696 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 proteid constituent of milk, 

 and has therefore 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 under the conditions that exist in the body after 

 it has been brought into a solid form. This has, however, been 

 doubted, and it has even been suggested that the process is a hin- 

 drance rather than an aid to the digestion of the casein. Until the 

 contrary is definitely demonstrated it is preferable to assume that 

 the process is of importance in the digestion of milk. The action of 

 rennin goes no further than the curdling; the digestion of the curd 

 is carried on by the pepsin, and later, in the intestines, by the 

 trypsin, with the formation of proteoses and peptones as in the 

 case of other proteids.* 



Rennin is found elsewhere than in the gastric mucosa. It has been 

 described in the pancreatic juice, in the testis, and in many other organs 

 as well as in the tissues of many plants. In fact, wherever proteolytic enzymes 

 are found there also some evidence of a curdling action on milk may be ob- 

 tained. For this reason some observers f have taken the view that the milk coag- 

 ulation is not due to a specific ferment, but is an action of the pepsin itself. 

 That is, the proteolytic enzyme is capable of causing the change from casein 

 to paracasein as well as the hydrolysis of the proteid. This view is opposed to 

 the prevalent opinion regarding the specificity of enzyme actions. 



Another interesting fact concerning rennin is that an animal may be im- 

 munized against it (see p. 387). If rennin be injected subcutaneously in 

 an animal an antirennin will be formed in its blood. This antirennin added 

 to milk prevents its curdling by rennin, giving a result, therefore, similar 

 to the reaction between toxins and antitoxins. 



The Digestive Changes Undergone by the Food in the 

 Stomach. In addition to the pepsin and rennin various observers 



* For references to the very abundant literature consult Oppenheimer, 

 loc. cit. 



f See Pawlow and Parastschuk, " Zeitschrift f . physiol. Chemie, " 42, 415. 



