (Columbia University Museum, No. 1466.)] + +In some forms the terminal portion of the pouch is reduced so as to +constitute an appendix. Many of these animals, in addition to the caecum +proper, have the proximal colon elongated and coiled in a spiral, and in +some this part of the large intestine is provided in the interior with a +spiral mucous fold. This latter structure functions again to increase +the extent of the mucous absorbing surface and to prolong the retention +of substances undergoing slow digestion and absorption. + +Typical examples of the capacious sacculated rodent caecum, with a +terminal pointed reduced segment, are afforded by _Castor fiber_, the +beaver (Figs. 381 and 382) and by _Erethizon dorsatus_, the Canadian +porcupine (Figs. 383 and 384). Figs. 385 and 386 show the ileo-colic +junction, caecum and appendix in _Lepus cuniculus_, the rabbit. The +interior of the caecal pouch and of the proximal segment of the colon is +provided with a complete spiral valve (Fig. 387), while the appendix is +differentiated by the histological character of its mucous membrane +which is studded with closely packed adenoid follicles (Fig. 388). A +similar aggregation of lymphoid tissue is found in this animal at the +ileo-colic junction forming the s. c. _saccus lymphaticus_ (Fig. 387). + +[Illustration: FIG. 381.--_Castor fiber_, beaver. Ileo-colic junction, +caecum, and proximal colon; ventral view. (Columbia University Museum, +No. 1607.)] + +[Illustration: FIG. 382.--_Castor fiber_, beaver. Ileo-colic junction, +caecum, and proximal colon; dorsal view. (Columbia University Museum, No. +1607.)] + +[Illustration: FIG. 383.--_Erethizon dorsatus_, Canadian porcupine. +Ileo-colic junction, caecum, and colon; ventral view. (Columbia +University, Study Collection.)] + +[Illustration: FIG. 384.--_Erethizon dorsatus_, Canadian porcupine. +Ileo-colic junction, caecum, and colon; dorsal view. (Columbia +University, Study Collection.)] + +[Illustration: FIG. 385.--_Lepus cuniculus_, rabbit. Ileo-colic junction +and caecum. (Columbia University Museum, No. 1568.)] + +[Illustration: FIG. 386.--_Lepus cuniculus_, rabbit. Ileo-colic junction +with saccus lymphaticus, isolated, (Columbia University, Study +Collection.)] + +[Illustration: FIG. 387.--_Lepus cuniculus_, rabbit. Ileo-colic junction +with saccus lymphaticus. Caecum and proximal segment of colon opened to +show spiral mucous fold in interior. (Columbia University Museum, No. +1587.)] + +[Illustration: FIG. 388.--_Lepus cuniculus_, rabbit. Caecum and appendix +inverted to show spiral fold and structure of mucosa. (Columbia +University Museum, No. 1588.)] + +The coils of the proximal colon encountered in many rodents are well +seen in _Dasyprocta agouti_, the agouti (Figs. 389 and 390), which +animal also illustrates a type of caecum found in several members of the +order. The pouch here is large, sacculated, uncinate, without reduction +of the terminal portion. + +[Illustration: FIG. 389.--_Dasyprocta agouti_, agouti. Ileo-colic +junction, caecum, and colon. (Columbia University Museum, No. 24/1576.)] + +[Illustration: FIG. 390.--_Dasyprocta agouti_, agouti. Ileo-colic +junction, caecum, and colon. (Drawing based on preparation shown in Fig. +388.)] + +[Illustration: FIG. 391.--_Lagomys pusillus._ Ileo-colic junction, +caecum, and colon. (After Pallas, from Oppel, "Lehrbuch d. Vergl. +mikrosk. Anat. d. Wirbelthiere," II., Jena, 1897, p. 577, Fig. 314.)] + +[Illustration: FIG. 392.--_Arvicola pennsylvanicus_, field mouse. +Alimentary canal. (Columbia University Museum, No. 815.)] + +[Illustration: FIG. 393.--_Mus decumanus_, white rat. Ileo-colic +junction, caecum, and colon. (Columbia University Museum, No. 1574.)] + +The relatively enormous size of the caecum in the _Muridae_ is shown in +Fig. 392, representing the entire visceral tract of _Arvicola +pennsylvanicus_, the meadow mouse. The pouch in these animals is large, +smooth and of uniform caliber (Fig. 393). + +In some the colon beyond the entrance of the small intestine is provided +with a spiral mucous valve (Fig. 394). + +[Illustration: FIG. 394.--_Arvicola riparius_, meadow mouse. Ileo-colic +junction, caecum, and colon. (Columbia University, Study Collection.)] + +In the single instance of _Myoxus_ among the rodents, the ileo-colic +junction is simple, without any caecal pouch (Fig. 380). + + +VIII. Order: Carnivora. + +A. PINNIPEDIA.--In the seals and walrus the caecum is very small with a +blunt termination. Fig. 395 shows its structure in _Zalophus +gillespiei_, Gillespie's sea-lion, and Fig. 396 in _Phoca vitulina_, the +harbor seal. + +[Illustration: FIG. 395.--_Zalophus gillespiei_, Gillespie's sea-lion. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 90.)] + +[Illustration: FIG. 396.--_Phoca vitulina_, harbor seal. Ileo-colic +junction and caecum. (Columbia University Museum, No. 762.)] + +B. FISSIPEDIA.--The _Cynoidea_, including the dogs, jackals, wolves and +foxes, form a well-marked central group with well-developed convoluted +caeca placed laterally to the ileo-colic junction (Figs. 397-399). + +[Illustration: FIG. 397.--_Vulpes fulvus_, red fox. Ileo-colic junction +and caecum; dried preparation. (Columbia University Museum, No. 114.)] + +[Illustration: FIG. 398.--_Canis familiaris_, dog. Ileo-colic junction +and caecum, Type I. (Columbia University Museum, No. 1550.)] + +[Illustration: FIG. 399.--_Canis familiaris_, dog. Ileo-colic junction +and caecum, Type II. (Columbia University Museum, No. 1551.)] + +From this type depart on the one hand the _Ailuroidea_, including the +civets, ichneumons and true cats, with the caecum uniformly present, but +short and markedly pointed, suggesting the degeneration of a formerly +better developed structure (Figs. 400-406), while on the other the +_Arctoidea_, including the bears, weasels and raccoons, constitute a +group united by many common fundamental peculiarities of structure, +among which is the entire absence of a caecal pouch (Figs. 407-415). + +[Illustration: FIG. 400.--_Genetta vulgaris_, genet. Ileo-colic junction +and caecum. (Columbia University Museum, No. 1625.)] + +[Illustration: FIG. 401.--_Felis concolor_, puma. Ileo-colic junction +and caecum; dried preparation. (Columbia University Museum, No. 119.)] + +[Illustration: FIG. 402.--_Felis borealis, var. rufus_, red lynx. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 177.)] + +[Illustration: FIG. 403.--_Paradoxurus typus_, paradoxure. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +112.)] + +[Illustration: FIG. 404.--_Herpestes sp.?_, ichneumon. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +120.)] + +[Illustration: FIG. 405.--_Herpestes griseus_, mongoose ichneumon. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 149.)] + +[Illustration: FIG. 406.--_Proteles lalandii_, aard-wolf. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1520.)] + +[Illustration: FIG. 407.--_Nasua rufa_, brown coati-mundi. Ileo-colic +junction. (Columbia University Museum, No. 1089.)] + +[Illustration: FIG. 408.--_Nasua rufa_, brown coati-mundi. Ileo-colic +junction, opened, showing pyloric-like ileo-colic valve. (Columbia +University Museum, No. 1581.)] + +[Illustration: FIG. 409.--_Bassaris astuta_, raccoon-fox. Ileo-colic +junction; dried preparation. (Columbia University Museum, No. 289.)] + +[Illustration: FIG. 410.--_Mustela sp.?_, marten. Ileo-colic junction; +dried preparation. (Columbia University Museum, No. 199.)] + +[Illustration: FIG. 411.--_Taxidea americana_, American badger. +Ileo-colic junction; dried preparation. (Columbia University Museum, No. +180.)] + +[Illustration: FIG. 412.--_Procyon lotor_, raccoon. Ileo-colic junction; +dried preparation. (Columbia University Museum, No. 230.)] + +[Illustration: FIG. 413.--_Cercoleptes caudivolvulus_, kinkajou. +Ileo-colic junction; dried preparation. (Columbia University Museum, No. +295.)] + +[Illustration: FIG. 414.--_Ursus americanus_, black bear. Ileo-colic +junction; dried preparation. (Columbia University Museum, No. 226.)] + +[Illustration: FIG. 415.--_Ursus maritimus_, polar bear. Ileo-colic +junction. (Columbia University Museum, No. 748.)] + +[Illustration: FIG. 416.--_Hyaena striata_, striped hyena. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +56.)] + +[Illustration: FIG. 417.--_Felis leo_, lion, Ileo-colic junction and +caecum. (Columbia University Museum, No. 1516.)] + +Among the ailuroid carnivora, the hyaena and the lion occupy an isolated +position in regard to the caecum. Both of these animals possess a +well-developed long caecal pouch with blunt extremity (Figs. 416 and +417). They probably afford examples of a persistent ancestral common +type from which the remaining carnivorous forms are derived by reduction +of the caecal apparatus in conformity with the food-habits of these +animals. The caecum of both the lion and hyaena resembles very closely the +pouch of the herbivorous marsupials, such as _Halmaturus_ or _Didelphis_ +(cf. Figs. 348 and 350, p. 205). + + +IX. Order: Cheiroptera. + +In the bats the alimentary canal is uniformly simple without caecum and +scarcely any differentiation between small and large intestine (Fig. +418). + +[Illustration: FIG. 418.--_Pteropus medius_, Indian fruit-bat. +Ileo-colon; dried preparation. (Columbia University Museum, No. 533.)] + + +X. Order: Insectivora. + +In the true Insectivora the caecum is also absent and the alimentary +canal a simple non-differentiated tube. + +In certain herbivorous animals included in this group on the other hand, +such as _Galeopithecus_ (Fig. 419), the caecum is present as an enormous +sacculated pouch with spiral convolutions. + +[Illustration: FIG. 419.--_Galeopithecus volans_, colugo. Ileo-colic +junction, caecum, and colon. (Columbia University Museum, No. 1844.)] + + +XI. Order: Primates. + +The caecum is uniformly present. In certain of the Lemuroidea the +terminal portion of the pouch is reduced, forming a species of appendix. +A typical vermiform appendix is regularly found in man and in the +anthropoid apes, orang, gibbon, chimpanzee and gorilla. + + +1. Suborder Lemuroidea. + +In the typical lemurs the caecum is long, frequently terminating in a +pointed appendage. The proximal segment of the colon is looped and +coiled, resembling the spiral colon of the Ungulates and Rodents. Fig. +420 shows the caecum of _Nycticebus tardigradus_, the slow lemur, with +the typical appendage, and Fig. 421 shows the spiral arrangement of the +proximal colon immediately beyond the ileo-colic junction in the same +animal. Fig. 422, taken from another specimen of the same animal shows +the caecum, appendix and spiral colon. Figs. 423, 424, 425 illustrate the +structure of the parts in three other members of the group, _Lemur +macaco_, _Lemur mongoz_ and _Otolicnus crassicaudatus_, all showing +terminal reduction of the caecal pouch and tendency to spiral coiling of +the proximal colon. In _Tarsius spectrum_ (Fig. 426) the caecum is +relatively well-developed, but forms a simple pouch of uniform diameter, +without terminal reduction. + +[Illustration: FIG. 420.--_Nycticebus tardigradus_, slow lemur. +Ileo-colic junction, caecum, appendix, and colon; dorsal view. (Columbia +University Museum, No. 20/1468.)] + +[Illustration: FIG. 421.--_Nycticebus tardigradus_, slow lemur. Same +preparation as Fig. 420; ventral view, showing spiral coiling of +proximal colon. (Columbia University Museum, No. 20/1468.)] + +[Illustration: FIG. 422.--_Nycticebus tardigradus_, slow lemur. +Ileo-colic junction, caecum, appendix, and spiral colon. (Columbia +University, Study Collection.)] + +[Illustration: FIG. 423.--_Lemur macaco_, lemur. Ileo-colic junction and +caecum. (Drawn from preparation.) (Columbia University Museum, No. +1623.)] + +[Illustration: FIG. 424.--_Lemur mongoz_, lemur. Ileo-colic junction and +caecum. (Drawn from preparation.) (Columbia University Museum, No. +1473.)] + +[Illustration: FIG. 425.--_Otolicnus crassicaudatus_, galago. Ileo-colic +junction and caecum. (Drawn from preparation.) (Columbia University +Museum, No. 1626.)] + +[Illustration: FIG. 426.--_Tarsius spectrum_, spectre lemur. (Drawn from +preparation.) (Columbia University Museum, No. 1521.)] + + +2. Suborder Anthropoidea. + +A. CYNOMORPHA. + +=1. Cynocephalus.=--The baboons have a well-developed capacious caecum. +The apex of the pouch is usually blunt and rounded, or only slightly +pointed. The caecum is sacculated, conforming in structure to the rest of +the large intestine. Two low vascular folds or ridges, a ventral and a +dorsal, carry the ventral and dorsal caecal branches of the ileo-colic +artery. The intermediate non-vascular fold is large, frequently fused +with the dorsal vascular fold (cf. p. 264). + +Figs. 427-433 show the structures in _Cynocephalus sphinx_, _porcarius_, +_babuin_, _anubis_ and in _Cercopithecus pogonias_, _sabaeus_ and +_campbellii_. + +[Illustration: FIG. 427.--_Cynocephalus sphinx_, Guinea baboon. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1082.)] + +[Illustration: FIG. 428.--_Cynocephalus porcarius_, Chacma baboon. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1071.)] + +[Illustration: FIG. 429.--_Cynocephalus babuin_, yellow baboon; dried +preparation. (Columbia University Museum, No. 89.)] + +[Illustration: FIG. 430.--_Cynocephalus anubis_, olive baboon. (Columbia +University Museum, No. 51/1618.)] + +[Illustration: FIG. 431.--_Cercopithecus pogonias_, bearded monkey. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 228.)] + +[Illustration: FIG. 432.--_Cercopithecus sabaeus_, green monkey. +Ileo-colic junction and caecum. (Drawn from preparation.) (Columbia +University Museum, No. 746.) + + 1. Ventral ileo-caecal vascular fold. + 2. Dorsal ileo-caecal vascular fold. + 3. Intermediate ileo-caecal non-vascular fold. +] + +[Illustration: FIG. 433.--_Cercopithecus campbellii_, cercopithecus +monkey. Ileo-colic junction and caecum. (Drawn from preparation.) +(Columbia University Museum, No. 55/1542.)] + +=2. Macacus.=--The caecum is of large caliber, blunt, or in some forms +slightly pointed at the apex, sacculated like the colon. + +The two vascular folds are narrow and low, studded with epiploic +appendages. The intermediate non-vascular fold is large, placed nearer +to the dorsal than to the ventral vascular fold. + +Figs. 434-439 show the structures in _Macacus cynomolgus_, _ochreatus_, +_rhesus_ and _pileatus_. + +Fig. 439 is from a formaline hardened situs preparation of the abdominal +viscera in _Macacus cynomolgus_, the Kra monkey. + +[Illustration: FIG. 434.--_Macacus cynomolgus_, Macaque monkey. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 19.)] + +[Illustration: FIG. 435.--_Macacus ochreatus_, ashy-black macaque. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 11.)] + +[Illustration: FIG. 436.--_Macacus rhesus_, rhesus monkey. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1126.)] + +[Illustration: FIG. 437.--_Macacus pileatus_, macaque. Ileo-colic +junction and caecum. (Columbia University Museum, No. 719.)] + +[Illustration: FIG. 438.--_Macacus sinicus_, bonnet macaque. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1072.)] + +[Illustration: FIG. 439.--_Macacus cynomolgus_, kra monkey. Abdominal +viscera, hardened _in situ_. (Columbia University Museum, No. 1801.)] + + +B. ARCTOPITHECINI. + +The marmosets have a long crescentic-shaped caecum, turning the concavity +of the curve upwards and to the left, terminating in a blunt point. + +Typical forms are shown in Fig. 440, _Hapale jacchus_, Fig. 441, _Midas +ursulus_, and Fig. 442, _Midas geoffrei_. + +[Illustration: FIG. 440.--_Hapule jacchus_, common marmoset. Ileo-colic +junction and caecum. (Columbia University Museum, No. 975.)] + +[Illustration: FIG. 441.--_Midas ursulus_, negro tamarin. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +235.)] + +[Illustration: FIG. 442.--_Midas geoffrei_, Geoffrey's marmoset. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 197.)] + + +C. CEBIDAE. + +=1. Ateles= and other howlers have a large caecum, crescentic in shape, +narrowed at the apex, separated from the colon by a sharp and deep +constriction, opposite the wedge-shaped ileo-colic junction. + +The ileo-caecal folds are well-developed and symmetrical, two equal +vascular folds, and a free intermediate non-vascular reduplication. + +Types: _Ateles ater_ (Figs. 443-445), _Chrysothrix sciureus_ (Fig. 447) +and _Nyctipithecus commersonii_ (Fig. 446). In _Mycetes_ (Figs. 448-450) +the pouch is shorter, less curved, with a slight reduction toward the +less distinctly pointed apex. + +[Illustration: FIG. 443.--_Ateles ater_, black-faced coaita. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +240.)] + +[Illustration: FIG. 444.--_Ateles ater_, black-faced coaita. Ileo-colic +junction and caecum, with ileo-caecal folds. (Columbia University Museum, +No. 720.)] + +[Illustration: FIG. 445.--_Ateles ater_, black-faced coaita. Ileo-colic +junction and caecum, with ileo-caecal folds. (Drawn from preparation.) +(Columbia University Museum, No. 300.) + + 1. Ventral vascular ileo-caecal fold. + 2. Intermediate non-vascular ileo-caecal fold. + 3. Dorsal vascular ileo-caecal fold. +] + +[Illustration: FIG. 446.--_Nyctipithecus commersonii_, Vitoe monkey. +Ileo-colic junction and caecum; dried preparation. (Columbia University +Museum, No. 238.)] + +[Illustration: FIG. 447.--_Chrysothrix sciureus_, Viti monkey. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1624.)] + +[Illustration: FIG. 448.--_Mycetes cavaya_, black howler. Ileo-colic +junction and caecum. (Columbia University Museum, No. 1136.)] + +[Illustration: FIG. 449.--_Mycetes fuscus_, brown howler. Ileo-colic +junction and caecum, with ileo-caecal folds; ventral view. (Columbia +University Museum, No. 674.) + + 1. Ventral vascular ileo-caecal fold. + 3. Intermediate non-vascular ileo-caecal fold. +] + +[Illustration: FIG. 450.--Drawn from the same preparation as Fig. 449; +dorsal view. + + 2. Dorsal vascular ileo-caecal fold. + 3. Intermediate non-vascular ileo-caecal fold. +] + +[Illustration: FIG. 451.--_Lagothrix humboldtii_, Humboldt's lagothrix. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1511.)] + +[Illustration: FIG. 452.--_Pithecia satanas_, black saki monkey. +Ileo-colic junction and caecum. (Columbia University Museum, No. 641.)] + +=2. Lagothrix.=--The caecum is very capacious and long, bent at a sharp +angle upwards and to the left toward the ileo-colic junction. + +Type: _Lagothrix humboldtii_ (Fig. 451). + +=3. Pithecia.=--The caecum resembles in general the type presented by +_Ateles_, but is less curved and less reduced and pointed at the +termination. + +Type: _Pithecia satanas_ (Fig. 452). + +In general the Arctopithecini and _Ateles_, _Mycetes_, _Lagothrix_ and +_Pithecia_ among the Cebidae form a group containing a series of caecal +transition types which lead up to the anthropomorphous type, +illustrating the following conditions: + +(_a_) The inherent crescentic curve of the caecum, with the concavity +directed toward the left, and carrying the apex of the pouch upward +toward the lower border of the ileum and the ileo-colic junction. +(_Hapalidae_, _Ateles_, _Lagothrix_.) + +(_b_) The reduction in caliber of the terminal part, foreshadowing by +the pointed and narrow extremity of the pouch the appearance of the +appendix in the anthropomorphous group. (_Hapalidae_, _Ateles_.) + +(_c_) The constriction at the level of the ileo-caecal junction, with the +corresponding well-marked differentiation between caecum and colon in the +interior. (_Ateles._) + +(_d_) The sharp bend in the pouch as it makes its turn upward and to the +left, repeated in certain types of adult human caeca (cf. p. 247). +(_Lagothrix._) + +(_e_) _Pithecia_ forms a transitive type between the blunt sacculated +caeca of the Cynomorpha and the curved pointed pouches of the Cebidae, +partaking of the characters of both. + +(_f_) The same character is seen in the caecum of _Mycetes fuscus_ the +brown howler monkey (Figs. 449 and 450). + +=4. Cebinae.=--In the typical genus _Cebus_ the caecum is placed laterad +to the small intestine which is in direct linear continuity with the +colon. The pouch is slightly convoluted toward its termination, +resembling in this respect and in its position relative to the lumen of +the intestinal canal, the disposition of the parts in the cynoid +carnivora. Figs. 453 and 454 show the structures in two typical species, +_Cebus monachus_ and _C. leucophaeus_. + +[Illustration: FIG. 453.--_Cebus monachus_, capuchin monkey. Ileo-colic +junction and caecum; dried preparation. (Columbia University Museum, No. +26.)] + +[Illustration: FIG. 454.--_Cebus leucophaeus_, capuchin monkey. +Ileo-colic junction and caecum. (Columbia University Museum, No. 1467.)] + + +D. ANTHROPOMORPHA. + +The caecum is large, sacculated, provided uniformly with a vermiform +appendix. + +The pouch of the four anthropoid apes agrees in curve, direction, +implantation of the appendix and the general arrangement of the vascular +and peritoneal folds with the structure in the human subject. + +=1. Hylobates hoolock, Gibbon.=--Figs. 455 and 456 represent +respectively the ileo-caecum of this animal in the ventral view, and from +the left side with the ileum turned forward. The caecum is a globular +rounded pouch of nearly uniform diameter, only slightly enlarged to the +right of the root of the appendix which arises from its lowest part and +is pendent. + +[Illustration: FIG. 455.--_Hylobates hoolock_, hoolock gibbon. +Ileo-colic junction and caecum; ventral view. (Drawn from Columbia +University Museum preparation No. 1530.)] + +[Illustration: FIG. 456.--Drawn from same preparation as Fig. 455; view +from left side, showing formation of posterior ileo-caecal fossa.] + +[Illustration: FIG. 457.--_Gorilla savagei_, gorilla. Ileo-colic +junction and caecum, with ileo-caecal folds. (Drawn from Columbia +University Museum preparation No. 1543.)] + +(For arrangement of the ileo-caecal folds and fossae in this form see p. +269.) + +=2. Gorilla savagei, Gorilla= (Fig. 457).--The caecum is large, +distinctly sacculated, presenting a decided curve with the concavity +directed toward the left. The appendix is implanted at the center of the +blunt apex of the pouch, the caecal sacculations on each side of the root +of the appendix being of nearly equal size (folds and fossae, cf. p. +269). + +[Illustration: FIG. 458.--_Simia satyrus_, orang. Caecum and ileo-colic +junction; ventral view. (Drawn from Columbia University Museum +preparation No. 716.) 1. Appendix. 2. Ventral vascular fold.] + +[Illustration: FIG. 459.--_Simia satyrus_, orang. Caecum and ileo-colic +junction; dorsal view. (Drawn from Columbia University Museum +preparation No. 716). 1. Appendix. 2. Intermediate non-vascular fold.] + +=3. Simia satyrus, Orang-outang.=--Figs. 458 and 459 represent +respectively the ventral and dorsal views of the caecum and ileo-colon in +a nearly adult male specimen of orang, about 41/2 feet high. + +The caecum is funnel-shaped, gradually narrowing to the origin of the +appendix from its apex, which is carried upwards to the left by the +well-marked crescentic curve of the pouch. The sweep of the funnel to +the left and upwards is characterized by the curved course of the +ventral longitudinal muscular band (Fig. 458), whose fibers spread out +over a surface 3 cm. wide. The apex is thus placed behind the terminal +ileum close to its entrance into the large intestine. + +At the level of the upper margin of the ileo-colic junction the narrow +pointed termination of the caecum passes gradually into the beginning of +the appendix (Fig. 459). + +The appendix measures along its free border 22.6 cm. It follows the +direction of the caecal curve for 2.7 cm., at which point it appears +somewhat constricted and takes an abrupt bend downwards for 4.3 cm.; +curving again upwards for 7.5 cm., it turns downward a second time for +5.4 cm. and terminates in a hook-like extremity 2.7 cm. long (Fig. 459). + +[Illustration: FIG. 460.--_Troglodytes niger_, chimpanzee. Ileo-colic +junction and caecum; ventral view. (Drawn from Columbia University Museum +preparation No. 675.) 1. Appendix. 2. Intermediate non-vascular +ileo-caecal fold. 3. Colon.] + +[Illustration: FIG. 461.--_Troglodytes niger_, chimpanzee. Dorsal view. +(Drawn from Columbia University Museum preparation No. 675.) 1. +Appendix. 2. Intermediate non-vascular ileo-caecal fold. 3. Dorsal +vascular fold.] + +=4. Chimpanzee, Troglodytes niger.=--Figs. 460 and 461 represent the +ventral and dorsal view respectively of the caecum and ileo-colon in a +young specimen. + +The caecum is curved to the left and the lowest point of the pouch is +formed by the right lateral and ventral wall of the gut, but the extreme +crescentic bend which carries the origin of the appendix up and to the +left behind the ileo-colic junction is not yet developed in the young +animal; on the other hand this character of the caecum is typically +apparent in Figs. 462 and 463, taken from an adult individual of the +same species. + +[Illustration: FIG. 462.--_Troglodytes niger_, chimpanzee. Ileo-colic +junction and caecum; ventral view. (Drawn from Columbia University Museum +preparation No. 1083.) 1. Ventral vascular ileo-caecal fold.] + +[Illustration: FIG. 463.--_Troglodytes niger_, chimpanzee. Ileo-colic +junction and caecum; dorsal view. (Drawn from Columbia University Museum +preparation No. 1083.) 1. Appendix.] + +[Illustration: FIG. 464.--_Troglodytes niger_, chimpanzee. Ileo-colic +junction and caecum. (Drawn from Columbia University Museum preparation +No. 1525.)] + +This extreme curve is well seen in the ventral view in Figs. 462 and +464, the latter taken from a large adult specimen. Seen from behind in +Fig. 463 the sharp bend or kink in the lumen of the caecal pouch produced +by this curve is striking and resembles the arrangement of certain types +of adult human caeca (p. 247). + + +II. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION AND CAECUM IN THE +VERTEBRATE SERIES. + +The segments of the alimentary canal illustrate very clearly the +adaptation of structure to function. Diversity of kind and quantity of +food habitually taken and variations in the rapidity of tissue +metabolism produce marked morphological modifications in different +forms. This is more especially the case with the junction of the mid- +and hindgut, the site of development of the caecal apparatus and of +structural alterations of the large intestine possessing a similar +physiological significance. No other portion of the visceral tract, +with the possible exception of the stomach, illustrates more completely +the result of physiological demand on the development of anatomical +structure and the morphological possibilities of departure, progressive +and retrograde, from a common primitive type in accordance with varying +conditions of alimentation. + +In cooerdinating, from the morphological standpoint, the structural +differences encountered in this segment of the alimentary canal, two +facts become apparent. + +1. In the first place the serial study of the ileo-colic junction, as we +can briefly define the region in question by borrowing the terminology +of anthropotomy, reveals a limited number of principal structural types +from which by successive gradations the vast variety of individual forms +may be derived. + +[Illustration: FIG. 465.