DIGESTION 327 



He states that when the comparison is instituted under proper 

 conditions there is an exact parallelism between the proteolytic 

 and the milk-curdling power of these secretions, no matter 

 what the circumstances may be in which they are collected, 

 or the influences to which they are exposed after collection. 

 He has found it impossible to separate from any one of them 

 a fraction which has milk-curdling power without proteolytic 

 power. 



The curdling of milk by the gastric ferment includes two pro- 

 cesses : (i) An action on caseinogen in the course of which a sub- 

 stance, whey-protein, not previously present in the milk, is pro- 

 duced. This substance is not capable of being converted into 

 casein, and remains in solution in the whey. (2) The altered 

 caseinogen is precipitated in the presence of calcium salts, but 

 not otherwise, as casein, which is insoluble, and forms the curd. 

 Dilute acid will of itself precipitate caseinogen, and the presence 

 of acid, and particularly hydrochloric acid, in the gastric juice 

 helps its milk-curdling action. But that a ferment is really 

 concerned is indicated 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- 315). 



As to the exact function of the milk-curdling ferment of 

 the gastric juice 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 produced 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 in the presence of calcium salts, 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, 

 or insures that it shall be more rapidly passed on into the duo- 

 denum. 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 pro- 

 portion of water in it might weaken the gastric juice too much for 

 rapid digestion of the proteins. But caution should be exercised 

 in giving a physiological value to all the details of the milk- 

 curdling action of the gastric juice. Milk-curdling ferments, or, 

 at any rate, ferments with a milk-curdling influence, have an 



