GASTRIC GLANDS 91 



secreted in twenty-four hours is from six to fourteen pounds. 

 Gastric juice will resist putrefaction for a long time, prob- 

 ably on account of the free acid. Its digestive properties 

 are due to the proteolytic enzyme, pepsin, the milk-curdling 

 enzyme rennin, and the free hydrochloric acid. 



Hydrochloric Acid The amount of free hydrochloric 

 acid present in normal gastric juice is from two-tenths to 

 three-tenths of one per cent. It has been frequently claimed 

 that the acidity of this secretion is due to lactic acid, but 

 while it cannot be denied that lactic acid, from the fermenta- 

 tion of carbohydrates is, or may be, normally in the stomach 

 during ingestion, yet hydrochloric acid is undoubtedly the 

 free acid proper to the gastric juice. Digestion, however, 

 will proceed under a proper (variable) degree of an acidity 

 from almost any acid. 



Beyond an insignificant effect in converting cane sugar 

 into dextrose, its function is a passive one, namely, that of 

 simply making the secretion acid, so that pepsin may act upon 

 the proteids. 



Pepsin. Pepsin is a proteolytic enzyme, the composition 

 of which has not been determined. From the definition, it 

 converts proteids into peptones. It operates only in an acid 

 medium. Hence its action is contingent upon the presence 

 of another constituent of the gastric juice, namely, hydro- 

 chloric acid. Pepsin is a typical enzyme, and reference to 

 the characteristics of those bodies will avoid repetition of its 

 properties here. 



Rennin. Rennin has the property of coagulating milk. 

 It acts upon the soluble proteid of milk (casein), changing it 

 into an insoluble product, which is precipitated. Acids also 

 will coagulate casein. Milk when left standing at ordinary 

 temperature has lactic acid produced by the action of bac- 

 teria upon the lactose in it, and this acid precipitates the 

 curd. The acid of the gastric juice might be sufficient to 

 bring about this result, but the quick coagulation of milk 

 when it is introduced into the stomach is probably not due 



