CH. VIII.] RENNIN. 207 



lamp. In this way the temperature of the mixture can be maintained con- 

 stant, an impossibility when the tube has to be repeatedly removed for in- 

 spection. A stop-watch is an added convenience. 



E. Reniiin and the Clotting of Milk. 



When warm milk is treated with a neutral or faintly 

 acid extract of the mucous membrane of the stomach a 

 clot forms after a certain time. The solid portion or curd 

 contains the fat held together by insoluble calcium para- 

 caseinate, which is formed from the soluble calcium 

 caseinate of the milk by ferment action. The fluid portion 

 or whey contains the lactalbumin, lactose, and inorganic 

 constituents. 



The nature of the enzyme responsible for the change 

 has been much discussed. The most probable view is that 

 in the stomach of infants and other sucklings a specific 

 enzyme called rennin is present. But it also seems to be 

 true that all enzymes that can hydrolyse casein can cause 

 milk to clot. Thus pepsin, trypsin, erepsin and many of 

 the proteolytic enzymes found in plants will clot milk in 

 the presence of a suitable concentration of calcium and of 

 hydrogen ions. Many workers, notably Pawlow, have 

 urged that the clotting of milk by extracts of the stomach 

 is due to pepsin alone, and that a specific rennetic enzyme 

 does not exist. Such results are probably due to the fact 

 that they only studied extracts of the stomach of dogs and 

 pigs, which do not^seem to contain rennin. The results 

 obtained by Bang and Hammersten on the comparison 

 of the various enzymatic activities of extracts of the gastric 

 mucosa of the calf and the pig are taken by the author as 

 conclusive evidence of the existence of two separate 

 enzymes. The results obtained by the author on the 

 relative heat destruction of the clotting powers of the two 

 extracts at a definite hydrogen-ion concentration can be 

 readily repeated, and are most convincing. (See Ex. 256.) 



