OIIAP. i.] TISSUES AND MECHANISMS OF DIGESTION. 373 



drochloric acid ('2 p.c.), or if the mixture be submitted to dialysis 

 to remove the peptones formed, and its acidity be kept up to the 

 normal, the action recommences. By removing the products of 

 digestion as fast as they are formed, and by keeping the acidity up 

 to the normal, a given amount of gastric juice may.be made to 

 digest a very large quantity of proteid material. Whether the 

 quantity is really unlimited is disputed ; but in any case the 

 energies of the juice are not rapidly exhausted by the act of 

 digestion. 



205. Nature of the action. All these facts go to shew that 

 the digestive action of gastric juice on proteids, like that of saliva on 

 starch, is a ferment-action ; in other words, that the solvent action 

 of gastric juice is essentially due to the presence in it of a ferment- 

 body. To this ferment-body, which as yet has been only ap- 

 proximately isolated, the name of pepsin has been given. It is 

 present not only in gastric juice but also in the glands of the 

 gastric mucous membrane, especially in certain parts and under 

 certain conditions which we shall study presently. The glycerine 

 extract of gastric mucous membrane, at any rate of that which has 

 been dehydrated, contains a minimal quantity of proteid matter, 

 -and yet is intensely peptic. Other methods, such as the elaborate 

 one of Briicke, give us a material which, though containing nitrogen, 

 exhibits none of the ordinary proteid reactions, and yet in concert 

 with normal dilute hydrochloric acid is peptic in a very high 

 degree. We seem therefore justified in asserting that pepsin is not 

 .a proteid, but it would be hazardous to make any dogmatic state- 

 ment concerning a substance, obtained in so small a quantity 

 .at a time that its exact chemical characters have not yet been 

 ascertained. At present the manifestation of peptic powers is our 

 only safe test of the presence of pepsin. 



In one important respect pepsin, the ferment of gastric juice, 

 differs from ptyalin, the ferment of saliva. Saliva is active in a per- 

 fectly neutral medium, and there seems to be no special connection 

 between the ferment and any alkali or acid. In gastric juice, 

 however, there is a strong tie between the acid and the ferment, 

 so strong that some writers speak of pepsin and hydrochloric 

 -acid as forming together a compound, pepto-hydrochloric acid. 



In the absence of exact knowledge of the constitution of 

 proteids, we cannot state distinctly what is the precise nature of 

 the change into peptone ; the various proteids differ from each 

 other in elementary composition quite as widely as does peptone 

 from any of them. Judging from the analogy with the action of 

 saliva on starch, we may fairty suppose that the process is at 

 bottom one of hydration ; and this view is further suggested by 

 the fact that peptone closely resembling, if not identical with, that 

 obtained by gastric digestion, may be obtained by the action of 

 strong acids, by the prolonged action of dilute acids especially at 

 a high temperature, or simply by digestion with super-heated 



