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 cæca 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 cæcum 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 cæcum 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 cæcum, 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 cæcum 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 cæcum downward into the iliac fossa. This leads to a backward bend of the cæcum 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 fœtus at term the lowest point of the large intestine in the right iliac fossa is formed by the proximal part of the cæcum 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 cæcum and colon, can therefore in part be referred to the mechanical conditions obtaining normally during the descent of the cæcum. Of course the shape of the cæcal pouch and the later development of the adult type of cæcum will modify this influence in individual cases. We have seen that this early adhesion and the resulting effects on the position of cæcum 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-cæcal 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 cæcal 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 cæcum 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 cæcum and the position of the appendix is the one above described as type Ia, Ib and Ic ([Fig. 509]).
[Fig. 510] from a fœtus at term, and [Figs. 515] and [516] representing infantile cæca, illustrate this form of the pouch, while the parts are shown in situ in [Fig. 543] taken from a preparation of a five-month fœtus.
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-cæcal 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 cæcum 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-cæcal ileal coils keeps the cæcum, already in the process of descent, in contact with the ventral abdominal wall. When the final rotation of the retro-cæcal 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 cæcum 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 cæcum and appendix are in contact with the dorsal prerenal parietal peritoneum. The escape of the dorsal set of ileal convolutions from the retro-cæcal position, by rotation downwards and to the left, is accomplished. The cæcum 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 cæcal pouch and appendix and the resulting direction of its axis.