290 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



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: Caseiu is a complex substance belonging to the 

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

 lytie 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 solu- 

 tion in the whey. 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. ( 1 asein constitutes the chief proteid constituent of milk, and has there- 

 lore 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 

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

 further than the curdling; the digestion of the curd is carried on by the pep- 

 sin, and later, in the intestines, by the trypsin, with the formation of proteoses 

 and peptones as in the case of other proteids. 



Action of Gastric Juice on Carbodydrates and Fats. — Human gastric 

 juice itself has no direct action upon carbohydrates; that is, it does not con- 

 tain an amylolytic enzyme. It is possible, nevertheless, that some digestion 

 of carbohydrates goes on in the stomach, for, as has been seen, the masticated 

 food is thoroughly mixed with saliva before it is swallowed. The portion 

 that enters the stomach in the beginning of digestion, when the acidity of the 

 total content- is small (see p. 289), may continue to be acted upon by the 

 ptyalin. According to a recent author, 1 the gastric juice of the dog contains 

 an amylolytic enzyme capable of acting on starch even in the presence of free 

 HC1 (0.5 per cent.). This statement needs confirmation, perhaps, and there is at 

 presenl no evidence of the existence of a similar enzyme in the human gastric 

 secretion. It should be added, however, that Lusk - has shown that cane-sugar 



can be inverted to dextrose and levulose in the stomach. The importance of 



this process of inversion, and the means by which it is accomplished, will be 



1 Friedenthal: Arehivfur Physiologie, 1899; Suppl. Bd. 383. 

 2 Voit: Zeitsehrifi fur Biohgie, 1891, Bd. xxviii. S. 269. 



