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 cæcum below the ileo-colic junction. It crosses the ventral surface of the broad dorsal vascular fold in passing to the cæcum, but remains entirely free and is not adherent to the same.
Consequently here again the dorsal or posterior ileo-cæcal 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-cæcum 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-cæcal fossa is formed, bounded ventrally by the intermediate fold at its accession to the cæcum, dorsally by the proximal part of the dorsal vascular fold, to the right by the left wall of the cæcum, 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 cæcum to the root of the appendix, fusing with the dorsal vascular fold and rounding out a deep posterior ileo-cæcal fossa.
The typical arrangement, as encountered in the human subject, corresponds closely to the conditions presented by these anthropoid apes.
Fig. 564.—Human adult. Cæcum and ileo-colic junction. (Drawn from preparation in Columbia University, Study Collection.)
1. Dorsal vascular fold at the beginning of the distal free portion, forming the appendicular mesentery.
2. Proximal segment of dorsal vascular fold, fusing with
3. Intermediate non-vascular fold.