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.
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.
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.
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).
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.
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).
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.
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.
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_).
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.
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_).
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.
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_).
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.
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.
=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.
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.
=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.
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).
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.
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.
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.
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.
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).
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).
(_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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
(_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.
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