GASTRIC SECRETION AND DIGESTION 85 



Digestion in the Stomach. The gastric juice is usually 

 said to contain two enzymes. Recent work indicates the 

 presence of a third. The two familiar ones are pepsin and 

 rennin. The third is the gastric lipase. Certain writers 

 have questioned whether we ought to speak of pepsin and 

 rennin as distinct individuals, suggesting rather that there 

 is in the secretion a single body having two sets of proper- 

 ties. We need not enter into such a discussion; we shall 

 for the present continue the convenient usage of speaking 

 of pepsin and rennin as two substances. 



Rennin. The fact that extracts of the stomach wall 

 cause the curdling or coagulation of milk has been known 

 from very early times. Such extracts, usually derived 

 from the stomach of the calf, have long been in use in the 

 manufacture of cheese. Rennet is the industrial term for 

 an extract with this property; rennin is the scientific term 

 for the supposed enzyme contained in it. Cheese curd 

 consists of the bulk of the protein of milk which has 

 undergone an obscure chemical change and has passed into 

 an insoluble form. From a physical standpoint this is an 

 anomaly among the digestive processes. We look to see 

 solids becoming liquids, while in this curious instance a 

 liquid becomes a solid. No very convincing explanation 

 of this occurrence has been offered. It may be suggested 

 that it prevents an unduly rapid passage to the small in- 

 testine, but so far as we know the mechanism of the 

 pylorus is entirely competent, even for liquid food. The 

 curd when formed has to undergo solution like any other 

 solid. 



The action of rennin becomes the more enigmatic when 

 it is noted that it is found in the stomachs of animals 

 which do not have milk in their normal diets. Milk is 

 curdled by extracts of various organs other than the digest- 

 ive glands and by some vegetable juices. In the human 

 stomach a very firm curd may be formed when a large 

 quantity of cows' milk is taken at one time. The dense 

 mass may be slow to digest. Human milk is said never to 

 set into such a tenacious coagulum, and this is natural, 



