CHAP, i.] DIGESTION. 245 



is impaired^ By boiling for a few minutes the activity of the most 

 powerful juice is irrevocably destroyed. The presence in a con- 

 centrated form of the products of digestion hinders the process. 

 If a large quantity of fibrin be placed in a small quantity of juice, 

 digestion is soon arrested; on dilution with the normal hydro- 

 chloric 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 di- 

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

 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 ener- 

 gies of the juice are not rapidly exhausted by the act of digestion. 



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, especially of that which has 

 been dehydrated, contains a minimal quantity of proteid matter, 

 and yet is intensely active. 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 the highest 

 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 small quantity only, 

 probably mixed with other bodies, and the chemical characters of 

 which we know as yet very little. 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. Judging from the analogy with the 

 action of saliva on starch, we may fairly suppose that the process 

 is at bottom one of hydration ; but we have no exact proof of 

 this, and it is at least quite as probable that peptone arises by a 



