DIGESTION 175 



This is accomplished by the solvent action of the gastric juice, which in 

 virtue of the chemic activity of its constituents on proteins, gradually disinte- 

 grates the food and reduces it to the liquid or semiliquid condition. 



The nature of this change and the respective influence which the acid 

 and pepsin exert can be studied with almost any form of protein. A most 

 convenient form, however, is fibrin obtained from blood by whipping and 

 thoroughly freed from corpuscles by washing under a stream of water. The 

 chemic features of proteins, as well as the typical forms contained in the 

 different articles of food, have been considered in connection with the chemic 

 composition of the body and the composition of foods (see pages 15 and 119). 

 For purposes of experimentation artificial gastric juice may be employed. 

 This is as effective as the normal secretion and in no essential respect differs 

 from it. A glycerin extract of the mucous membrane acidulated with 0.2 

 per cent, hydrochloric acid is probably the best. 



If the small pieces of fibrin be suspended in clear gastric juice and kept 

 at a temperature of io4F. (4oC.) for an hour or two, they will be dissolved 

 and will entirely disappear, giving rise to a slightly opalescent mixture. In 

 the early stages of the process the fibrin becomes swollen and transparent 

 and partly dissolved. If at this time the solution be carefully neutralized, 

 the dissolved portion can be regained in the form of acid fibrin a fact which 

 indicates that the first effect of the gastric juice is the acidification of the 

 protein. This having been accomplished, the pepsin becomes operative, 

 and in a varying length of time transforms the acid-protein into a new form 

 of protein, termed peptone which differs from all other forms of protein 

 in being soluble in both acids and alkalies and non-coagulable by heat. 

 In the transformation of acid-protein into peptone it is possible to isolate 

 by the addition of magnesium sulphate and ammonium sulphate inter- 

 mediate bodies to which the term proteases has been given, and which 

 differ somewhat in their solubility. The proteoses are termed, from the 

 order in which they make their appearance, primary and secondary. The 

 primary proteoses are precipitated by magnesium sulphate, the secondary 

 by ammonium sulphate. This supposed change produced by gastric juice 

 is represented by the following scheme: 



Protein Acid-protein Proteose Proteose Peptone 

 (Primary) (Secondary) 



From the fact that when peptones are subjected to the prolonged action 

 of pancreatic juice there arise compounds such as leucin, tyrosin, aspartic 

 acid, arginin, etc., it was believed that two kinds of peptones were formed 

 out of a simple protein one of which succumbed to the destructive action of 

 pancreatic juice, while the other resisted it; for this reason the latter was 

 termed anil- and the former hemi-peptone. The two were included under 

 the term ampho-peptone. It is generally admitted now, however, that there 

 is but one kind of peptone formed from any given protein, which under the 

 influence of pancreatic, and intestinal juice as well, is reduced by hydrolysis, 

 through successive stages to amino-acids or perhaps only to the antecedent 

 stage, in which two or more amino-acids yet remain united forming sub- 

 stances known as peptids. 



Nearly all forms of protein are in a similar manner transformed into 

 peptones by gastric juice. Beyond this stage, however, there does not seem 



