DIGESTION IN THE STOMACH. 229 



From these experiments, we may understand the structural 

 changes which the chief alimentary substances undergo in their 

 conversion into chyme; and the proportions of each which are 

 not reducible to chyme, nor capable of any further act of di- 

 gestion. The chemical changes undergone in and by the proxi- 

 mate principles are less easily traced. 



Of the albuminous principles, some, as the casein of milk, 

 are coagulated by the acid of the gastric fluid; and thus, be- 

 fore they are digested, come into the condition of the other 

 solid principles of the food. These, including solid albumen 

 and fibrin, in the same proportion that they are broken up and 

 anatomically disorganized by the gastric fluid, appear to be 

 reduced or lowered in their chemical composition. This chemi- 

 cal change is probaby produced, as suggested by Dr. Prout, by 

 the principles entering into combination with water. It is suf- 

 ficient to conceal nearly all their characteristic properties ; the 

 albumen is rendered scarcely coagulable by heat; the gelatin, 

 even when its solution is evaporated, does not congeal in cool- 

 ing; the fibrin and casein cannot be found by their character- 

 istic tests. It would seem, indeed, that all these various sub- 

 stances are converted into one and the same principle, a low 

 form of albumen, not precipitable by nitric acid or heat, and 

 now generally termed albuminose or peptone, from which, after 

 being absorbed, they are again raised, in the elaboration of the 

 blood, to which they are ultimately assimilated. 



The change of molecular constitution suffered by the albu- 

 minous parts of the food, in consequence of the action of the 

 gastric juice, has an important relation to their absorption by 

 the bloodvessels of the stomach. From the condition of " col- 

 loids," or substances, so named by Professor Graham, which 

 are absorbed with extreme difficulty, they appear, from ex- 

 periments of Funke, to assume to a great degree the char- 

 acter of " crystalloids," which can pass through animal mem- 

 branes with ease. 1 



Whatever be the mode in which the gastric secretion 

 affects these principles, it, or something like it, appears essen- 

 tial, in order that they may be assimilated to the blood and 

 tissues. For, when Bernard and Barreswil injected albumen 

 dissolved in water into the jugular veins of dogs, they always 

 in about three hours after, found it in the urine. But if, pre- 

 vious to injection, it was mixed with gastric fluid, no trace of 

 it could be detected in the urine. The influence of the liver 

 seems to be almost as efficacious as that of the gastric fluid, in 



1 These terms will be further explained and illustrated in the 

 chapter on Absorption. 



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