--Schematic table of the vertebrate types of +ileo-colic junction.] + +(In the schematic Fig. 465 the fundamental types and their derivatives +are indicated. In the following the individual forms illustrating these +types are referred to this schema in brackets.) + +2. The observer will be impressed by the fact that representatives of +all the main types of ileo-colic junction are found within a very +limited zoological range, as within the confines of a single order. +Examples of this are furnished by the Marsupialia and, to a lesser +extent, by the Edentata. The members of these zoological groups, while +united by certain common anatomical characters, such as the reproductive +system and dentition, differ widely in habit and in the kind and +quantity of the food normally taken. These differences in the method of +nutrition have impressed their influence on the structure of the +alimentary canal and have led to the evolution of varying and divergent +types of ileo-colic junction. The study of this segment of the +intestinal tract can therefore elucidate the mutual relationship of the +vertebrate groups only to a limited degree and in special cases. On the +other hand, it renders very clear the fundamental structural ground-plan +common to all vertebrates and accentuates the specialized modifications +of this plan which develop in response to the physiological environment. +Moreover, such a review serves to reveal the significance of +rudimentary and vestigial structures, such as the human vermiform +appendix and the serous and vascular folds connected with the same. +Throughout the entire vertebrate series the alimentary canal is found to +respond with great readiness in its structure to varying grades of +functional demand. This fact becomes still more apparent if the inquiry +is not limited strictly to the region of the ileo-colic junction but +takes into account likewise the structural modifications of similar +physiological significance in other segments of the alimentary tract. + +A caecal pouch or diverticulum in some form at the junction of mid- and +hindgut is a very common and widely distributed mammalian character. The +activity of the tissue-changes in warm-blooded animals, and the +consequent necessity for a rapid and complete digestive process, account +for the structural modifications of the alimentary tract so commonly +encountered among these forms. On the other hand, in the lower +cold-blooded vertebrates, notably in fishes and amphibians, the +metabolism is slow and the alimentary canal usually simple. + +Specifically, the caecum appears as a pouch or diverticulum in which +food-substances, already partially digested and mixed with the +secretions of the small intestine, are retained until their elaboration +is completed and the nutritive value of the food ingested is secured for +the organism. Consequently the most complicated and highly developed +caecal apparatus is found among mammalia in the Herbivora, such as the +Ungulates and Rodents, whose food contains a comparatively small amount +of nutriment in ratio to its bulk, and hence requires considerable +elaboration before absorption. On the other hand the caecum appears as a +reduced or even rudimentary organ, or defaults entirely, in Carnivora +whose food is concentrated and easily assimilated, containing only a +small amount of non-nutritive material. + +The function of the caecal apparatus may be defined as follows: + +1. It provides space for the retention of partly digested substances, +and of such as are difficult of digestion, mixed with the secretions of +the preceding intestinal segment, until the digestive elaboration is +completed. + +2. It increases the intestinal mucous surface for absorption, and may +develop, in certain cases, special localized areas of lymphoid tissue. + +These two functional characters may be shared by other segments of the +intestinal tract, which undergo corresponding structural modifications. +It is only necessary to refer in this connection to the extreme +morphological variations encountered in the stomach. The intestinal +canal proper, however, in many instances exhibits structural +peculiarities which possess the functional significance of the caecal +apparatus. Thus the projection into the lumen of the canal of a series +of mucous folds, or the development of a continuous spiral mucous valve, +evidently serves the double purpose of prolonging the period during +which the intestinal contents are retained, and of increasing the +intestinal mucous surface for absorption. + +[Illustration: FIG. 466.--_Squalus acanthius_, dog-fish. Alimentary +tract, spleen, pancreas. (Drawn from Columbia University Museum +preparation No. 1405.)] + +[Illustration: FIG. 467.--_Galeus canis_, dog-shark. Alimentary tract +opened, showing spiral intestinal valve. (Drawn from Columbia University +Museum preparation No. 1429.)] + +[Illustration: FIG. 468.--_Ceratodus forsteri_, Australian lung-fish. +Intestinal canal with spiral valve. (Columbia University Museum, No. +1645.)] + +[Illustration: FIG. 469.--_Python molurus_, Indian python. Mid-gut, +distended and fenestrated to show spiral course of lumen. (Columbia +University Museum, No. 725.)] + +This spiral mucous fold is encountered in the straight intestinal canal +of the Cyclostomata (Fig. 465, _IV_, 1, and Fig. 310), Selachians (Figs. +466 and 467) and Dipnoeans (Fig. 468). Phylogenetically it is a very old +structure, for evidences of its existence are found in the fossil +remains of some Elasmobranchs. In the Ostrich (Fig. 341) the enormously +developed caeca possess the same spiral mucous fold in the interior. The +direct combination of the caecum and spiral fold is again seen in certain +mammalia, as in _Lepus_ (Fig. 387). In some Ophidians the same +physiological purpose is served by the manner in which the convolutions +of the long intestine are bound together by a subperitoneal arachnoid +membrane. The lumen of the canal is thus made to assume a spiral course +(Figs. 331 and 469). The mucous folds of the human intestine, both the +valvulae conniventes and the crescentic folds of the large intestine, +represent the same spiral valve, perhaps modified and influenced by the +erect posture of man (Figs. 470-475). + +[Illustration: FIG. 470.--Human small intestine, opened to show valvulae +conniventes. (Columbia University Museum, No. 1841.)] + +[Illustration: FIG. 471.--Human large intestine, showing colic taenia and +plica. (Columbia University Museum, No. 1848.)] + +[Illustration: FIG. 472.--Human large intestine, opened and in section, +showing colic plicae. (Columbia University Museum, No. 1847.)] + +[Illustration: FIG. 473.--_Cynocephalus anubis_, olive baboon. Large +intestine, with cross-section showing colic taenia and plicae. (Columbia +University Museum, No. 26/1168.)] + +[Illustration: FIG. 474.--Comparison of portion of human transverse +colon with distal segment of rabbit's large intestine, showing same +arrangement of longitudinal muscular bands (colic taenia) and colic +sacculations. (Columbia University Museum, No. 1589.)] + +[Illustration: FIG. 475.--_Felis leo_, lion. Large intestine, with +transverse section, showing smooth carnivore lumen, without sacculations +or plicae. (Columbia University Museum, No. 1600.)] + +A second modification of the intestinal canal, suggesting the same +physiological interpretation as the ileo-colic caecum, is presented by +the so-called pyloric caeca or appendices of many Teleosts and Ganoids +already referred to (p. 119). While these structures in some forms very +probably have assumed a secretory function (Figs. 476 and 477), they +evidently act in others as diverticula in which material undergoing +digestion is retained, while they increase at the same time the +intestinal mucous secretory and absorbing surface (Figs. 478 and 479). +They thus correspond physiologically to the ileo-colic caecum. In this +connection it is interesting to note that in Ganoids, which possess both +the pyloric appendices and the spiral valve, the two structures develop +in inverse ratio to each other, indicating their functional identity. In +the serial review of the structure and significance of the vertebrate +caecum and ileo-colic junction these functionally allied modifications of +other segments of the intestinal canal deserve notice. + +[Illustration: FIG. 476.--Pyloric caeca of _Gadus callarias_, codfish. +(Columbia University Museum, No. 1825.) + +_A._ Bound together by connective tissue and blood-vessels. + +_B._ Dissected to show confluence of caeca to form a smaller number of +terminal tubes of larger calibre entering the intestine.] + +[Illustration: FIG. 477.--Alimentary canal of _Accipenser sturio_, +sturgeon. Numerous pyloric caeca are bound together to form a gland-like +organ. + +In the smaller upper figure on the left the stomach, mid-gut, and +pyloric caeca are seen in section, showing the lumen of the latter and +their openings into the mid-gut. + +The lower left-hand figure shows the mid- and end-gut in section, the +latter provided with a spiral mucous valve. (Columbia University Museum, +Nos. 1826, 1827, and 1828.)] + +[Illustration: FIG. 478.--Stomach, duodenum, and pyloric caeca of +_Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)] + +[Illustration: FIG. 479.--_Pleuronectes maculatus_, window-pane. Stomach +and mid-gut with pyloric caeca and hepatic duct. (Columbia University +Museum, No. 1432.)] + +The study of the vertebrate ileo-colic junction proper begins both +ontogenetically and phylogenetically with the consideration of the +primitive type in which the alimentary tube is not differentiated into +successive segments and in which consequently no distinction between +mid- and hindgut is found (Fig. 465). An example of this primitive +condition is presented by the Cyclostomata, in whom the alimentary canal +traverses the coelom cavity as a straight non-differentiated cylindrical +tube. Fig. 310 shows the alimentary canal of the Lamprey, _Petromyzon +marinus_, and it will be observed that the intestine is provided with +the spiral mucous fold above mentioned. + +From this fundamental type the following main groups are to be derived: + +I. Symmetrical Form of Ileo-colic Junction. Mid- and Endgut in Direct +Linear Continuity. (Fig. 465, I.) + +1. _Ileo-colic junction marked externally by an annular constriction, +corresponding to a ring-valve with central circular opening in the +interior_ (Fig. 465, _I_, 1). + +This form is encountered in many Teleosts. The projecting annular mucous +fold resembles the pyloro-duodenal valve. + +Figs. 311-315 illustrate the structures in representative Teleosts. + +Among the higher forms this type of ileo-colic junction is encountered +in the simple alimentary canal of many Amphibians (Figs. 318-320). Among +Reptiles it is found in certain lizards, as in _Heloderma suspectum_, +the gila monster (Fig. 322). This animal lives almost entirely upon +bird's eggs, and its simple and reduced ileo-colic junction contrasts +strongly with the highly developed and complicated caecal apparatus of +the phytophagous lizards, as _Iguana_ (Figs. 326-330), affording one of +the most striking illustrations of the effect which the character of the +food habitually taken has on the structure of the alimentary canal in +forms otherwise closely allied. + +The same type of ileo-colic junction, as a reduction form, occurs in the +arctoid group of Carnivora among Mammalia (cf. p. 212). + +2. _Differentiation in caliber of large and small intestine. +Funnel-shaped ileo-colic transition._ + +This type, compared with the preceding, is characterized (Fig. 465, _I_, +2) by the greatly increased caliber of the large intestine, resulting in +a funnel-shaped transition between mid- and hindgut, the small intestine +continuing into the colon at the apex of the funnel. + +Examples of this type are presented by several Edentates, _Myrmecophaga +jubata_, the great ant-eater (Fig. 356), and _Choloepus didactylus_, the +two-toed sloth (Fig. 357). + +3. _Abrupt demarcation of small and large intestine with caliber +differentiation_ (Fig. 465, _I_, 3). + +The small intestine is still central at the ileo-colic junction, +_i. e._, the axis of its lumen is continuous with the central axis of +the colic lumen. In place of the gradual funnel-shaped transition of +the preceding type the demarcation is abrupt. + +An example of this form is furnished by another Edentate, _Tatusia +peba_, the nine-banded armadillo (Fig. 358). + +Among reptiles a similar well-marked ileo-colic transition is +encountered in _Alligator mississippiensis_ (Fig. 321). + +4. _Colic pouch prolonged back on each side of the ileo-colic junction, +producing symmetrical colic caeca_ (Fig. 465, _I_, 4). + +A growth of the colic tube cephalad, on each side of the junction with +the midgut, leads to the formation of this type, characterized by the +presence of two symmetrical globular caecal pouches. In its simplest form +this condition is illustrated by the double colic caeca of another +armadillo, _Dasypus sexcinctus_ (Fig. 359). + +The bifid caecal apparatus of the American manatee (Fig. 366) belongs to +the same group. + +5. _Caecal pouches of the birds_ (Fig. 465, _I_, 5).--A continuation of +the backward extension of the bilateral colic pouches leads to the +production of the typical double avian caeca in a greater or lesser +degree of development. Frequently the caeca differentiate more completely +from the colon, appearing as pouches of varying capacity joined to the +large intestine by a narrower neck. + +Figs. 334-341 show the well-developed pouches as they appear in +representative avian types, while Fig. 333 illustrates the reduction of +the caecal apparatus encountered in many carnivorous birds. + +6. Among mammalia _Cyclothurus didactylus_ (Fig. 360), the little +ant-eater, furnishes an example of double symmetrical globular caeca, +connected with the colon by a narrow neck (Fig. 465, _I_, 6). Reference +to the schema given in Fig. 465 will show that the types heretofore +examined all have the following common character: + +They appear derived from the primitive type by a differentiation in the +caliber of the gut and by the gradual development of _symmetrical +bilateral_ caecal pouches, resulting in central median implantation of +the small intestine and its direct continuity with the colon. + + +II. Asymmetrical Development of a Single Caecal Pouch, Lateral to the +Ileo-colic Junction, Mid- and Endgut Preserving Their Linear Continuity. +(Fig. 465, II.) + +In the second general group the symmetry of the ileo-colic junction is +disturbed. The following types are encountered, forming a series of +successive stages: + +1. The increase in the caliber of the large intestine is chiefly marked +along the border opposite to the mesenteric attachment, resulting in a +greater degree of convexity in this part of the intestinal wall (Fig. +465, _II_, 1). Among Reptilia this condition is found in the ileo-colic +junction of some of the pond-turtles, as _Pseudemys elegans_ (Fig. 323), +while a mammalian example is furnished by the three-toed sloth, +_Arctopithecus marmoratus_ (Fig. 363). + +2. An increase of this lateral extension of the colon leads to the +formation of a single lateral caecal pouch (Fig. 465, _II_, 2) such as is +seen in another Edentate, _Tamandua bivittata_ (Fig. 364), among +Mammalia, and in certain Ophidians among Reptiles, as in the _Anaconda_ +(Figs. 331 and 332). + +3. Prolongation of the pouch and reduction in caliber lead to the +formation of the slender lateral caecum found in all the Monotremes +(Figs. 345-347, Fig. 465, _I_, 3). In its general appearance the caecum +of these singular animals bears a close resemblance to the caecal pouches +of many birds. + +4. Direct continuity of small and large intestine, with lateral colic +caecum, extending along the convex free border of the terminal ileum and +slightly convoluted at the extremity (Fig. 465, _II_, 4), characterizes +the entire group of the _Cebidae_ among the new-world monkeys. The caecum +in these animals is a comparatively long pouch, nearly equalling in +caliber the remainder of the intestine, occupying a distinctly _lateral_ +position, with the terminal portion rounded and slightly recurved (Figs. +453 and 454). + +5. The _Cynoid group_ of Carnivora, including the dogs, wolves, jackals +and foxes, presents a similar relative position of small and large +intestine and caecum (Fig. 465, _II_, 5). The caecum, compared with that +of _Cebus_, is longer and more highly convoluted (Fig. 397). Variations +encountered in certain forms indicate reversions to a more primitive +type. Thus Fig. 398 shows the usual form in the dog, while Fig. 399 +exhibits an occasional type in the same animal. The caecum here is less +twisted and indicates the probable derivation of the more commonly +encountered type. + +III. Rectangular Ileo-colic Junction with Direct Linear Continuity of +Caecum and Colon. (Fig. 465, III.) + +The third general group, to which the large majority of Mammalia belong, +is characterized in its typical form by a right-angled entrance of ileum +into large intestine and by the direct caudal prolongation of the colon +into a caecal pouch of nearly uniform caliber with globular termination. +The axes of the small and large intestine are not in the same line as in +the two former groups, but are placed nearly at right angles to each +other. With this change in the direction of the main intestinal segments +the caecum ceases to be a lateral appendage to the canal and appears as a +caudal prolongation of the colon beyond the ileo-colic junction (Fig. +465, _III_). The type-form of this group is encountered among the +herbivorous Marsupialia, such as the kangaroos and opossums. Fig. 350 +shows the ileo-colic junction and caecum in the rock wallaby, _Halmaturus +derbyanus_, and Fig. 348 the same structures in our common opossum, +_Didelphis virginiana_. The majority of the remaining mammalian forms +depend upon modifications of this type, either in the direction of +reduction of the caecal apparatus, or of increased development with +concomitant structural changes of similar physiological import in the +proximal portion of the colon. + +The following subdivisions of the general group may be established. + +A. 1. The caecum is long, markedly curved or uncinate, with the +crescentic medial margin turned toward the free border of the terminal +ileum. The entire pouch usually diminishes gradually in caliber to its +termination (Fig. 465, _III_, _A_, 1). This type is encountered in a +large group of new-world monkeys, including the marmosets and howlers. + +Fig. 440 shows the structures in _Hapale jacchus_, one of the marmosets, +and Fig. 443 illustrates the typical caecum of this form in _Ateles +ater_, the black-handed spider monkey. + +2. The caecum and appendix of man and of the anthropoid apes can be +regarded as a reduction form of this type (Fig. 465, _III_, _A_, 2). +Arrest of development of the terminal portion converts the distal +segment of the caecal pouch into an appendix whose relation to the apex +of the funnel-shaped proximal segment or caecum proper is seen in its +pure form in the human embryo (Figs. 512 and 525). With the further +development of the caecum the sharper demarcation between it and the +appendix results (Figs. 517 and 518). The displacement of the root of +the appendix cephalad and to the left, toward the lower margin of the +ileo-colic junction, as it is usually seen in adults, is due to the +relatively greater growth of the right terminal sacculation of the caecum +compared with the left (cf. types of caeca, p. 248). Throughout these +changes the initial crescentic curve of the caecum, turning its concavity +upwards and to the left, can be recognized by tracing the course of the +longitudinal colic muscular bands. The caeca and appendices of the +anthropoid apes present the same characters. The structures in the +orang, chimpanzee, gorilla and gibbon are shown in Figs. 455-464. + +B. The AEluroid and Arctoid groups of the Carnivora and the Pinnipedia +constitute a very complete and instructive series illustrating the +gradual reduction of the caecum from the capacious pouch of the primitive +type and its final complete elimination from the organism (Fig. 465, +_III_, _B_). + +In _Hyaena_ (Fig. 416), the large caecum with undiminished caliber of the +terminal portion persists in its full development, as seen in the +Marsupials furnishing the fundamental type (Fig. 465, _III_). The same +type of caecum is found in the lion (Fig. 417), the only true cat in +which the caecal apparatus has not undergone extensive reduction. +Phylogenetically the presence of a capacious and uniform caecal pouch in +these two animals is exceedingly important and indicates that this type +of caecum represents the ancestral form common to the aeluroid carnivore +group, which, in the remaining living representatives, has become +reduced in response to the influence which the character of the food has +on the structure of this portion of the intestinal canal. The two +instances of persistence of the primal type are all the more important +as exceptions to the rule which is otherwise universal throughout the +group. + +1. The first example of this reduction (Fig. 465, _III_, _B_, 1) is +encountered in the Aard-Wolf, _Proteles lalandii_, a near relative of +hyaena (Fig. 406). The caecum in this animal is considerably shortened, +although still of fairly large and uniform caliber. + +A similar type of caecal reduction is encountered in the Pinnipede +Carnivora. Fig. 396 shows the ileo-colic junction and the short blunt +caecum of the harbor seal, _Phoca vitulina_. + +2. The caecum of the typical Felidae, other than the lion, is short and +the terminal portion much reduced in caliber, constituting in many forms +a species of pointed rudimentary appendix (Fig. 465, _III_, _B_, 2). +Fig. 401 represents the typical feline caecum as seen in the puma, _Felis +concolor_. Among the smaller AEluroid Carnivora related to the true cats, +as the civets and ichneumons, the terminal reduction of the short caecum +is still more marked, as seen for example in _Herpestes griseus_ (Figs. +404 and 405). + +3. In the Arctoid group of Carnivora (Fig. 465, _III_, _B_, 3 and 4) the +reduction of the caecal apparatus has been carried to the complete +elimination of the pouch, restoring the primitive type of a straight +intestinal tube without diverticulum as encountered above in some of the +Edentates (Figs. 356 and 357). + +In some forms allied to the true bears, such as _Procyon_, _Bassaris_, +_Cercoleptes_, _Taxidea_ and _Nasua_, the ileo-colic junction is marked +externally by a slight constriction and internally by the projection of +an annular pylorus-like valve (Figs. 407-409). The transition from the +thin-walled ileum to the thick muscular walls of the large intestine is +abrupt. The latter is very short and usually increases in caliber as it +approaches the anal orifice. The mucosa of the terminal ileum presents +very commonly one or two large oval areas of agminated follicles near +the ileo-colic junction. The mucous membrane of the large intestine is +thrown into prominent longitudinal folds. Fig. 408 shows the intestine +of the brown coati, _Nasua rufa_, opened on each side of the ileo-colic +transition. + +In some of the Arctoidea, as _Procyon_ and _Nasua_, the beginning of the +colon just beyond the ileo-colic valve is bowed out opposite the +mesenteric border indicating the original site of the eliminated caecum, +and recalling the arrangement of the intestine encountered above in +_Arctopithecus_ among the Edentates (Figs. 363, 407, 412, and 465, +_III_, _B_, 3). Moreover, in the same forms rudimentary vascular and +serous folds around the ileo-colic junction, corresponding to similar +structures found in connection with a well-developed caecal apparatus in +other mammalia, point to the former existence of a caecum. + +4. In the typical Ursidae even these remnants and traces of a caecal pouch +have disappeared and the intestinal canal preserves a uniform caliber, +without any differentiation of large and small intestine (Figs. 414 and +415, Fig. 465, _III_, _B_, 4). + +C. The last subdivision of the third main group contains forms in which +the large uniform pouch of the primal type appears moderately reduced in +length and sacculated, terminating either in a blunt extremity or +carrying a distal constricted and rudimentary segment as an appendage. + +1. The first of these types is encountered in the Old World cynomorphous +monkeys. In all of these animals the caecal pouch is wide but +comparatively short, of nearly uniform caliber and sacculated like the +rest of the colon, of which it forms the direct caudal continuation +(Fig. 465, _III_, _C_, 1). The terminal portion of the pouch is usually +blunt, globular and rounded (Figs. 428, 430 and 431), in a comparatively +small number of forms slightly pointed (Figs. 427 and 437). + +2. In the second group the terminal reduced portion persists either as a +fairly distinct appendage, or in the form of a tapering pointed +extremity into which the caecal pouch proper is continued (Fig. 465, +_III_, _C_, 2). This type is encountered in certain non-ruminant +Ungulates. An example of the first condition is furnished by the caecal +apparatus of the peccary (_Dicotyles torquatus_) (Fig. 370), while the +structures in _Tapirus americanus_ (Fig. 377) illustrate the second +form. + + +=IV. Caecal Apparatus Combined with Structural Modifications of the +Proximal Colon of Similar Physiological Significance. (Fig. 465, IV.)