258 THE TISSUES AND MECHANISMS OF DIGESTION. 



Since, if sufficient time be allowed, even a small quantity of gastric juice 

 will dissolve at least a very large if not an indefinite quantity of fibrin, we 

 are led to take, as a measure of the activity of a specimen of gastric juice, 

 not the quantity of fibrin which it will ultimately dissolve, but the rapidity 

 with which it dissolves a given quantity. 



The greater the surface presented to the action of the juice, the more 

 rapid the solution ; hence minute division and constant movement favor diges- 

 tion. And this is probably, in part at least, the reason why a fragment of 

 spongy filamentous fibrin is more readily dissolved than a solid clump of 

 boiled white of egg of the same size. Neutralization of the juice wholly 

 arrests digestion ; fibrin may be submitted for an almost indefinite time to 

 the action of neutralized gastric juice without being digested. If the neu- 

 tralized juice be properly acidified, it may again become active ; when gas- 

 tric juice, however, has been made alkaline, and kept for some time at a 

 temperature of 35 C., its solvent powers are not only suspended but actually 

 destroyed. Digestion is most rapid with dilute hydrochloric acid of 0.2 per 

 cent, (the acidity of natural gastric juice). If the juice contains much more 

 or much less free acid than this, its activity is distinctly impaired. Other 

 acids, lactic, phosphoric, etc., may be substituted for hydrochloric ; but they 

 are not so effectual, and the degree of acidity most useful varies with the 

 different acids. The presence of neutral salts, such as sodium chloride, in 

 excess is injurious. The action of mammalian gastric juice is most rapid at 

 35-40 C. ; at the ordinary temperature it is much slower, and at about 

 C. ceases altogether. The juice may be kept, however, at C. for an 

 indefinite period without injury to its powers. The gastric juice of cold- 

 blooded vertebrates is relatively more active at low temperatures than that 

 of warm-blooded mammals or birds. 



At temperatures much above 40 or 45 the action of the juice is im- 

 paired. By boiling for a few minutes the activity of the most powerful 

 juice is irrevocably destroyed. The presence in a concentrated form of the 

 products of digestion hinders the process of solution. If a large quantity 

 of fibrin be placed in a small quantity of juice, digestion is soon arrested; 

 on dilution with the normal hydrochloric acid (0.2 per cent.), or if the mix- 

 ture be submitted to dialysis to remove the peptones formed, and its acidity 

 be kept up to the normal, the action recommences. By removing the prod- 

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

 to the normal, a given amount of gastric juice maybe made to digest a very 

 large quantity of proteid material. Whether the quantity is really un- 

 limited is disputed ; but in any case the energies of the juice are not rapidly 

 exhausted by the act of digestion. 



191. Nature of the action. All these facts go to show 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 approximately 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 glycerin 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 statement con- 



