THE ANATOMY
OF THE
HUMAN PERITONEUM
AND
ABDOMINAL CAVITY

CONSIDERED FROM THE STANDPOINT OF DEVELOPMENT
AND COMPARATIVE ANATOMY

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PREFACE.

In the following pages an attempt has been made to emphasize the value of Embryology and Comparative Anatomy in elucidating the difficult and often complicated morphological problems encountered in the study of human adult anatomy.

Moreover, in addition to the direct advance in the method and scope of anatomical teaching afforded by these aids, it is further hoped that the broader interpretation, both of structure and function, obtained by ontogenetic and phylogenetic comparison, will impart an interest to the study of adult human morphology, such as the subject, considered solely in the narrow field of its own limitations, could never arouse.

The book represents part of the course in visceral anatomy as developed during the past fourteen years at Columbia University. The sections dealing with the morphology of the vertebrate ileo-colic junction and with the structural details of the human cæcum and appendix are considered somewhat more fully, as warranted by the extensive material available. The illustrations are for the greater part taken from preparations in the Morphological Museum of the University. Wherever practicable the direct photographic reproduction of the actual preparation is given. In the case of preparations not suitable for this purpose, careful drawings have been made which offer in every instance a faithful and correct interpretation of the conditions presented by the actual object. A number of the embryonic illustrations are taken from the standard text-books on the subject, due credit being given to their source. I desire to express my sincere thanks to Dr. Edward Leaming, of the Department of Photography and to Mr. M. Petersen, artist of the Anatomical Department of the University, for their skilful and thoroughly reliable work in the preparation of the illustrations.

George S. Huntington.

Columbia University, in the City of New York,
December, 1902.

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CONTENTS.

colspan="3">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 Cæca 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 Cæcum 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       Valvulæ 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 Cæcum 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 Cæcal 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 Cæcum and Colon 225   IV. Structure of Cæcal Apparatus and Specialized Morphological Characters of Colon in Rodents and Ungulates 229     1. Cæcum Proper

class="pginternal" tag="{http://www.w3.org/1999/xhtml}a">229     2. Structural Modifications of Proximal Segment of Colon analogous in their Functional Significance to the Cæcal Apparatus 230   V. Cæcal Apparatus and Colon in Hyrax. 234 Part IV. Morphology of the Human Cæcum and Vermiform Appendix 237   I. Development of the Cæcum and Appendix 237   II. Changes in the Position of the Cæcum and Appendix during normal Development, depending upon the Rotation of the Intestine and the subsequent Descent of the Cæcum 239   III. Variations of Adult Cæcum and Appendix 244     A. Shape of Cæcum and Origin of Appendix. Types and Variations of Adult Cæcum and Appendix 245     B. Position and Peritoneal Relations of Appendix 250     C. Ileo-Cæcal Folds and Fossæ 260

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INTRODUCTION.

In considering the anatomy of the human abdominal cavity and peritoneum in the following pages the explanation of the adult conditions encountered is based upon the development of the parts, and the successive human embryonal stages are illustrated by the examination of the lower vertebrates presenting permanent adult structural conditions which appear as merely temporary embryonal stages in the development of the higher mammalian alimentary tract.

For the sake of clearness and brevity all discussion of the theories of peritoneal development has been designedly omitted. The assumption of peritoneal adhesion, and consequent obliteration of serous areas, offers many advantages in considering the adult human abdominal cavity, especially from the standpoint of comparative anatomy. The same has consequently been adopted without reference to divergent views and theories.

In studying the descriptive text and the diagrams the student should remember that the volume offers in no sense a complete or detailed account of the development of the abdominal cavity and its contents. The purpose is not to present the embryology of this portion of the vertebrate body, but to utilize certain embryological facts in order to explain the complicated adult conditions encountered. To avoid confusion, and to bring the salient points into strong relief, the majority of the diagrams illustrating human embryonal stages are purely schematic.

Moreover, in order to avoid confusing and unnecessary details it is often desirable to disregard developmental chronology entirely. Many of the diagrams combine several successive developmental stages, showing different degrees of development in different portions of the same drawing. Again it is frequently necessary, for the sake of brevity and clearness, to actually depart from known embryological conditions. If, for example, the stomach and liver are treated as if they were from their inception abdominal organs, the student of systematic embryology will recall the fact that this position is only obtained after their primitive differentiation by growth and migration.