= + +The fourth general mammalian group comprises forms in which the caecal +pouch is large, with or without terminal appendage, while in addition +the large intestine develops structural modifications which possess the +general functional significance of the caecal apparatus. This highly +developed and complicated structure of the alimentary canal indicates +that the habitual food of these animals is bulky and difficult of +digestion. Accordingly we find the group composed in main of the +majority of the Ungulates and Rodents (with the exception of _Myoxus_), +forms in which the diet under natural conditions is purely herbivorous. +Other mammalian orders, however, also furnish representatives of this +type of caecal apparatus, the conditions as regards character and +quantity of food habitually taken corresponding to those encountered +among the Ungulates and Rodents. Thus the _Phalangers_ among Marsupials +(Fig. 352), _Galeopithecus_ (Fig. 419) as an exceptional form among the +Insectivora, and certain lemurs among Primates (Figs. 420-425) present +examples of a highly developed and specialized type of caecal apparatus. + +The intestinal tract of these forms must therefore be considered from +two points of view: + +I. The caecum proper. + +II. The analogous structural modifications of the proximal segment of +the colon. + + +=I. CAECUM PROPER.= + +The pouch of the Ungulates and Rodents, taking these forms as the +typical representatives of the entire group, is usually of very large +size compared with the rest of the alimentary canal. Two types are +found: + +1. Large capacious smooth caecal pouch of uniform caliber (Fig. 465, +_IV_, 2). This form is met with in the Muridae among Rodents and is +illustrated in Fig. 393 showing the caecum of _Mus decumanus_, var. +_albinus_, the white rat. Fig. 392 represents the entire alimentary +canal of the meadow mouse, _Arvicola pennsylvanicus_, and indicates the +proportion which the caecal apparatus bears to the remainder of the +intestinal tract. The typical caecum of the Ungulates is shown in Fig. +371, taken from _Capra aegagrus_, the bezoar goat, and in Fig. 372, taken +from a preparation of _Boselaphus tragocamelus_, the Nilghai. + +2. The caecal pouch is large, markedly crescentic in shape, sacculated, +or provided in the interior with a more or less complete spiral valve, +and reduced in caliber in the terminal segment, forming at times a +pointed appendix (Fig. 465, _IV_, 3). This form is encountered typically +among certain Rodents, as in _Castor fiber_, the beaver (Figs. 381 and +382), and _Erethizon dorsatus_, the Canadian porcupine (Figs. 383 and +384), but is not confined to this order. Thus caeca of very similar +structure are found among the Marsupials, as in _Phascolarctos_ and +_Cuscus_ (Fig. 352). In some of these forms the terminal reduction of +the caecum is very marked, resulting in a long narrow segment of the +pouch tapering to a sharp point. It is significant to note in this +connection that in one member of the marsupial order, the wombat +(_Phascolomys_), this tendency to terminal reduction of the pouch has +led to the development of a caecum and appendix identical in structure +and arrangement with the corresponding parts of man and the anthropoid +apes (Fig. 354). This is merely another illustration of the fact, +evidenced throughout the entire vertebrate series, that a primal +type-form of caecal apparatus, in responding to the conditions which +influence the development of structural modifications, will produce +identical specific types in animals otherwise widely separated in the +zoological series. + +Thus again the form of caecum under discussion, found in many Rodents and +certain Marsupials, is encountered in the only Insectivore possessing a +caecum (_Galeopithecus_) (Fig. 419), and in several _Lemuroidea_ among +Primates (Figs. 420-425). + + +II. Structural Modifications of the Proximal Segment of the Colon +Analogous in Their Functional Significance to the Caecal Apparatus. + +In these forms, in addition to the caecal apparatus proper, certain +accessory structural modifications of the adjacent large intestine are +developed which possess the physiological significance of the caecal +apparatus in general, since they serve to increase the extent of the +intestinal mucous surface and to prolong the period during which the +contents of the canal are retained for elaboration and absorption. These +modifications, which appear most fully developed in certain Rodents and +Ungulates, are of two kinds. + +1. The development of the colic mucous membrane in the form of a +projecting fold or valve usually surrounding the lumen spirally (Fig. +465, _IV_, 1). The significance and phylogeny of this spiral fold has +been considered above (cf. p. 193). Functionally this reduplication must +be regarded as in general equivalent to the caecal apparatus proper, in +producing an increased surface for secretion and absorption and in +retarding the movement of intestinal contents. The caecal pouch evidently +acts as a reservoir in which partly digested substances, mixed with the +secretions of the small intestine, are retained while the slow processes +of digestion and absorption, already inaugurated in the antecedent +segment of the canal, are completed. It is reasonable to suppose that +the system of projecting mucous folds and reduplications encountered in +the colon beyond the caecum have a similar physiological import. +Moreover, in certain forms the caecum itself is provided with a similar +spiral mucous fold, as in the instances already mentioned of _Lepus_ +among mammalia (Fig. 381) and of the Ostrich among birds (Fig. 341). We +have seen above (cf. p. 193) that the spiral intestinal valve is +encountered very early in the vertebrate series, in forms in which the +alimentary canal is but slightly, or not at all differentiated, short +and straight in its course. In these forms the evident purpose of the +spiral fold is to retard the movement of the intestinal contents and to +increase the area of the secretory and absorbing surface. As a +structural modification possessing this character we saw the fold in +the Cyclostomata, Selachians and Dipnoeans (Figs. 310, 466, 467 and 468) +and in certain Ophidians (_Python_ and _Anaconda_, Figs. 331 and 469). +Among Mammals it is found in certain Rodentia in two forms: + +(_a_) In some of the Muridae, as _Arvicola_ (Fig. 394), the mucous +membrane of the large globular caecal pouch is smooth, but the proximal +segment of the colon, immediately beyond the ileo-colic junction, +develops the spiral fold (Fig. 465, _IV_, 2). + +(_b_) In other forms, as in the hares (Fig. 465, _IV_, 3), the greater +part of the caecum carries a typical spiral fold, continued up to the +root of the terminal appendage (Fig. 388), in which segment the mucous +membrane is devoid of folds, but studded thickly with lymphoid +follicles. Beyond the caecum proper the spiral fold is continued in the +opposite direction into the proximal segment of the colon, which is +large and capacious and evidently shares both the physiological and +morphological characters of the caecum proper, forming so to speak an +accessory caecal chamber. Beyond what we thus might term the caecal +division of the colon the large intestine becomes reduced in caliber, +and the previously continuous spiral fold becomes broken up into +separate semilunar haustral plicae, corresponding to the superficial +constrictions between the colic cells. In structure this distal segment +of the rabbit colon closely resembles the human large intestine (Fig. +474). + +One of the most marked examples of this secondary modification of the +colon is presented by the intestinal canal of another Rodent, _Lagomys +pusillus_ (Fig. 391). + +The caecum of this animal is long, curved, provided with a well-developed +spiral fold. The terminal segment of the pouch is reduced to an +appendix, with smooth mucosa containing adenoid tissue, as in the +rabbit. A second adenoid appendix, representing the globular saccus +lymphaticus of the rabbit, is derived from the caecum at the ileo-colic +junction. The first segment of the colon beyond the ileo-colic junction +is dilated and sacculated, the caecal mucous fold being prolonged into +it. This is succeeded by a narrow smooth-walled second segment. The +third division of the colon is again dilated and sacculated, +followed by a short fourth smooth-walled section. A fifth stretch is +again provided with colic cells, beyond which the terminal segment +continues of uniform caliber and with smooth walls to the vent. The +colon therefore presents three distinct sacculated portions whose +structural modifications suggest that they function in the same sense as +the caecal pouch proper. In man and in other Primates the crescentic +colic plicae are disposed in a more or less evident spiral manner around +the axis of the intestine, and it is not difficult to recognize in them +the modified remnants of the typical spiral valve of lower forms. On the +other hand, in conformity with the general reduction of the caecal +apparatus, the mucous membrane of the large intestine in Carnivora is +smooth and devoid of any trace of the spiral fold (Fig. 475). + +2. The second structural modification of the large intestine, associated +in functional significance with the caecal apparatus, depends upon the +increase in the length of the proximal segment of the colon beyond the +ileo-colic junction and the twisting or coiling of this segment in a +more or less complicated definite manner, usually in the form of a +spiral, the individual turns of the coil being held in place by the +peritoneal connections. The proximal colon thus modified is admirably +adapted to retard the movement of contents not yet completely digested +and to increase the absorbing surface of the intestine, and hence is +functionally allied to the caecal apparatus. + +This colic modification is found in its highest degree of development in +the ruminant Ungulates, whose caecal pouch proper is also enormously +developed. In these animals the colon immediately beyond the ileo-caecal +junction is arranged in the form of a double spiral, the afferent +(caecal) and efferent (colic) tubes alternating, and continuous with each +other in the center of the coil (Fig. 465, _IV_, 5). Examples of this +type of spiral colon are shown in Fig. 373 (_Bos indicus_), Fig. 374 +(_Cervus sika_), Fig. 375 (_Ovis aries_), Fig. 376 (_Oryx leucoryx_). +Ontogenetically the complicated spiral colon of the ruminants starts as +a simple loop of the proximal colon, which, with the further rapid +growth of this segment of the intestine, is bent to produce the turns of +the coil as shown in the schematic Figs. 480-482. Phylogenetically the +same gradual development can be traced in the vertebrate series. Perhaps +the earliest tendency to structurally modify the intestine in the +direction named is found in the manner in which the intestinal coils are +bound together by the subperitoneal arachnoid in many Ophidians (Fig. +331). Further in the Manidae among the Edentates there is no caecal pouch, +but the intestine at the ileo-colic junction is twisted into a figure 8 +and held in this position by the peritoneal connections (Figs. 362 and +465, _IV_, 4). In certain Marsupials with well-developed caecal pouches, +such as _Phascolarctos_ and the Vulpine Phalangers (Figs. 351 and 352), +the colon immediately beyond the ileo-colic entrance is sacculated and +bent in the form of a short loop. In the tapir (Fig. 377), the proximal +segment of the colon forms a simple loop, whose afferent and efferent +limbs are closely bound together. The arrangement of the large intestine +in this animal illustrates the early embryonal stage in the development +of the complete ruminant spiral coil (cf. Fig. 480). + +[Illustration: FIGS. 480-482.--Schematic representation of three stages +in the development of the ungulate spiral colon.] + +The condition encountered in some Rodents presents a more advanced +stage. Thus the large intestine in the agouti (_Dasyprocta agouti_), +shows the development of the spiral coil advanced as far as the second +turn of the original loop (Figs. 389 and 390). It is readily seen that +continued growth of this segment of the intestine leads to the formation +of the complete colic spiral as found in the typical Ungulates. + +The same arrangement of the large intestine obtains in certain Lemurs +among the Primates. Thus the proximal colon of the Slow Lemur +(_Nycticebus tardigradus_) is seen in Figs. 421 and 422 to present a +typical spiral coil, and similar conditions are encountered in other +members of the suborder. + + +=V. Caecal Apparatus and Colon in Hyrax.= + +We have left for our final consideration the aberrant and unique +mammalian type found in _Hyrax_ (Fig. 378). In this remarkable little +animal the large intestine develops a typical mammalian sacculated caecum +at the ileo-colic junction, and in addition is provided further on with +two symmetrical pointed lateral colic caeca of large size. It is quite +true that this arrangement is unique among Mammalia, confined entirely +to the members of the suborder formed by the single family of _Hyrax_, +and that no strictly analogous disposition of the alimentary canal is +encountered in the entire vertebrate series. Yet these aberrant +structures are possibly capable of explanation, in regard to the method +of their development, by reference to the caecal apparatus of certain +phytophagous saurians, as _Iguana_ and _Cyclura_. In these forms (Fig. +326-330) the small intestine enters the colon somewhat asymmetrically, +the opening being guarded by a well developed annular valve. + +The proximal segment of the large intestine forms an extensive +sacculated pouch. If this is opened (Figs. 328-330) it is seen that the +small intestine leads into a compartment which is separated from the +remainder of the pouch by a valvular diaphragm with central circular +opening. Beyond this primary compartment the colic pouch is incompletely +subdivided by a series of gradually diminishing crescentic folds, +corresponding to the external constrictions between the sacculations. +The entire pouch gradually diminishes in caliber until it passes with a +sharp angular bend into the terminal portion of the endgut. This +terminal segment is differentiated from the elongated colic pouch by the +greater thickness of its muscular walls and by a slight annular +projecting fold in the interior. In considering the intestinal tract of +_Hyrax_ it is conceivable that the unique condition presented by this +animal may be derived from some type conforming in general structure to +the reptilian arrangement of the parts just detailed, as indicated in +the schematic Figs. 483-485. The proximal typical caecal pouch of _Hyrax_ +would then correspond to the similar colic pouch of _Iguana_. To explain +the supplementary colic caeca it is necessary to suppose that the +transition of the colic pouch into the terminal hindgut had become well +differentiated, and that on each side of this junction the colic tube +had extended backwards, resulting in the production of the supplementary +bilateral caecal pouches of _Hyrax_. + +[Illustration: FIGS. 483-485.--Schematic figures illustrating possible +line of derivation of aberrant mammalian type of alimentary canal +encountered in _Hyrax_.] + + + + +PART IV. + +MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX. + + +Not only is the anatomy of this portion of the alimentary tract of great +interest in relation to the evolution of the human structure, but in +addition the pathological and surgical importance of the region warrants +a very careful study of the caecum and appendix. This is more especially +the case since a number of variations in the arrangement of the +structures are encountered. These departures from what we consider the +normal human type have an important bearing on the development and +progress of the pathological conditions prone to involve the appendix. +We may consider the subject under the following subdivisions: + + +I. DEVELOPMENT OF THE CAECUM AND APPENDIX. + +Much light is thrown on the adult anatomy of the parts and on the origin +of the variations observed by the study of their embryonic history. In +considering the factors which determine the variations in the position, +size, and shape of the appendix it must be remembered that the +rudimentary character of this structure is responsible for many of the +aberrant conditions encountered. + +As a part of the general caecal pouch which persists in an early +developmental stage and which we can regard as destined for further +reduction and ultimate elimination in the course of evolution, the +appendix shares with other vestigial structures a wide range of +variation. Consequently the study of the development of this portion of +the alimentary tract enables us to gain a clearer view of the primary +arrangement of the structures and to trace the causes which are active +in determining the adult conditions most frequently encountered. + +[Illustration: FIG. 486.--Alimentary canal and appendages of human +embryo of 12.5 mm. x 12. (Kollmann, after His.)] + +[Illustration: FIG. 487.--_A_, schematic representation of alimentary +canal, with umbilical loop and mesenteric attachments in human embryo of +about six weeks. _B_ and _C_, stages in the intestinal rotation.] + +At the time when the umbilical loop of the intestine has formed and has +begun to protrude into the cavity of the umbilical cord (fifth to sixth +week), the first indication of the future caecum appears as a +circumscribed thickening of the returning or ascending limb of the +intestinal loop a short distance from the apex (Figs. 486-488). This +rudiment indicates the derivation of the future definite intestinal +segments from the elements of the loop. The descending limb, apex (site +of embryonic vitelline duct, Meckel's diverticulum of adult) and a short +succeeding portion of the ascending limb furnish the ileum and jejunum. +The rest of the ascending limb develops into caecum and appendix, +ascending and transverse colon. The increase in the length of the +intestine is not uniform. The formation of convolutions begins in the +seventh week in the apex and subsequently in the descending limb. By the +eighth week a considerable number of jejuno-ileal coils have resulted +from the growth in length of these parts of the original umbilical loop, +while the growth of the segment which furnishes the colon is at this +time still inconsiderable (Fig. 489). In the meanwhile the thickening of +the tube which forms the first rudiment of the caecum has developed into +a small sac-like enlargement of the gut, budding from the left and +dorsal aspect of the ascending limb, crescentic in shape, turning its +concavity toward the parent tube. In the majority of instances examined +the small outgrowth is packed closely between the incipient ileal +convolutions, lying under cover of the more prominent bulging coils of +the umbilical protrusion, between them and a single coil of larger arc +situated dorsally and belonging to the jejunal or proximal portion of +the small intestine (Fig. 490). Fig. 497, taken from an embryo of 11 mm. +cervico-coccygeal length, represents this stage in the development of +the umbilical loop. The arrangement of the caecum which we can assume as +the typical condition at this stage and which determines in part the +subsequent final arrangement of the structures, is illustrated by this +relation of the caecal bud to the surrounding incipient convolutions of +the small intestine, with the larger part of these coils situated +ventrad of the caecum and only a single coil of larger curve placed +dorsally; the caecal pouch, derived from the ascending limb of the +umbilical loop, is situated between these two divisions, turning its +concave border to the right and embracing the parent tube. At the time +when the human caecum first appears as a distinct structure it forms a +small conical pouch with blunt extremity whose shape is well illustrated +by the caecum of some of the new-world monkeys, as _Mycetes fuscus_, the +brown howler monkey (Figs. 449 and 450). The outgrowth develops rapidly +in length and very soon assumes a distinct crescentic shape, gradually +tapering toward the extremity, a type which is found reproduced in the +caecum of _Ateles ater_, the black-handed spider monkey (Fig. 443). There +is as yet no constriction or demarcation separating the distal segment +(future appendix) from the proximal part (caecum proper) but the entire +pouch gradually narrows funnel-like to its termination. + +[Illustration: FIGS. 488-496.--Series of schematic figures illustrating +stages in the rotation of the intestinal canal.] + + +II. CHANGES IN THE POSITION OF THE CAECUM AND APPENDIX DURING NORMAL +DEVELOPMENT, DEPENDING UPON THE ROTATION OF THE INTESTINE AND THE +SUBSEQUENT DESCENT OF THE CAECUM. + +The primary cause leading to the rotation of the intestinal canal and +inaugurating the successive stages which produce the adult disposition +of the tube is to be found in the rapid increase in length of the small +intestine. Numerous convolutions of this tube succeed to the few primary +coils noted in the first stages. This condition is illustrated in Fig. +498, taken from an embryo of 4.4 cm. cervico-coccygeal measure, and the +arrangement of the intestine is indicated in schema, Fig. 491. The caecum +is found nearly in the median line imbedded among the surrounding coils +of the small intestine, which by their rapid increase have pushed the +pouch cephalad nearly into contact with the caudal surface of the liver. + +[Illustration: FIG. 497.--Human embryo of 11 mm. cervico-coccygeal +measure. Enlarged view of ventral and left aspect of intestinal canal. +(Columbia University, Study Collection.)] + +[Illustration: FIG. 498.--Human embryo of 4.4 cm. cervico-coccygeal +measure. Intestinal canal; Liver removed. (Columbia University, Study +Collection.)] + +Three main divisions of the convolutions of the small intestine +can be made out, slightly separated from each other in the figure +to exhibit the caecum between them. The proximal (jejunal) set of +these convolutions occupy the upper and left part of the abdominal +cavity. They are the product of the single larger coil which in the +earlier stage (Fig. 497, schema Fig. 490) appeared dorsad of the +caecal diverticulum. The distal (ileal) division of small intestinal +convolutions has become greatly augmented and lies to the right of the +caecum. The concavity of the pouch is still, as in the earlier stages, +directed to the right and the entrance of ileum into colon takes +place from right to left. The caudal part of the abdominal cavity is +occupied by an intermediate set of transition convolutions which join +the proximal and distal divisions. In the two stages just described +(Figs. 497 and 498, Schema Figs. 490 and 491), the initial step in the +intestinal rotation has been taken, _i. e._, the beginning of the colon +has been displaced cephalad from its original position in the caudal +and left part of the abdominal cavity by the pressure of the rapidly +growing coils of the small intestine and now lies transversely ventrad +of the duodenum, having crossed the duodeno-colic neck or isthmus of +the primitive umbilical loop (cf. Fig. 487, _C_). + +At first the distal coils of the small intestine occupy a position +_behind_ as well as to the right of the caecum, forming a dorsal +retro-caecal division connected by intermediate convolutions with the +ventral division occupying the lower and left portion of the abdominal +cavity. The apex of the caecum is frequently imbedded among these +terminal coils of the ileum. With the continued growth of the small +intestines a further displacement of the caecum cephalad and to the right +takes place, while at the same time the terminal ileal coils pass +downwards and to the left, from a retro-caecal into a subcaecal position, +thus permitting a direct apposition of the caecum to the dorsal parietal +(prerenal) peritoneum. The last steps in this process of withdrawal of +the original voluminous dorsal (retro-caecal) division of ileal +convolutions are well seen in the preparation shown in Fig. 499, taken +from an embryo of 6.7 cm. vertex-coccygeal measure, and corresponding to +the schematic stages represented in Figs. 490 and 491. +The caecum in this preparation has not yet completed its rotation and +still turns its concavity upwards and to the right, with the apex +imbedded among the terminal convolutions of the ileum. + +The ileo-caecal junction takes place from right to left in a downward +direction. Nearly the entire mass of the small intestine is situated +below and to the left of caecum and colon, but a terminal ileal coil +still occupies, although evidently in the process of withdrawal, the +retro-caecal position, separating the caecum from direct contact with the +dorsal parietal peritoneum. The withdrawal of this terminal coil of the +small intestine is accompanied, or immediately followed, by a further +turn of the colon cephalad and to the right, which brings it into +contact with the caudal surface of the liver and completes the rotation, +producing a change in the relative positions of the terminal ileal coils +and the caecum. In the stages illustrated in Figs. 498 and 499 and shown +schematically in Figs. 490 and 491, the terminal coils of the ileum pass +from right to left behind the caecum to enter the colon, and the +concavity of the caecal pouch is directed upwards and to the right. After +the final rotation has occurred (schema, Fig. 492) the ileum enters the +large intestine from the left and from below, and the concave border of +the caecum is directed caudad and to the left. This change in relative +position has been accomplished by a revolution of the colon and caecum +through an arc of 180 deg. around its own long axis carrying the caecum above +and behind the small intestine and bringing it into contact with the +dorsal prerenal parietal peritoneum. At the same time the terminal coils +of the ileum turn downwards and to the left. If this final step in the +rotation of the large intestine fails to occur, with otherwise normal +development of the parts, the ileum will persist in entering the large +intestine from right to left after the caecum has obtained its final +lodgment in the right iliac fossa. We have had occasion to refer +previously to the significance of these instances of partially arrested +development (cf. p. 61, Figs. 123, 127 and 128). + +[Illustration: FIG. 499.--Human embryo of 6.7 cm. vertex-coccygeal +measure. Liver removed. (Columbia University, Study Collection.)] + +[Illustration: FIG. 500.--Human embryo of 4.9 cm. vertex-coccygeal +measure. Ventral view of abdominal cavity, with liver partially removed. +(Columbia University, Study Collection.)] + +[Illustration: FIG. 501.