STRUCTURE OF THE SMALL INTESTINE. 221 



INTESTINAL DIGESTION. 



The intestinal canal extends from the stomach to the anus, and 

 is divided into the duodenum, jejunum, and ileum, which constitute 

 the small intestine, which is about 8 meters in length, and the 

 cecum, colon, and rectum, constituting the large intestine, having 

 together a length of 1.68 meters. 



Structure of the Small Intestine. The duodenum is the 

 portion of the small intestine into which the food enters after 

 leaving the stomach. It is about 30 cm. in length and 5 cm. in 

 diameter, and passes into the jejunum, and this in turn into the 

 ileum, the opening between the ileum and the beginning of the 

 large intestine being guarded by the ileocecal valve. 



Coats of the SmaU Intestine. Like the stomach, the small 

 intestine is composed of four coats: 1, serous; 2, muscular; 

 3, submucous ; 4, mucous. As in the stomach, the two coats 

 which have special physiologic interest are the muscular and the 

 mucous. The muscular coat is made up of two layers : an ex- 

 ternal or longitudinal and an inner or circular, between which are 

 lymphatic vessels and the plexus myentericus of Auerbach (Fig. 

 115). 



In the submucous coat are blood-vessels, lymphatics, and the 

 plexus of Meissner. 



FIG. 115. A portion of the plexus of Auerbach from stomach of cat, stained with 

 methylene-blue (intra vitam), as seen under low magnification (Huber). 



The mucous, or most internal, coat contains the following 

 structures, a knowledge of which is essential to an understanding 

 of the physiology of this portion of the digestive apparatus : (1) 

 valvula? conniventes ; (2) villi ; (3) glands of Brunner ; (4) glands 

 of Lieberkiihn ; (5) solitary glands ; (6) agminated glands or 

 Peyer's patches. 



