DIGESTION IN THE STOMACH 387 



view that these bodies form the central nucleus round which the rest 

 of the protein molecule is built. 



The Action of Rennet. The rennet enzyme runs parallel in secre- 

 tion to pepsin. There is a prorennin in the mucous membrane, like 

 pepsinogen, which is not destroyed by a weak sodium hydrate solu- 

 tion, and is converted by hydrochloric acid ; but rennin, unlike pepsin, 

 can act also in neutral and faintly alkaline media. The action of 

 rennin is upon the caseinogen of milk. The molecule of this soluble 

 protein is rearranged by the action of the rennin, so that a body 

 called '' soluble casein " is formed. Here the action of the rennin 

 ceases. This soluble casein, in the presence of calcium salts, forms 

 insoluble casein, or the clot of milk. 



(1) Caseinogen + Rennin = Soluble casein. 



(2) Soluble casein + Ca salts = Insoluble casein, or clot. 



Thus, if in a test-tube experiment rennin be added to some milk, 

 from which the calcium salts have been removed by the addition of 

 a soluble oxalate, its action proceeds to stage (1), and no clot forms. 

 If after, say, fifteen minutes the milk be heated to 100 C., the rennet 

 enzyme is destroyed. The addition of calcium salts now causes the 

 clot to form, thus showing that the rennet enzyme is merely concerned 

 in the rearrangement of the molecule, and not in the formation of the 

 insoluble clot. 



Why a rennet enzyme should be provided to clot milk is somewhat 

 a mystery, since unclotted milk is digested by pepsin and trypsin. 

 It is noteworthy that in plants, as well as animals, a rennet enzyme 

 is found running parallel in secretion with proteolytic enzymes. The 

 clotting may be merely a result of the action of the proteolytic 

 enzyme on the protein caseinogen. 



The Lipase of the Gastric Juice. The presence of a gastric lipase 

 has been established : at one time it was held to be due to a reflux 

 of pancreatic juice through the pylorus. It exerts its maximum 

 action in neutral or faintly acid medium. Therefore, normally its 

 action in the stomach is not at all potent. Nevertheless, it is probably 

 important, inasmuch as by its action neutral fats will be rendered 

 slightly rancid, and these fats, on entering the small intestine, owing 

 to this rancidity, will be far more easily and more finely emulsified 

 than would otherwise be the case, and their digestion thereby greatly 

 facilitated (see later, p. 395). 



The Action of Gastric Juice upon Starches and Sugars. In some 

 animals e.g., the, pig the starches will be digested by the amy- 

 lopsin of the cardiac juice. By the stomach of man and the dog there 

 is probably no amylopsin secreted. Nevertheless, the fact must not 

 be overlooked that in the cardiac reservoir salivary digestion normally 

 proceeds for thirty to forty minutes. 



The hydrochloric acid of the gastric juice exerts but little or no 

 hydro lyzing effect upon starches, dextrins, and the disaccharides. 



