The dorsal vascular fold (2), forming the broad appendicular mesentery (1), is well developed and free in its distal portion, extending, with gradually diminishing width, to the apex of the appendix. The proximal segment of this fold (between 1 and 2) descends over the dorsal surface of the ileo-colic junction and meets (at 4) the intermediate non-vascular fold (3) which extends between the ileum and caecum, rounding out a crescentic ridge (4) which bounds the entrance into the posterior ileo-caecal fossa (between 2 and 3). The influence of the folds and of the blood vessels on the position and curves of the appendix is quite apparent in this preparation. The dorsal larger branch of the ileo-colic artery, supplying caecum and appendix, passes over the dorsal surface of the ileo-colic junction (2) where the same, as well as the adjacent dorsal surface of the colon, is adherent to the parietal peritoneum. At the point where the dorsal vascular fold intersects and fuses with the intermediate non-vascular fold (4) the artery divides into a proximal and distal branch. The former proceeds to the caecum and root of the appendix, reaching this tube at the point marked 5. The latter continues (from 1 on) in the free border of the appendicular mesentery to the beginning of the distal third of the appendix, from which point on the fold extends as a narrow reduplication to the tip of the tube. The segment of the appendix situated between these two main arterial branches is thrown into several coils, the expression of the continued growth between two points relatively fixed by the accession of the two arterial branches. The pathological significance of these bends is apparent when we consider the effect which the kinking of the tube would have on catarrhal and other inflammations accompanied by distension of the appendix. Typical examples of the posterior ileo-caecal fossa and of the mutual relationship of the limiting folds are seen in Figs. 565 and 566, both taken from adult human subjects. [Illustration: FIG. 565.--Human adult, Caecum and ileo-colic junction with large intermediate non-vascular fold and deep posterior ileo-caecal fossa. (Columbia University Museum, No. 1546.)] [Illustration: FIG. 566.--Human adult. Ileo-colic junction and caecum. (Columbia University Museum, No. 1659.)] The significance and mutual relations of the folds seen in the preparations just considered--which illustrate the typical adult human arrangement of the structures--will perhaps be best understood by comparison with an adult caecum in which the infantile condition, as seen in Fig. 563, has become further developed. [Illustration: FIG. 567.--Human adult. Ileo-colic junction and caecum; dorsal view. (Drawn from preparation in Columbia University, Study Collection.) 1. Dorsal vascular fold, carrying the distal appendicular branch of the dorsal caecal artery in the mesentery of the appendix.