(_a_) _Vascular Mesenteric Folds._--The peritoneal vascular folds, carrying the blood vessels to supply the caecum, are two in number, a ventral (1) and dorsal (3). They are of nearly equal size and extent, passing from the ventral and dorsal aspect of the ileo-colic junction nearly to the apex of the caecum. Each contains a branch of the ileo-colic artery, which forks in the ileo-colic mesentery, in the angle between ileum and large intestine. The ventral branch continues in the ventral mesenteric fold (Fig. 445) downward across the ventral surface of the ileo-colic junction to supply the ventral part of the caecum, while the dorsal branch descends behind the ileo-colic junction, preserving a similar course in the dorsal mesenteric fold. The dorsal arterial branch is somewhat larger than the ventral and its distribution extends a little further down to the actual apex of the caecum. (_b_) _Non-vascular Ileo-caecal Serous Reduplication._--Between the two vascular mesenteric folds a third serous reduplication, carrying no blood vessels, is found passing between the ileum and caecum. This fold begins, in the preparation from which the figure is taken, on the ileum opposite the attached mesenteric border, 2.7 cm. from the ileo-colic junction, and passes for exactly the same distance down on the adjacent left concave surface of the caecum. It is placed a little nearer to the dorsal than to the ventral vascular fold, so that it passes, if the distance between the two vascular folds on the caecum be divided into three parts, at the junction of the dorsal third with the ventral two thirds. The production of this intermediate non-vascular ileo-caecal reduplication, which is of very constant occurrence in the mammalian series, is to be led back to the development of the caecum. When the pouch protrudes from the smooth surface of the embryonic intestine opposite the mesenteric border, it extends backward along the future small intestine and lifts off the serous investment of the gut in the form of a small peritoneal plate filling the interval between itself and the adjacent ileum. A very perfect illustration of this process can be seen in the instance of Meckel's diverticulum shown in Fig. 561. The proximal portion of the diverticulum is here still closely connected to the small intestine along which it extends, both being surrounded by the common visceral peritoneum. The distal part of the diverticulum has separated more completely from the intestine, and in so doing has drawn out the serous investment in the form of the triangular fold which is seen to pass between the free margin of the intestine and the adjacent surface of the pouch. The same process can be followed in its different stages in certain normal mammalian caecal types. [Illustration: FIG. 561.--Human adult ileum with Meckel's diverticulum. Ileo-diverticular serous fold and persistent omphalo-mesenteric artery. (Columbia University Museum, No. 1803.)]