76 ANATOMY OF THE RAT 



duodenum be slit lengthwise, spread out, thoroughly 

 cleaned, and hardened with formalin. The mucous mem- 

 brane 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 prob- 

 ably 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 resem- 

 bling 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 intes- 

 tines. 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 sur- 

 face 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 ascend- 

 ing colon. This follows an irregular route forward, ven- 

 tral to the duodenum but dorsal to loops of the jejunum 



