220 PRACTICAL PHYSIOLOGY 



constant movement of the muscular wall of the stomach, whereby its 

 contents are kept in motion; and when these have been sufficiently 

 digested, they are collected into the funnel-shaped pyloric portion of 

 the stomach, and passed through the pyloric sphincter, which mean- 

 while relaxes to allow of their passage. On entering the stomach the 

 food is very little changed, except that it has been masticated. On 

 leaving it, however, its appearance is quite altered, being now a thick, 

 more or less coloured fluid called chyme. 



Various methods have been adopted for studying gastric digestion 

 e.g. observing the process through a gastric fistula, removing samples 

 of the gastric contents by means of a stomach tube, etc. 



In order to obtain pure gastric juice, the most reliable method is 

 that of Pawlow, which consists in resecting a portion of the fundus of 

 the stomach, and sewing it up so as to form a bag, which is then 

 sutured to an abdominal wound. This isolated sac of stomach secretes 

 pure gastric juice along with the main stomach, and the juice may 

 be collected and analysed. 



The Composition of Gastric Juice. Pure gastric juice obtained from 

 the sac in Pawlow's experiment is a clear, colourless fluid, with a 

 specific gravity of 1003-1006, and of an acid reaction. 



Its percentage composition varies in different animals, that of the 

 dog and of man being as follows : 



Man. Dog. 



Water, 99'44 97'3 



Organic matter, chiefly pepsin, - - - - 0*32 1*71 

 Inorganic matter 



(a) free hydrochloric acid, 0'2-0'3 G'3 1 



(&) chlorides and phosphates of alkalies and 



alkaline earths, ... - - (H-0'2 0'66 



The most important features to be considered in connection with 

 this table are: (1) the presence of free hydrochloric acid and (2) the 

 nature of the organic matter. 



The Acidity of the Gastric Juice. This might be caused either by 

 a free acid or by an acid salt, such as acid sodium phosphate (NaH 2 P0 4 ). 

 We can decide which it is by testing with Congo red, a red pigment 

 which is turned blue by a free acid, but is unaffected by an acid salt. 



EXPERIMENT I. To a 0-2 per cent, solution of HC1 add a few drops 

 of a solution of Congo red. The red colour is at once changed to blue. 

 Repeat with a solution of acid potassium phosphate. This turns 

 blue litmus red, but a solution of Congo red is unaffected. 



The acidity is therefore due to a free acid. We must now find out 

 if this acid be organic or inorganic. 



1 Pawlow always found more than 0'2 per cent. namely 0'5-0'G per cent. 





