334 HUMAN PHYSIOLOGY 



in the mucous membrane. The gastric juice is a clear 

 watery fluid, which is markedly acid from the presence of 

 free hydrochloric acid. In the dog the free acid may 

 amount to O2 per cent., but in man it is less abundant, 

 and when the gastric juice is mixed with food the acid 

 rapidly combines with alkalies and with proteids, and is no 

 longer free. In addition to the HC1 small quantities of 

 organic salts are present. Traces of proteids may also be 

 demonstrated, and with these two enzymes are associated- 

 one a proteolytic or proteid-digesting enzyme, pepsin, the 

 other a milk-curdling enzyme, rennin. 



(b) Course of Gastric Digestion (1) Amylolytic Period. 

 The action of the gastric juice does not at once become 

 manifest. For half-an-hour after the food is swallowed the 

 ptyalin of the saliva goes on acting, and the various micro- 

 organisms swallowed with the food grow and multiply, and 

 thus there is a continuance of the conversion of starch to 

 sugar which was started in the mouth, and at the same time 

 the micro-organisms go on splitting the sugar to form lactic 

 acid, which may thus be regarded as a normal constituent 

 of the stomach during the first half-hour after a mixed 

 meal. 



(2) Proteolytic Period. Before the amylolytic period is 

 completed, the gastric juice has begun its special action on 

 proteids. This may be readily studied by placing some 

 coagulated proteid in gastric juice, or in an extract of the 

 mucous membrane of the stomach made with dilute hydro- 

 chloric acid, and keeping it at the temperature of the body. 

 The proteid swells, becomes transparent, and dissolves. The 

 solution is coagulated on boiling a soluble native proteid 

 has been formed. Very soon it is found that, if the soluble 

 native proteid is filtered off, the filtrate gives a precipitate 

 on neutralising, showing that an acid proteate has been 

 produced. If the action is allowed to continue and the acid 

 proteate precipitated and filtered off, it will be found that 

 the filtrate gives a precipitate on saturating with common 

 salt, showing that a proto-proteose has been formed. Along 

 with this a certain amount of hetero-proteose is also formed. 



