36 PHYSIOLOGY OF ALIMENTATION. 



anti-peristaltic waves in the colon to the region of the slowly 

 advancing rings is not yet entirely understood. In part the 

 later portions of the food push that which has gone before 

 ahead of it. But the ascending and transverse portions of the 

 colon can get rid of most of their contents even in starvation, 

 though they are, perhaps, never entirely emptied even under 

 these conditions. Apparently strong tonic peristaltic waves 

 at times pass over the ascending and transverse colon and 

 force the contents of these portions of the large bowel into the 

 region of the descending colon, where peristaltic waves are the 

 rule. 



9. The Movement of Food through the Alimentary Tract 

 as a Whole. The process of mastication takes a varying 

 length of time, depending upon the nature of the food and to 

 a large extent upon the individual. Dry food takes longest 

 to become mixed with saliva sufficiently to allow of its being 

 swallowed, while liquids are at once passed backward toward 

 the O3sophagus. The entire length of time required for the 

 food to fall into the stomach after mastication is completed 

 varies with different animals, the extremes being represented 

 by four and twelve seconds. In man from four to seven 

 seconds are required from the initiation of the act of deglu- 

 tition to the time the food falls into the stomach, 



The food remains in the stomach a variable number of 

 hours, depending upon the quantity and the quality of the 

 food ingested. Other things being equal, a small amount of 

 food will escape into the small intestine in shorter time than 

 a larger amount. The time that food remains in the stomach 

 is probably greater than is generally believed to be the case. 

 An average meal does not ordinarily leave the stomach en- 

 tirely in less than six hours. The quality of the food plays 

 an important role by determining the frequency with which 

 the pyloric sphincter relaxes and allows the food to be forced 

 onward into the duodenum by the contractile waves which 

 pass over the antrum. As was shown above, not every wave 

 forces food out of the stomach. 