--The same embryo represented in Fig. 500. The +colic coil further depressed and turned to the left; seen from the right +side.] + +In Figs. 500 and 501, taken from an embryo of 4.9 cm. vertex-coccygeal +measure, the final rotation of the caecum from the position occupied in +Fig. 498 has occurred and the concavity of the pouch is directed caudad +and towards the left. At the same time the escape of the terminal ileal +coils from behind the caecum and beginning of the colon has not yet taken +place and hence the colon is still kept by these coils from direct +opposition to the dorsal prerenal parietal peritoneum. The condition +presented by this preparation can be schematically indicated by Figs. +492 and 493. The rotation has carried the beginning of the colon (Fig. +500), with the caecal bud and appendix curved on itself and turning its +concavity to the left, into the subhepatic position. The greater part of +the small intestinal coils lie now below and to the left of the caecum, +but the terminal ileal convolutions (Fig. 500) still occupy a +retro-caecal position, separating the pouch and the colon from the dorsal +parietal peritoneum. In Fig. 501 the right lateral view of the same +embryo is shown with the caecum and colon depressed and turned to the +left. The termination of the ileum reaches the ileo-colic junction by +passing behind the caecum, and the immediately adjacent ileal coils are +still retro-caecal, intervening between the pouch and the dorsal parietal +peritoneum. + +In the next succeeding stage (schema, Fig. 494) these coils of the ileum +turn downward and to the left so as to lie below and mesad to the caecum +and colon, thus permitting the direct apposition of the large intestine +to the parietal prerenal peritoneum. The terminal ileum now passes from +below and to the left upwards and to the right to its junction with the +colon. This freeing of the dorsal surface of caecum and colon from +contact with the coils of the small intestines, and the consequent +direct apposition of the same to the dorsal parietal peritoneum +influences to a great extent the subsequent arrangement of the parts, +because it affords the conditions necessary to the fixation of the colon +and mesocolon by adhesion to the parietal peritoneum (cf. p. 81). + +Fig. 499, taken from an embryo of 6.7 cm. vertex-coccygeal measure, +illustrates this stage, which is encountered in the majority of +instances and during which the retro-caecal coils of the terminal ileum +are withdrawn (schema, Fig. 493). The convolutions of the small +intestine have greatly increased in size and number. The retro-caecal +ileal coils, compared with Fig. 500, have shifted their position caudad +and to the left, so as to lie below and ventrad of the beginning of the +colon. Only a single coil remains behind the caecum and appendix, +intervening between these structures and the ventral surface of the +right kidney, and this coil is in the process of withdrawal from the +dorsal position as indicated by the superficial and short course of the +coil which connects it with the remaining ventral convolutions. As soon +as the withdrawal of this single remaining dorsal coil is completed the +entire mass of the small intestines will occupy a position ventrad, +caudad and to the left of the caecum and colon (Fig. 494), which will +then rest directly against the dorsal parietal peritoneum investing the +ventral surface of the right kidney. + +This stage is illustrated in Fig. 502, taken from an embryo of 6.6 cm. +vertex-coccygeal measure. The caecum and appendix here occupy the +subhepatic position, well to the right of the median line and in the +background of the abdominal cavity. The terminal retro-caecal ileal coils +of the embryo shown in Figs. 500 and 501 have descended caudad and to +the left, thus freeing the dorsal surface of caecum and colon and +permitting direct contact with the prerenal parietal peritoneum. + +[Illustration: FIG. 502.--Human embryo, 6.6 cm. vertex-coccygeal +measure. Liver removed. (Columbia University, Study Collection.)] + +In the succeeding stages the caecum gradually descends along the +background of the right lumbar region from the subhepatic position to +the right iliac fossa, producing by this descent the ascending colon as +a distinct segment of the large intestine. + +It will be observed that in the stage shown in Fig. 502 (schema, Fig. +494) the large intestine passes from the caecum to the splenic flexure +transversely from right to left across the upper part of the abdominal +cavity, caudad and ventrad of the stomach and cephalad of the coils of +the small intestine. + +In the following stages the disproportionately large size of the +embryonic liver compels the colon, as the caecum descends, to assume an +oblique position. When the caecal descent is completed the colon +traverses the abdominal cavity in contact with the caudal surface of the +liver passing from the right iliac fossa obliquely cephalad and to the +left to the splenic flexure where it becomes continuous with the +descending colon, which segment has early assumed its definite position +in the background of the abdominal cavity on the left side (Fig. 495). +This oblique position of the colon is seen in Figs. 503 and 504. During +this stage the increase in the length of the colon may lead to the +arrangement seen in Fig. 505, where the future transverse segment of the +large intestine is bent caudad in form of an arch whose summit extends +nearly to the pelvis. This condition at times persists in the adult, in +cases of unusually long large intestine, and recalls the normal +arrangement found in many of the cynomorphous monkeys in whom the +transverse colon forms an extensive V-or U-shaped loop, with the apex +directed caudad toward the pubic symphysis (Fig. 506). In other +instances in the human foetus this part of the large intestine is thrown +into a number of shorter irregular coils (Fig. 507). + +[Illustration: FIG. 503.--Human embryo, 7.6 cm. vertex-coccygeal +measure. Liver and small intestine from the duodeno-jejunal to the +ileo-colic junction removed. (Columbia University, Study Collection.)] + +[Illustration: FIG. 504.--Human foetus, 10.6 cm. vertex-coccygeal +measure. Liver and greater part of small intestine removed. (Columbia +University, Study Collection.)] + +[Illustration: FIG. 505.--Human foetus, 20.4 cm. vertex-coccygeal +measure. (Columbia University, Study Collection.)] + +[Illustration: FIG. 506.--Abdominal viscera of _Macacus rhesus_, rhesus +monkey, hardened _in situ_. (Columbia University Museum, No 1817.)] + +[Illustration: FIG. 507.--Abdominal viscera of human foetus at term, +hardened _in situ_; hepatic flexure formed, and ascending and transverse +colon differentiated. (Columbia University Museum, No. 1816.)] + +[Illustration: FIG. 508.--Human foetus of 10.7 cm. vertex-coccygeal +measure. Liver and small intestine from the duodeno-jejunal to the +ileo-colic junction removed. The colon already presents an ascending, +transverse and descending segment. The appendix is retro-caecal, curved, +with the tip turned down, under cover of the ileo-colic junction and +mesentery. (Columbia University, Study Collection.)] + +Normally, however, in the process of further development and with the +relative decrease in the size of the liver, the hepatic flexure (Fig. +505) becomes defined and passes cephalad and to the right, taking up the +slack of the bent segment and establishing the typical ascending and +transverse colon as seen in Fig. 508 (schema, Fig. 496). + + +III. VARIATIONS OF ADULT CAECUM AND APPENDIX. + +The study of the variations of the adult caecum and appendix involves the +consideration of the following points: + +(_a_) Shape of caecum and origin of appendix. (_Type of adult caecum._) + +(_b_) Position, direction and peritoneal relations of the appendix. + +(_c_) Arrangement of the vascular and serous ileo-caecal folds. + +The peculiarities encountered in any individual case usually depend upon +the combination of all three of these factors, which together influence +and determine the arrangement of the structures in the adult. Hence the +examination of each case should be made with reference to these three +points, which we will now consider in detail. + + +A. SHAPE OF CAECUM AND ORIGIN OF APPENDIX. TYPES AND VARIATIONS OF ADULT +CAECUM AND APPENDIX. + +The various forms of the adult caecum are all derived by modifications +from the foetal type of the pouch. + +In the embryo the caecum is funnel-shaped, narrowing gradually and +symmetrically in caliber to the root of the appendix, at which point the +three colic taenia or longitudinal muscular bands of the large intestine +meet. The appendix arises from the apex of the funnel, the lateral walls +of which are equally and symmetrically developed. The entire pouch is of +a crescentic shape, the concavity of the curve turned to the left and +directed toward the caudal margin of the terminal ileum. Two +subdivisions of the foetal type are found: + +I. The crescentic curve of the caecum is only slightly marked; the +appendix arises from the most pendent part of the pouch and hangs +downward (schema, Fig. 509, _I, a_). + +This form, which is encountered only occasionally in the foetus and +infant, is illustrated by the preparation shown in Fig. 510, taken from +a foetus at term. + +[Illustration: FIG. 509.--Schematic table of types of human caeca.] + +II. In the majority of cases the inherent crescentic shape of the caecal +pouch is pronounced and carries the termination of the funnel with the +root of the appendix cephalad and to the left toward the caudal margin +of the ileo-colic junction (schema, Fig. 509, _II, a_). + +At birth this typical arrangement of the caecum frequently places the +pouch in a nearly transverse position, with the apex and the root of the +appendix turned to the left, in contact with, or under cover of the +terminal piece of the ileum at its junction with the large intestine. + +Figs. 511 and 512 represent the parts in the ventral view in the foetus +at term. + +[Illustration: FIG. 510.--Human foetus at term. Caecum and ileo-colic +junction; ventral view. (Columbia University, Study Collection.) + + 1. Appendix. + 2. Reduced intermediate non-vascular fold. + 3. Ventral vascular fold. +] + +[Illustration: FIG. 511.--Human foetus at term. Caecum and ileo-colic +junction; ventral view. (Columbia University, Study Collection.) + + 1. Appendix, coiled spirally behind terminal ileum. + 2. Non-vascular intermediate fold. +] + +[Illustration: FIG. 512.--Human foetus at term (negro). Caecum and +ileo-colic junction; ventral view. (Columbia University Museum, No. +692.)] + +[Illustration: FIG. 513.--Human foetus at term. Caecum and ileo-colic +junction; dorsal view. (Columbia University, Study Collection.)] + +[Illustration: FIG. 514.--Human foetus at term. Caecum and ileo-colic +junction; dorsal view. (Columbia University Museum, No. 1715.)] + +Figs. 513 and 514, also taken from the foetus at term, show the caecum +from the dorsal aspect and illustrate well the sharp character of the +curve which carries the apex of the pouch up and to the left. + +All the variations observed in the adult caecum are derived from these +two foetal types by a subsequent and usually asymmetrical enlargement and +dilatation of the pouch. + +We can consider the derivatives of each form separately. + +=I. Adult Caeca Derived From Type I.= (schema, Fig. 509, _I^a_, Fig. +510). + +1. Further development leads to an enlargement of the caecal pouch and a +sharper demarcation between the same and the appendix. The resulting +caecum is symmetrical, with equally developed lateral sacculi, between +which the termination of the longitudinal muscular bands and the root of +the appendix is situated (schema, Fig. 509, _I^b_). + +[Illustration: FIG. 515.--Human foetus at term. Caecum and ileo-colic +junction; ventral view. (Columbia University Museum, No. 1510.)] + +[Illustration: FIG. 516.--Human foetus at term. Caecum and ileo-colic +junction; ventral view. (Columbia University Museum, No. 1548.)] + +In Figs. 515 and 516 two infantile caeca are shown which illustrate this +form. The narrow and pointed apex of the foetal conical caecum is replaced +by the capacious pouch which is differentiated sharply from the +appendix. Among the anthropoid apes the same type is seen in the caecum +of the gibbon (Figs. 455 and 456), and of the young chimpanzee shown in +Fig. 460. + +2. An increased development of the caecal pouch in the adult leads to the +protrusion caudad of two symmetrical sacculations on each side of the +root of the appendix which appears between them. The original apex of +the caecal pouch is still marked by the implantation of the appendix and +by the termination of the longitudinal muscular bands, but the lowest +level of the pouch is found on each side of this point at the fundus of +the secondary lateral sacculi (schema, Fig. 509, _I^c_). Treves, to whom +belongs the credit of first accurately describing and classifying the +forms of the adult caecum based on the development, found this type in +three of a series of 100 cases examined. + +[Illustration: FIG. 517.--Adult human caecum and ileo-colic junction. +(Columbia University, Study Collection.)] + +[Illustration: FIG. 518.--Adult human caecum and ileo-colic junction. +(Columbia University Museum, No. 234.)] + +Figs. 517 and 518 illustrate this form of the pouch, which, in our +experience, is frequently associated with the retro-caecal erect +position of the appendix (cf. infra, p. 251). Fig. 472 shows this type +in the adult with pendent appendix. + +=II. Adult Caeca Derived from Type II.= (schema, Figs. 509 and +511).--From this more commonly observed type of foetal caecum the +following adult forms are developed: + +1. The general shape and trend of the foetal caecum is preserved. The +pouch turns sharply to the left, carrying the apex with the root of the +appendix upward toward the ileum, the appendix itself being frequently +placed under cover of the terminal coil of the small intestine (schema, +Fig. 509, _II^b_). + +[Illustration: FIG. 519.--Human adult (Smith's Sound Eskimo). Ileo-colic +junction and caecum. (Columbia University Museum, No. 59/1483.)] + +[Illustration: FIG. 520.--Human adult. Ileo-colic junction and caecum. +(Columbia University, Study Collection.)] + +[Illustration: FIG. 521.--Human adult. Ileo-colic junction and caecum. +(Columbia University, Study Collection.)] + +[Illustration: FIG. 522.--Human adult (Smith's Sound Eskimo). Ileo-colic +junction and caecum. (Columbia University Museum, No. 56/1571.)] + +The apex of the caecal pouch is either conical, narrowing gradually +toward the root of the appendix (Figs. 520 and 521), or blunt and more +sharply defined against the appendix (Fig. 522). Mr. Treves encountered +this "persistent foetal type" in two per cent. of his series. + +The caecum is frequently sharply bent on itself in making the turn upward +and to the left, resulting in a deep indentation of the concave border +and producing a corresponding projecting fold in the interior of the +pouch (Fig. 523). The ventral longitudinal muscular band follows the +crescentic sweep of the caecum to the root of the appendix. + +[Illustration: FIG. 523.--Human adult. Ileo-colic junction and caecum; +dorsal view; dried preparation. (Columbia University Museum, No. 200.)] + +[Illustration: FIG. 524.--Human foetus at term. Ileo-colic junction and +caecum; ventral view. (Columbia University Museum, No. 1717.)] + +[Illustration: FIG. 525.--Same preparation as Fig. 524; dorsal view.] + +Figs. 524_a_ and 525_b_, representing the caecum of a foetus at term in +the ventral and dorsal view respectively, show very clearly the +arrangement of the foetal pouch from which the adult type with sharp +angular bend is derived. This type of adult caecum is found in certain of +the anthropoid apes. + +In the orang (Figs. 458 and 459) the caecum turns sharply upward and to +the left, gradually narrowing in caliber to the root of the appendix +which is coiled behind the termination of the ileum. + +The same type is seen in Figs. 462 and 463, taken from a preparation of +the adult chimpanzee. Fig. 463 shows especially well the sharp bend +between the caecum and colon by means of which the apex of the pouch is +carried cephalad behind the ileo-colic junction. + +Fig. 431, taken from another specimen of the same animal, shows the +characteristic crescentic curve of the caecum and the corresponding +course of the longitudinal muscular band. The apex of the pouch in this +preparation is more rounded and blunt. + +The same blunt termination of the caecum of this type, with a +corresponding sharper demarcation of the appendix, is seen in the +gorilla (Fig. 457) recalling the conditions found in certain instances +in the human subject (Fig. 522). + +2. In by far the larger proportion of cases (ninety per cent. in Treves' +series) the adult caecum obtains its characteristic form by an unequal +development of the walls of the intestine. The right segment between the +ventral and dorso-lateral muscular bands dilates, forming a sacculation +which projects caudad and constitutes the secondary caput coli, while +the segment between the lower border of the ileum and the original apex, +marked by the origin of the appendix, remains stationary or is further +reduced. This unequal development produces a relative displacement of +the root of the appendix upward and to the left toward the ileo-colic +junction. + +In some cases the primitive crescentic curve of the caecum, as indicated +by the direction of the ventral longitudinal muscular band, is still +perceptible. + +The right wall of the foetal caecum, forming the most pendent portion of +the pouch, dilates uniformly and thus constitutes the adult caput coli. +The left wall appears as a small sacculation separating the root of the +appendix from the ileo-colic junction (schema, Fig. 509, _II, c_). This +type of the adult caecum is illustrated by the preparations shown in +Figs. 526-528. In other cases part of the right wall of the caecum +between the ventral and dorso-lateral colic taenia, dilates abruptly +forming a very prominent rounded sacculation which carries the lowest +part of the pouch caudad in a sharper curve than in the preceding form +as indicated by its deviation from the direction of the longitudinal +muscular band (schema, Fig. 509, _II, d_). + +[Illustration: FIG. 526.--Human adult (Smith's Sound Eskimo). Ileo-colic +junction and caecum; dorsal view. (Columbia University Museum, No. +61/1461.)] + +[Illustration: FIG. 527.--Human adult. Ileo-colic junction and caecum; +ventral view. (Columbia University, Study Collection.)] + +[Illustration: FIG. 528.--Human adult. Ileo-colic junction and caecum; +ventral view. (Columbia University, Study Collection.)] + +Figs. 529-531 afford examples of this type, while Fig. 532, taken from +an infantile preparation, shows that the same may begin to develop at a +very early age. + +[Illustration: FIG. 529.--Human juvenile. Ileo-colic junction and caecum; +dorsal view. (Columbia University, Study Collection.)] + +[Illustration: FIG. 530.--Human adult. Ileo-colic junction and caecum; +dorsal view. (Columbia University Museum, No. 115.)] + +[Illustration: FIG. 531.--Human adult. Ileo-colic junction and caecum; +dorsal view. (Columbia University, Study Collection.)] + +3. Finally, in about four per cent. to five per cent., adult caeca, the +reduction of the wall to the left of the root of the appendix, between +this point and the ileo-colic junction, is complete. The entire caecal +pouch is formed by the dilated right wall between the ventral and +dorsolateral muscular bands. The ventral band terminates at the lower +border of the ileo-colic junction, from which the appendix appears to +arise, indicating the original apex of the foetal caecum (schema, Fig. +509, _II^e_). + +This type is illustrated in the specimens shown in Figs. 533 and 534. + +[Illustration: FIG. 532.--Human infant, Ileo-colic junction and caecum, +with secondary terminal sacculation. (Columbia University Museum, No. +1632.)] + +[Illustration: FIG. 533.--Human adult. Ileo-colic junction and caecum; +dorsal view; dried preparation. (Columbia University Museum, No. 124.)] + +[Illustration: FIG. 534.--Human adult. Ileo-colic junction and caecum; +ventral view; dried preparation. (Columbia University Museum, No. 14.)] + +=III. Adult Caeca in Cases of Absence of the Appendix.=--A few instances +of congenital absence of the appendix have been observed. + +A. v. Haller[9] describes the condition in the following words: +"Defuisse visa est in homine appendicula, ut tuberculum minimum +superesset." + +[9] A. v. Haller, Elements physiologiae, Tom. 7, Liber 24, Sect. 3. + +Fr. Arnold,[10] without describing any individual case, states that +"very rarely the appendix is entirely wanting." + +[10] Fr. Arnold, Handbuch der Anat. d. Menschen. 1847. II. Bd., cloth, +p. 84. + +E. Zuckerkandl,[11] reports having observed one case of absence of the +appendix. + +[11] E. Zuckerkandl, "Ueber die Obliteration des Darmfortsatzes beim +Menschen." Anat. Hefte XI. (Bd. IV., Heft 1), 1894, p. 107. + +J. D. Bryant,[12] reports a case in which he operated for appendicitis +but found "absolutely no appendix." "The point of tenderness was found +to be a glandular growth located posterior to the usual site of the +appendix." + +[12] N. Y. Med. Journal, Vol. LXIX., No. 14, p. 508. + +[Illustration: FIG. 535.--Human adult. Ileo-colic junction and caecum; +absence of appendix. (Columbia University Museum, No. 1077.)] + +[Illustration: FIG. 536.--Human adult. Ileo-colic junction and caecum, +hardened _in situ_; absence of appendix. (Columbia University Museum, +No. 715.)] + +Two instances of this variation are shown in Figs. 535 and 536, taken +from preparations in the Morphological Museum of Columbia University. In +both careful examination of the external as well as of the mucous +surface of the caecum demonstrated the entire absence of the appendix, +and the subjects from which they were obtained presented no scars or +other evidences of operative removal or of pathological processes. They +are both, therefore, authentic instances of complete congenital absence +of the appendix, not of so-called retro-peritoneal or hidden +appendix.[13] + +[13] Cf. Quain. + +The two examples differ from each other in some details. In the first +case (Fig. 535, schema, Fig. 509, _III^a_) the caecum is rounded and +globular. The ventral longitudinal muscular band is vertical and +continued to the lowest point of the pouch, which greatly resembles the +caecum of a typical cynomorphous monkey. + +In the second case (Fig. 536, schema, Fig. 509, _III^b_) the caecum turns +upwards and to the left, terminating in a sharp point, to which several +lobes of epiploic fat are attached. + +We must assume that in these cases the embryonic portion of the caecal +bud was developed just sufficiently to yield the required adult pouch +with nothing to spare, so to speak, which could remain rudimentary in +the form of an appendix. + +Instances of exceedingly rudimentary and reduced appendix are also +encountered. + +In the case illustrated in Fig. 537 the appendix formed a small conical +elevation without distinct lumen, measuring only 0.5 cm. in length. + +[Illustration: FIG. 537.--Human adult. Ileo-colic junction and caecum, +with rudimentary appendix. (Columbia University Museum, No. 1655.)] + +=B. Position and Peritoneal Relations of the Appendix.=--Statistical +records of the position of the appendix indicate a wide range of +variation. In general the results obtained by different observers show +that certain positions of the appendix are encountered in a sufficiently +large percentage of the cases to enable us to adopt a classification, +but that a very extensive series of records are required in order to +determine even approximately the preponderant relations of the appendix. +The following are the most frequently observed positions: + +1. The appendix is directed upward, inward and to the left, the terminal +portion being frequently coiled under cover of the ileum and mesentery. +This position of the appendix is largely due to the normal crescentic +curve of the caecum, which carries the apex of the pouch and the root of +the appendix upward and to the left. Its production is, moreover, +favored by the tendency of the adult caecum to develop by dilatation of +the ventral and right wall at the expense of the left side of the pouch, +thus relatively shortening the interval between the origin of the +appendix and the ileo-colic junction. + +Examples of this commonly encountered position of the appendix are given +in Figs. 512, 513, 514, 520, 521, 523 and 526. + +2. The appendix is erected vertically behind the caecum and ascending +colon and closely attached to the dorsal wall of the large intestine. In +some instances the caecum and colon, with the adherent vertical appendix, +possess a free serous dorsal surface, not adherent to the parietal +peritoneum (Figs. 529, 538, 539 and 540). In other cases the ascending +colon is fixed and the greater part of the retro-colic appendix is +buried in the connective tissue which attaches the large intestine to +the abdominal parietes (Fig. 517). Even in these cases, however, the +dorsal surface of the caecum and the root of the appendix retain their +free serous investment. + +[Illustration: FIG. 538.--Human juvenile. Caecum _in situ_ lifted up to +show vertical course of appendix, situated behind caecum and ascending +colon. The large intestine has a free peritoneal dorsal surface, and the +appendix is held in position by adhesion to the large intestine. +(Columbia University, Study Collection.)] + +[Illustration: FIG. 539.--Human adult. Ileo-colic junction and caecum; +dorsal view. (Columbia University Museum, No. 1594.)] + +[Illustration: FIG. 540.--Human adult. Ileo-colic junction and caecum; +dorsal view. (Columbia University Museum, No. 1850.)] + +3. The proximal part of the appendix turns upward and to the left in +continuation of the caecal curve, but the distal portion is directed +downward and inward, hanging over the brim of the pelvis (Figs. 505, 541 +and 542). + +4. The appendix is directed downward, pendent from the lowest point of +the conical caecal pouch, and hangs free over the pelvic brim. + +This type is encountered at times in foetal and infantile subjects (Figs. +516 and 543). + +[Illustration: FIG. 541.--Human infant. Ileo-colic junction and caecum; +ventral view. (Columbia University, Study Collection.)] + +[Illustration: FIG. 542.--Human infant. Ileo-colic junction and caecum; +dorsal view. The dorsal surface of caecum as far as root of the appendix +is adherent to the parietal peritoneum of the iliac fossa. (Columbia +University Museum, No. 394.)] + +[Illustration: FIG. 543.