34-8 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



coagulating milk, while the pepsin remains unaffected. In neutral solu- 

 tions, on the other hand, the milk-curdling ferment may be heated up to 70 

 C., or even may be boiled for a moment without being entirely destroyed. 

 Alcohol only slowly interferes with the milk-curdling ferment ; caustic 

 alkalies rapidly destroy it. Even .02 per cent, of caustic soda is suffi- 

 cient to cause a previously active ferment solution to become entirely 

 inactive. Salicylic acid does not interfere with its action. In common 

 with the other ferments, an almost infinitely small amount of this ferment 

 will coagulate an immense volume of milk. Hammarsten precipitated a 

 gtycerin extract of milk-curdling ferment with alcohol, dissolved the 

 resulting precipitate in water, and, since the percentage of solid in this 

 solution could be readily determined, was able to estimate that one part 

 by weight of milk-curdling ferment would coagulate at least from 

 four hundred thousand to eight hundred thousand parts of casein. The 

 milk-curdling ferment is entirely without action on sugar solutions and 

 is without influence in the digestion of albumen. It has been found that 

 milk-curdling ferment is principally secreted by the glands of the fundus 

 of the stomach, while the pyloric region furnishes but a small amount of 

 this ferment. Milk-curdling ferment may almost invariably be found 

 in wateiy extracts of the stomach of the calf and sheep, while in other 

 mammals and birds it is usually absent, and is scarcely ever to be de- 

 tected in the stomach of the fish, even although watery extracts of the 

 stomachs of these animals become effective after being first acidulated 

 and then after twenty-four hours again neutralized. This would seem to 

 show that the acid serves to develop milk-curdling ferment out of some 

 previously inactive body. The coagulation of milk by the milk-curdling 

 ferment is more analogous to the process of coagulation of the blood 

 than to our generally accepted ideas as to the processes of fermentation ; 

 for the casein, a soluble albuminoid body, through the action of rennet 

 simply becomes insoluble without undergoing any other change. Its 

 action is, therefore, directly opposed to that of pepsin, which converts 

 an insoluble albuminoid into a soluble bod t y. 



A difference also exists in the result of coagulation of milk, accord- 

 ing as this coagulum has been produced through the action of the 

 ferment or by the development of acid. In the latter case the precipi- 

 tate is still casein, in the former case it is cheese. In the former instance 

 the casein is precipitated in fine, tender flocculi, which are readily soluble 

 in dilute acid, but solutions that are coagulated by rennet are very much 

 less soluble. 



The process differs still further in that the casein precipitated by 

 acids, if carefully washed, may be obtained perfectly free from ash. 

 Casein precipitated by a milk-curdling ferment, on the other hand, always 

 contains phosphate of lime, and this salt seems to be essential to the 



