188 MANUAL OF ANIMAL BIOLOGY 



the length of the alimentary canal is brought about by peristalsis 

 which consists of a series of progressive waves of reduction in the 

 diameter of the tube as a result of the contraction of the layer of 

 circular muscle fibers in the walls. The peristaltic movement be- 

 gins anteriorly and sweeps posteriorly, forcing the food materials 

 along with it. 



The movements of the muscular walls of the stomach are of such 

 a character as thoroughly to mix the food with the secreted en- 

 zyme, pepsin. This ferment, working in a slightly acid medium, 

 due to the presence of a small amount of hydrochloric acid, is able 

 to begin the process of digestion of the protein materials and to 

 change them into peptones. Both the pepsin and the hydro- 

 chloric acid are secreted by the endodermal glands noted above. 



When the proper condition of the food has been obtained, the 

 pyloric valve opens, and the partially digested food is passed 

 through it and into the duodenum. Here it is acted upon by the 

 digestive fluids secreted by the pancreas and carried to that region 

 by the common bile duct, and also by those present in the intestinal 

 juices. The pancreatic juice contains three principal digestive 

 agents as follows : (1) trypsin, which further acts on the peptones 

 and completes their digestion ; (2) amylopsin, which acts on the 

 starches and changes them into sugar, and (3) lipase, which acts 

 on the fats, splitting them into a fatty acid and glycerol. The 

 digestive action of the pancreatic enzymes is dependent upon an 

 alkaline condition of the food, and this is largely brought about 

 by the strongly alkaline pancreatic juice. The food materials from 

 the stomach with an acid reaction are mixed with this alkaline 

 secretion before the final phases of digestion take place. When 

 digestion is completed, the liquid food is absorbed by the cells of 

 the mucosa lining the intestine and then turned over to the vascu- 

 lar system for transportation to all regions of the body. 



The essential features of digestion are the same throughout the 

 vertebrate series, but certain variations are to be noted in the 

 regions utilized and the enzymes employed. Thus, in Man and 

 other Mammals, the digestion of starchy foods begins in the mouth 

 through the action of the enzyme ptyalin, which is present in the 

 saliva. The final stage of starch digestion occurs in the small 

 intestine just as in the Frog. Other important enzymes present in 

 the intestinal secretion are erepsin which completes the digestion 

 of the proteins begun in the stomach ; and maltase, sucrase, and 