--Human foetus at fifth month. Abdominal cavity +and viscera; liver and greater part of small intestine removed. (Drawn +from preparation in Columbia University Museum, No. 1814.)] + +[Illustration: FIG. 544.--Human foetus at term. Abdominal cavity and +viscera; greater part of small intestine removed. (Drawn from +preparation in Columbia University Museum, No. 1813.)] + +[Illustration: FIG. 545.--Human foetus at term. Ileo-colic junction and +caecum. (Columbia University Museum, No. 998.)] + +5. The position of the appendix is variant and abnormal, as _e. g._ +placed to the right of caecum and colon (Fig. 544) or turned up ventrad +of the ileo-colic junction (Fig. 545). + +These variations in the position of the appendix and the resulting +peritoneal relations of the structure depend upon the following +factors. + +1. The influence of peritoneal adhesions established during the descent +of the caecum from the subhepatic position to the iliac fossa. + +2. The inherent curve of the caecal pouch. + +3. The subsequent alterations in the caliber of the intestine and the +unequal development of the pouch leading to the formation of the types +of adult caeca above considered. + +In determining the causes which lead to the establishment of any given +position of the appendix all three of the factors above enumerated must +be taken into account, although their influence is not exerted in every +case to an equal degree. + +We have seen that normally, after completed rotation of the intestine, +the caecum with the appendix and the beginning of the colon are lodged in +the upper and right part of the abdomen, below the liver and in contact +with the prerenal parietal peritoneum (schema, Figs. 493, 502). During +the subsequent stages the caecum descends into the right iliac fossa, +thus producing the ascending colon. It is immaterial whether this change +in position is regarded as an actual descent of the pouch over the +ventral surface of the right kidney, which seems more probable, or as a +growing away from the iliac region of the remainder of the abdominal +wall, with a concomitant relative reduction in the size of the liver, +producing a relatively lower position of the caecum, or as a combination +of these processes. In either case during this period the dorsal surface +of the ascending colon and mesocolon normally becomes adherent to the +dorsal parietal peritoneum, connective tissue developing between the +opposed serous areas and leading to the usual fixation of the ascending +colon and obliteration of the free ascending mesocolon. If this process +of adhesion is inaugurated at an early stage, _i. e._, before the +descent of the caecum has been accomplished, it will act as a drag on the +dorsal surface of the colic tube during the subsequent change in +position, which carries the caecum downward into the iliac fossa. This +leads to a backward bend of the caecum and appendix which parts will in +the ventral view appear under cover of the protruding free ventral and +lateral walls of the colon. Hence in many late embryos and foetus at term +the lowest point of the large intestine in the right iliac fossa is +formed by the proximal part of the caecum or by the adjacent segment of +the colon, while the original termination of the pouch, with the root of +the appendix, is turned backward and upward, and, as we have seen, by +reason of the inherent shape of the pouch, also to the left, carrying +the beginning of the appendix frequently behind the terminal ileum and +the ileo-colic junction. + +Two of the more common positions of the appendix, viz., backwards, +upwards and inwards behind the ileo-colic junction, and directly +backward, erected vertically behind caecum and colon, can therefore in +part be referred to the mechanical conditions obtaining normally during +the descent of the caecum. Of course the shape of the caecal pouch and the +later development of the adult type of caecum will modify this influence +in individual cases. We have seen that this early adhesion and the +resulting effects on the position of caecum and appendix depend on the +direct apposition of the colic tube and mesocolon to the dorsal parietal +peritoneum. Any condition which will prevent or delay this apposition +will likewise perpetuate the original embryonal condition of the tube, +completely invested by peritoneum and with a free mesocolon. + +Such an element is found in the persistence of the dorsal set of ileal +convolutions in the original retro-caecal position beyond the usual +period, as indicated in the schematic Fig. 492, _IV, a_. If the turn +downward and to the left of these coils is for any reason delayed beyond +the usual time the caecal extremity of the colon will descend from the +subhepatic to the iliac position without coming directly into contact +with the dorsal parietal peritoneum, and therefore without the usual +peritoneal adhesion and obliteration of the apposed serous surfaces. The +caecum under these conditions descends without making the backward bend, +and the origin of the appendix is found at the lowest point of the +pendent funnel-shaped pouch, causing it finally to hang downward or +downward and inward over the pelvic brim. The resulting form of the +caecum and the position of the appendix is the one above described as +type _Ia_, _Ib_ and _Ic_ (Fig. 509). + +Fig. 510 from a foetus at term, and Figs. 515 and 516 representing +infantile caeca, illustrate this form of the pouch, while the parts are +shown in situ in Fig. 543 taken from a preparation of a five-month +foetus. + +[Illustration: FIG. 546.--Human embryo, 6.5 cm. cervico-coccygeal +measure. Abdominal cavity, with liver removed, seen from the right side. +(Columbia University, Study Collection.)] + +Fig. 546 exhibits the condition obtaining during the development of this +type in the more exceptional instances of delayed apposition of the +colon to the parietal peritoneum and of increased development of the +terminal ileal coils in the original retro-caecal position. In this +embryo, measuring 6.5 cm. in vertex-coccygeal length, the development +has progressed sufficiently to establish a distinct transverse colon and +to bring the caecum and appendix into the subhepatic position. But in +place of lying in contact with the dorsal parietal peritoneum, as in the +embryo, shown in Fig. 502, over the ventral surface of the right kidney, +the increased mass of the retro-caecal ileal coils keeps the caecum, +already in the process of descent, in contact with the ventral abdominal +wall. When the final rotation of the retro-caecal small intestinal coils +downward and to the left occurs, placing the ileo-colic junction (_C_) +to the left of the large intestine (schema. Fig. 494), the ascending +colon and caecum are not yet fixed by adhesion to the dorsal parietal +peritoneum, and the appendix will present downward and to the left, +affording the necessary conditions for the establishment of the +permanent pendent position of the tube or causing the same to be +directed downward and inward over the brim of the pelvis. + +[Illustration: FIG. 547.--Human embryo, 5.9 cm. vertex-coccygeal +measure. (Columbia University, Study Collection.)] + +In contrast with the preceding is the condition shown in Fig. 547, taken +from an embryo of 5.9 cm. vertex-coccygeal measure. The transverse colon +in this preparation has already begun to assume an oblique position, +passing down and to the right from the splenic flexure. The caecum and +appendix are in contact with the dorsal prerenal parietal peritoneum. +The escape of the dorsal set of ileal convolutions from the retro-caecal +position, by rotation downwards and to the left, is accomplished. The +caecum and appendix are placed in the position which they would have +occupied in the embryo shown in Fig. 546 if the dorsal ileal coils had +not prevented, in the latter preparation, the apposition of the colon to +the dorsal parietal peritoneum. + +In considering the effect of these variant conditions on the adult +arrangement of the structures it is necessary to bear in mind the second +of the above-mentioned factors, namely, the inherent shape of the caecal +pouch and appendix and the resulting direction of its axis. + +As previously stated the normal type of the human embryonal caecum is +represented by the pouch of some of the new-world monkeys, as _Ateles_ +(Fig. 443) or of certain lemurs, of which _Nycticebus_ (Fig. 420) +furnishes an excellent example. The caecum is distinctly crescentic, +turning its concave margin, after completed intestinal rotation, upwards +and to the left, toward the lower margin of the ileum. The distal +diminished segment of the pouch in _Ateles_ has already assumed the +character of a caecal appendage in _Nycticebus_ and becomes by further +reduction the typical appendix in man and the anthropoid apes, while the +proximal portion develops into the capacious sacculated caecum proper. +Consequently the initial curve of the caecum tends to carry the root of +the appendix upward and to the left toward the ileo-colic junction. This +curve of the pouch, combined with the mechanical effects produced by the +adhesion of the colon during the caecal descent, accounts for the +frequency with which the caecum in the later months of foetal life and at +birth is found curved backward, upward and to the left, placing the root +of the appendix under cover of the terminal ileal convolutions (Fig. +548). We have seen that this disposition of the structures accounts for +the preponderance of that type of adult caecum which results from the +further and unequal development and dilatation of the segment of the +pouch situated to the right of the origin of the appendix. + +[Illustration: FIG. 548.--Human foetus at term. Ileo-colic junction and +caecum. Early colic and caecal adhesion with retroverted appendix. +(Columbia University Museum, Study Collection.)] + +Bearing in mind the three elements just considered, viz., the effect of +adhesion during the caecal descent, the inherent shape of the pouch and +the unequal alterations in caliber in the development of the adult type, +we can at once take up the resulting variations in the peritoneal +relations of the adult caecum and appendix which have an important +influence on the progress of pathological processes in this region. It +should be remembered that in the following schematic figures the colon, +caecum and appendix are represented in the profile view in a straight +line, without indicating the characteristic turn of the crescentic caecal +pouch upwards and to the left. + +Fig. 549 shows the arrangement in unimpeded caecal descent without +adhesion of colon and mesocolon to the parietal peritoneum. This +disposition of the structures, if carried into adult life, would produce +the permanently free ascending colon and mesocolon which we encountered +exceptionally in the human subject (cf. p. 82) and normally in certain +of the cynomorphous monkeys (p. 83). The ascending colon and mesocolon +can, under these conditions, be turned mesad, lifting them away from the +primary parietal peritoneum investing the ventral surface of the right +kidney. Caecum and appendix have, of course, a complete serous +investment. + +[Illustration: FIG. 549.] + +[Illustration: FIG. 550.] + +[Illustration: FIG. 551.] + +[Illustration: FIG. 552.] + +[Illustration: FIG. 553.] + +[Illustration: FIG. 554.] + +[Illustration: FIGS. 549-554.--Schematic series illustrating the +variations in the arrangement of the caecal and colic peritoneum.] + +Normally, however, in the human subject, even if the obliteration of the +apposed serous surfaces and the resulting fixation of the ascending +colon has been delayed beyond the usual period, as above indicated, +adhesion takes place subsequently, involving the dorsal surface of the +ascending colon between the ileo-colic junction and the hepatic flexure +(schema, Fig. 550). The dorsal surface of the caecum usually retains its +free serous surface in whole or in greater part. The appendix is +pendent, entirely invested by peritoneum and hangs free in the abdominal +cavity, directed toward the pelvic brim, illustrating the effect of +delayed fixation of the colon on the position of the appendix. + +Examples of this condition are not frequent, and are confined almost +exclusively to foetal and juvenile subjects. Illustrations are afforded +by Figs. 515 and 516. + +We have already noted (p. 246) the resulting foetal type of pendent caecum +(Fig. 510). + +More commonly colic adhesion before the caecum obtains its final iliac +position results in imparting a backward turn to the pouch, leading to +the peritoneal disposition shown in schema, Fig. 551, in which the root +of the appendix is involved in the area of obliteration, while the +terminal segment remains free. An example of this condition is furnished +by the embryo shown in Fig. 508 (10.7 cm. vertex-coccygeal measure). The +colon is already segmented into an ascending, transverse and descending +portion. The caecum is retroverted and its apex with the appendix is +placed under cover of the terminal ileum which enters the large +intestine in the direction from below upward and to the right. In the +side-figure the divided end of the ileum is displaced upward to show +caecum and appendix and their relation to the ileal mesentery. + +The disposition of the structures illustrated by this example probably +depends upon delayed adhesion of the colic embryonal tube to the dorsal +parietal peritoneum. The caecum and appendix appear to have descended +freely until the final position in the right iliac fossa has been nearly +attained, adhesion and fixation of the colon taking place just before +the descent is completed, and thus producing the backward turn of the +caecal end of the tube. Further development of the caecum to form the +adult caput coli in these cases leads to the unequal and exaggerated +expansion of the ventral and lateral walls of the pouch, as compared +with the fixed and adherent dorsal wall. The former are distended and +pushed downwards, producing a relative recession of the root of the +appendix upward and to the left, until it comes into relation with, or +even under cover of, the ileo-colic junction and of the terminal ileal +coil entering the colon at this point. + +The resulting characteristic adult position of the appendix in these +cases is as follows: + +The termination of the caecum proper, and the root of the appendix are +under cover of the terminal ileum and frequently adherent to the +parietal peritoneum of the iliac fossa (Fig. 555). The distal portion of +the appendix remains free, either hanging down and in over the brim of +the pelvis (Fig. 542), or turned upwards and to the left and coiled in +several turns (Figs. 504, 555 and 556). + +[Illustration: FIG. 555.--Human foetus at term. Ileo-colic junction and +caecum; dorsal view. The area of peritoneal adhesion is seen to involve +the dorsal aspect of the caecum as far as the root of the appendix. +(Columbia University Museum, No. 1549.)] + +[Illustration: FIG. 556.--Human infant. Ileo-colic junction and caecum, +with extensive adhesion to parietal peritoneum. (Columbia University +Museum, No. 301.)] + +Finally the _erect vertical retro-caecal_ position of the appendix +presents several important variations in the disposition of the +peritoneal investment. In Fig. 503, taken from an embryo of 7.6 cm. +vertex-coccygeal length, the early complete recession of the retro-caecal +ileal convolutions has probably permitted an early apposition and +adhesion of the beginning of the colon to the dorsal prerenal parietal +peritoneum. The subsequent descent into the iliac fossa produces a bend +in the ventral wall of the colic tube, with a marked convexity directed +downwards and forwards, the apex of the bend situated at or near the +level of the ileo-colic junction, while the dorsal colic wall is held by +the adhesion to the parietal peritoneum, thus giving a backward +inclination to the entire caecum and appendix. During the subsequent +descent of the caecum proper this bend in the colon is gradually +diminished and the tube becomes straightened but the apex of the caecum +remains turned back and the appendix is placed in a more or less +vertical erect position behind caecum and ascending colon. + +As regards the disposition of the peritoneal membrane in this type of +appendix the following conditions are to be noted: + +(_a_) (Schema, Fig. 552.)--The apex of the caecum and the entire appendix +are extraperitoneal, imbedded in the loose connective tissue which +occupies the area of serous obliteration. The line of peritoneal +reflection from the dorsal wall of the secondary caput coli to the +parietal peritoneum of the right iliac fossa is placed transversely +below the true apex of the foetal caecum and the root of the appendix. The +latter tube, imbedded in connective tissue, passes vertically upwards +behind the ascending colon, its tip frequently reaching the ventral +surface of the right kidney. A well-marked example of this arrangement +in the adult is shown in Figs. 557 and 558 (ventral and dorsal view, +with peritoneal reflection and vertical retro-colic appendix). + +[Illustration: FIG. 557.--Human adult. Ileo-colic junction and caecum; +ventral view. (Columbia University Museum, No. 1612.)] + +[Illustration: FIG. 558.--Same preparation as Fig. 557; dorsal view. The +appendix, erected vertically between caecum and colon, is completely +imbedded in connective tissue.] + +(_b_) (Schema, Fig. 553.)--In other cases, with the same position of the +appendix, the entire caecum and greater part of the ascending colon +remains free. The vertically erected appendix is closely attached to the +dorsal surface of the ascending colon, included within the serous +investment of the large intestine. The adhesion of the latter is +confined to a limited area near the hepatic flexure. Consequently caecum +and greater part of ascending colon can be turned up, away from the +parietal peritoneum of the iliac fossa, and the dorsal surface of the +appendix shows the free serous covering of the adjacent large intestine. + +We may assume that this type of the peritoneal relations of the appendix +is produced in one of two ways: + +1. Either the retro-colic appendix has become early attached to the +adjacent large intestine, whose dorsal surface in large part remains +free, or + +2. The arrangement of the peritoneum indicated in schema, Fig. 552, may +be subsequently changed into that shown in schema, Fig. 553, by a +continued downward displacement of the caecum, producing a secondary +serous investment of the dorsal surface of appendix and part of +ascending colon. + +Examples of this type are found both in infantile and adult subjects. + +In Fig. 538, taken from an infant three years of age, the caecum is +lifted up to show the vertical position of the appendix behind the caecum +and ascending colon, the dorsal surface of the large intestine retaining +its free serous covering. Another illustration of this arrangement in a +juvenile subject is shown in Fig. 529. The same condition in the adult +subject is illustrated in Figs. 539 and 540. + +(_c_) (Schema, Fig. 554.)--Occasionally, with the appendix erected +vertically behind the ascending colon, the apex of the caecum and the +proximal portion of the appendix are invested by peritoneum for a short +distance and the tip of the appendix likewise obtains a free serous +investment, while the intermediate greater portion of the appendix and +the corresponding segment of the dorsal surface of the ascending colon +are extraperitoneal, adherent to the abdominal parietes. Examples of +this peritoneal relation of the appendix in an infant are shown in Figs. +559 and 560, while Fig. 509 represents the same arrangement in an adult +specimen. The condition is produced from the arrangement of schema, Fig. +554, by secondary adhesion and obliteration of the serous surfaces over +the intermediate portion of the retroverted appendix and the adjacent +dorsal surface of the ascending colon. + +[Illustration: FIG. 559.--Human infant. Ileo-colic junction and caecum; +dorsal view. Retro-colic appendix, adherent to the free dorsal serous +surface of the large intestine, with intermediate extraperitoneal +segment. (Columbia University Museum, No. 1638.)] + +[Illustration: FIG. 560.--Human adult. Ileo-colic junction and caecum; +dorsal view. Appendix with intermediate non-peritoneal segment, while +the proximal portion and the tip are covered by serous investment. +(Columbia University Museum, No. 1615.)] + + +C. ILEO-CAECAL FOLDS AND FOSSAE. + +Certain peritoneal folds, either mesenteric in character, _i. e._, +containing blood vessels, or non-vascular, pass between the terminal +ileum and the caecum and appendix, modifying in some instances very +markedly the position and peritoneal relations of the structures. + +In considering the influence which these vascular mesenteric and +non-vascular serous folds exert in producing further changes in the +shape, position and relations of the human appendix it is necessary to +remember that in the early embryonal stages these bands and folds of the +peritoneum appear only slightly marked, but that they gain their +importance and influence on the final adult configuration of the caecal +pouch and appendix in the course of the further development of these +structures. + +For this reason the comparative study of the corresponding parts in +other vertebrates, especially in certain mammalia, is of the utmost +value, if we seek to explain and understand the derivation, significance +and typical arrangement of these folds. We have seen that the caecum as +found in the large majority of mammalian forms is equivalent to the +caecum and appendix of the human subject and anthropoid apes; that in +other words the vermiform appendix represents the distal segment of a +caecal pouch, originally uniform in caliber, which has remained +undeveloped, while the proximal portion has progressed evenly with the +general development of the alimentary canal to form the caecum proper. We +have seen that this tendency to retain the distal portion of the pouch +in a rudimentary condition, _i. e._, the production of an appendage to +the caecum proper, is encountered in several of the lower forms, as +certain Marsupials, Carnivores, Ungulates and Lemurs. The morphology of +the ileo-caecal folds is hence best understood by considering these +structures as they appear in connection with the various caecal types +presented by the lower mammalia. Their arrangement and significance can +here be readily made out. On the other hand, in studying these +structures in the human appendix we are following lines which are +already becoming indistinct on account of the rudimentary character of +the organ, which we must regard as undergoing an exceedingly slow +process of reduction, with a view to its ultimate elimination from the +body. We have seen that the structural uncertainty impressed on caecum +and appendix by this evolutionary influence finds its expression in the +wide range of variation in size and arrangement which these parts +present. Necessarily, of course, this tendency to variation is shared, +and even exhibited to a more marked degree, by what we can term the +accessory structures connected with caecum and appendix, viz., the +mesenteric vascular and non-vascular serous folds passing to them from +the ileum. + +We can most profitably begin our consideration of these folds in a form +in which they are preserved in their entire and original development, +and then successively trace the changes leading up to the normal +disposition in the human subject. Such a type is presented by the caecum +of _Ateles ater_, the black-handed spider monkey (Figs. 444 and 445). +The caecum of this animal presents a uniform crescentic curve, with the +concavity directed upward and to the left, and the gradual diminution in +the caliber of the pouch, from the ileo-colic junction to the apex, +denotes the tendency to retain the distal segment in a rudimentary +condition, foreshadowing the eventual formation of a vermiform appendix. + +In the ventral view, with the terminal ileum lifted up, the following +arrangement of folds passing between ileum and caecum is noted (Figs. 444 +and 445). + +(_a_) _Vascular Mesenteric Folds._--The peritoneal vascular folds, +carrying the blood vessels to supply the caecum, are two in number, a +ventral (1) and dorsal (3). They are of nearly equal size and extent, +passing from the ventral and dorsal aspect of the ileo-colic junction +nearly to the apex of the caecum. Each contains a branch of the +ileo-colic artery, which forks in the ileo-colic mesentery, in the angle +between ileum and large intestine. The ventral branch continues in the +ventral mesenteric fold (Fig. 445) downward across the ventral surface +of the ileo-colic junction to supply the ventral part of the caecum, +while the dorsal branch descends behind the ileo-colic junction, +preserving a similar course in the dorsal mesenteric fold. The dorsal +arterial branch is somewhat larger than the ventral and its distribution +extends a little further down to the actual apex of the caecum. + +(_b_) _Non-vascular Ileo-caecal Serous Reduplication._--Between the two +vascular mesenteric folds a third serous reduplication, carrying no +blood vessels, is found passing between the ileum and caecum. This fold +begins, in the preparation from which the figure is taken, on the ileum +opposite the attached mesenteric border, 2.7 cm. from the ileo-colic +junction, and passes for exactly the same distance down on the adjacent +left concave surface of the caecum. It is placed a little nearer to the +dorsal than to the ventral vascular fold, so that it passes, if the +distance between the two vascular folds on the caecum be divided into +three parts, at the junction of the dorsal third with the ventral two +thirds. The production of this intermediate non-vascular ileo-caecal +reduplication, which is of very constant occurrence in the mammalian +series, is to be led back to the development of the caecum. When the +pouch protrudes from the smooth surface of the embryonic intestine +opposite the mesenteric border, it extends backward along the future +small intestine and lifts off the serous investment of the gut in the +form of a small peritoneal plate filling the interval between itself and +the adjacent ileum. A very perfect illustration of this process can be +seen in the instance of Meckel's diverticulum shown in Fig. 561. The +proximal portion of the diverticulum is here still closely connected to +the small intestine along which it extends, both being surrounded by the +common visceral peritoneum. The distal part of the diverticulum has +separated more completely from the intestine, and in so doing has drawn +out the serous investment in the form of the triangular fold which is +seen to pass between the free margin of the intestine and the adjacent +surface of the pouch. The same process can be followed in its different +stages in certain normal mammalian caecal types. + +[Illustration: FIG. 561.