328 A MANUAL OF PHYSIOLOGY . 



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. 



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, however, been recently shown that both in the stomach 

 and in vitro (with glycerin extracts of the gastric mucous 

 membrane) a considerable amount of well-emulsified fat may be 

 split up. 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 different form from that in which it exists in the juice 

 namely, as a zymogen or mother-substance. As regards the 

 carbo-hydrates the swallowed saliva will continue to act on starch 

 in the stomach, so long as the acidity is not too great ; while the 

 hydrochloric acid of the gastric juice is able to invert cane-sugar, 

 changing it into a mixture of dextrose and levulose,* and also, 

 doubtless, to hydrolyse to dextrose a portion of the maltose 

 formed by the saliva. Altogether, there is no doubt that the 

 proportion of the carbo-hydrates of the food digested in the 

 stomach is far from insignificant. 



The Antiseptic Function of the Gastric Juice. The 

 stomach, with its acid contents, forms during the greater part 

 of gastric digestion a valve or trap to cut off the upper end of 



* These are both reducing sugars, but, as their names indicate, they 

 rotate the plane of polarization in opposite directions. The specific 

 rotatory power of levulose is greater than that of dextrose, so that when 

 cane-sugar is completely inverted, although equal quantities of dextrose 

 and levulose are produced, the plane of polarization is rotated to the left. 

 Cane-sugar itself rotates it to the right. The term ' inversion ' has been 

 extended to include the similar hydrolysis of other sugars of the disacchar- 

 ride group e.g., maltose to dextrose, and lactose to a mixture of dextrose 

 and galactose, even although the products are not levo-rotatory. 



