DIGESTION AND ABSORPTION IN THE STOMACH. 695 



five minutes in water at 95 C. After some time the tube is cut into 

 lengths of 10 to 15 rams, and these are used to test the digestive action 

 or amount of pepsin. One or more of the tubes are placed in the 

 solution to be measured and kept for ten hours at body temperature. 

 The digestive power is measured in terms of the length in millimeters 

 of the column of egg-albumin that is dissolved. The relative amounts 

 of pepsin in solutions compared in this way are determined by the 

 law of Schiitz, according to which the digestive power is proportional 

 to the square root of the amount of pepsin. If in two specimens of 

 gastric juice the number of millimeters of egg albumin digested 

 was in one case two and in the other three, the digestive powers of the 

 two solutions would be as the squares of the numbers, as 4 to 9. 



The Rennin Enzyme (Rennet, Chymosin). The property 

 possessed by the mucous membrane of the calf's stomach of curdling 

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

 manufacture of cheeze 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 action 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 it 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 setting 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. 



