260 THE TISSUES AND MECHANISMS OF DIGESTION. 



But this is not the case, for neutralized gastric juice, or neutral rennet, is 

 equally efficacious. 



The curdling action of rennet is closely dependent on temperature, being 

 like the peptic action of gastric juice favored by a rise of temperature up to 

 about 40 C. Moreover the curdling action is destroyed by previous boiling 

 of the juice or rennet. These facts suggest that a ferment is at the bottom 

 of the matter; and, indeed, all the features of the action support this view. 

 Moreover, as a matter of fact, a curdling ferment may be extracted by 

 glycerin and by the other methods used for preparing ferments. The fer- 

 ment, however, is not pepsin, but some other body ; and the two may be 

 separated from each other. 



It might be thought that the rennet-ferment, rennin we may call it, acted 

 by inducing a fermentation in the sugar of milk, giving rise to lactic acid 

 which precipitated the casein by virtue of its being an acid. But this view 

 is disproved by the following facts which show that the ferment produces its 

 curdling effect by acting directly on the natural casein itself. Casein maybe 

 precipitated unchanged, that is, capable of redissolving in water (the presence 

 of calcic phosphate being assumed) by saturating milk with neutral saline 

 bodies (such as sodium chloride or magnesium sulphate) ; and by being pre- 

 cipitated and redissolved more than once may be obtained largely free from 

 fat and wholly free from milk-sugar. Such solutions of isolated'casein freed 

 from milk-sugar may be made to curdle like natural milk by the addition 

 of rennin, showing that the milk-sugar has nothing to do with the matter. 

 Moreover, the precipitate thrown down from milk by dilute acids, lactic acid 

 included, is itself unaltered or very slightly altered casein, not curd, and 

 with care may be so prepared as to be redissolved into solutions which 

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

 salts. 



When isolated casein is curdled by means of reunin, two proteids, it is 

 stated, make their appearance, one 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 somewhat analogous formation of the fibrin, that this curdling action 

 will not take place if calcic phosphate be wholly absent from the mixture. 

 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, 1 becomes insoluble. 



Rennin is abundant in the gastric juice and in the gastric mucous mem- 

 brane 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. 



THE ACT OF SECRETION OF SALIVA AND GASTRIC JUICE AND THE 

 NERVOUS MECHANISMS WHICH REGULATE IT. 



194. The saliva and gastric juice whose properties we have studied, 

 though so different from each other, are both drawn ultimately from one 



1 It might be useful, in order to distinguish the curd from the natural soluble 

 casein, to call the former tyre/in (rvpoi, cheese), and so reserve the name of casein for 

 the latter. 



