160 PHYSIOLOGICAL CHEMISTRY 



paracasein is insoluble. When renniii has transformed the case- 

 inogen into paracasein, the calcium salt of the latter precipitates, 

 and the milk is said to be clotted. Removal of the calcium by 

 precipitation as oxalate will prevent clotting, but will not inter- 

 fere with the formation of paracasein. When this has been 

 formed by the action of rennin in oxalated milk, the rennin may 

 be destroyed by boiling. Now, on adding a soluble calcium salt 

 to the milk it will coagulate, showing that the transformation of 

 caseinogen into paracasein has taken place in the absence of 

 calcium. 



Are pepsin and rennin identical? Not only does gastric juice 

 clot milk, but extracts of a very large number of substances con- 

 taining proteolytic enzymes show a similar behavior. The ques- 

 tion has thus arisen, are pepsin and rennin identical, or are they 

 different enzymes? This question is still in dispute. The trend 

 of evidence seems to indicate, however, that they are not identi- 

 cal. Solutions have been prepared which showed peptic but not 

 rennin action, and also the reverse is true. Also, peptic digestion 

 takes place only in acid solution, whereas rennin will act also 

 in neutral or weakly alkaline solution. Pepsin itself undoubt- 

 edly can clot milk, however. There is still no uniformity of 

 opinion as to whether the two enzymes are, or are not identical, 

 although as already stated, opinion is tending toward the latter 

 view. 



The value of rennin to the young animal depends on the fact 

 that by clotting, the chief protein of the milk is retained in the 

 stomach instead of running on through into the small intestine. 

 It is thus subjected to the action of the enzyme pepsin. 



Gastric juice contains a lipase, which acts on fats. The action 

 is not extensive however, except in the case of emulsified fats, 

 such as those of milk, which are digested to a considerable ex- 

 tent. 



The contents of the small intestine may occasionally regurgi- 

 tate into the stomach. Ordinarily this takes place only to a 

 slight extent. In this way the digestive enzymes of the intestine 



