324 A MANUAL OF PHYSIOLOGY . 



from any admixture with saliva or with a pathological secretion 

 of mucus, it would show as high a percentage of acid as the dog's 

 juice. 



In cases of cancer, whether the growth is situated in the 

 stomach or not, the free hydrochloric acid of the gastric juice 

 is usually much reduced, and often absent. Under such con- 

 ditions some lactic acid may be present in the stomach, being 

 produced from the carbo-hydrates by the action of bacteria 

 (Bacillus acidi lactici), which are normally held in check by the 

 hydrochloric acid, although not rendered incapable of growth 

 when they have passed on into the intestine. Even in the 

 strength of 0-07 to 0-08 per cent, hydrochloric acid prevents the 

 formation of lactic acid from dextrose. Indeed, when all the 

 hydrochloric acid of the gastric juice is combined with proteins, 

 the protein-acid compound still inhibits the growth of bacteria 

 in the stomach, although not so efficiently as the same amount 

 of free acid. That in normal gastric juice the acidity is not due 

 to lactic acid can be shown by shaking the juice with ether, 

 which takes up lactic acid, and then applying Uffelmann's test 

 to the ethereal extract (Practical Exercises, p. 428). 



More than this, it is not due to an organic, but to an inorganic 

 acid, for healthy gastric juice causes such an alteration in the 

 colour of aniline dyes like congo-red and methyl violet as would 

 be produced by dilute mineral acids, and not by organic acids, 

 even when present in much greater strength.* Finally, when the 

 bases and acid radicles of the juice are quantitatively compared, 

 it is found that there is more chlorine than is required to combine 

 with the bases ; the excess must be present as free hydrochloric 

 acid. In the pure gastric juice of fishes like the dogfish and skate, 

 however, the acid is said not to be hydrochloric but an organic 

 acid. The quantity of gastric juice secreted depends upon the 

 nature and amount of the food. It has been estimated at as much 

 as 5 litres in twenty-four hours, or several times the quantity of 

 saliva secreted in the same time. With sham feeding a dog may 

 yield 200-300 c.c. in an hour. 



The great action of gastric juice is upon proteins. In this two 

 of its constituents have a share, the pepsin and the free acid. 

 One member of this chemical copartnery cannot act without the 

 other ; peptic digestion requires the presence both of pepsin 

 and of acid ; and, indeed, an active artificial juice can be obtained 

 by digesting the gastric mucous membrane with dilute (0*2 to 

 0-4 per cent.) hydrochloric acid. A glycerin extract of a stomach 



* A dilute solution of congo-red is turned violet by organic and blue by 

 inorganic acids ; the gastric juice turns it blue. Methyl violet is rendered 

 blue by an inorganic acid like hydrochloric acid, and green if more of the 

 acid be added. It is not altered by organic acids. Gastric juice turns it 

 blue. 



