Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 329 



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. Concerning 

 its exact nature we cannot make any definite statement ; we have 

 no absolute proof that it is a proteid, probable as this may seem. 

 We are as yet unable to define it by any chemical characters. 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 con- 

 nection between the ferment and any alkali or acid. In gastric 

 juice, however, there is a strong tie between the acid and the fer- 

 ment, 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 

 siliva 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 super-heated 

 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. 



§ 184. All proteids, so far as we know, are converted by pep- 

 sin into peptone. Concerning the action of gastric juice on other 

 nitrogenous substances 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 dis- 

 solved 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 ; moreover 

 both prepared gelatine 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 gela- 

 tinisation is entirely lost. Chondrin and the elastic tissues 

 undergo a similar change. 



§ 185. Action of gastric juice on milk. It has long been 



