GASTRIC GLANDS 9! 



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 fermentation of carbonydrates 

 is, or may be, normally in the stomach during digestion, yet 

 hydrochloric acid is undoubtedly the free acid proper to the gas- 

 tric 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 con- 

 verts proteids into peptones. It operates only in an acid me- 

 dium. Hence its action is contingent upon the presence of an- 

 other constituent of the gastric juice, namely, hydrochloric 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 tem- 

 perature has lactic acid produced by the action of bacteria 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 to the acid, since neutral ex- 

 tracts of the gastric mucous membrane will themselves curdle 

 milk. After coagulation the action of pepsin begins and the 