--Human adult ileum with Meckel's diverticulum. +Ileo-diverticular serous fold and persistent omphalo-mesenteric artery. +(Columbia University Museum, No. 1803.)] + +[Illustration: FIG. 562.--Human adult. Ileum with Meckel's diverticulum, +131.5 cm. from ileo-colic junction; a distinct vascular fold is +prolonged from the ileal mesentery to the margin of the diverticulum. +(Columbia University Museum, No. 1849.)] + +In this connection it may be noted that the production of the caecal +vascular folds and their relation to the mesentery is also very +perfectly illustrated in some forms of Meckel's diverticulum. Thus in +the preparation shown in Fig. 562, a broad triangular serous fold passes +from the ileal mesentery to the margin of the diverticulum, carrying the +blood vessels which supply the pouch. If the section of the intestine to +the left of the figure is regarded as representing the terminal ileum, +that to the right the colon, and the diverticulum the caecal pouch, the +formation of the fold and its relation to the mesentery, blood vessels +and intestine will correspond closely to the ileo-caecal vascular folds. + +Fig. 350 shows the ileo-colic junction and caecum of _Halmaturus +derbyanus_, the rock kangaroo. The caecum here extends backwards along +the free border of the ileum to which it is closely bound by the common +investing visceral peritoneum for the greater part of its extent. In +another marsupial form, a small species of opossum from Trinidad (Fig. +349), the caecum has separated itself more completely from the adjacent +small intestine--thus drawing out the peritoneum into a narrow +connecting fold. Finally, in the Virginia opossum (Fig. 348), the ileum +has attained the usual position at right angles to caecum and colon. The +former pouch is separated from the small intestine by a considerable +interval and the angle between the two is filled out by a well-developed +triangular serous fold, connecting the free margin of the terminal ileum +and the adjacent left border of the caecum. + +This is the "intermediate non-vascular" ileo-caecal fold. + +Passing now from the condition presented by _Ateles_, with three fully +developed and distinct ileo-caecal folds, to the next stage leading up to +the normal human arrangement, we find the same illustrated in the caecum +of another new-world monkey, _Mycetes fuscus_, the brown howler monkey, +shown in the ventral and dorsal views in Figs. 449 and 450. The ventral +vascular fold (Fig. 449, 1) is still well developed, the contained +ventral branch of the ileo-colic artery descending over the ventral wall +of the ileo-colic junction and caecum and supplying both. The dorsal +vascular fold (Fig. 450, 2), on the other hand, is nearly completely +fused with the intermediate non-vascular reduplication (Figs. 449 and +450, 3), the approximation between these structures exhibited by +_Ateles_ having in _Mycetes_ reached the point of actual union, so that +the larger dorsal branch of the ileo-caecal artery descends to the apex +of the caecum in the following manner: The main post-caecal artery passes +over the dorsal surface of the ileo-colic junction included in a short +serous fold which corresponds to the dorsal vascular fold of _Ateles_. +Beyond the lower border of the ileo-colic junction this fold fuses with +the intermediate non-vascular fold, one arterial branch descending along +the line of attachment of this fold to the caecum, the other distributed +over the dorsal surface of the pouch. + +A third type, also taken from the lower Primates, is presented by +the caecum of a cynomorphous monkey, _Cercopithecus sabaeus_, the +African green monkey, shown in Fig. 432, in the ventral and left +aspect with the terminal ileum lifted up. The caecum of this animal +is comparatively short, somewhat conical, terminating in a blunt +apex. The vascular supply is arranged on the same type as in _Ateles_ +and _Mycetes_, _i. e._, a trunk of the ileo-colic artery divides +at the ileo-colic notch, one branch descending ventrad, the other +dorsad of the ileo-colic junction. The slightly larger size of the +dorsal vessel, noted in _Ateles_ and _Mycetes_, has been increased in +_Cercopithecus_ until the ventral artery (1) supplies merely the front +of the ileo-colic junction and the upper part of the adjoining ventral +wall of the caecum, while the larger dorsal vessel (2) descends behind +the ileo-colic junction, supplying the same and the entire dorsal and +apical portions of the caecum. The relation of these caecal arteries to +the peritoneum is moreover different from that encountered in _Ateles_. +In place of running in distinct mesenteric folds, as in the latter +species, the vessels pass close to the surface of the intestine, merely +covered and partly surrounded by slightly redundant visceral peritoneum +containing numerous pads of epiploic fat, which bead the course of the +vessels at regular intervals. Between the two arteries the intermediate +non-vascular fold (2) is seen, presenting much the same arrangement as +in _Ateles_ and passing between the left border of the caecum and the +adjacent margin of the ileum, nearer to the dorsal larger than to the +ventral smaller caecal artery. + +We have, therefore, in the three types just considered, the following +variations in the arrangement of the vascular and non-vascular folds: + + 1. (_a_) Ventral and dorsal vascular folds distinct } + and free. Ventral and dorsal caecal } + arteries of nearly equal size. } + (_b_) Intermediate non-vascular fold free on }_Ateles._ + both surfaces, placed nearer to the } + dorsal than to the ventral vascular fold. } + 2. (_a_) Ventral vascular fold distinct. Ventral } + caecal artery somewhat further reduced } + in size. Dorsal vascular fold distinct } + only over the dorsal surface of the ileo-colic} + junction. At the lower border of } + the ileo-colic junction the dorsal vascular }_Mycetes._ + fold fuses with the intermediate non-vascular } + fold. } + (_b_) Intermediate non-vascular fold free only } + on ventral surface, the dorsal surface } + below the ileo-colic junction being fused } + with the dorsal vascular fold. } + 3. (_a_) Dorsal and ventral vascular folds reduced. } + Dorsal artery much larger than ventral. }_Cercopithecus._ + (_b_) Intermediate non-vascular fold well developed,} + free on both surfaces. } + +We may judge from this series that the following factors are capable of +materially modifying the definite arrangement of the structures: + +1. The vascular folds are capable of reduction until the vessels run +close to the intestinal surface, merely covered by somewhat redundant +peritoneum containing epiploic appendages. (_Cercopithecus._) + +2. The dorsal caecal artery tends to assume in all three forms the +greater share in the caecal vascular supply. This tendency is slightly +developed in _Ateles_, becomes more pronounced in _Mycetes_, and is well +marked in _Cercopithecus_, in which animal the dorsal vessel nearly +replaces the ventral branch, the latter confining itself to the ventral +surface of the ileo-colic junction and the adjacent ventral parts of the +caecal wall. + +3. The intermediate non-vascular fold is placed nearer to the dorsal +larger than to the ventral smaller caecal artery. This condition, present +in both _Ateles_ and _Cercopithecus_, foreshadows the fusion of the +intermediate and dorsal vascular folds at the lower border of the +ileo-colic junction, as seen in _Mycetes_. + +4. This fusion of the two folds named in _Mycetes_ results in giving +different values to the dorsal vascular fold in its proximal and distal +segments. The proximal segment descends from the ileo-colic notch behind +the ileo-colic junction to its lower border as a distinct fold. Beyond +this point its fusion with the distal (caecal) segment of the +intermediate fold rounds out a fossa, the inferior or posterior +ileo-caecal, which is consequently bounded in front by the intermediate +vascular fold, behind by the proximal segment of the dorsal vascular +fold, to the right side by the inner wall of the caecum, between the +intermediate and dorsal vascular folds, above by the lower border of +ileum and ileo-colic junction, and below by the fusion of the two +folds. + +This pocket or fossa which is the most important and constant of the +peritoneal recesses in the neighborhood of the caecum, opens upward and +to the left. + +5. A superior or anterior ileo-caecal fossa, formed in cases of +well-developed ventral vascular fold between the same and the ventral +wall of the ileo-colic junction, is of small size and shallow. + +The cause of the greater development of the dorsal as compared with the +ventral caecal artery is probably to be sought in the adhesion of the +colon to the dorsal parietal peritoneum. In _Cercopithecus_ the dorsal +surface of the ascending colon is adherent to the parietal peritoneum +down as far as the iliac region and beginning of the caecum, whereas in +_Mycetes_ the entire caecum, as well as the ascending colon, are free and +non-adherent to the abdominal parietes. The influence of this adhesion +on the arrangement of the vascular supply of the lower portion of the +ascending colon and caecum appears to be important. Some of the +departures from the _Ateles_ type presented by _Cercopithecus_ become +still better developed in the human subject, where the adhesion of the +ascending colon and the obliteration of the apposed serous surfaces of +ascending mesocolon and parietal peritoneum is normally complete, even +if the caecum remains entirely free, or only adheres to the iliac +parietal peritoneum in the proximal part of its dorsal surface. +Comparison with forms presenting non-adherent colic and caecal tubes +indicates that the adhesion determines the relative size and arrangement +of the ileo-colic vessels. + +Thus the partially adherent colon and caecum of _Cercopithecus_ presents, +compared with the free tube of _Ateles_ and _Mycetes_, a marked +reduction of the ventral and a corresponding enlargement of the dorsal +caecal artery. Further progress in the same direction is noted in the +human subject where normally the ascending colon and at times the +proximal portion of the caecum are adherent to the dorsal parietal +peritoneum. + +It appears that in the adhesion of the colic tube to the parietal +peritoneum the dorsal ileo-colic vessels find an element favorable to +their more complete development and extension, replacing in part or +entirely the ventral caecal artery which becomes limited in distribution +to the region of the ileo-colic junction. The adhesion and fixation of +the dorsal wall of the intestine seems to afford an advantage to the +dorsal vessel, whereas the greater mobility and the alternating +conditions of distension and contraction, with variations of intracaecal +pressure, depending upon the contents of the pouch, appear to operate +unfavorably upon the development of the ventral vessel. + +This view is borne out by the conditions observed in the exceptional +instances in which in the human subject the ventral artery assumes the +large share in the supply of caecum and appendix (cf. p. 276). In all the +cases observed the type of the caecum indicated delayed or imperfect +colic adhesion, and the ascending mesocolon remained partially free. + +If we now compare the conditions above described for _Ateles_, +_Mycetes_, _Cercopithecus_ with those usually found in man and in the +anthropoid apes, we may appreciate the significance of the structures +encountered by beginning the investigation with a type in which the +derivation of the different parts is still quite evident. Such a +condition is presented by the preparation shown in Fig. 563, taken from +a child one year of age. Here the descent of the caecum has evidently +been quite rapid and uniform without dorsal adhesion. The caecum and +ascending colon remain free and can still be lifted away from the +ventral facies of the right kidney and turned toward the median line to +a point somewhat beyond the renal hilus. The caecum hangs downward +vertically and the appendix arises from the funnel-shaped apex of the +pouch. + +[Illustration: FIG. 563.--Human; child one year old. Caecum and +ileo-colic junction; ventral view. (Columbia University, Study +Collection.) + +1. Ventral caecal artery, surrounded by epiploic appendages. + +2. Dorsal vascular fold, forming appendicular mesentery. + +3. Intermediate non-vascular fold.] + +The ventral caecal branch of the ileo-colic artery is slightly developed, +(1) as a small vessel descending in an epiploic fold over the ventral +surface of the ileo-colic junction as far as the root of the appendix. +The intermediate non-vascular fold (3) is well marked, measuring 2.9 cm. +in length, extending from the free border of the terminal ileum to the +caecum and appendix and crossing over the well-developed dorsal vascular +fold (2), which descends, as the appendicular mesenterolium, to the tip +of the appendix, carrying the dorsal artery. In studying the conditions +presented by this specimen, it is not difficult to trace the analogous +structures in the caeca of _Cercopithecus_, _Ateles_ and _Mycetes_. The +same vascular and non-vascular serous reduplications are found passing +between the ileum and caecum. In accordance with the type presented by +_Cercopithecus_ the ventral artery is much reduced and runs in a short +serous fold loaded with epiploic appendages. The dorsal artery, on the +other hand, is well developed and the intermediate non-vascular fold is +distinct. In their relative arrangement these folds follow the _Ateles_ +type. The dorsal vascular fold forms the true mesentery of the appendix, +and, although close to and crossed by the intermediate non-vascular +reduplication, remains still quite separable and distinct from the same; +consequently the lower limit of the usual posterior ileo-caecal fossa, +produced by the fusion of the dorsal vascular and the intermediate +non-vascular fold, is absent. + +A very perfect illustration of this type of the human ileo-caecal fold is +presented by the preparation of _Gorilla savagei_ shown in Fig. 457. The +ventral fold and artery appear reduced in this animal. The dorsal +vascular fold forms a broad triangular plate of serous membrane carrying +the dorsal artery in its free border and extending to the tip of the +appendix. + +The intermediate non-vascular fold is narrow but distinct, continued for +a considerable distance along the ileum, opposite to the attached +border, but only for a short extent along the left border of the caecum +below the ileo-colic junction. It crosses the ventral surface of the +broad dorsal vascular fold in passing to the caecum, but remains entirely +free and is not adherent to the same. + +Consequently here again the dorsal or posterior ileo-caecal fossa loses +its distal limitation. The usual arrangement of the parts, as found in +the human subject and derived from the preceding, is well illustrated by +another anthropoid ape, _Hylobates hoolock_. Fig. 455 shows the +ileo-caecum of this animal in the ventral view and the homologous parts, +as compared with _Gorilla_, are readily recognized. On turning the +terminal ileum ventrad and cephalad (Fig. 456), it is, however, seen +that the intermediate non-vascular fold does not merely cross the dorsal +vascular reduplication, as in _Gorilla_, but that it has begun to adhere +to the same at the point of intersection. Consequently a well-marked and +clearly limited posterior or dorsal ileo-caecal fossa is formed, bounded +ventrally by the intermediate fold at its accession to the caecum, +dorsally by the proximal part of the dorsal vascular fold, to the right +by the left wall of the caecum, behind by the attachment of the +intermediate fold, below by the confluence of the two folds, and above +by the lower border of ileum and ileo-colic junction. + +The open mouth of the fossa looks to the left. Fig. 464, taken from an +adult specimen of the chimpanzee, _Troglodytes niger_, shows the extent +of the dorsal vascular fold and of its connection with the mesentery of +the terminal ileum. + +The intermediate non-vascular fold extends from the ileum downwards +along the entire left border of the caecum to the root of the appendix, +fusing with the dorsal vascular fold and rounding out a deep posterior +ileo-caecal fossa. + +The typical arrangement, as encountered in the human subject, +corresponds closely to the conditions presented by these anthropoid +apes. + +[Illustration: FIG. 564.--Human adult. Caecum and ileo-colic junction. +(Drawn from preparation in Columbia University, Study Collection.) + +1. Dorsal vascular fold at the beginning of the distal free portion, +forming the appendicular mesentery. + +2. Proximal segment of dorsal vascular fold, fusing with + +3. Intermediate non-vascular fold. + +4. Rounded edge of union of dorsal vascular and intermediate folds +bounding the ileo-caecal fossa caudad. + +5. Point of accession to appendix of proximal branch of appendicular +artery derived from posterior ileo-caecal artery.] + +In Fig. 564, taken from an adult male human subject, the dorsal surface +of the ascending colon and of the ileo-colic junction is adherent to the +parietal peritoneum. The distention of the caecum is nearly uniform, the +right sacculation being only slightly larger than the left. The +appendix, measuring 18.4 cm. in length, arises from the dorsal surface +of the caput coli, 1.7 cm. from the point where the ventral longitudinal +muscular band turns around the caudal end of the pouch between the two +sacculations, and 3.7 cm. below the caudal margin of the ileo-colic +junction. + +The dorsal vascular fold (2), forming the broad appendicular mesentery +(1), is well developed and free in its distal portion, extending, with +gradually diminishing width, to the apex of the appendix. The proximal +segment of this fold (between 1 and 2) descends over the dorsal surface +of the ileo-colic junction and meets (at 4) the intermediate +non-vascular fold (3) which extends between the ileum and caecum, +rounding out a crescentic ridge (4) which bounds the entrance into the +posterior ileo-caecal fossa (between 2 and 3). The influence of the folds +and of the blood vessels on the position and curves of the appendix is +quite apparent in this preparation. + +The dorsal larger branch of the ileo-colic artery, supplying caecum and +appendix, passes over the dorsal surface of the ileo-colic junction (2) +where the same, as well as the adjacent dorsal surface of the colon, is +adherent to the parietal peritoneum. At the point where the dorsal +vascular fold intersects and fuses with the intermediate non-vascular +fold (4) the artery divides into a proximal and distal branch. The +former proceeds to the caecum and root of the appendix, reaching this +tube at the point marked 5. The latter continues (from 1 on) in the free +border of the appendicular mesentery to the beginning of the distal +third of the appendix, from which point on the fold extends as a narrow +reduplication to the tip of the tube. The segment of the appendix +situated between these two main arterial branches is thrown into several +coils, the expression of the continued growth between two points +relatively fixed by the accession of the two arterial branches. The +pathological significance of these bends is apparent when we consider +the effect which the kinking of the tube would have on catarrhal and +other inflammations accompanied by distension of the appendix. + +Typical examples of the posterior ileo-caecal fossa and of the mutual +relationship of the limiting folds are seen in Figs. 565 and 566, both +taken from adult human subjects. + +[Illustration: FIG. 565.--Human adult, Caecum and ileo-colic junction +with large intermediate non-vascular fold and deep posterior ileo-caecal +fossa. (Columbia University Museum, No. 1546.)] + +[Illustration: FIG. 566.--Human adult. Ileo-colic junction and caecum. +(Columbia University Museum, No. 1659.)] + +The significance and mutual relations of the folds seen in the +preparations just considered--which illustrate the typical adult human +arrangement of the structures--will perhaps be best understood by +comparison with an adult caecum in which the infantile condition, as seen +in Fig. 563, has become further developed. + +[Illustration: FIG. 567.--Human adult. Ileo-colic junction and caecum; +dorsal view. (Drawn from preparation in Columbia University, Study +Collection.) + +1. Dorsal vascular fold, carrying the distal appendicular branch of the +dorsal caecal artery in the mesentery of the appendix. + +2. Proximal branch of the same vessel, turning downward to caecum and +root of appendix. + +3. Intermediate non-vascular fold.] + +Fig. 567 shows the dorsal view of such a preparation. The caecum is +funnel-shaped with the apex, carrying the root of the appendix, turned +upward and to the left, the sacculation to the right of the ventral +muscular band being somewhat dilated. The appendix--7.2 cm. long--turns +sharply upward and to the left, closely applied to the left caecal +sacculation, passes dorsad to the ileo-colic junction and lies in its +terminal part under cover of the ileo-colic mesentery. The ventral +branch of the ileo-colic artery descends over the ileo-colic junction, +supplying the ventral wall of the caecum. The intermediate non-vascular +fold (3) is 3.9 cm. long and entirely free. + +The dorsal vascular fold contains the large dorsal branch of the +ileo-colic artery, dividing into two main branches. The first of these +(1) passes distally in the free edge of the fold to the terminal part of +the appendix. The other proximal branch (2) turns downward to the root +of the appendix and the adjacent wall of the caecum, aiding materially in +holding the proximal upturned segment of the appendix in contact with +the left caecal sacculation. + +The intermediate fold, short in its caecal attachment, does not meet the +dorsal vascular fold at any point, consequently the ileo-caecal fossa is +not limited caudad toward the root of the appendix. The conditions +presented by this specimen correspond exactly to those found in the +gorilla (Fig. 457) and in the human infantile preparation (Fig. 563). + +In comparing Figs. 564 and 567 it will be noticed that the line of +fusion between the intermediate fold and the dorsal vascular fold (Fig. +564, 4) corresponds to the point where the dorsal ileo-caecal artery +divides into its proximal and distal branches (Fig. 567, angle between 1 +and 2). Fig. 567 shows that the proximal arterial twig, even without +fusion with the intermediate fold, suffices to influence to a +considerable degree the curves and position of the appendix, inasmuch as +it serves to hold the proximal segment of the tube closely applied in +the erected position to the surface of the left caecal sacculation. The +intermediate segment of the appendix, between the points of accession of +the two arterial branches, is most prone to develop spiral +twists and bends, especially when the usual fusion of the two folds +takes place and still further fixes the parts, while the distal segment, +carrying the narrow crescentic terminal appendicular mesentery, remains +free. + +[Illustration: FIG. 568.--Human adult. Ileo-colic junction and caecum. +(Drawn from preparation in Columbia University, Study Collection.) + +1. Distal and + +2. Proximal branch of dorsal ileo-caecal artery running in dorsal +vascular fold. + +3. Intermediate non-vascular fold fusing with 2 and forming a narrow +caudal limit to the posterior ileo-caecal fossa.] + +Finally, in a certain number of cases, an intermediate condition between +the types presented by Figs. 564 and 567 is encountered. In Fig. 568 the +general arrangement of the parts corresponds pretty accurately to that +seen in Fig. 566, but the transition from a completely free intermediate +non-vascular fold to one which has begun to fuse with the dorsal +vascular fold is evident. The caecum is bent upward and to the left, the +caput coli being formed by the right sacculation. The appendix, 7.8 cm. +long, takes a wide {~WREATH PRODUCT~}-shaped curve. The convexity of the proximal curve +corresponds to the point where the proximal appendicular artery (2) +passes to the tube. The non-vascular intermediate fold (3), measuring +2.2 cm., fuses with the dorsal vascular fold at this point. + +The three preparations illustrate serially the share which the +peritoneal folds take in the formation of the posterior ileo-caecal +fossa. + +In Fig. 566 the failure of the intermediate fold to meet and fuse with +the dorsal vascular fold has left the caudal boundary of the fossa +(between 2 and 3) incomplete, the ventral and dorsal walls being formed +by the folds in question. Fig. 568, in which fusion between the +non-vascular and the dorsal vascular folds has commenced, shows the +shallow form of the complete fossa under these conditions, while in Fig. +567, with extensive union of the folds, the fossa has correspondingly +increased in depth. + +[Illustration: FIG. 569.--Human adult. Ileo-colic junction and caecum. +(Columbia University Museum, No. 1610.)] + +[Illustration: FIG. 570.--Human infant, four days old. Ileo-colic +junction and caecum. (Columbia University Museum, No. 879.)] + +[Illustration: FIG. 571.--Human infant. Ileo-colic junction and caecum. +(Columbia University, Study Collection.)] + +A similar series is shown in Figs. 569, 570 and 571. In Fig. 569, taken +from an adult subject, the intermediate non-vascular fold is entirely +free, the dorsal branch of the ileo-caecal artery passes to caecum and +appendix in an area of adhesion between parietal peritoneum and the +intestine which includes the dorsal vascular fold. There is consequently +no caudal boundary to the ileo-caecal fossa. Figs. 570 and 571 are both +taken from infantile preparations. + +In Fig. 570 the dorsal vascular and the intermediate folds nearly meet +at the root of the appendix. They serve to outline the fossa, which +appears completed in Fig. 571 by the actual meeting and fusion of the +folds. + +_The Ileo-caecal Folds in the Anthropoid Apes.