

304 A MANUAL OF PHYSIOLOGY 



It can be separated from pepsin by precipitating an acid 

 extract of calf's stomach with magnesium carbonate in 

 powder, and some neutral acetate of lead. The pepsin is. 

 mechanically carried down with the precipitate, but most of 

 the rennin remains in solution. The curdling of milk by 

 rennin is essentially a coagulation of casein. It seems to 

 be produced by the splitting up of a more complex body, 

 caseinogen, into two substances, one of which, casein, is 

 insoluble (in the presence of calcium phosphate, but not 

 otherwise), and forms the curd ; while the other, wliey- 

 proteid, is soluble, and passes into the whey. Dilute acid 

 will of itself precipitate casein, and the presence of acid, 

 and particularly hydrochloric acid, in the gastric juice helps 

 the action of the milk-curdling ferment. That a ferment is 

 really concerned in the process is, however, shown by the 

 fact that the juice, after being made neutral or alkaline, still 

 curdles milk, and that this power is destroyed by boiling. 

 The optimum temperature is the same as that of the other 

 ferments of the digestive tract, about 40 C. (p. 379). 



As to the exact function which the milk-curdling ferment 

 of the gastric juice performs in digestion, we have no precise 

 knowledge. It seems superfluous if we suppose that the 

 free acid is able of itself to do all that the ferment does 

 along with it. But there is evidence that the curd pro- 

 duced by the ferment is more profoundly changed than the 

 precipitate caused by dilute acids ; for the latter may be 

 redissolved, and then again curdled by rennin, while this 

 cannot be done with the former. We may suppose, then, 

 that the ferment is capable of effecting changes more 

 favourable to the subsequent action of the pepsin upon the 

 casein than those which the acid alone would effect. Or it 

 may be that the ferment acts in the early stages of digestion 

 before much acid has been secreted. We do not know 

 whether the curdling of milk renders it easier for the 

 watery portion to be absorbed by the walls of the stomach. 

 If this were the case, it would be a raison d'etre for early 

 curdling, since milk is a very dilute food, and the immense 

 proportion of water in it might weaken the gastric juice too 

 much for rapid digestion of the proteids. 



On fats gastric juice has no action, although it will dissolve 

 the proteid constituents of fat-cells, and the proteid substances 



