OHAP. i ] DIGESTION. 243 



neutralisation, in the form of what is called peptone, and, on the 

 whole, the longer the digestion is carried on, the greater is the 

 proportion borne by the peptone to the precipitate thrown down 

 on neutralisation; indeed, in some cases at all events, all the 

 proteids are brought into the condition of peptone. 



Peptone is a proteid, having the same approximate elementary 

 composition as other proteids, and giving most of the usual proteid 

 reactions. 



It is distinguished from other proteids by the following marked 

 features : 



1st. Though soluble in" distilled water and in neutral saline 

 solutions, even the most dilute, and therefore not precipitated 

 from its acid or alkaline solutions by neutralisation, it is not, like 

 the other similarly soluble proteids, coagulated by heat. 



2nd. It is diffusible, passing through membranes with consider- 

 able ease. The diffusion is not so rapid as that of salts, sugar, and 

 other similar substances, but is very marked as compared with that 

 of other proteids ; these pass through membranes with the greatest 

 difficulty. (For the other less important reactions see Appendix.) 



The neutralisation precipitate resembles, in its general charac- 

 ters, acid-albumin, or syntonin. Since, however, it probably is 

 distinguishable from the body or bodies produced by the action of 

 simple acid on muscle or white of egg, it is best to reserve for it 

 the name of parapeptone. Thus the digestion by gastric juice of 

 white of egg results in the conversion of all the proteids present 

 into peptone and parapeptone, of which the former must be 

 considered as the final and chief product, the latter a bye product 

 or initial product of variable occurrence and importance. The 

 gastric digestion of fibrin, either raw or boiled, and of all foims of 

 coagulated albumin, gives rise to the same products, peptone and 

 parapeptone. Milk when treated with gastric juice is first of all 

 coagulated or curdled. This is the result partly of the action of 

 the free acid but chiefly of the special action of a particular 

 constituent of gastric juice, of which we shall speak hereafter. 

 The coaguluna consists of a proteid, casein, and of fat ; and the casein 

 is subsequently dissolved with the same appearance of peptone and 

 parapeptone as in the case of other proteids. In fact, the digestion 

 by gastric juice of all the varieties of proteids consists in the con- 

 version of the proteid into peptone, with the concomitant appear- 

 ance of a certain variable amount of parapeptone. 



Circumstances affecting gastric digestion. The solvent action 

 of gastric juice on proteids is modified by a variety of circum- 

 stances. The nature of the proteid itself makes a difference, 

 though this is determined probably by physical rather than by 

 chemical characters. Hence in making a series of comparative 



162 



