DIGESTION 311 



are confined to this portion of the stomach, it may be con- 

 chided that they are the producers of the acid. The NaCl of 

 the blood plasma must be the source of the HCl. 



Pepsin and Rennin are produced in the chief or 

 peptic cells which line the glands both of the cardiac and of 

 the pyloric parts of the stomach. Daring fasting, granules are 

 seen to accumulate in these cells, and when the stomach is 

 active they are discharged. These granules are not pepsin 

 but the forerunner of pepsin — pepsinogen (p. 36). 



(3) Course of Gastric Digestion. — (a) Amylolytic Period. — 

 The action of the gastric juice does not at once become 

 manifest. In the pig for about two hours 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 whicli was started in the mouth, and, at 

 the same time, the micro-organisms go on splitting sugar to 

 form lactic acid, which mav thus be req-arded as a normal 

 constituent of the oesophageal end of the pig's stomach 

 during the first two hours after a meal. 



(6) Proteolytic Period. — Before the amylolytic period is 

 completed, the gastric juice has begun its special action on 

 Proteins. This may be readily studied by placing some 

 coagulated protein 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 protein swells, becomes transparent, and dissolves. The 

 solution is coagulated on boiling — a soluble native protein 

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

 native protein is filtered off, the filtrate gives a precipitate 

 on neutralising, showing that an aci<l compound — a meta- 

 protein — has been produced. If the action is allowed to 

 continue and the meta-protein precipitated and filtered off, 

 it will be found that the filtrate gives a precipitate on half 

 saturation with ammonium sulphate, showing that proto- 

 proteo-ies have been formed. These differ somewhat in their 

 reaction, and apparently differ in the proportion of their 

 constituent amino-acids. On filtering off these proteoses, 

 the filtrate yields a precipitate on saturating with ammonium 



