206 LECTURE X. 



cleavage-products was due to the peculiar nature of casein, which we will 

 soon consider. In this case it would be more correct not to speak of a 

 milk-coagulating function, but of that of the proteolytic ferment, pepsin. 

 We are inclined to look upon the pepsin and rennin ferment as probably 

 identical, not from the fact that these have never been isolated in a satis- 

 factorily pure condition, but more especially because of the interesting 

 observations of Pawlow and Parastschuk. They found that the secretions 

 of the pancreatic gland act towards casein in exactly the same manner as 

 does the gastric juice, with this modification, that the proteolytic ferment 

 of the pancreatic juice, trypsin, acts only in alkaline, neutral, or weakly 

 acid solutions. The establishment of this fact has settled one thing. We 

 must either assume that different ferments exist which will coagulate milk, 

 i.e., one which acts in distinctly acid reaction, and another which is efficient 

 in neutral or alkaline solution (the rennin of the stomach and the trypsin 

 from the pancreas are activated by entirely different agents) ; or, as is far 

 simpler, we must assume that only one process takes place, namely, a 

 hydrolysis. Coagulation occurs as a secondary effect in the general 

 decomposition of casein. It is caused by the precipitation of the early 

 cleavage-products. It is possible that this stage of decomposition, which 

 probably takes place before the formation of peptones, is common to 

 all proteins. On the other hand, it is also possible that casein occupies 

 a unique position, and that perhaps, corresponding to its functions, it 

 represents a particularly complicated protein. We should like to place 

 stress upon the above observations of Pawlow rather than upon the 

 established similar behavior of the two ferments, and will state once more 

 that we do not wish to speak of two different actions of one ferment. 

 Until we have an accurate knowledge of these relations we will retain 

 the conception of two separate ferments, pepsin and rennin, in our pre- 

 sentation of the subject. 



Rennin is very widely distributed in the whole animal kingdom, and 

 ferments acting in an analogous manner are undoubtedly found in the 

 vegetable world. It has been found in animals which chew their cud, 

 and especially in the fourth, so-called rennet-stomach of the calf. Its 

 main function has generally been considered to be the curdling of milk. 

 It has been found, however, that this precipitation of casein is not the 

 primary process. The nature of casein is changed first of all by the action 

 of the rennin. Another protein with different properties is produced. 

 Curdling depends on the formation of insoluble calcium salts, arising from 

 the casein, which has been changed by the rennin. That this conception 

 is correct is evident from the fact that casein will not be curdled by rennin 

 if the solution is free from calcium salts, but it will, nevertheless, undergo 

 a change. If the casein, treated in the above manner, is then boiled, 

 thus destroying the rennin, it will curdle on the addition of calcium 



