252 DIGESTION IN THE LARGE INTESTINE. 



tine possesses no digestive power, it has considerable power of 

 absorption. 



Movements of the I/arge Intestine. In the cecum, as- 

 cending and transverse colon, the normal activity, according to 

 Cannon, is an antiperistalsis. Constriction waves start in the 

 transverse colon and pass backward toward the cecum. These 

 waves, unlike those of the stomach, do not run in continuous 

 rhythm ; they occur usually in periods lasting about five minutes, 

 and recur every fifteen or thirty minutes. He says : " The anti- 

 peristalsis in this part of the large intestine seems, indeed, to 

 give a reason for the presence of a valvular structure at the en- 

 trance of the small intestine into the large. Hundreds of anti- 

 peristaltic waves have been seen coursing toward the cecum, and 

 only twice has food been observed to be pressed back into the 

 ileum through the ileocolic valve. Inasmuch as the valve is com- 

 petent for the food which has gone from the small into the large 

 intestine, the antiperistaltic waves have the same effect here as the 

 peristaltic waves have in the stomach when the pylorus remains 

 closed. For, when a constriction occurs, some of the mucous sur- 

 face of the colon becomes enclosed by the narrowed muscular ring; 

 and, as this constriction passes on, fresh areas of this surface are 

 continually pressed inward around the narrow orifice, while a thin 

 stream of food is passing in the opposite direction. As waves 

 recur about five times a minute, twenty-five waves or more aifect 

 every particle of food in the cecum and in the ascending colon in 

 this churning manner during each normal period of antiperistalsis. 

 The result must be again a thorough mixing of the contents with 

 the digestive secretions brought from the small intestine and an 

 exposure of the digested food to the absorbing wall. 



" Immediately after food passes from the ileum into the large 

 intestine a strong tonic contraction of the cecum and the ascending 

 colon is commonly observed, which serves to press onward to- 

 ward the rectum the contents of these parts. Antiperistaltic waves 

 follow at once the general contraction, so that much of the food 

 which has been pressed onward is returned into the cecum. With 

 the repetition of this process, however, more and more material 

 appears at the end of the transverse colon, and on its appearance 

 there a persistent ring of contraction cuts it off from the region of 

 antiperistalsis ; as still more food appears in the large intestine this 

 ring moves slowly onward toward the rectum, pressing a mass before 

 it, and is followed by other similar rings carrying onward similar 

 masses by very slow peristalsis. 



" Thus, in the large intestine the function of mixing the food with 

 the digestive secretions and of exposing this food to the absorbing 

 walls is performed by an antiperistalsis at the beginning of the 

 large intestine. It is here that the last valuable constituents of the 

 food are worked over and taken into the body. The remnant of 

 unused material is propelled onward by occasional strong contrac- 