--(1) Chimpanzee, +Troglodytes niger._ + +The structures in a juvenile specimen of this animal are shown in Figs. +460 and 461. + +The ventral vascular fold (Fig. 460, 3), containing epiploic fat, +descends over the ileo-colic junction nearly to the level of the lower +ileal margin. The intermediate non-vascular fold (Figs. 460 and 461, 2), +derived from the ileum opposite to the mesenteric border, passes to the +ventral and left aspects of the caecum and meets, near the root of the +appendix, the dorsal vascular fold (Fig. 461, 3) carrying the dorsal +caecal branch of the ileo-colic artery, which ramifies over the caecum and +supplies the appendix. + +The appendix measures 12.3 cm. and presents a terminal hook, slightly +dilated. + +The appendicular mesentery terminates within the concavity of this hook +and measures 1.5 cm. in width at the broadest part, about 4.5 cm. from +the root of the appendix. + +Figs. 462 and 463 show the caecum of the adult chimpanzee in the ventral +and dorsal view. The ventral vascular fold (Fig. 462, 1) is well +developed, heavily fringed with epiploic appendages. + +The non-vascular fold is extremely short and tense, fusing with the +short appendicular mesentery near the point where in the dorsal view +(Fig. 463) the appendix is seen bent at its origin sharply to the right. + +Fig. 464, also taken from an adult specimen of the same animal, shows a +very well-developed dorsal vascular fold, which fuses with the +intermediate fold to limit a distinct ileo-caecal recess. + +The chimpanzee, therefore, agrees closely with the human subject in the +arrangement of the folds. + +(2) _Orang, Simia satyrus._ + +In Figs. 458 and 459 the arrangement of the folds in an adult specimen +of the orang is shown. + +The ventral caecal artery (Fig. 458) is well developed, forming with the +peritoneal fold and epiploic appendages surrounding it, a sharp +sickle-shaped edge which descends over the ventral surface of the +ileo-colic junction following the curve of the left caecal margin, and +turning its concavity to the left toward the entering ileum. + +The ventral caecal artery follows the left margin of the caecum below the +ileo-caecal junction and passes for 0.5 cm. upon the portion of the pouch +which turns up behind the terminal ileum. + +The dorsal caecal artery is a vessel of large size, supplying branches to +the narrow appendicular mesentery which extends, with many epiploic +appendages, to within 9 mm. of the blunt apex of the appendix. 2.5 cm. +beyond the first bend in the appendix the fold is narrowed to a fringe +not more than 0.75 cm. wide. Up to this point the dorsal vascular fold +measures 1.5 cm. in width, and just where it narrows it is joined by the +intermediate non-vascular fold (Fig. 459), which forms a membranous +band, 3.3 cm. wide in the middle, spread out in the angle between the +lower and dorsal surfaces of the ileum and the dorsal surface of the +caecum which turns up behind the ileo-colic junction. Between this fold +and the dorsal vascular fold is seen the deep recess of the posterior +ileo-caecal fossa--which by reason of the sharp curve of the caecum looks +not only to the left but also upward and backward. + +Direct comparison of the preparations of these two anthropoid apes just +described with the conditions found in many adult human caeca shows the +close correspondence in the arrangement of these folds and of their +influence on the configuration of the parts. + +Figs. 572 and 573--taken from an adult human subject--show a caecum and +appendix which almost reproduces that of the chimpanzee illustrated in +Figs. 462 and 463 and closely resembles that of the orang. + +[Illustration: FIG. 572. Human adult. Ileo-colic junction and caecum; +ventral view. (Drawn from preparation in Columbia University, Study +Collection.) + +1. Ventral vascular fold.] + +[Illustration: FIG. 573.--Dorsal view of the same preparation. + +1. Appendix. + +2. Intermediate non-vascular fold.] + +Fig. 572, giving the ventral view, shows, by the course of the ventral +longitudinal muscular band, the turn of the caecum upwards and to the +left. The ventral caecal artery runs in a fold (1) loaded with epiploic +appendages. + +The non-vascular intermediate fold (Fig. 573, 2) passes to the root of +the appendix, joining the proximal segment of the dorsal vascular fold +in which the dorsal branch of the ileo-colic artery runs to the tip of +the appendix. The distal two thirds of the appendicular mesentery are +free. + +3. _Gibbon, Hylobates hoolock_ (Figs. 455 and 456).--In the gibbon the +folds appear well developed. The intermediate and dorsal vascular folds +are quite distinct structures, although fusion (Fig. 456) has begun at +one point, thus limiting a typical posterior ileo-caecal fossa. + +4. _Gorilla, Gorilla savagei_ (Fig. 457).--Finally in the gorilla all +three folds appear quite distinct and separate from each other, the +dorsal vascular fold being especially well developed. + +_Unusual and Aberrant Types of Ileo-caecal Folds and Fossae.--(A) Ventral +caecal artery larger than the dorsal, supplying the greater part of the +caecum and the appendix._ + +This condition is occasionally encountered. Dr. Martin, in a recent +examination of the vascular supply of caecum and appendix in one hundred +subjects, found it to obtain in six instances. + +Apparently the dorsal wall of the caecum and of the proximal segment of +the ascending colon remains free in these cases and does not become +adherent to the parietal peritoneum. The shape of the pouch, moreover, +indicates a free and unimpeded embryonal caecal descent. The normal +relative size of the two vascular folds is reversed. A good example of +this variation, in the caecum of an infant, is seen in Fig. 516. The same +arrangement in an adult specimen is seen in Fig. 574. + +[Illustration: FIG. 574.--Human adult. Caecum and ileo-colic junction; +well-developed ventral vascular fold, carrying appendicular artery. +(Columbia University Museum, No. 1613.)] + +In the Slow Lemur (_Nycticebus tardigradus_) (Fig. 420) the ventral +artery is normally the larger of the two, extending in the ventral fold +to the tip of the reduced appendix of the caecal pouch. + +(_B_) _Fusion of ventral vascular fold with the intermediate fold, +resulting in the production of a well-defined superior or +ventral ileo-caecal fossa._ + +Normally the reduced ventral artery crosses the ileo-colic junction in a +slightly developed ventral vascular fold, closely adherent to the +intestine, with a very narrow free margin. The superior or ventral +ileo-caecal fossa in these cases is very shallow and confined (Fig. 574) +to the ventral surface of the ileo-colic junction. Occasionally the fold +is better developed and fuses with the intermediate non-vascular fold, +producing a fossa of greater extent, which is bounded dorsad by the +ileum, ventrad and cephalad by the ventral fold, caudad by the fusion of +this fold with the intermediate reduplication, and to the right by the +left wall of the caecum. Figs. 576, 577, 578 and 579 show this aberrant +disposition of the structures in a series of adult human caeca. + +[Illustration: FIG. 575.--Human foetus at term. Ileo-colic junction and +caecum; ventral view. (Columbia University Museum, No. 1715.)] + +[Illustration: FIG. 576.--Human adult. Ileo-colic junction and caecum; +ventral appendicular artery and ileo-caecal fossa. (Columbia University +Museum, No. 1614.)] + +[Illustration: FIG. 577.--Human adult. Ileo-colic junction and caecum; +ventral appendicular artery and ileo-caecal fossa. (Columbia University +Museum, No. 1657.)] + +[Illustration: FIG. 578.--Human adult. Ileo-colic junction and caecum; +ventral appendicular artery and ileo-caecal fossa. (Columbia University +Museum, No. 1856.)] + +[Illustration: FIG. 579.--Human adult. Ileo-colic junction and caecum; +ventral appendicular artery and ileo-caecal fossa. (Columbia University +Museum, Study Collection.)] + +A corresponding arrangement is noted in the preparation of the caecum of +_Cercopithecus campbellii_ (Fig. 433). The large intermediate fold is +joined by the ventral vascular fold, thus defining the lower boundary of +ventral ileo-caecal fossa. + +(_C_) _Union of both vascular folds with the intermediate non-vascular +fold._ + +I have encountered one instance of this arrangement in an infant, whose +caecum and ileo-colic junction is shown in Fig. 580. Both the ventral and +dorsal arteries in this case were equally developed, and shared equally +in the supply of caecum and appendix. Both vascular folds fused with the +intermediate fold, thus producing two typical ileo-caecal fossae, one +ventral, the other dorsal. + +[Illustration: FIG. 580.--Human infant. Ileo-colic junction and caecum; +fusion of ventral and dorsal vascular folds, with intermediate fold. +(Columbia University Museum, No. 1663.)] + +(_D_) _Abnormal positions of the appendix due to variations in the +arrangement and tension of the intermediate fold._ + +Fig. 510 shows a foetal caecum in the ventral view. The ventral vascular +fold (3) is well developed. The non-vascular fold is short, arising from +the ventral surface of the ileum, instead of from the free border of the +intestine opposite to the mesenteric attachment. It fuses with the +ventral vascular fold a short distance below the ileo-colic junction, +thus limiting a small ventral ileo-caecal fossa. The dorsal caecal artery +in this specimen was large, but the fold carrying it extremely narrow. + +The preparation illustrates the type resulting from the reduction in +size and extent of the non-vascular and mesenteric folds. The +intermediate fold is reduced to a short and narrow band. Compared with +the usual infantile type the caecum lacks the characteristic turn upwards +and to the left, possibly in consequence of the slight traction caused +by the rudimentary intermediate fold. The pouch occupies a nearly +vertical pendent position, which the appendix, arising from the lowest +point of the caecal funnel, shares. The appendix is not drawn into the +retro-ileal position by the dorsal vascular fold, which is much reduced. + +In Fig. 511, representing the caecum and appendix of a foetus at term, the +effect of the tense non-vascular intermediate fold (2) is seen in the +sharp turn to the left which it imparts to the nearly transversely +directed funnel-shaped caecum. The appendix (1) is coiled spirally for +13/4 turns behind the ileo-colic junction, with the tip directed upward +behind the mesentery of the terminal ileum. The non-vascular +intermediate fold (2) extends to the rest of the appendix. It appears +short in its caecal attachment, on account of the turn of the caecum +backwards and to the left and the close connection between the adjacent +margins of the ileum and caecum. + +[Illustration: FIG. 581.--Human foetus at term. Ileo-colic junction and +caecum. (Columbia University, Study Collection.) + +1. Appendix, terminal portion turned ventrad of ileo-colic junction. + +2. Intermediate non-vascular fold.] + +[Illustration: FIG. 582.--Human infant. Ileo-colic junction and caecum; +ventral position of appendix. (Columbia University Museum, No. 693.)] + +In Fig. 581--a foetal preparation at term--the caecum is turned to the +left, below and behind the terminal ileum. The non-vascular fold (2) is +well developed as regards _length_ of _ileal_ attachment, but is very +narrow and tense, passing between ileum and the proximal curve of the +caecum behind the ileo-colic junction, where it merges with the dorsal +vascular fold. The appendix takes a sudden turn caudad at this point and +then continues up _ventrad_ to the ileo-colic junction, the proximal +portion being kept firmly in contact with the dorsal and caudal +circumference of the ileum by the tension of the non-vascular band. It +is quite evident that this peculiar turn of the appendix is directly due +to the confining influence of the non-vascular band--which passes from +its ileal attachment almost directly dorsad to the point of fusion with +the dorsal vascular fold, causing the sharp downward and forward turn of +the proximal segment of the appendix. Similar cases with ventral +position of the appendix are shown in Figs. 545 and 582. + + + + +INDEX. + + + Abdominal vein in Anure Amphibian, 158 + in Reptilia, 167 + in _Iguana_, 160 + in Urodele Amphibian, 157 + viscera of _Macacus rhesus_, 77 + + Abnormal positions of appendix, 277 + + Abomasum, 49 + + _Accipenser sturio_, biliary ducts in, 145 + pyloric appendices in, 120 + ileo-colic junction of, 212 + + Ailuroidea, ileo-colic junction of, 212 + + Alimentary canal of _Ammocoetes_, 42 + of _Amphioxus_, 42 + of _Belone_, 40 + of _Chelydra_, 58 + of Cyclostomata, 40, 42 + derivation of epithelium, 30 + of muscular and connective tissue, 30 + differentiation from body-cavity, 21, 29 + divisions of, 38 + early developmental stages, 21, 29 + mammalian embryonal stages, 40 + of _Esox_, 40 + of _Echelus conger_, 54 + of _Necturus_, 40 + of _Petromyzon_, 200 + of _Proteus_, 40 + primitive type, 40, 42 + of _Pseudemys elegans_, 55 + of _Rana_, 55 + separation from yolk-sac, 22 + tract of _Necturus maculatus_, 52 + of _Tamandua_, 56 + + Allantois in Amniota, 36 + arteries of, 63, 146 + derivation from alimentary canal, 35 + function of, 36 + relation to placenta, 36 + to primitive intestine, 24 + to urinary bladder, 24 + + _Alligator mississippiensis_, ileo-colic junction of, 201 + stomach of, 51 + + _Ammocoetes_, alimentary canal of, 42 + pancreas in, 117 + + _Ammodytes_, pyloric appendix in, 120 + + Amnion, definition of, 36 + + Amniota, development of liver in, 143 + + Amphibia, development of pancreas, 115 + folds of intestine in, 196 + ileo-colic junction of, 201 + + Amphibians, biliary ducts in, 145 + intestinal canal of, 191 + + _Amphioxus_, alimentary canal of, 42 + hepatic diverticulum of, 43 + intestinal canal of, 191 + + Anthropoid apes, ileo-caecal folds of, 274 + + Anthropoidea, ileo-colic junction of, 213 + + Anthropomorpha, ileo-colic junction of, 216 + + _Anguilla anguilla_, stomach of, 47 + + Anure Amphibian, abdominal vein of, 158 + cardiac vein in, 158 + musculo-cutaneous vein in, 158 + pelvic vein in, 158 + post-cava in, 158 + pre-cava in, 158 + venous system in, 158 + + Aorta, early condition of intestinal branches, 32 + + Aortal arterial system, development of, 63 + + Aplacentalia, definition of, 36 + + Appendix, abnormal positions of, 277 + absence of, 249 + influence of dorsal vascular fold on shape of, 271 + origin of, and shape of caecum, 245 + position and peritoneal relations of, 250 + variations of peritoneal relations, 258 + + Arctoidea, ileo-colic junction of, 212 + + _Arctopithecus marmoratus_, ileo-colic junction of, 208 + + Arctopithecini, ileo-colic junction of, 214 + + Arrest of development before intestinal rotation, 60 + + Arteries, of allantois, 64, 146 + + Artery, caudal, 64 + ileo-colica, 66 + colica dextra, 66 + media, 66 + coronary, 181 + external iliac, 64 + gastro-epiploica sinistra, 108 + hepatic, 65, 179 + ileo-colic, 262 + inferior mesenteric, 67 + internal iliac, 64 + pancreatico-duodenalis inferior, 66 + omphalo-mesenteric, 64, 146 + sacralis media, 64 + splenic, 65, 108 + superior mesenteric, 64, 65 + umbilical, 64 + vitelline, 64, 146 + + Artiodactyla, ileo-colic junction of, 209 + + _Arvicola pennsylvanicus_, ileo-colic junction and caecum of, 211 + + Asymmetrical type of ileo-colic junction, 223 + + _Ateles_, ileo-caecal folds of, 261 + _ater_, ileo-colic junction and caecum of, 214 + + Atresia ani, 24, 28 + + Axial mesoderm, connection with splanchnic and somatic mesoderm, 22 + + + _Bassaris astuta_, ileo-colic junction of, 212 + + Batrachians, stomach of, 44, 46 + + _Belone_, alimentary canal of, 40 + + Biliary ducts in _Accipenser_, 145 + in Amphibians, 145 + arrangement of, 145 + in birds, 145 + in _Buceros_, 145 + in calf, 145 + in dog, 145 + in _Galeopithecus_, 145 + in _Lophius_, 145 + in _Lutra_, 145 + in Monotremes, 145 + in _Phoca_, 145 + in Reptilia, 145 + in sheep, 145 + in _Tarsius_, 145 + in _Trigla_, 145 + in _Xiphias_, 145 + + Birds, folds of intestine in, 196 + glandular stomach of, 50 + biliary ducts in, 145 + ileo-colic junction of, 203 + muscular stomach of, 50 + venous system of, 161 + + Blastoderm, 20 + layers of, 21 + primitive, 20 + + Blastodermic vesicle, 20 + + Blastomeres, 20 + + Blastula, 20 + + Blastosphere, 20 + + Body-cavity, development of, 21 + primitive condition of, 29 + + Body-wall, 22 + + _Boselaphus tragocamelus_, ileo-colic junction and caecum of, 210 + + _Bos indicus_, ileo-colic junction and caecum of, 210 + spiral colon of, 233 + + _Bradypus marmoratus_, ileo-colic junction of, 208 + stomach of, 51 + + Brunner's glands, 194 + + _Buceros_, biliary ducts in, 145 + + Bursa epiploica in lower forms, 187 + + + Caeca of the anthropoidea, compared with the human, 247 + + Caecal gastric appendices of _Dicotyles_, 48 + + Caecum and appendix, changes in position during development, 239 + development of, 237 + morphology of, 237 + variations of, 244 + descent of, 76, 243 + of embryo, shape of, 245 + first appearance in human embryo of, 53 + function of, 219 + non-descent in adult, 75 + persistent subhepatic position in adult, 75 + in the Rodentia, 229 + shape of, and origin of appendix, 245 + types of, 245 + in the Ungulata, 229 + + Calf, biliary ducts in, 145 + + Camel, gastric water-cells, 49 + + Canal, medullary, 21, 28 + neuro-enteric, 23 + + _Canis familiaris_, ileo-colic junction and caecum of, 212 + + _Capra aegagrus_, ileo-colic junction and caecum of, 209 + + Cardiac vein in Anure Amphibian, 158 + + Cardinal veins, anterior, 147 + posterior, 147 + + Carnivora, gastric diverticula of, 48 + ileo-colic junction of, 212 + stomach of, 46, 47 + + Carnivorous birds, stomach of, 50 + + _Casuarius_, duodenum, biliary and pancreatic ducts of, 115 + intestinal villi of, 195 + + _Castor fiber_, ileo-colic junction and caecum of, 211 + stomach of, 46 + + Cat, development of pancreas, 115 + dorsal mesogastrium, spleen and pancreas, 126 + lesser peritoneal sac, 128 + spleen, pancreas and great omentum, 127 + + Caudal artery, 64 + vein in Selachian, 154 + in Urodele Amphibian, 156 + + Caudate lobe, 170 + + Cebidae, ileo-colic junction of, 214 + + _Cebus leucophaeus_, ileo-colic junction and caecum of, 216 + _monachus_, ileo-colic junction and caecum of, 216 + + Cell-body, 19 + + Cellulae coli, 199 + + _Ceratodus_, spiral intestinal valve in, 119 + + _Cercoleptes caudivolvulus_, ileo-colic junction of, 212 + + _Cercopithecus campbellii_, ileo-colic junction and caecum of, 214 + _pogonias_, ileo-colic junction and caecum of, 214 + _sabaeus_, ileo-caecal folds of, 264 + ileo-colic junction and caecum of, 214 + + _Cervicapra_, intestinal folds of, 196 + + _Cervus sika_, ileo-colic junction and caecum of, 210 + spiral colon of, 233 + + Cetacea, ileo-colic junction of, 209 + + Cetaceans, stomach of, 49 + + Changes in position during development of caecum and appendix, 239 + + Cheek pouches, 48 + of _Macacus nemestrinus_, 48 + + Cheiroptera, ileo-colic junction of, 212 + + Chelonians, liver of, 144 + stomach of, 45, 46 + + _Chelydra_, alimentary canal of, 58 + pancreas of, 117 + _serpentaria_, ileo-colic junction of, 201 + + Chick, development of liver in, 143 + development of pancreas in, 115 + + _Chlamydophorus_, ileo-colic junction of, 207 + + _Choloepus didactylus_, ileo-colic junction of, 207 + + _Chrysothrix sciureus_, ileo-colic junction and caecum of, 214 + + Cleft, uro-genital, 27 + + Cloaca, development of, 24 + division of, in higher vertebrates, 27 + in human embryos, 26 + in _Platypus anatinus_, 26 + structure of, in lower vertebrates, 25 + in _Iguana tuberculata_, 25 + + Cloacal membrane, 24 + anal segment, 28 + uro-genital segment, 28 + + Coeliac axis, 65 + + Coelom, composition and derivation of walls, 29 + development of, 21 + primitive condition of, 29 + + Colic bend of the Manidae, 234 + loop in _Phascolarctos_, 234 + + Colico-phrenic ligament, 109 + + Colon, ascending, adhesions of, 81 + position of, in foetus, 84 + and caecum of _Lagomys pusillus_, 232 + descending, adhesion of, 81 + relation of, to left kidney, 83 + position as influenced by foetal liver, 77 + spiral coil of, 233 + structural modifications of, 230 + + _Coluber natrix_, stomach of, 44 + + Common bile duct, 145 + + Comparative anatomy of hepatic venous circulation, 154 + of liver, 144 + + Comparison of human and anthropoid caeca, 247 + + Connective tissue and muscular fibers, derivation of, 30 + + Coprodaeum, 25 + + Coronary artery, 181 + ligaments of liver, 173 + + _Corvus_, caeca of, 203 + + Costo-colic ligament, 109 + + Crocodiles, stomach of, 46, 51 + + Crop, 48 + + _Cryptobranchus alleghaniensis_, ileo-colic junction of, 201 + + Cyclostomata, divisions of alimentary canal of, 40, 42 + intestinal canal of, 191 + spiral intestinal valve of, 119 + + _Cyclothurus didactylus_, ileo-colic junction and caeca of, 207 + + _Cyclura teres_, ileo-colic junction and caecum of, 202 + + _Cynocephalus anubis_, ileo-colic junction and caecum of, 214 + _babuin_, ileo-colic junction and caecum of, 214 + _porcarius_, ileo-colic junction and caecum of, 214 + _sphinx_, ileo-colic junction and caecum of, 214 + + Cynoidea, ileo-colic junction of, 212 + + Cynomorpha, ileo-colic junction of, 213 + + _Cyprini_, stomach of, 44 + + Cystic duct, 146 + development of, 142 + + Cysto-enteric duct, 145 + + + _Dasyprocta agouti_, ileo-colic junction and caecum of, 211 + spiral colon of, 234 + + _Dasypus sexcinctus_, ileo-colic junction and caeca of, 207 + + _Dasyurus viverinus_, ileo-colic junction of, 206 + + Descent of caecum, 243 + + Derivatives of entodermal intestinal tube, 34 + + Deuteroplasm, 19 + + Development of caecum and appendix, 237 + of cystic duct, 142 + of gall-bladder, 142 + of liver, 141 + in amniota, 143 + in chick, 143 + in Elasmobranchs, 143 + in Teleosts, 143 + of portal circulation, 147 + of spiral colon, 233 + of transverse colon, 244 + of vascular system of liver, 145 + + _Dicotyles_, caecal gastric appendices of, 48 + _torquatus_, ileo-colic junction and caecum of, 209 + + _Didelphis_, ileo-caecal folds of, 263 + _virginiana_, ileo-colic junction and caecum of, 205 + + Digitiform gland of Selachians, 201 + + Dipnoeans, intestinal canal of, 191 + spiral intestinal valve in, 119 + + Diverticulum caecum vitelli in birds, 35 + in _Urinator imber_, 35 + _lumme_, 35 + vateri, 114 + + Dog, biliary ducts in, 145 + + Dorsal mesentery, early condition and derivation, 32 + in lower vertebrates, 32 + smooth muscular fiber of, 33 + mesogastrium, area of adhesion to parietal peritoneum, 106 + developmental changes in direction and extent, 103 + definition of, 100, 101 + gastro-splenic segment, 108 + redundant omental growth, 105 + spleen and pancreas in cat, 126 + vertebro-splenic segment, 108 + vascular ileo-caecal fold, 262 + fold, influence on shape of appendix, 271 + + Double caecal pouches of birds, 203 + + Ducts of Cuvier, 147 + in Selachian, 155 + in Urodele amphibian, 156 + omphalo-mesenteric, 22 + of Santorini, 111 + vitello-intestinal, 22 + of Wirsung, 111 + development of, 112 + + Ductus venosus, 149 + changes after birth in, 152 + + Duodenal antrum, 194 + fold of cat, 92 + of _Hapale vulgaris_, 93 + inferior, 95 + of _Nasua rufa_, 92 + superior, 95 + fossae, 92 + superior, 94 + vascular relations, 95, 96 + loop, 54 + + Duodeno-colic neck, 57 + + Duodeno-jejunal fossa in the cat, 93 + + Duodenum, adhesion of, 67 + development of, 53 + peritoneal relations of infra-colic segment, 81 + of supra-colic segment, 81 + suspensory muscle of, 33 + with biliary and pancreatic ducts, of _Casuarius_, 115 + + + _Echidna hystrix_, ileo-colic junction and caecum of, 204 + + _Echelus conger_, alimentary canal of, 54 + ileo-colic junction of, 200 + intestinal mucosa of, 197 + endgut of, 199 + pyloric appendix of, 120 + + Ectoderm, 21 + + Edentata, ileo-colic junction of, 206 + types of ileo-colic junction and caecum in, 218 + + Egg, development of, 20 + structure of, 19 + + Elasmobranchs, development of liver in, 143 + + _Elephas indicus_, ileo-colic junction and caecum of, 210 + + Embryonal intestinal hernia, 52 + + Embryonic shield, 20 + + Embryo, separation of, 20 + + Endgut of _Echelus_, 199 + extent and contained segments, 38 + function of, 198 + in lower vertebrates, 199 + + Enteric canal, primitive condition of, 29 + + Entoderm, 21 + derivatives of, 28 + + Entodermal intestinal tube, derivatives of, 34 + + Epiblast, 21 + + Epiploic bursa, 107 + early stages of, 104 + + Epithelium of alimentary canal, derivation of, 30 + + _Erethizon dorsatus_, ileo-colic junction and caecum of, 211 + + _Esox_, alimentary canal of, 40 + + _Eunectes marinus_, ileo-colic junction and caecum of, 203 + + External iliac artery, 63 + perineal folds, 28 + + + Foetus at term, venous system of, 162 + + Falciform ligament, as part of ventral mesogastrium, 165 + + _Felis_, ileo-colic junction and caecum of, 212 + _leo_, ileo-colic junction and caecum of, 212 + + Fish, development of pancreas, 115 + folds of intestine, 196 + ileo-colic junction of, 200 + + Fissipedia, ileo-colic junction of, 212 + + Fissure, transverse anal, 27 + + Folds, ileo-caecal, 260 + + Follicles, solitary, 196 + + Foramen of Winslow, 174 + boundaries in adult human subject, 184 + caudal boundary, 178 + in lower mammals, 183 + relation to duodenal adhesion, 184 + in _Tamandua bivittata_, 183 + + Foregut, comparative anatomy of, 42 + divisions of, 191 + extent and contained segments, 38 + + Formative yolk, 19 + + Fossa duodeno-jejunalis, 96 + ileo-caecal, 260 + intersigmoidea, 97 + of Treitz, 92, 96 + + Function of caecum, 219 + of pyloric appendices, 221 + of pyloric caeca, 221 + of spiral fold of intestinal mucous membrane, 220 + + Furrow, primitive intestinal, 22 + + + _Gadus callarias_, ileo-colic junction of, 201 + pyloric appendices in, 120 + + _Galeopithecus_, biliary ducts in, 145 + ileo-colic junction and caecum of, 213 + + Gall-bladder, development of, 142 + occurrence of, 144 + + Gastric diverticula of Carnivora, 48 + of Herbivora, 48 + of Omnivora, 48 + + Gastro-hepatic omentum, as part of ventral mesogastrium, 165 + + Gastro-splenic omentum, 109 + + Genito-urinary sinus, 27 + tract, male, in _Platypus anatinus_, 26 + + Germinal area, 20 + membrane, 20 + spot, 19 + vesicle, 19 + + Glands of Lieberkuehn, 194 + + Glandular stomach of birds, 50 + + _Gobius_, stomach of, 45 + + _Gorilla savagei_, ileo-colic junction and caecum of, 216 + + Graafian follicle, 19 + + Greater curvature, first appearance of, 41 + + Groove, medullary, 21 + primitive intestinal, 22 + + + _Halicore_, ileo-colic junction of, 208 + + _Halmaturus derbyanus_, ileo-caecal folds of, 263 + ileo-colic junction and caecum of, 205 + stomach of, 47 + + _Hapale jacchus_, ileo-colic junction and caecum of, 214 + _vulgaris_, duodenal fold, 93 + + _Heloderma suspectum_, ileo-colic junction of, 211 + + Hepatic antrum of lesser sac, 170 + artery, 65 + development of, 179 + in relation to foramen of Winslow, 180 + relation