

CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 397 



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 Brlicke, 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 fairly 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 superheated 

 water in a Papin's digester, that is to say by means of agents 

 which, in other cases, produce their effects by bringing about 

 hydrolytic changes ; beyond this we cannot at present go. We 

 may add however, as supporting the same view, the statement 

 of some observers that peptone when treated with dehydrating 

 agents or when simply heated to 140 170C. is in part recon- 

 verted into a body or bodies resembling acid-albumin or globulin. 



206. All proteids, so far as we know, are converted by gastric 

 juice into peptone. The gelatiniferous tissues are also dissolved 

 by it ; and the bundles and membranes of connective tissue are 

 very speedily so far affected by it, that at a very early stage of 

 digestion the bundles and elementary fibres of muscle which are 

 bound together by connective tissue fall asunder. The changes 

 thus brought about by gastric juice, by pepsin and acid, on the 

 gelatiniferous tissues or on gelatin prepared from them are in 

 many ways analogous to those produced on proteids. The gelatin 

 is converted into a substance which no longer gelatinises, and 



