SALIVA AND GASTRIC JUICE. 259 



cerning 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 perfectly 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 pep- 

 tone ; 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 sug- 

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

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

 acids, by the prolonged action of dilute acids especially at a high tempera- 

 ture, 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. 



192. All proteids, as far as we know, are converted by pepsin into pep- 

 tone. Concerning the action of gastric juice on other nitrogenous sub- 

 stances more or less allied to proteids, but not truly proteid in nature, our 

 knowledge is at present imperfect. Mucin, nuclein, and the chemical basis 

 of horny tissues are wholly unaffected by gastric juice. The gelatiniferous 

 tissues are 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 muscles which are bound 

 together by connective tissue fall asunder ; moreover, both prepared gelatin 

 and the gelatiniferous basis of connective tissue in its natural condition, 

 that is without being previously heated with water, are by it changed into 

 a substance so far analogous with peptone that the characteristic property 

 of gelatinization is entirely lost. Chondrin and the elastic tissues undergo 

 a similar change. 



193. Action of gastric juice on milk. It has long been known that ah 

 infusion of calves' stomach, called rennet, has a remarkable effect in rapidly 

 curdling milk, and this property is made use of in the manufacture of 

 cheese. Gastric juice has a similar effect: milk when subjected to the 

 action of gastric juice is first curdled and then digested. If a few drops of 

 gastric juice be added to a little milk in a test-tube, and the mixture ex- 

 posed to a temperature of 40 C.,the milk will curdle into a complete clot in 

 a very short time. If the action be continued the curd or clot will be ulti- 

 mately dissolved and digested. Milk contains, besides a peculiar form, or 

 peculiar forms of albumin, fats, milk-sugar, and various salines, the peculiar 

 proteid casein. In natural milk casein is present in solution, and " curd- 

 ling" consists essentially in the soluble casein being converted (or more 

 probably as we shall see presently, split up) into an insoluble modification 

 of casein, which as it is being precipitated carries down with it a great deal 

 of the fat and so forms the " curd." Now casein is readily precipitated 

 from milk upon the addition of a small quantity of acid, and it might be 

 supposed that the curdling effect of gastric juice was due to its acid reaction. 



