DIGESTION AND ABSORPTION IN THE STOMACH. 717 



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 proteins.* 



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 proteplytic 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. 399). If rennin be injected subcutanepusly 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 

 have described other enzymes in the gastric juice or gastric mem- 

 brane a lipase or fat-splitting enzyme (Volhard), an amylolytic 

 or starch-splitting enzyme (Friedenthal) , and an inverting enzyme 

 (Widdicombe) but the normal existence or at least the normal 

 action of these latter enzymes in digestion is a matter about which 

 little is known. As was said above, it is probable that the ptyalin 

 swallowed with the food continues to exert its action upon the 

 starchy materials in the fundus for a long time, so that in this way 

 the starch digestion in the stomach may be important. Regarding 

 the fats, it is usually believed that they undergo no truly digestive 

 change in the stomach. They are set free from their intimate 

 mixture with other foodstuffs by the dissolving action of the gastric 

 juice upon proteins, they are liquefied by the heat of the body, and 

 they are disseminated through the chyme in a coarse emulsion by 



* For references to the very abundant literature consult Oppenheimer, 

 loc. cit. 



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



