THE GASTRIC JUICE AND STOMACH DIGESTION. 163 



Both the essential constituents of the gastric juice, namely, the pep- 

 sine and the free acid, are produced by the glandular mucous membrane 

 of the stomach. It would appear, however, that the mode of their 

 production is somewhat different. Pepsine is an albuminoid substance 

 formed by the nutritive process in the glands themselves. It probably 

 accumulates in the intervals of digestion, and may therefore be extracted 

 from the substance of the mucous membrane in the manner already de- 

 scribed. On the other hand, the free acid appears in quantity only at 

 the time of digestion, and is poured out with the watery constituents 

 of the secretion. There is evidence, however, that the acid is not imme- 

 diately formed by the glandular cells, but is produced by a subsequent, 

 though very rapid, change after the fluid has been secreted. The acid 

 reaction of the gastric fluids is never strongly pronounced in the deeper 

 and middle parts of the mucous membrane, but only upon its free sur- 

 face. This was shown by Bernard, 1 who injected into the jugular vein 

 of a rabbit two successive solutions, one of iron lactate, the other of 

 potassium ferrocyanide. These two salts would remain unaltered in 

 neutral or alkaline fluids, but in the 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 glands. As the two salts must have both 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 a sufficient quantity of free 

 acid to cause their combination. According to Dr. Lauder Brunton, 2 

 moreover, a horizontal section through the lower part of the gastric 

 glands of the pigeon, if tested by litmus paper, will be found to have a 

 neutral or extremely weak acid reaction, while the inner surface of the 

 stomach presents a strongly marked acidity. At the same time, the 

 deeper parts of the mucous membrane contain pepsine in sufficient 

 quantity to form a digestive fluid, if extracted and acidulated in the 

 usual way. Finally, the free acid continues to be formed during a certain 

 time after death; for it has been found that if the fresh gastric mucous 

 membrane of a rabbit or a pig be cut in small pieces and washed with 

 distilled water until all trace of acidity is removed, it will again acquire 

 an acid reaction after being left to itself for some hours. The materials 

 of the free acid of the gastric juice are therefore furnished during life 

 by the alkaline fluids of the circulating blood ; but the acid itself origi- 

 nates subsequently by some change taking place in the products of 

 exudation. 



Self-digestion of the Stomach after Death. Notwithstanding that the 

 gastric juice has the power, at the temperature of the living body, of 

 digesting all soft tissues composed of albuminous matter, yet owing to 



1 Liquides de TOrganisme. Paris, 1859, torn. ii. p. 375. 



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



