234 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



in a preparatory form (rennin-zymogen) in stomachs of all mammals. This 

 enzyme has been given several names ; rennin seems preferable to any other, 

 and is the term most commonly employed. Rennin may be obtained from 

 the stomach by self-digestion of the mucous membrane or by extracting it 

 with glycerin. Such extracts usually contain both pepsin and rennin, but the 

 two have been separated successfully, most easily by the prolonged action of 

 a temperature of 40 C. in acid solutions, which destroys the rennin, but not 

 the pepsin. Good extracts of rennin cause clotting of milk with great rapidity 

 at a temperature of 40 C., the milk (cow's milk), if undisturbed, setting first 

 into a solid clot, which afterward shrinks and presses out a clear yellowish 

 liquid, the whey; with human milk, however, the curd is much less firm, 

 being deposited in the form of loose flocculi. The whole process resembles the 

 clotting of blood not only in the superficial phenomena, but also in the 

 character of the chemical changes. Briefly, what happens is that the rennin 

 acts upon a soluble proteid in the milk known usually as casein, but by some 

 called " caseinogen," and changes this proteid to an insoluble modification which 

 is precipitated as the curd. The chemistry of the change is not completely 

 understood, and there is an unfortunate want of agreement in the terminology 

 used to designate the products of the action. It has been shown that, as in 

 the case of blood, curdling cannot take place unless lime salts are present. What 

 seems to occur is as follows : Casein is a complex substance belonging to the 

 group of nucleo-albumins, and when acted upon by rennin it undergoes hydro- 

 lytic cleavage, with the formation of two proteid bodies, paracasein and whey 

 proteid. The first of these bodies forms with calcium salts an insoluble com- 

 pound which is precipitated as the curd ; the second remains behind in solution 

 in the whey. It will be seen that this theory supposes the action to be parallel 

 with that occurring in blood-coagulation, where fibrin ferment causes a cleavage 

 of the fibrinogen molecule, a part uniting with calcium to form the insoluble 

 fibrin, and a part much the smaller part remaining in solution in the serum 

 as fibrin-globulin. 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. One might suppose that the curdling 

 of milk in the stomach is caused by the acid present in the gastric secretion, 

 but it has been shown that perfectly neutral .extracts of the gastric mucous 

 membrane will curdle milk quite readily. 



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

 milk. Casein constitutes the chief proteid constituent of milk, and has there- 

 fore an important nutritive value. It is interesting to find that before its 

 peptic digestion begins the casein is acted upon by an altogether different 

 enzyme. The value of the curdling action is not at once apparent, but we 

 may suppose that casein is more easily digested by the proteolytic enzymes 

 after it .has been brought into a solid form. The action of rennin goes no 



