114 digestion 



Digestion in the Stomach. 



Gastric juice. — The digestive action of the stomach is 

 due mainly to the action of the digestive fluid produced by 

 the gastric glands. This gastric juice is a thin, nearly 

 colorless fluid, consisting mainly of water and holding in 

 solution a small proportion of hydrochloric acid and two 

 enzymes, pepsin and rennin. 



As a result of the acid contamed it gives a red color to 

 blue litmus, and, since ptyalin can act only in an alkaline 

 medium, the saliva swallowed with the food can exert no 

 digestive action upon starch after the food is acidified. 



The flow of the gastric juice begins with the entrance of 

 food into the stomach, and the flow increases with the 

 amount of food taken in. This flow may also be stimu- 

 lated by certain food accessories, by the alkaline saliva, 

 and by the flavor of the food. Normally, about three 

 quarts of gastric juice is produced by the glands in twenty- 

 four hours. 



Digestive action of the enzymes. — Pepsin. This enzyme 

 has the power to transform proteids to a soluble form 

 called peptone. It acts only in an acid medium, and what 

 is known as peptic digestion is therefore the result of the 

 combined action of pepsin and hydrochloric acid. Peptic 

 digestion varies with temperature. Low temperatures 

 retard, while very high temperatures may stop completely 

 the action of the pepsin. Pepsin can be obtained in a 

 fairly pure state (see Ex. XXXV., b) and with the product 

 so obtained it is possible to prepare artificial gastric 

 juice whose action upon various foods can be observed 

 with great accuracy. (See Ex. XXXVII.) In such experi- 

 ments the artificial juice is allowed to act upon food prep- 



