Consequently here again the dorsal or posterior ileo-caecal 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-caecum 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-caecal fossa is formed, bounded ventrally by the intermediate fold at its accession to the caecum, dorsally by the proximal part of the dorsal vascular fold, to the right by the left wall of the caecum, 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 caecum to the root of the appendix, fusing with the dorsal vascular fold and rounding out a deep posterior ileo-caecal fossa. The typical arrangement, as encountered in the human subject, corresponds closely to the conditions presented by these anthropoid apes. [Illustration: FIG. 564.--Human adult. Caecum 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. 4. Rounded edge of union of dorsal vascular and intermediate folds bounding the ileo-caecal fossa caudad. 5. Point of accession to appendix of proximal branch of appendicular artery derived from posterior ileo-caecal artery.] In Fig. 564, taken from an adult male human subject, the dorsal surface of the ascending colon and of the ileo-colic junction is adherent to the parietal peritoneum. The distention of the caecum is nearly uniform, the right sacculation being only slightly larger than the left. The appendix, measuring 18.4 cm. in length, arises from the dorsal surface of the caput coli, 1.7 cm. from the point where the ventral longitudinal muscular band turns around the caudal end of the pouch between the two sacculations, and 3.7 cm. below the caudal margin of the ileo-colic junction.