4. Rounded edge of union of dorsal vascular and intermediate folds bounding the ileo-cæcal fossa caudad.
5. Point of accession to appendix of proximal branch of appendicular artery derived from posterior ileo-cæcal 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 cæcum 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.
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 cæcum, rounding out a crescentic ridge (4) which bounds the entrance into the posterior ileo-cæcal 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 cæcum 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 cæcum 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-cæcal fossa and of the mutual relationship of the limiting folds are seen in Figs. 565 and 566, both taken from adult human subjects.
Fig. 565.—Human adult, Cæcum and ileo-colic junction with large intermediate non-vascular fold and deep posterior ileo-cæcal fossa. (Columbia University Museum, No. 1546.)
Fig. 566.—Human adult. Ileo-colic junction and cæcum. (Columbia University Museum, No. 1659.)