CHAP. XXIV.] THE GASTKIC JUICE. 207 



stated the leading facts which observation and experiment have 

 developed respecting the act of digestion in the stomach, it remains 

 to enquire what is the real nature of the digesting power of the 

 gastric fluid. 



Two questions present themselves for consideration : is the digest- 

 ing power of the stomach a true solvent power, producing simply 

 a solution of the matters submitted to its action, without effect- 

 ing any change in their chemical constitution ? or does the digestive 

 fluid exercise a catalytic action on the substances submitted to it, 

 whereby it effects a chemical decomposition of them, similar to that 

 produced in barley by diastase, whereby the starch of the grain is 

 converted into sugar, or like the action of yeast upon sugar, whereby 

 the latter is decomposed into carbonic acid and alcohol ! 



To decide these questions, it is necessary to examine the exact 

 nature of the changes produced in the food by stomach digestion. 



Milk. If milk be introduced into the stomach, its caseine is first 

 coagulated and afterwards apparently dissolved. The solidified 

 caseine seems gradually to melt down and becomes absorbed. In 

 overfed infants, milk-curd appears in the stools in considerable 

 quantity, the child having received so much milk that its stomach 

 is unable to digest the caseine precipitated from it. When, how- 

 ever, the quantity of milk is in just proportion to the digestive 

 power of the stomach, all the curd is digested, and therefore none 

 is found in the stools. 



Albumen. White of egg (ovalbumen), if swallowed raw, is 

 immediately coagulated by the gastric juice and then dissolved. 

 Tiedemann and Gmelin found that after three hours' sojourn in the 

 stomach of a dog albumen was dissolved, forming " a yellowish 

 mucous liquor," which coagulated readily by heat. 



Coagulated albumen becomes softened down and dissolved in the 

 fluids of the stomach, from which it may again be precipitated 

 by heat or nitric acid. 



In experiments with the artificial digestive fluid, we find that if 

 the fluid in which albumen had been digested be carefully filtered 

 and subjected to heat and nitric acid, a copious precipitate of 

 albumen will take place. 



Meat is softened, gelatinized, and dissolved, and albumen may 

 be precipitated by heat, nitric acid, or ferro-cyanate of potass from 

 the liquids obtained from the stomach. 



Vegetable Substances. In all the experiments upon animals of 

 the carnivorous kind (cats, dog), bread, potatoes, and other vege- 

 table substances underwent change much more slowly than animal 



