LARGE INTESTINE 117 



ileocecal valve. Here it is that typical peristalsis is found. 

 The main factor in the passage is the layer of circular fibers. 

 Contraction of these fibers in the upper duodenum may at 

 least be conceived to begin upon the introduction of chyme. 

 The contraction passes down the gut in a wave-like manner, 

 the wave being produced by the contraction of segment after 

 segment of the circular fibers with relaxation just behind the 

 advancing contraction. The tendency of such a movement is 

 to force the alimentary mass along the canal. The longitu- 

 dinal fibers are probably chiefly concerned in changing the 

 position of the intestine and in shortening the tube, and thus 

 slipping the mucous membrane above the bolus, so that it 

 can be grasped by the circular fibers. A continuation and 

 repetition of these movements, which are slow, gentle and 

 gradual in character, is finally effectual in passing the con- 

 tents into the colon. It is not probable that antiperistaltic 

 movements take place normally. 



Nerve Supply. Very probably the intestinal movements 

 are naturally excited by the food and by the bile. It is prob- 

 able also that these stimuli exert their influence through the 

 ganglia of the plexuses of Auerbach and Meissner. The in- 

 testine receives fibers from the right vagus and the sympa- 

 thetic. The former are probably motor (contractors) and 

 the latter inhibitory (dilators). Here, as in the stomach, 

 they are probably only regulators of the movements, without 

 being actually necessary to peristalsis. 



The Large Intestine. 



Anatomy. The Jarge intestine, known as the colon, is 

 about five feet in length and is divided into ascending, trans- 

 verse and descending portions. The sigmoid flexure is the 

 terminal extremity of the descending colon and empties into 

 the rectum. The small intestine communicates with the 

 colon at right angles a little above the beginning of the latter, 

 leaving below the opening a blind pouch, the cecum, or 



