THE CHEMISTRY OF THE DIGESTIVE JUICES 345 



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. 454). 



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 which is not 

 too fresh also possesses peptic power, the zymogen or mother 

 substance, pepsinogen, having been activated to pepsin. Free 

 acid very readily effects this activation, but this is far from being 

 the only function of the hydrochloric acid, for active pepsin still 

 requires the addition of a sufficient quantity of acid to render its 

 proteolytic power available. 



Well-washed fibrin obtained from blood is a convenient protein 

 for use in experiments on digestion; although, of course, for many 

 purposes only isolated purified proteins can be employed. Since the 

 blood contains traces of pepsin, the fibrin should be boiled to destroy 

 any which may be present (see also p. 447). 



If we place a little fibrin in a beaker, cover it with gastric juice 

 obtained from a dog or with 0-4 per cent, hydrochloric acid, to which a 

 small quantity of pepsin or of a gastric extract has been added, and 

 put the beaker in a water-bath at 40 C., the fibrin soon swells up and 

 becomes translucent, then begins to be dissolved, and in a short time 

 has disappeared (see Practical Exercises, p. 452). If we examine the 



* 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. 



