716 PHYSIOLOGY OF DIGESTION AND SECRETION. 



milk has been known from remote times, and has been utilized in the 

 manufacture of cheese and curds. This action takes place with 

 remarkable rapidity under favorable conditions, a large mass of milk 

 setting to a firm coagulum within a very brief time. It .has been 

 shown that this effect is due to an enzyme rennin or rennet. The 

 rennin. like the pepsin, is supposed to be formed in the chief cells of 

 the gastric tubules and to be present in the glands in a zymogen 

 form, the prorennin or prochymosin, which after secretion is con- 

 verted to the active enzyme. This conversion takes place very 

 readily under the influence of acid. Rennin (or its zymogen) may be 

 obtained easily from the mucous membrane of the stomach '(with 

 the exception of the pyloric end) by extracting with glycerin or water 

 or by digesting with dilute acid. Good extracts of rennin cause 

 the milk to clot with great rapidity at a temperature of 40 C.; the 

 milk (cows' milk), if undisturbed, sets at first into a solid clot, which 

 afterward shrinks and presses out a clear, yellowish liquid the 

 whey. With human milk the curd is much less firm, and takes the 

 form of loose flocculi. The whole process resembles much the clotting 

 of blood. The rapidity of clotting is said to vary inversely as the 

 amount of rennin, or, in other words, the product of the amount of 

 rennin and the time necessary for clotting is a constant. The 

 curdling of the milk involves two apparently independent proc- 

 esses : First, the rennin acts upon the casein of the milk and converts 

 it into a substance known as paracasein. The paracasein then 

 reacts with the lime salts of the milk, forming an insoluble calcium 

 salt, which constitutes the curd or coagulum. According to this 

 view, the enzyme does not cause clotting directly. What takes place 

 when the casein is changed to paracasein is not understood. Ham- 

 marsten originally regarded the change as a cleavage process, but 

 this view has not been supported. Others have supposed that a 

 transformation or rearrangement of molecular structure occurs. 

 Indeed, the differences in properties between casein and paracasein 

 are not great, the most marked difference being that the calcium 

 salts of the latter are insoluble. If soluble calcium salts are removed 

 from milk by the addition of oxalate solutions it does not curdle upon 

 the addition of rennin. Addition of lime salts restores this property. 

 It should be added that casein is also precipitated from milk by the 

 addition of an excess of acid. The curdling of sour milk in the 

 formation of bonnyclabber is a well-known illustration of this fact. 

 When milk stands for some time the action of bacteria upon the milk- 

 sugar leads to the formation of lactic acid, and when this acid 

 reaches a certain concentration it causes the precipitation of the 

 casein. 



So far as our positive knowledge goes, the action of rennin is 

 confined to milk. Casein is the chief protein constituent of milk, 



