GASTRIC DIGESTION. 157 



cus, free hydrochloric acid (about .02 per cent.), and a 

 substance called pepsin, which in acid liquids has the power 

 of converting ordinary proteids into closely allied bodies 

 called peptones. It also dissolves solid proteids, changing 

 them at the same time into peptones. 



Peptones. Ordinary proteids are typical examples of 

 what are called "colloids;" that is to say, substances which 

 do not readily pass through moist animal membranes; pep- 

 tones are a kind of proteid which does readily pass through 

 such membranes, and are, therefore, capable of absorption 

 from the alimentary canal. (See Dialysis, p. 186.) 



Gastric Digestion. In the stomach the onward progress 

 of the food is stayed for some time. The pyloric sphincter 

 remaining contracted closes the aperture leading into the 

 intestine, and the irregularly disposed muscular layers of 

 the stomach keep its semi-liquid contents in constant 

 movement, by which all portions are thoroughly mixed 

 with the secretion of its glands. In the stomach part of 

 the proteid of the food is dissolved and turned into pep- 

 tones. Certain mineral salts (as phosphate of lime, of 

 which there is always some in bread), which are insoluble 

 in water but soluble in dilute acids, are also dissolved in 

 the stomach. On the other hand, the gastric juice has no 

 action upon starch, nor does it digest oily substances. By 

 fche solution of the white fibrous connective tissues the 

 .disintegration of animal foodg, commenced by the teeth, is 



Name its more important constituents. What powers does pepsin 



What are colloids? Give examples. How do peptones differ 

 from other proteids? 



Does food pass on immediately from the stomach to the intestine? 

 How is it kept back? How is it mixed with the gastric juice? What 

 happens to proteid foods in the stomach? Name another substance 

 dissolved in the stomach. Name foodstuffs which are not changed 

 in the stomach. How are animal foods broken up in the stomach? 



