376 PRACTICAL PHYSIOLOGY 



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, and removing 

 samples of the gastric contents by means of a stomach tube. In order 

 to obtain pure gastric juice the most reliable method is that introduced 

 by 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 fistula. This isolated sac of stomach secretes pure 

 gastric juice, which may be collected from the fistula. The juice 

 secreted by this miniature stomach has been shown to be identical in 

 amount and strength with that secreted in the main stomach. 



The Composition of Gastric Juice. Pure gastric juice obtained by 

 Pawlow's method is a clear, colourless fluid, with a specific gravity of 

 1003-1006, and of 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 



(6) salts, - 0-1-0-2 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. 



In marked contrast to most of the other fluids of the animal body, 

 the gastric juice has a strong acid reaction towards all indicators. This 

 acidity performs a very important rdle in the process of digestion in 

 the stomach, which makes it of interest and importance to study 

 carefully. The urine also reacts acid towards certain indicators, but 

 the acidity in this case can readily be shown to be due to quite another 

 cause from that of the acidity of the gastric juice. In certain diseased 

 conditions, alterations take place in the degree and nature of the 

 acidity of gastric juice, and these alterations are sometimes of value in 

 assisting in the diagnosis of the pathological condition. 



From a chemical standpoint acidity is invariably due to the presence 

 of excess of hydrogen ions in the solution. For the presence of these 



1 Pawlow always found more than 0'3 per cent. namely, 0'5-0*6 per cent. 



