THE CHEMISTRY OF THE DIGESTIVE JUICES 349 



that the ferment does along with it. But there is evidence that the 

 curd produced by the ferment is more profoundly changed than the 

 precipitate caused by dilute acids; for the latter may be redissolved, 

 and then again curdled by rennin in the presence of calcium salts, 

 while this cannot be done with the former. We may suppose, then, 

 that the ferment is capable of effecting changes more favourable to 

 the subsequent action of the pepsin upon the casein than those 

 which the acid alone would effect. Or it may be that the ferment 

 acts in the early stages of digestion before much acid has been 

 secreted. The curdling of milk probably plays a part in ensuring 

 the retention of this food, the proper digestion of which is all-impor- 

 tant for the suckling, for a sufficient length of time in the stomach. 

 Otherwise, like water and watery liquids in general, it would be 

 rapidly passed into the duodenum. Even if this were not the case, 

 there is another reason for early curdling. Milk is a very dilute 

 food, and the immense proportion of water in it might weaken the 

 gastric juice too much for rapid digestion of the proteins. The 

 separation of the whey and its prompt escape through the pylorus 

 would obviate this. But caution should be exercised in giving a 

 physiological value to all the details of the milk-curdling action of 

 the gastric juice. Milk-curdling ferments, or, at any rate, ferments 

 with a milk-curdling influence, have an extremely wide distribution, 

 both in secretions which in normal circumstances can never come 

 into contact with milk, and in the tissues of animals and plants. 

 Many bacteria produce them. And it appears that in the suckling, 

 where it might be expected, if anywhere, to have a definite and 

 important office, the rennet action of the gastric juice is distinctly 

 less than in the adult. It is worthy of note that the curd formed by 

 rennet from human milk is more finely divided than that formed 

 from cow's milk, and therefore is more easily digested. The addition 

 of lime-water or barley-water to cow's milk keeps the curd from 

 adhering in large masses, and thus aids its digestion a fact which 

 is sometimes usefully applied in the artificial feeding of infants. 



Gastric Lipase. On fats gastric juice has usually been supposed 

 to have no action, although everybody admits that it will dissolve 

 the protein constituents of fat-cells and the protein substances which 

 keep the fat-globules of milk apart from each other. It has, how- 

 ever, been recently shown that both in the stomach and in vitro 

 (with glycerin extracts of the gastric mucous membrane) a consider- 

 able amount of well -emulsified fat may be split up, and that this is 

 due to a ferment which is different in several respects from the 

 lipase of pancreatic juice. Gastric juice splits up fat, both in 

 neutral and in weakly acid solutions. The slightest excess of alkali 

 checks the action. The glycerin extract is much more resistant to 

 alkali, while very sensitive to hydrochloric acid. This indicates 

 that the fat-splitting ferment exists in the mucous membrane in a 



