DIGESTION. 157 



Both the essential constituents of the gastric juice are produced by 

 the mucous membrane, but their mode of production is different. Pep- 

 sine is continuously formed by the nutritive process, and accumulates 

 during the intervals of digestion in the glandular cells. The free acid, 

 on the other hand, appears in quantity only at the time of digestion, 

 and is poured out with the watery constituents of the secretion. There 

 is evidence that it is not present in the immediate product of the glan- 

 dular cells, but is produced by a rapid change in the fluid after secretion. 

 The mucous membrane is never distinctly acid in its deeper and middle 

 parts, but only on its free surface. This was shown by Bernard,* who 

 injected into the jugular vein of a rabbit two solutions, one of iron lac- 

 tate, the other of potassium ferrocyanide. These salts would remain 

 unaltered in neutral or alkaline fluids, but in presence of a free acid 

 would unite to form Prussian blue (iron ferrocyanide). On killing 

 the animal, three-quarters of an hour afterward, no blue coloration, 

 was found anywhere excepting in the stomach ; and in this organ it 

 was confined to the free surface of the mucous membrane, not being 

 perceptible in the substance of the glandules. As both salts must 

 have exuded from the blood-vessels of the mucous membrane, it is 

 evident that it was only at or near its upper surface that they met 

 with sufficient free acid to cause their combination. According to 

 Bruntonf a horizontal section through the lower part of the gastric 

 glands of the pigeon, if tested by litmus-paper, shows a neutral or 

 extremely weak acid reaction, while the inner surface of the stomach 

 is strongly acid. The materials of the free acid of the gastric juice 

 are therefore furnished by the alkaline blood ; but the acid itself origi- 

 nates by some change in the products of exudation. 



A necessary condition for the action of the gastric juice is a certain 

 temperature. It may go on more or less rapidly within varying limits, 

 but its most favorable temperature is that of the living body. It is 

 suspended at or near the freezing-point, becomes more active with the 

 increase of warmth, and is at its maximum about 38 C. ; above which 

 it again diminishes, and is totally arrested at the boiling temperature. 

 The favorable influence of moderate warmth has been shown by Schiff, J 

 who made two acidulated digestive infusions, and placed in each the 

 same quantity of coagulated albumen ; one of the infusions being 

 allowed to remain at a temperature varying from 10 to 17 C., the 

 other being introduced, in a closed glass tube, into the stomach of a 

 living dog. The second was found to have digested in six hours as* 

 much albumen as the first at the end of three weeks. 



A further peculiarity of the gastric juice is its resistance to putre- 

 faction. While other animal fluids, as the saliva, bile, pancreatic juice, 

 mucus, and blood, enter into putrefaction with great readiness, gastric 

 juice may remain exposed to the air at ordinary temperatures for 



* Liquides de I'Organisme. Paris, 1859, tome ii., p. 375. 



f Handbook for the Physiological Laboratory. Philadelphia, 1873, p. 491. 



% Lecons sur la Physiologic de la Digestion. Paris, 1867, tome ii., p. 19. 