to duodenal adhesion, 182 + relation to primitive dorsal mesentery, 182 + cylinders, 143 + ducts, 145 + flexure, formation of, 76 + recess of lesser sac, 177 + ridge, 142 + veins, 148 + venous circulation, comparative anatomy of, 154 + direction of current, 152 + summary of development, 153 + + Hepatic-portal system in Selachian, 155 + in Urodele Amphibian, 157 + vein in _Iguana_, 160 + + Hepato-cystic duct, 145 + + Hepato-enteric duct, 145 + + Herbivora, gastric diverticula of, 48 + stomach of, 46, 47 + + Herbivorous birds, stomach of, 50 + + Herons, caecum of, 204 + stomach of, 50 + + _Hippopotamus_, ileo-colic junction of, 209 + + Human caeca compared with those of the Anthropoidea, 247 + + _Herpestes griseus_, ileo-colic junction and caecum of, 212 + _ichneumon_, ileo-colic junction and caecum of, 212 + + _Hyaena striata_, ileo-colic junction and caecum of, 212 + + _Hylobates hoolock_, ileo-colic junction and caecum of, 216 + + Hypoblast, 21 + + Hyracoidea, ileo-colic junction of, 210 + + _Hyrax capensis_, ileo-colon, ileo-caecum and colic caeca of, 210 + large intestine and caeca of, 234 + + + Iguana, abdominal vein of, 160 + caecal pouch and valves of, 202 + hepatic-portal vein of, 160 + post-cava of, 159 + renal-portal system of, 159 + sciatic vein of, 160 + segmental veins of, 161 + _tuberculata_, ileo-colic junction and caecum of, 201 + cloaca in, 25 + ventral mesogastrium of, 166 + + Ileo-caecal folds, aberrant types of, 276 + of the anthropoid apes, 274 + of _Ateles_, 261 + of _Cercopithecus sabaeus_, 264 + of _Didelphis_, 263 + and fossae, 260 + of _Halmaturus derbyanus_, 263 + of _Mycetes fuscus_, 264 + smooth muscular fibers of, 33 + fossa, anterior, 267 + posterior, 271 + fossae, aberrant types of, 276 + + Ileo-colic artery, 262 + junction of _Accipenser sturio_, 201 + of _Alligator mississippiensis_, 201 + of Amphibia, 201 + of the Arctoidea, 212 + of the Ailuroidea, 212 + of _Arvicola pennsylvanicus_, 211 + of the Anthropoidea, 213 + of the Anthropomorpha, 216 + of the Artiodactyla, 209 + of the Arctopithecini, 214 + of _Arctopithecus marmoratus_, 208 + of _Ateles ater_, 214 + of _Bassaris astuta_, 212 + in birds, 203 + of _Boselaphus tragocamelus_, 210 + of _Bos indicus_, 210 + in the Carnivora, 212 + of _Canis familiaris_, 212 + of _Capra aegagrus_, 209 + in cases of arrested intestinal rotation, 241 + of _Castor fiber_, 211 + of the Cebidae, 214 + of _Cebus_, 215 + _leucophaeus_, 216 + _monachus_, 216 + of _Cercoleptes caudivolvulus_, 212 + of _Cercopithecus campbellii_, 214 + _pogonias_, 214 + _sabaeus_, 214 + of _Cervus sika_, 210 + of the Cetacea, 209 + of _Chlamydophorus_, 207 + of Cheiroptera, 212 + of _Chelydra serpentaria_, 201 + of _Choloepus didactylus_, 207 + of _Chrysothrix sciureus_, 214 + of _Corvus_, 203 + of _Cryptobranchus alleghaniensis_, 201 + of _Cyclothurus didactylus_, 207 + of _Cyclura teres_, 202 + of _Cynocephalus_, 213 + _anubis_, 214 + _babuin_, 214 + _porcarius_, 214 + _sphinx_, 214 + of the Cynoidea, 212 + of the Cynomorpha, 213 + of _Dasyprocta agouti_, 211 + of _Dasypus sexcinctus_, 207 + of _Dasyurus viverinus_, 206 + of _Dicotyles torquatus_, 209 + of _Didelphis virginiana_, 205 + of _Echelus conger_, 200 + of _Echidna hystrix_, 204 + of the Edentata, 206 + effect of rotation on position of, 59 + of _Elephas indicus_, 210 + of _Erethizon dorsatus_, 211 + of _Eunectes marinus_, 203 + of _Felis_, 212 + leo, 212 + in fish, 200 + of the Fissipedia, 212 + of _Gadus callarias_, 201 + of _Galeopithecus_, 213 + of _Gorilla savagei_, 216 + of _Halicore_, 208 + of _Halmaturus derbyanus_, 205 + of _Hapale jacchus_, 214 + of _Heloderma suspectum_, 204 + of the herons, 204 + of _Herpestes ichneumon_, 212 + of _Hippopotamus_, 209 + of _Hyaena striata_, 212 + of _Hylobates hoolock_, 216 + of _Hyracoidea_, 210 + of _Hyrax capensis_, 210 + of _Iguana tuberculata_, 202 + of the Insectivora, 213 + of _Lagothrix humboldtii_, 215 + of Lamellirostra, 203 + of _Lemur macaco_, 213 + _mongoz_, 213 + of the Lemuroidea, 213 + of _Lepus cuniculus_, 211 + of _Macacus_, 214 + _cynomolgus_, 214 + _ochreatus_, 214 + _pileatus_, 214 + _rhesus_, 214 + of mammalia, 204 + of _Manatus americanus_, 208 + of _Manis longicauda_, 208 + of Marsupialia, 204 + of Monotremata, 204 + of _Midas geoffrei_, 214 + _ursulus_, 214 + of _Monodon_, 209 + of _Mustela_, 212 + of _Mycetes cabaya_, 214 + _fuscus_, 215 + of _Myoxus_, 211, 212 + of _Myrmecophaga jubata_, 207 + of _Nasua rufa_, 212 + of _Necturus maculatus_, 201 + non-vascular serous folds, 262 + of _Nycticebus tardigradus_, 213 + of _Nyctipithecus commersonii_, 214 + of _Ornithorhynchus anatinus_, 204 + of _Orycteropus_, 208 + of _Oryx leucoryx_, 210 + of _Otolicnus crassicaudatus_, 273 + of _Paradoxurus typus_, 212 + of _Perameles nasuta_, 206 + of the Perissodactyla, 210 + of _Phascolarctos cinereus_, 205 + of _Phascolomys wombat_, 206 + of _Phocaena_, 209 + of _Phoca vitulina_, 212 + of _Physeter_, 209 + of the Pinnipedia, 212 + of the piscivorous divers, 203 + of _Pithecia satanas_, 215 + of _Pleuronectes maculatus_, 201 + of the Primates, 213 + of the Proboscidea, 210 + of _Proteles lalandii_, 212 + of _Pseudemys elegans_, 201 + of _Pteropus medius_, 212 + of _Rana catesbiana_, 201 + of the Ratitae, 203 + of Reptilia, 201 + of the Rodentia, 211 + serial review in Vertebrata, 200 + of the Sirenia, 208 + of _Simia satyrus_, 216 + of _Strix_, 203 + of _Struthio africanus_, 204 + of _Sus scrofa_, 209 + asymmetrical type, 223 + symmetrical type, 221 + of _Tamandua bivittata_, 208 + of _Tapirus americanus_, 210 + of _Tarsius spectrum_, 213 + of _Tatusia novemcincta_, 207 + of _Taxidea americana_, 212 + of _Tolypeutes_, 207 + of _Trichosurus vulpinus_, 205 + of _Troglodytes niger_, 217 + types of, and caecum, 217 + in Edentata, 218 + in Marsupialia, 218 + of _Ursus_, 212 + of the Ungulata, 209 + of _Vulpes fulvus_, 212 + vascular mesenteric folds of, 262 + of _Xenurus_, 207 + of _Zalophus gillespiei_, 212 + + Iliac vein in Urodele Amphibian, 157 + + Inferior mesenteric artery, 67 + + Infra-colic compartment, secondary parietal peritoneum of, 85, 86 + + Insectivora, ileo-colic junction of, 213 + + Intermediate duodenal fold, 96 + non-vascular ileo-caecal fold, 262 + + Internal iliac artery, 63 + perineal folds, 27 + + Intestinal blood vessels, effect of intestinal rotation on, 59 + canal of _Amphioxus_, 191 + of Amphibians, 191 + of Cyclostomata, 191 + diverticula, 193 + of Dipnoeans, 191 + of Teleosts, 191 + folds of mucosa, 193 + non-differentiated, of lower vertebrates, 191 + folds in Amphibia, 196 + in birds, 195 + in fish, 196 + furrow, primitive, 22 + glandular apparatus in lower vertebrates, 195 + groove, primitive, 22 + juice, function of, 194 + mucous membrane of _Cervicapra_, 196 + of _Echelus conger_, 197 + _Lophius_, 197 + lymphoid tissue, 196 + of _Phocaena_, 196 + of _Thalassochelys_, 197 + rotation, 58 + arrest of development, 60 + demonstration in cat, 67 + spiral fold, function of, 193 + vascular supply, 63 + in cases of non-rotation, 67 + villi of Carnivora, 195 + of _Casuarius_, 195 + of Ophidia, 195 + of _Ursus maritimus_, 195 + + Intestine in early human embryo, 52 + general consideration of, 51 + large, and caeca of _Hyrax_, 234 + functions of, 198 + length of, 199 + of monkeys, 199 + of rodents, 199 + width of, 199 + small, 192 + absorbing apparatus, 195 + divisions of, 194 + length of, 192 + secretory apparatus, 194 + structure of, 194 + villi, 195 + + Isthmus, duodeno-colic, 57 + + + Jejuno-ileum, development of, 54 + + + _Labrus_, stomach of, 44 + + _Lagomys pusillus_, colon and caecum of, 232 + + _Lagothrix humboldtii_, ileo-colic junction and caecum of, 215 + + Lamellirostra, ileo-colic junction and caeca of, 203 + + Lateral vein in Selachian, 155 + + _Lemur macaco_, ileo-colic junction and caecum of, 213 + _mongoz_, ileo-colic junction and caecum of, 213 + + Lemuroidea, ileo-colic junction of, 213 + + _Lepus cuniculus_, ileo-colic junction and caecum of, 211 + saccus lymphaticus of, 211 + + Lesser curvature, first appearance of, 41 + + Ligament, colico-lienale, 109 + of ductus venosus, 152 + gastro-lienale, 110 + lieno-renale, 109 + phrenico-lienale, 109 + + Ligamenta coli, 199 + + Liver in Chelonians, 144 + comparative anatomy of, 144 + derivation of, 34 + development of, 141 + function of, 195 + lobation of, 144 + in Ophidia, 144 + peritoneal lines of reflection in embryo, 167 + in foetus at term, 171 + peritoneal relations of, 167 + of _Petromyzon_, 141 + unilobar type, 144 + + _Lophius_, biliary ducts in, 145 + _piscatorius_, mucosa of midgut, 197 + pyloric appendices in, 120 + stomach of, 46 + + Lungs, derivation of, 34 + + _Lutra_, biliary ducts in, 145 + stomach of, 48 + + Lymphoid tissue of intestinal mucosa, 196 + + + _Macacus cynomolgus_, ileo-colic junction and caecum of, 214 + descending mesocolon of, 140 + lesser omentum of, 176 + mesosigmoidea in, 140 + _nemestrinus_, cheek-pouches of, 48 + _ochreatus_, ileo-colic junction and caecum of, 214 + peritoneum of infra-colic compartment, 136 + _pileatus_, ileo-colic junction and caecum of, 214 + relations of spleen and great omentum, 139 + _rhesus_, abdominal viscera, 77 + ileo-colic junction and caecum of, 214 + + Mammalia, ileo-colic junction of, 204 + pancreatic ducts in, 117 + + _Manatus americanus_, ileo-colic junction and bifid caecum of, 208 + stomach of, 48 + + _Manis longicauda_, ileo-colic junction of, 208 + + Manidae, colic bend of, 234 + + Marsupalia, ileo-colic junction of, 204 + types of ileo-colic junction and caecum in, 218 + + Meckel's diverticulum, 35 + serous folds connected with, 262, 263 + + Medullary canal, 21 + groove, 21 + plates, 21 + + Membrane, cloacal, 24 + + Mesenchyma, 30 + + Mesenteric peritoneum, definition of, 32 + + Mesentery, absorption of, 33 + definition of, 101 + jejuno-ileal, line of attachment, 86 + of umbilical loop, development of, 56 + relation to adult mesocolon and mesentery, 72 + + Mesoblast, 21 + + Mesocola in cat, 86 + + Mesocolic fossa, 97 + + Mesocolon, ascending, adhesion of, 82 + in monkeys, 83 + relation of, to right kidney, 83 + definition of, 101 + descending, adhesion of, 82 + in foetus, 83 + line of attachment of, 83 + in lower mammals, 83 + in _Macacus_, 140 + in monkeys, 83 + transverse, root of, 85 + + Mesoderm, 21 + derivatives of, 28 + + Mesoduodenum, adhesion of, 67 + definition of, 101 + + Mesorectum, definition of, 101 + + Mesosigmoidea, definition of, 101 + in _Macacus_, 140 + + Mesothelium, 21 + + Metanephros, 24 + + _Midas geoffrei_, ileo-colic junction and caecum of, 214 + _ursulus_, ileo-colic junction and caecum of, 214 + + Midgut, 192 + extent of, 38 + + _Monodon_, ileo-colic junction of, 209 + + Monotremata, ileo-colic junction of, 204 + + Monotreme, structure of penis, 26 + + Monotremes, biliary ducts in, 145 + + Morphology, general, of vertebrate intestine, 190 + of human caecum and appendix, 237 + + Morula, 20 + + _Moschus_, stomach of, 49 + + Muscular stomach of birds, 50 + + Musculo-cutaneous vein in Anure Amphibian, 158 + + _Mustela_, ileo-colic junction of, 212 + + _Mycetes cobaya_, ileo-colic junction and caecum of, 214 + _fuscus_, ileo-caecal folds of, 264 + ileo-colic junction and caecum of, 215 + + _Myoxus_, alimentary canal of, 211, 212 + stomach of, 46 + + _Myrmecophaga jubata_, ileo-colic junction of, 207 + + _Myxinoids_, pancreas in, 117 + + + _Nasua rufa_, duodenal fold of, 92 + ileo-colic junction of, 212 + pancreatico-gastric folds of, 181 + + _Necturus_, alimentary canal of, 40 + _maculatus_, alimentary tract of, 52 + ileo-colic junction of, 201 + stomach of, 43 + venous system of, 158 + + Neuro-enteric canal, 23 + + Non-vascular ileo-caecal folds, 262 + + Nucleolus, 19 + + Nucleus, 19 + + Nutritive yolk, 19 + + _Nycticebus tardigradus_, ileo-colic junction and caecum of, 213 + spiral colon of, 234 + + _Nyctipithecus commersonii_, ileo-colic junction and caecum of, 214 + + + OEsophageal gutter of ruminants, 49 + + OEsophageo-gastric junction, 45 + + Omega loop, development of, 77 + + Omental bursa, 107 + early stages of, 104 + + Omentum, great, 107 + layers of, 107 + peritoneal adhesions in adult, 131 + relation of, to transverse colon, transverse colon and duodenum, + 129 + lesser, as part of ventral mesogastrium, 165 + divisions of, 172 + in _Macacus_, 176 + + Omnivora, gastric diverticula of, 48 + + Omphalo-mesenteric arteries, 64, 146 + artery, persistence of rudiments of, 65 + duct, 22 + veins, 146 + + Ophidia, intestinal villi of, 195 + liver in, 144 + stomach of, 44, 46 + + Oral pouches, 48 + + _Ornithorhynchus anatinus_, ileo-colic junction and caecum of, 204 + + _Orycteropus_, ileo-colic junction and caecum of, 208 + + _Oryx leucoryx_, ileo-colic junction and caecum of, 210 + spiral colon of, 233 + + _Otolicnus crassicaudatus_, ileo-colic junction and caecum of, 213 + + _Ovis aries_, spiral colon of, 233 + + Ovum, structure of, 19 + + Owl, stomach of, 50 + + + Pancreas, adhesion of, 67 + adhesion of mesoduodenal segment, 123 + in _Ammocoetes_, 117 + in Chelydra, 117 + comparative anatomy of, 116 + concealed, of Teleosts, 117 + derivation of, 34 + development of, in Amphibia, 115 + of, in cat, 115 + of, in chick, 115 + of, in fish, 115 + of, in lower vertebrates, 115 + of, in man, 111, 115 + of, in sheep, 115 + in foetal pig, 123 + function of, 195 + in _Myxinoids_, 117 + peritoneal relations, 122 + vascular and visceral relations of adult, 125 + in _Protopterus_, 117 + relation to dorsal mesogastrium, 123 + to mesoduodenum, 122 + to omental bursa, 123 + in Selachians, 116 + + Pancreatic ducts in Mammalia, 117 + normal arrangement, 113 + secondary, 111 + variations, 114 + + Pancreatico-gastric folds in man, 187 + in _Nasua rufa_, 181 + + Papilla Vateri, 114 + + _Paradoxurus typus_, ileo-colic junction of, 212 + + _Paralichthys_, pyloric appendices in, 120 + + Parietal mesoderm, 21 + peritoneum, definition of, 32 + + _Pelamys_, pyloric appendices in, 120 + + Pelvic vein in Anure Amphibian, 158 + + _Perameles nasuta_, ileo-colic junction and caecum of, 206 + + _Perca_, pyloric appendices in, 120 + + Perennibranchiates, stomach of, 44, 46 + + Perissodactyla, ileo-colic junction of, 210 + + Peritoneal cavity, lesser, summary and development of structure, 180 + relations and position of appendix, 250 + of appendix, variations of, 258 + sac, lesser, of cat, 128 + + Peritoneum, arrangement in infra-colic compartment, 78 + of cat, compared with human arrangement, 73 + of infra-colic compartment in adult human subject, 88 + lesser cavity of, in relation to liver, 174 + of liver, in relation to lesser sac, 174 + secondary lines of reflection, 73 + of supra-colic compartment, general considerations, 99 + + _Petromyzon_, alimentary canal of, 200 + liver of, 141 + spiral intestinal valve of, 119 + + Peyer's patches, 196 + + _Phascolarctos cinereus_, ileo-colic junction and caecum of, 205 + colic loop in, 234 + + _Phascolomys wombat_, ileo-colic junction, caecum and appendix of, 206 + + _Pteropus medius_, ileo-colic junction of, 212 + + _Phoca_, biliary ducts in, 145 + _vitulina_, ileo-colic junction and caecum of, 212 + stomach of, 45 + + _Phocaena communis_, ileo-colic junction of, 209 + intestinal folds, 196 + + Phylogeny of types of ileo-colic junction and caecum, 217 + + _Physeter_, ileo-colic junction of, 209 + + Physiology of vertebrate intestine, 190 + + Pickerel, pyloric valve of, 45 + stomach of, 44 + + Pinnipedia, ileo-colic junction of, 212 + + _Pipa_, stomach of, 46 + + Piscivorous divers, caeca of, 203 + + _Pithecia satanas_, ileo-colic junction and caecum of, 215 + + Placental circulation, 146 + + Placentalia, definition of, 36 + + Plates, medullary, 21, 28 + + _Platypus anatinus_, male genito-urinary tract and cloaca, 26 + + _Pleuronectes maculatus_, ileo-colic junction of, 201 + pyloric appendices in, 120 + + Pleuro-peritoneal cavity, 28 + + Plicae coli, 199 + + _Polypterus_, pyloric appendix in, 120 + + Portal circulation, development of, 147 + vein, development of, 148 + + Position and peritoneal relations of appendix, 250 + + Post-anal gut, 23 + + Post-cardinal veins in Urodele Amphibian, 157 + + Post-cava in Anure Amphibian, 158 + in _Iguana_, 159 + in Urodele Amphibian, 157 + + Post-caval vein, 151 + + Pre-cava in Anure Amphibian, 158 + + Primates, ileo-colic junction of, 213 + types of ileo-caecal folds in, 265 + + Primitive aortae, 63 + common dorsal mesentery, 33 + dorsal mesentery after rotation, 79 + mesentery, effect of intestinal rotation on, 59 + mesenteric segment, 72 + mesocolic segment, 72 + jugular veins, 147 + + Proboscidea, ileo-colic junction of, 210 + + Proctodaeum, 24, 26 + in human embryos, 27 + + Protoplasm, 19 + + _Protopterus_, pancreas in, 117 + + _Proteus_, alimentary canal of, 40 + _anguineus_, stomach of, 43 + + _Proteles lalandii_, ileo-colic junction and caecum of, 212 + + Proventriculus, 50 + + Psalterium, 49 + + _Pseudemys elegans_, alimentary canal of, 55 + ileo-colic junction and caecum of, 201 + + Pyloric appendices, 119 + function of, 221 + in _Accipenser_, 120 + in _Gadus_, 120 + in _Lophius_, 120 + in _Paralichthys_, 120 + in _Pelamys_, 120 + in _Perca_, 120 + in _Pleuronectes_, 120 + in _Rhombus_, 120 + in _Scomber_, 120 + in _Thynnus_, 120 + relation to pancreas, 121 + significance of, 120 + appendix in _Ammodytes_, 120 + in _Echelus_, 120 + in _Polypterus_, 120 + caeca, 119 + function of, 221 + stomach, 50 + valve, 44 + in fishes, 45 + of loon, 45 + + + _Rana_, alimentary canal of, 55 + _catesbiana_, ileo-colic junction of, 201 + _esculenta_, venous system of, 158 + + Ratitae, ileo-colic junction and caeca of, 203 + + Rectal gland of Selachians, 201 + + Rectangular ileo-colic junction, 225 + + Recto-coccygeal muscles, 33 + + Recto-uterine muscles, 33 + + Rectum, development of, 54 + separation from genito-urinary sinus, 27 + + Renal-portal circulation in Urodele Amphibian, 156 + system in Selachian, 154 + in _Iguana_, 159 + + Reptilia, abdominal vein, 167 + biliary ducts in, 145 + ileo-colic junction of, 201 + + Retro-gastric peritoneal space, boundaries of, 175 + space, rudimentary form of, 105 + + Retro-peritoneal hernia, 92 + + Reticulum, 49 + + _Rhombus_, pyloric appendices in, 120 + + Rodentia, caecal pouch of, 229 + compound stomach of, 49 + ileo-colic junction of, 211 + spiral colic valve of, 231 + + Rodents, saccus lymphaticus of, 196 + + Round ligament of liver, 152 + + Rumen, 49 + + Ruminantia, structure of stomach in, 49 + + + Saccus lymphaticus of _Lepus cuniculus_, 211 + of Rodents, 196 + + _Salamandra maculosa_, venous system of, 158 + + Salivary glands, derivation of, 34 + + Saurians, stomach of, 44, 46 + + Sciatic vein in _Iguana_, 160 + + _Scincus ocellatus_, stomach of, 45 + + _Scomber_, pyloric appendices in, 120 + + Segmental veins in _Iguana_, 160 + + Segmentation, 20 + + Segmentation-cavity, 20 + + Selachian, caudal vein in, 154 + digitiform gland of, 201 + duct of Cuvier, 155 + hepatic portal system of, 155 + lateral vein of, 155 + pancreas in, 116 + rectal gland of, 201 + renal portal system of, 154 + spiral intestinal valve in, 119 + venous system, 154 + + _Semnopithecus_, stomach of, 47 + + Septum urogenitale, 27 + transversum, 142 + + Serous folds in cases of Meckel's diverticulum, 262, 263 + membrane, derivation of, 31 + + Shape of caecum and origin of appendix, 245 + of embryonic caecum, 245 + + Sheep, biliary ducts in, 145 + development of pancreas, 115 + + Sigmoid flexure, development of, 54, 77 + + _Simia satyrus_, ileo-colic junction and caecum of, 216 + + Sinus venosus, 146 + + Sirenia, ileo-colic junction of, 208 + + Soft palate, 42 + + Somatic mesoderm, 21 + + Somatopleure, 22, 29 + + Spigelian lobe, boundaries of, 170 + development of, 169 + recess of lesser sac, 177 + + Spiral coil of colon, 233 + colic valve of Rodentia, 231 + colon of _Bos indicus_, 233 + of _Cervus sika_, 233 + of _Dasyprocta agouti_, 234 + development of, 233 + of _Nycticebus tardigradus_, 234 + of _Oryx leucoryx_, 233 + of _Ovis aries_, 233 + fold of intestinal mucous membrane, function of, 220 + intestinal valve in _Ceratodus_, 119 + in Cyclostomata, 119 + in Dipnoeans, 119 + in _Petromyzon_, 119 + valve of gastric diverticulum in _Sus_, 48 + + Splanchnic mesoderm, 21 + + Splanchnopleure, 22, 29 + + Spleen, development and relation to dorsal mesogastrium, 108 + and great omentum in _Macacus_, 139 + pancreas and great omentum in cat, 127 + peritoneal relations, 110 + vascular connections, 108 + + Splenic artery, 65, 108 + flexure, development of, 54, 76 + vessels, peritoneal relations, 109 + + Stomach of _Alligator_, 51 + assumption of special functions modifying form of, 48 + of _Anguilla anguilla_, 47 + of Batrachians, 44, 46 + of _Bradypus_, 51 + caecal diverticula of, 47 + of Carnivora, 46, 47 + of carnivore birds, 50 + of _Castor_, 46 + cellular structures connected with, 47 + of Cetaceans, 49 + changes in position during development, 102 + of Chelonians, 45, 46 + of _Coluber natrix_, 44 + comparative anatomy of, 42 + of Crocodiles, 46, 51 + of the _Cyprini_, 44 + definition of, as segment of foregut, 43 + embryonic borders and surfaces, 41 + factors modifying form of, 43 + first differentiation in human embryos, 40 + further development in human embryos, 40 + glandular, of birds, 46 + of _Gobius_, 45 + of _Halmaturus_, 47 + of heron, 50 + of Herbivora, 46, 47 + of herbivorous birds, 50 + influence of habitual amount of food on form of, 44 + of size and shape of abdominal cavity on form of, 46 + of volume and character of food on form of, 46 + of Teleosts, 46, 47 + of _Labrus_, 44 + of _Lophius_, 46 + of _Lutra_, 48 + of _Manatus americanus_, 48 + of _Moschus_, 49 + masticating surfaces of, 48 + of _Myoxus_, 46 + of _Necturus maculatus_, 43 + of Ophidia, 44, 46 + of owl, 50 + of Perennibranchiates, 44, 46 + of _Phoca vitulina_, 45 + of the pickerels, 44 + of _Pipa_, 46 + of _Proteus anguineus_, 43 + relation to vagus nerve, 43 + ruminant type of, 43 + of Saurians, 44, 46 + of _Scincus ocellatus_, 45 + of _Semnopithecus_, 47 + storage compartments of, 48 + structural modifications of, increasing action of gastric juice, 46 + of _Tamandua_, 51 + transverse position of, 45 + type-form of, 43 + + Stomadaeum, 24 + in human embryos, 27 + + _Strix_, caeca of, 203 + + Structural modifications of colon, 230 + + _Struthio africanus_, ileo-colic junction and caeca of, 204 + + Sturgeon, pyloric valve of, 45 + + Subintestinal veins, 147 + + Submucosa, derivation of, 30 + + Superior mesenteric artery, 64, 65 + relation to umbilical loop, 66 + + Suspensory ligament of liver, derived from ventral mesogastrium, 165 + + _Sus scrofa_, ileo-colic junction and caecum of, 209 + spiral valve of gastric diverticulum in, 48 + + Symmetrical type of ileo-colic junction, 221 + + + Taenia coli-, 199 + + _Tamandua_, alimentary tract of, 56 + _bivittata_, foramen of Winslow in, 183 + ileo-colic junction and caeca of, 208 + stomach of, 51 + + _Tapirus americanus_, ileo-colic junction and caecum of, 210 + + _Tarsius_, biliary ducts in, 145 + _spectrum_, ileo-colic junction and caecum of, 213 + + _Taxidea americana_, ileo-colic junction of, 212 + + _Tatusia novemcincta_, ileo-colic junction of, 207 + + Teleosts, anal and genito-urinary orifices in, 25 + concealed pancreas of, 117 + development of liver in, 143 + gastric diverticula of, 47 + intestinal canal of, 191 + stomach of, 46, 47 + without pyloric appendices, 120 + + _Thalassochelys_, intestinal folds of, 197 + + Thymus, derivation of, 34 + + _Thynnus_, pyloric appendices in, 120 + + Thyroid, derivation of, 34 + + _Tolypeutes_, ileo-colic junction of, 207 + + Transverse anal fissure, 27 + colon, development of, 54, 244 + differentiation of, 76 + mesocolon, development of, 80 + + _Trichosurus vulpinus_, ileo-colic junction and caecum of, 205 + + _Trigla_, biliary ducts in, 145 + + _Troglodytes niger_, ileo-colic junction and caecum of, 217 + + Types of ileo-caecal folds in Primates, 265 + ileo-colic junction and caecum, phylogeny of, 217 + + + Umbilical arteries, 63 + hernia of embryo, 52 + loop, derivation of adult intestinal segments from, 53 + divisions of, 52 + of embryonic intestine, 52 + relation of vitello-intestinal duct to, 52 + veins, 147 + changes after birth in, 152 + final arrangement, 151 + further changes in, 149 + intra-hepatic distribution, 152 + vesicle, 22 + + Umbilicus, 21 + + Ungulata, caecal pouch of, 229 + ileo-colic junction of, 209 + + Urinary bladder, in human embryos, 27 + relation to allantois, 24 + + _Urinator imber_, diverticulum caecum vitelli, 35 + _lumme_, diverticulum caecum vitelli, 35 + pyloric valve of, 45 + + Urodaeum, 25 + + Urodele Amphibian, abdominal vein in, 157 + caudal vein in, 156 + ducts of Cuvier in, 156 + hepatic-portal system of, 157 + iliac vein in, 157 + post-cardinal veins in, 157 + post-cava in, 157 + renal-portal system in, 156 + venous system of, 156 + + Uro-genital cleft, 27 + + _Ursus_, ileo-colic junction of, 212 + _maritimus_, intestinal villi, 195 + + Uvula, 42 + + + Vagus, gastric distribution of, 43 + + Valves of Kerkring, 196, 197 + + Valvulae conniventes, 196, 197 + + Variations of caecum and appendix, 244 + in the peritoneal relations of the appendix, 258 + + Vasa intestini tenuis, 66 + + Vascular mesenteric folds of ileo-colic junction, 262 + system of liver, development of, 145 + + Vein, portal, development of, 148 + + Veins, anterior cardinal, 147 + hepatic, 148 + omphalo-mesenteric, 146 + posterior cardinal, 147 + primitive jugular, 147 + subintestinal, 147 + umbilical, 147 + vitelline, 146 + hepaticae advehentes, 147 + revehentes, 147 + + Venous system in Anure Amphibian, 158 + of bird, 161 + of human foetus at term, 162 + of _Necturus maculatus_, 158 + of _Rana esculenta_, 158 + in Selachian, 154 + of _Salamandra maculosa_, 158 + of Urodele Amphibian, 156 + + Ventral mesentery, early condition and derivation, 31 + mesogastrium, 163 + in _Iguana_, 166 + and liver, 140 + relation to duodenum, 164 + relation to liver, 105 + to umbilical vein, 165 + vascular ileo-caecal fold, 262 + + Vertebrate intestine, general morphology and physiology, 190 + + Visceral mesoderm, 21 + peritoneum, definition of, 32 + + Vitelline arteries, 64, 146 + membrane, 19 + sac, 20 + veins, 146 + anastomosis of, 147 + + Vitello-intestinal duct, 22 + + Vitellus, 19 + + _Vulpes fulvus_, ileo-colic junction and caecum of, 212 + + + Water-cells of camel's stomach, 49 + + Wolffian duct, relation to primitive intestine, 24 + + + _Xenurus_, ileo-colic junction of, 207 + + _Xiphias_, biliary ducts in, 145 + + + Yolk, 19 + + Yolk-sac, 20 + + + _Zalophus gillespiei_, ileo-colic junction and caecum of, 212 + + Zona pellucida, 19 + + + + + + + +End of the Project Gutenberg EBook of The Anatomy of the Human Peritoneum +and Abdominal Cavity, by George. S. Huntington + +*** END OF THE PROJECT GUTENBERG EBOOK 43350 *** |