Chapter XV: Part IV (1)

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.

1. Appendix.
2. Reduced intermediate non-vascular fold.
3. Ventral vascular fold.
]

1. Appendix, coiled spirally behind terminal ileum.
2. Non-vascular intermediate fold.
]

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.