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 cæcal 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 intracæcal 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 cæcum and appendix (cf. [p. 276]). In all the cases observed the type of the cæcum indicated delayed or imperfect colic adhesion, and the ascending mesocolon remained partially free.
Fig. 563.—Human; child one year old. Cæcum and ileo-colic junction; ventral view. (Columbia University, Study Collection.)
1. Ventral cæcal artery, surrounded by epiploic appendages.
2. Dorsal vascular fold, forming appendicular mesentery.
3. Intermediate non-vascular fold.
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 cæcum has evidently been quite rapid and uniform without dorsal adhesion. The cæcum 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 cæcum hangs downward vertically and the appendix arises from the funnel-shaped apex of the pouch.
The ventral cæcal 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 cæcum 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 cæca of Cercopithecus, Ateles and Mycetes. The same vascular and non-vascular serous reduplications are found passing between the ileum and cæcum. 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-cæcal 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-cæcal 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.