The dorsal caecal artery is a vessel of large size, supplying branches to the narrow appendicular mesentery which extends, with many epiploic appendages, to within 9 mm. of the blunt apex of the appendix. 2.5 cm. beyond the first bend in the appendix the fold is narrowed to a fringe not more than 0.75 cm. wide. Up to this point the dorsal vascular fold measures 1.5 cm. in width, and just where it narrows it is joined by the intermediate non-vascular fold (Fig. 459), which forms a membranous band, 3.3 cm. wide in the middle, spread out in the angle between the lower and dorsal surfaces of the ileum and the dorsal surface of the caecum which turns up behind the ileo-colic junction. Between this fold and the dorsal vascular fold is seen the deep recess of the posterior ileo-caecal fossa--which by reason of the sharp curve of the caecum looks not only to the left but also upward and backward. Direct comparison of the preparations of these two anthropoid apes just described with the conditions found in many adult human caeca shows the close correspondence in the arrangement of these folds and of their influence on the configuration of the parts. Figs. 572 and 573--taken from an adult human subject--show a caecum and appendix which almost reproduces that of the chimpanzee illustrated in Figs. 462 and 463 and closely resembles that of the orang. [Illustration: FIG. 572. Human adult. Ileo-colic junction and caecum; ventral view. (Drawn from preparation in Columbia University, Study Collection.) 1. Ventral vascular fold.] [Illustration: FIG. 573.--Dorsal view of the same preparation. 1. Appendix. 2. Intermediate non-vascular fold.] Fig. 572, giving the ventral view, shows, by the course of the ventral longitudinal muscular band, the turn of the caecum upwards and to the left. The ventral caecal artery runs in a fold (1) loaded with epiploic appendages. The non-vascular intermediate fold (Fig. 573, 2) passes to the root of the appendix, joining the proximal segment of the dorsal vascular fold in which the dorsal branch of the ileo-colic artery runs to the tip of the appendix. The distal two thirds of the appendicular mesentery are free. 3. _Gibbon, Hylobates hoolock_ (Figs. 455 and 456).--In the gibbon the folds appear well developed. The intermediate and dorsal vascular folds are quite distinct structures, although fusion (Fig. 456) has begun at one point, thus limiting a typical posterior ileo-caecal fossa. 4. _Gorilla, Gorilla savagei_ (Fig. 457).--Finally in the gorilla all three folds appear quite distinct and separate from each other, the dorsal vascular fold being especially well developed. _Unusual and Aberrant Types of Ileo-caecal Folds and Fossae.--(A) Ventral caecal artery larger than the dorsal, supplying the greater part of the caecum and the appendix._