LARGE INTESTINE II 7 



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 con- 

 traction. The tendency of such a movement is to force the 

 alimentary mass along the canal. The longitudinal fibers are 

 probably chiefly concerned in changing the position of the intes- 

 tine 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 move- 

 ments, which are slow, gentle and gradual in character, is finally 

 effectual in passing the contents 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 probable also 

 that these stimuli exert their influence through the ganglia of the 

 plexuses of Auerbach and Meissner. The intestine receives 

 fibers from the right vagus and the sympathetic. 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 large 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 caput coli. 



