772 PHYSIOLOGY 



On PHOSPHO-PROTEINS gastric juice acts in a somewhat similar 

 manner. The protein of milk, caseinogen, undergoes special changes 

 in the stomach. The first effect of gastric juice, even in neutral 

 medium, is to convert the caseinogen into an insoluble casein. This 

 action is generally ascribed to the presence of a distinct ferment 

 of the gastric juice, named rennin, or rennet ferment. But, according 

 to some authorities, it is due directly to the pepsin, i.e. rennin and 

 pepsin are identical. For the conversion of caseinogen into the solid clot 

 of casein the presence of lime salts is necessary. The addition of rennet 

 to an oxalated milk apparently produces no effect, but clotting ensues 

 if a soluble lime salt, such as calcium chloride, is then added to the 

 mixture. Under the action of the acid gastric juice the solid clot 

 of casein is dissolved, but a precipitate is left containing a small 

 proportion of the original phosphorus of the caseinogen. This pre- 

 cipitate is sometimes spoken of as para-nuclein, or pseudo-nuclein. 

 It does not yield the typical purin bases on hydrolysis with acids, 

 but contains phosphoric acid in organic combination. By prolonged 

 digestion with strong gastric juice it is possible to dissolve the whole 

 of this precipitate. It is therefore thought that in the clotting of 

 milk the caseinogen under the action of the rennet first undergoes 

 a conversion into a soluble casein, or perhaps a splitting into a 

 soluble casein and some other globulin-like body. The soluble 

 casein then, under the influence of the lime salts, forms an insoluble 

 casein which is precipitated, and causes the solidification of the milk, 

 In the absence of lime salts, the conversion or splitting of caseino- 

 gen takes place, but the second stage of the process cannot occur 

 until the lime salts are added. 



THE EFFECT OF GASTRIC JUICE ON CARBOHYDRATES 



On account of the fact that cane sugar undergoes inversion into 

 equal molecules of glucose and fructose in the stomach, it has been 

 sometimes thought that gastric juice contains a ferment invertase. 

 It seems, however, that the inversion which takes place in the stomach 

 can be completely accounted for by the action of hydrochloric acid 

 present, and that there is no need to assume the presence of a special 

 ferment. 



In the same way inulin, the variety of starch which gives rise 

 to the laevorotatory sugar fructose on hydrolysis, and is found 

 in dahlia tubers and certain other reserve structures of plants, is 

 converted by the acid of gastric juice into fructose. The inulin is 

 therefore completely utilised in the alimentary canal of animals, 

 although there is no definite ferment inukse provided for its hydrolysis, 



