198 STOMACH DIGESTION. 



that it decomposes the alkaline phosphates, without which the 

 bacteria cannot live. 



Pepsin The chief or central cells of the cardiac glands present 

 a granular appearance ; this is due to granules which are the product 

 of the cells, and consist of a zyraogen, pepsinogen. During gastric 

 digestion they diminish, so that the outer portions of the cells 

 become quite clear, losing / the granular appearance, while the 

 inner portions, or those near the lumen of the tube, retain it. 

 While the chief cells of the cardiac glands doubtless produce 

 most of the pepsinogen, still it has been abundantly proved 

 that the same variety of cells of the pyloric glands also produces 

 this zymogen. The pepsinogen thus formed becomes converted 

 into pepsin, which exists in the human stomach at birth. 



Pepsin is a proteolytic enzyme which acts only in an acid 

 medium, so that the presence of the acid is as essential to stomach- 

 digestion as is that of the enzyme. While hydrochloric acid gives 

 the best results, some other acids may be substituted ; thus nitric 

 and lactic, and even oxalic and tartaric acids, will exert a pro- 

 teolytic action. 



In the case of pepsin, as also of ptyalin, and indeed of all 

 enzymes, the effect of temperature upon the zymolytic process 

 must always be considered. The optimum temperature for the 

 action of pepsin is from 37 to 40 C. ; while if exposed to 80 C. 

 in a moist state, the enzyme loses its proteolytic power. Low 

 temperatures inhibit its action, but it still acts, though feebly, at 

 0C. 



It has already been stated that the enzymes have not as yet 

 been obtained in sufficient quantity or sufficiently separated from 

 other substances to analyze, so that the composition of pepsin is 

 still undetermined. 



Pepsin-hydrochloric Acid. It is held by some authorities that 

 the pepsin and the hydrochloric acid exist in gastric juice in 

 a state of combination, to which the name of pepsin-hydrochloric 

 acid has been given, but this cannot be said to have been demon- 

 strated. 



Rennin. This enzyme exists in human gastric juice at birth, 

 and it appears to be more abundantly produced in the fundus 

 than in the pyloric region, though the exact seat of its formation 

 is not determined. Its action is to coagulate the caseinogen ; the 

 changes which this undergoes in the process of coagulation have 

 already been fully discussed (p. 112), and need not be repeated 

 here. Its optimum temperature is between 38 and 40 C., while 

 at 63 C. in an acid medium it is destroyed. The curdling proc- 

 ess precedes the action of the hydrochloric acid and pepsin during 

 the gastric digestion of milk. 



Mothers are sometimes alarmed when their children, seemingly 

 in perfect health, vomit curdled milk ; but, as has been stated, this 