Again the mesenteries are treated here as if they formed definite and well-defined membranes from the beginning—without reference to the abdominal organs with which they are associated. We speak of the liver as growing into and between the layers of the ventral mesogastrium, because this conception offers the opportunity of more clearly explaining the adult condition. Actually, however, the membrane develops, as a new structure, after the first differentiation of liver and stomach, as these organs descend into the abdominal cavity.

Similar discrepancies between fact and schema are encountered throughout. Consequently, while the purpose of the volume is to facilitate the study and comprehension of the adult peritoneal cavity and its contents, the reader should guard against receiving the developmental illustration as a correct successive and detailed account of the embryology of the parts concerned.

In like manner the comparative anatomical facts adduced form in no sense even approximately a complete serial morphological account of the vertebrate alimentary tract.

To the student of human anatomy the zoölogical position of the forms which help him to understand complicated human structural conditions is immaterial. He can draw on all the vertebrate classes independently of their mutual relations. Hence neither ontogeny nor phylogeny are here introduced, except as aids to the study of adult human anatomy. The following pages offer neither an embryology nor a comparative anatomy of the alimentary tract, but an attempt has been made in them to illustrate the significance of the complicated anatomical details presented by the adult human abdominal cavity by reference to the simpler antecedent conditions encountered during the early developmental stages of the higher forms and permanently in the structure of the lower vertebrates.

While, as just stated, a complete presentation of the development of the abdominal cavity is not required, yet the student will find it of advantage to rehearse the main facts of vertebrate embryology, for the purpose of bringing a clear understanding of the manner in which the vertebrate body is built up to bear upon the problems which the special organs and structures of the body-cavity present for his consideration. This purpose can be accomplished by a very brief and condensed consideration of the cardinal facts.

The entire vertebrate body is the product of developmental changes taking place after fertilization in a single primitive CELL, the EGG or OVUM (Fig. 1).

In structure the ovum corresponds to other animal cells. On account of their special significance during development the different component parts of the egg-cell have received special distinctive names. The cell-body is known as the vitellus or yolk. It is composed of two substances, the protoplasm or formative yolk and the deuteroplasm or nutritive yolk, which vary in their relative proportions in the ova of different animals.

The protoplasm represents the material from which in the course of development the cells forming the body of the individual are derived, while the deuteroplasm serves for the nutrition of the ovum during the earliest stages of development.

The nucleus of the egg-cell is distinguished as the germinal vesicle, and its nucleolus as the germinal spot.

The cell-body or vitellus is surrounded by a condensed portion of the cell contents to which the name of the vitelline membrane has been applied, which in turn is enclosed by a transparent and elastic cover, the zona pellucida, presenting a radially striated appearance.

The ovum is contained in the cortical portion of the ovary, enclosed in the Graafian follicle, a vesicle 4-8 mm. in diameter, whose fibrous walls are lined by several layers of epithelial cells, which surround the ovum, forming the discus proligerus.

After impregnation the egg-cell, by a process of repeated division or cleavage, undergoes segmentation, the cell-body being divided successively into two, four, eight, sixteen, thirty-two, etc., cells, called blastomeres (Figs. 2 and 3). The mass of cells finally resulting from this process of segmentation forms the ground work of the future body. A vertebrate ovum in this stage of complete segmentation is called the morula from its resemblance to a mulberry (Fig. 4).

After segmentation is completed a cavity filled with fluid and surrounded by the developing cells is gradually formed in the interior of the mass. This cavity is known as the segmentation-cavity. The egg is now called the blastula, blastosphere or blastodermic vesicle and the cellular membrane enclosing the segmentation-cavity forms the germinal membrane or blastoderm (Figs. 5 and 6). The cells of the blastoderm become aggregated at one point on the circumference of the vesicle (dorsal pole of blastosphere) forming, when viewed from above, a thickened biscuit or disk-shaped opaque area. This is known as the germinal area, or primitive blastoderm or embryonic shield (Figs. 7 and 12).

   

This is the first indication of the coming division of the entire egg-cell into the embryo proper and the vitelline or yolk-sac (Figs. 8 and 9). The entire future individual develops from the cells of the germinal area. This area comprises both the embryo proper and the region immediately surrounding it.

 

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

 

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.)