DIGESTION AND THE DIGESTIVE AGENTS. 335 



found in a much more concentrated form in the stomachs 

 of sucking animals, for which reason these stomachs are 

 used in the preparation of cheese, but in diluted and weaker 

 forms it maybe extracted from the stomachs of all mammals. 

 No doubt the curdling of milk in the adult stomach is 

 largely due to the presence of the acid in the stomach ; but 

 it can be shown that gastric juice may be perfectly neutral 

 and still retain its power of curdling milk. The chemical 

 nature of this process is not known. Possibly it is not very 

 unlike the coagulation of blood, a fact which seems to gain 

 credence in the observation that the milk will not clot or 

 curdle unless lime salts are present. The action of the 

 rennet, however, goes no further than the curdling of the 

 milk. As soon as this is effected its action ceases, and by 

 the pepsin the milk is converted into soluble peptones. 



The mineral salts are the chlorides and phosphates of 

 potassium, calcium, magnesium and iron, but as they figure 

 in no integral way in gastric digestion we are not specially 

 concerned with them here. 



GASTRIC DIGESTION OF CARBOHYDRATES AND 

 HYDROCARBONS. 



Gastric juice exerts no digestive action upon the 

 starches. The starches in a perfectly unaltered condition 

 are passed into the intestine, and their digestion there 

 turned over to the pancreas. Nor are the sugars affected, 

 although there would be no special reason for a change in 

 sugars, as all the sugars are soluble and dialyzable to begin 

 with. Neither are the fats affected. Bits of tallow, lard 

 or butter suffer no digestive changes in the stomach, save 

 that by the general mixing of the stomach they are broken 

 up into small bits. However, very frequently the fats are 

 eaten not as pure fat, but in the form of droplets of fat en- 

 closed in albuminous coverings. Thus in ordinary milk and 

 cream, the butter is held in the form of minute droplets, 

 each surrounded by a thin albuminous envelope. To get 

 the butter in a pure form it is of course necessary to break 



