226 GASTEIC DIGESTION. 



away in the faeces. The nitrogenized constituents of bread are probably acted 

 upon in the stomach in the same way and to the same extent as albumen, 

 fibrin and caseine. 



Peptones. It has been shown that gastric digestion is not merely a solu- 

 tion of certain alimentary matters, but that these substances undergo very 

 marked changes and lose the properties by which they are generally recog- 

 nized. That the different products of this transformation resemble each 

 other very closely is also undoubted ; but there are certain differences in the 

 chemical composition of the products of digestion of the different constitu- 

 ents of food, as well as differences, which have lately been noted, as regards 

 their behavior with reagents. 



The peptones in solution form colorless liquids, having a feeble odor re- 

 sembling that of meat. They are not coagulable by heat or by most acids, a 

 property which distinguishes them from almost all of the nitrogenized con- 

 stituents of food. They are coagulated, however, by many of the metallic 

 salts, by chlorine, and by tannin, in slightly acidulated solutions. On evapo- 

 rating peptones to dryness, the residue consists of a yellowish- white substance, 

 resembling desiccated white of egg. This is soluble in water, when it regains 

 its characteristic properties, but is entirely insoluble in alcohol. 



It is evident that the gastric juice, aside from its action in preparing cer- 

 tain articles for digestion by the intestinal fluids, does not simply liquefy 

 certain of the alimentary matters, but changes them in such a way as to ren- 

 der them osmotic and provides against the coagulation which is so readily 

 induced in ordinary nitrogenized bodies. Peptones pass through membranes 

 with great facility. 



Another, the most important and the essential change which is exerted 

 by the gastric juice upon the albuminoids, is that by which they are rendered 

 capable of assimilation by the system after their absorption. Pure albumen 

 and gelatine, when injected into the blood, are not assimilable and are 

 rejected by the kidneys ; but albumen and gelatine which have been digested 

 in gastric juice are assimilated in the same way as though they had pene- 

 trated by the natural process of absorption from the alimentary canal (Ber- 

 nard and Barreswil). The same is true of caseine and fibrin. These facts, 

 showing that something more is necessary in gastric digestion than mere 

 solution, point to pepsine as the important agent in producing the peculiar 

 modifications so necessary to proper assimilation of nitrogenized alimentary 

 substances. The action of pepsine is essential to the changes which occur in 

 the albuminoid alimentary matters, resulting in the formation of what are 

 known as peptones ; and the change into peptones takes place in all nitro- 

 genized substances that are dissolved in the stomach. This may occur even 

 when the albuminoid matters are somewhat advanced in putrefaction ; and 

 the gastric juice possesses antiseptic properties, which fact accounts for the 

 frequent innocuousness of animal substances in various stages of decomposi- 

 tion when taken into the stomach. 



The change of the albuminoids into peptones in the stomach is not direct. 

 The intermediate processes probably are the following : The albuminoids are 



