DIGESTION 



199 



FK 



by way of the sympathetic chain, the great and small splanchnics, the 

 solar and mesenteric plexuses, and especially from the vagus. 



THE MOVEMENTS OF THE SMALL INTESTINE, for descriptive purposes 

 only, may be analyzed into four varieties. These are peristalsis, longi- 

 tudinal contractions, swinging movements, and, according to Cannon, 

 "segmentation." The three first-mentioned are but aspects of one 

 complex movement; the last is more or less separate, and as described by 

 Cannon, only recently discovered. 



Intestinal peristalsis, like peristalsis 

 elsewhere (esophagus, ureter, etc.), 

 is a slow progressive contraction of 

 the circular muscular fibers forming 

 the major part of the musculature of 

 the gut. This successive contraction 

 of the rings of muscle causes a pro- 

 gressive narrowing of the lumen of 

 the tube a process well adapted to 

 squeezing its contents slowly along. 

 The progression is much aided by the 

 automatic contraction of the fibers 

 above a mass of food, while those 

 below it are simultaneously relaxed 

 (Bayliss and Starling). The rate of 

 movement is slow. In one case a 

 marble was pushed along only a trifle 

 more than 1 cm. in a minute, but the 

 normal propulsion of the chyle is 

 doubtless often at a much greater 

 rate. There is no direct evidence 

 that antiperistalsis normally occurs 

 in the small intestine, although it 

 does occur in the large. It probably 

 takes place, however, in certain ab- 

 normal conditions, and in the small 

 Crustacea (for example, Daphnia) 

 antiperistalsis is the normal direction 

 of the wave. 



Lessening of the length of the 

 gut by means of contraction of the 



longitudinal fibers is an almost indispensable part of the peristaltic 

 movements, for by this means the intestinal tube is drawn over the 

 contained mass, so leaving the latter, as it were, farther down the intestine 

 than before. Thus, peristalsis and occasional shortening of a loop of the 

 intestine together squeeze the contained mass slowly along. The speed 

 of these movements is hastened several times when the intestines . are 

 exposed to the irritating air, as during an operation or a demonstration. 

 Normally they are very slow a gentle complex squirming movement 

 hard to describe but easy to understand. 



Magnification of the intestinal area by 

 the villi. Were the villi not present the 

 absorptive area of the gut would be only 

 about one-nineteenth of what it is. This 

 ratio is that of the smaller rectangle to the 

 larger. 



