CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 331 



with rennin, like solutions of casein prepared by means of neutral 

 salts. 



When isolated casein is curdled by means of rennin two pro- 

 teids, it is stated, make their appearance, one of which is soluble 

 and allied to albumin, and another, which is insoluble and forms 

 the curd. Curdling, therefore, according to this result appears to 

 be the splitting up by a ferment of a more complex body ; and it 

 is interesting to observe, as perhaps throwing light on the some- 

 what analogous formation of fibrin, that this curdling action will 

 not take place if calcic phosphate be wholly absent from the mix- 

 ture. The calcic phosphate appears to play a peculiar part in 

 determining the insolubility of the curd, for there is evidence 

 that in the absence of calcic phosphate the ferment has power to 

 attack the casein and split it up, but that both products remain 

 in solution ; if calcic phosphate be present, the one, viz. the curd, 

 becomes insoluble. 



Eennin is abundant in the gastric juice and in the gastric 

 mucous membrane of ruminants, but is also found in the gastric 

 juice of other animals, and either it, or what we shall presently 

 have occasion to speak of as the antecedent of the ferment or 

 zymogen, is present also in the mucous membrane of the stomach 

 of most animals. A very similar if not identical ferment has 

 also been found in many plants. 



