CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 399 



cipitation ceases to be caused by it, and the mixture be allowed 

 to stand, the clear fluid left above the precipitate will be found 

 to curdle milk readily, but even when acidified to have no peptic 

 action on proteids, shewing that the precipitate caused by the 

 addition of the magnesium carbonate has carried down all the 

 pepsin but left behind at least a good deal of the 'curdling' or 

 rennet-ferment. 



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 shew that the ferment produces its curdling effect by 

 acting directly on the natural casein itself. Casein may be pre- 

 cipitated 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 precipitated 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, shewing 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 pre- 

 pared 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 rennin two 

 proteids, 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 somewhat analogous formation of fibrin, that this curdling 

 action will not take place except in the presence of a calcic salt ; 

 in natural milk the calcic salt is the phosphate. The calcic salt 

 appears to play a peculiar part in determining the insolubility of 

 the curd, for there is evidence that in the absence of calcic salts 

 the ferment has power to attack the casein and split it up, but 

 that both products remain in solution ; if a calcic salt be present, 

 the one, viz. the curd, becomes insoluble. The term ' casein ' has 

 been used to denote on the one hand the more complex body 

 present in the natural milk, and on the other hand the simpler 

 body, the curd. It may be reserved for the latter, if the term 

 caseinogen (in analogy with fibrinogen) be used for the former. 

 We may here remark that though caseinogen has certain re- 

 semblances to alkali-albumin, it differs materially from that body. 

 For instance, unlike that body and most proteids, it contains a 

 considerable quantity of phosphorus, and nuclein may by digestion 



