As previously stated the normal type of the human embryonal cæcum 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 cæcum 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 cæcal 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 cæcum proper. Consequently the initial curve of the cæcum 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 cæcal descent, accounts for the frequency with which the cæcum in the later months of fœtal 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 cæcum 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 cæcal 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 cæcum 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, cæcum and appendix are represented in the profile view in a straight line, without indicating the characteristic turn of the crescentic cæcal pouch upwards and to the left.
Fig. 549 shows the arrangement in unimpeded cæcal 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. Cæcum and appendix have, of course, a complete serous investment.
Figs. 549-554.—Schematic series illustrating the variations in the arrangement of the cæcaland colic peritoneum. | |
Normally, however, in the human subject, even if the obliteration of the apposed serous surfaces and the resulting fixation of the ascending colon has been delayed beyond the usual period, as above indicated, adhesion takes place subsequently, involving the dorsal surface of the ascending colon between the ileo-colic junction and the hepatic flexure (schema, Fig. 550). The dorsal surface of the cæcum 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 fœtal and juvenile subjects. Illustrations are afforded by [Figs. 515] and [516].
We have already noted ([p. 246]) the resulting fœtal type of pendent cæcum ([Fig. 510]).
More commonly colic adhesion before the cæcum 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 cæcum 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 cæcum 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 cæcum 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 cæcal end of the tube. Further development of the cæcum 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.