The =ileum= is the portion of the small intestine between the jejunum and colon. It lies suspended by its mesentery in numerous folds in the caudal part of the abdominal cavity, separated from the ventral abdominal wall only by the great omentum. It is of nearly uniform diameter, but its caudal portion is thinner-walled than its cranial portion. Its walls have a microscopic structure like that of the duodenum and jejunum. On its inner surface and on the inner surface of the jejunum are seen close-set villi, but these become rather sparser toward the caudal end of the ileum and disappear about one centimeter from the opening into the colon. Among the villi of the caudal end of the ileum are numerous rounded elongations free from villi. These are the solitary follicles or solitary glands (lymphatic) of the intestine. These glands when aggregated together form the agminated glands or patches of Peyer. The ileum passes at the caudal end into the colon, the opening being guarded by the =ileocolic valve= (Fig. 99). This is formed by a marked projection of the mucosa (_f_) and transverse muscle layer (_e_) of the ileum into the colon. Its surface is free from villi. 6. =The Large Intestine. Intestinum crassum=.--The large intestine is divided into =colon= and =rectum=. The =colon= or first part of the large intestine lies against the dorsal body wall and is separated from the ventral body wall by the folds of the ileum. It has a diameter about three times that of the ileum. The opening of the ileum into it is on its side between one and two centimeters from its cranial end (Fig. 98). The blind pouch thus formed by the cranial end of the colon is the =cæcum= (Fig. 98, _c_; Fig. 99, _a_). The cæcum ends in a slight conical projection which may be considered as the rudiment of a vermiform appendix. The colon lies at first on the right side and passes at first craniad; then transversely to the left, then caudad, lying nearly in the middle line and next to the dorsal abdominal wall. The colon may thus be distinguished according to its direction into =ascending, transverse,= and =descending= colon. At its caudal end the colon passes without sharp limit into the rectum. [Illustration: FIG. 98.--JUNCTION OF SMALL AND LARGE INTESTINE. FIG. 99.--SECTION OF THE ILEOCOLIC VALVE. Fig. 98.--_a_, ileum; =b=, ascending colon; _c_, cæcum; _d_, position of ileocolic valve. Fig. 99.--_a_, cæcum; _b_, colon; _c_, ileum; _d_, longitudinal muscle layer; _e_, transverse muscle layer; _f_, mucosa; _g_, ileocolic valve (opened, as when material is passing into the colon).] At the bottom of the cæcum on its inner surface is seen a collection of solitary glands forming one of the agminated glands of Peyer, or Peyer’s patches. The mucous membrane is without villi. It presents a few considerable elevations, probably solitary glands.