DIGESTION. 209 



contractions, known as the vermicular or peristaltic, are extremely 

 slow, creeping along the canal in a progressive manner from above 

 downward. The peristalsis is produced by the contraction of both 

 the circular and longitudinal muscle-fibers. By the contraction 

 of the former the caliber of the canal is constricted behind a portion 

 of the food, and as the wave of constriction advances it carries the 

 food with it. At the same time, the longitudinal fibers contracting, 

 the segment of the canal is shortened and drawn up over the ad- 

 vancing portion of the food. The result of these movements is to 

 push the food through the canal, to turn it over and to incorporate 

 the digestive fluids with it. 



Though the general movement of the intestinal contents is gradu- 

 ally downward, Cannon has shown that the result of the peristaltic 

 movements is to divide the mass into segments, and that the mere 

 presence of a segment in the lumen of the intestine seems to excite the 

 overlying circular fibers to activity in such a way that each segment is 

 again divided. The lower half of each segment then unites with the 

 upper half of the segment below, to commingle with it and to expose 

 new surfaces of the food mass to contact with the actively absorbing 

 mucosa. A continual repetition of this process results in a thorough 

 mixing of the food with the digestive juices and materially aids the 

 process of absorption. 



Influence of the Nervous System. The peristaltic movements 

 of the small intestine, though independent of the central nervous 

 system, are nevertheless at times largely influenced by it /^Stimu- 

 lation of the vagus nerve generally increases the vigor of the contrac- 

 tion; stimulation of splanchnic nerves inhibits or diminishes the 

 contraction. ^These nerve pathways serve to transmit to the intestinal 

 walls nerve impulses, which increase or diminish the force and extent 

 of their contractions. 



An excised portion of intestine (removed from the body) will 

 exhibit rhythmic movements for some time. Stimulation either by 

 electric or chemic agencies will also develop contractions which will 

 be propagated in the usual direction. As to the manner in which 

 the contraction wave is propagated to adjoining fibers, practically 

 nothing is known. It may be connected with the nerve-fiber plexuses 

 (Meissner's and Auerbach's) in the walls of the intestine. 



The Large Intestine. The large intestine is that portion of the 

 alimentary canal situated between the termination of the ileum and 

 the anus. It varies in length from four and a half to five feet, in 

 diameter from one and a half to two and a half inches. It is divided 

 into the cecum, the colon (subdivided into an ascending, transverse, 

 and descending portion, including the sigmoid flexure), and the 

 rectum. 



The cecum is situated in the right iliac fossa. It is that dilated 

 14 



