duodenum be slit lengthwise, spread out, thoroughly cleaned, and hardened with formalin. The mucous membrane is folded into very closely packed, relatively short, flat, transverse ridges. The longer ridges follow a zig-zag course and their distal edges are lobed. The ridges probably facilitate digestion by increasing the secretory and absorptive surface of the intestine.
The jejunum follows the duodenum. It constitutes in man two-fifths of the small intestine, exclusive of the duodenum. Its inner surface, like that of the duodenum, is thickly covered with short, flat, transverse ridges.
The ileum is not sharply separated from the jejunum in man. It connects the jejunum and the colon. In the rat it is characterized by its dark color, due to the color of the contained feces. The mucous membrane of the ileum is also thrown into numerous closely packed transverse ridges.
The contents of the ileum pass through the ileocolic valve into a small chamber which communicates by wide openings both with the colon and the caecum. The latter is a sac measuring about two and one-half centimeters on its longest axis. It varies in shape, sometimes resembling an interrogation mark, or assuming the form of a short spiral. Thus it has a greater, or outer, and a lesser, or inner, curvature. At one end it opens into the intestines. A mesentery connects the lesser curvature with the ileum. The caecum contains a mass of dark fecal matter. When this is removed it will be seen that the inner surface presents smooth areas of considerable extent, but bears folds also. These folds are transverse along the inner curvature, but near the proximal end they radiate, in general, from the opening of the caecum.
From the caecum and ileum the feces enter the ascending colon. This follows an irregular route forward, ventral to the duodenum but dorsal to loops of the jejunum
