DIGESTION 193 



been noted already, rennin is secreted by the central cells of the gastric 

 mucous glands coming apparently from different zymogenic granules 

 in the same cells that produce pepsin, these needing only reaction with 

 acid to produce pepsin and rennin respectively. Like pepsin, the f urn lie 

 glands produce it in much larger proportion than do the glands of the 

 antrum. Rennin has never been isolated; its optimum temperature is 

 about 40 C., and its action ceases at somewhat above zero. It is 

 destroyed by a lower degree of heat than are most enzymes, namely at 

 63 in acid medium and at 70 in a neutral medium. Rennet, the dried 

 fourth stomach of the calf, has been used for many centuries to curdle 

 milk for the purpose of making cheese. Why the caseinogen, the soluble 

 proteid of milk, requires coagulating before being hydrolyzed by the 

 proteolytic enzymes is quite unknown, and it is especially hard to under- 

 stand because the acidity of the gastric juice is sufficient after a while 

 to bring about this coagulation. Perhaps the coagulation is required 

 promptly on the entrance of food into the stomach, more promptly than 

 the acid could do it in the relatively undisturbed fundus. But it is more 

 likely that the rennin in some undiscovered way actually starts the 

 hydrolysis of the milk-pro teid, and still more likely, as already has been 

 noted, that rennin has important functions as yet quite unguessed. 

 The coagulation of milk in the stomach may even be a defect produced 

 by the pepsin, since Pawlow claims that all proteolytic enzymes coagulate 

 caseinogen. As has been said, rennin is perhaps only some chemical 

 aspect of pepsin. We may possibly be even mistaking rennin for what 

 is really an anti-rennin (an opponent of coagulation) in process of evolu- 

 tion. In the present ignorance of proteids themselves and of their 

 metabolism such numerous doubts in description and in theory are 

 inevitable. 



The mode of action of rennin in coagulating caseinogen to casein 

 is partly homologous to the coagulation of blood and some other body- 

 liquids by thrombin, but is not wholly similar. Caseinogen is the 

 protein of milk, and is apparently a nucleo-proteid soluble in the normal 

 fluid. On the addition of rennin to milk, calcium phosphate always 

 being present, this nucleo-proteid is made to absorb water and to split 

 up into paracaseinic acid and an albumin by the usual process of hydro- 

 lytic cleavage. At least two new proteins are produced by this splitting 

 process. One of these, casein (calcium paracaseinate), being insoluble, 

 falls as a precipitate, the curd. The other is a whey-globulin which 

 remains in solution, possibly with a third, an albumose, in small 

 amount. Whether the reaction in the case of human milk is quite like 

 this or not is in some slight degree of doubt, as nearly all the literature 

 on the subject relates to the curdling of cows' milk. If the calcium 

 phosphate be removed from the milk entirely, casein is not thrown 

 down. Some have thought that in this coagulating process the action 

 of the calcium was not to help the first reaction, changing the caseinogen 

 to casein, so much as to assist in the separation of the two cleavage- 

 proteids, casein and whey-proteid. The details of the process are, in- 

 13 



