148 DIGESTION. 



incessantly restored to it by the other. A very large quantity, 

 therefore, of the secretion may be poured out during the digestion 

 of a meal, at an expense to the blood, at any one time, of only two 

 or three ounces of fluid. The simplest investigation shows that 

 the gastric juice does not accumulate in the stomach in any con- 

 siderable quantity during digestion ; but that it is gradually 

 secreted so long as any food remains undissolved, each portion, as 

 it is digested, being disposed of by reabsorption, together with its 

 solvent fluid. There is accordingly, during digestion, a constant 

 circulation of the digestive fluids from the bloodvessels to the ali- 

 mentary canal, and from the alimentary canal back again to the 

 bloodvessels. 



That this circulation really takes place is proved by the fol- 

 lowing facts: First, if a dog be killed some hours after feeding, 

 there is never more than a very small quantity of fluid found in 

 the stomach, just sufficient to smear over and penetrate the half 

 digested pieces of meat ; and, secondly, in the living animal gastric 

 juice, drawn from the fistula five or six hours after digestion has 

 been going on, contains little or no more organic matter in solution 

 than that extracted fifteen to thirty minutes after the introduction 

 of food. It has evidently been freshly secreted ; and, in order to 

 obtain gastric juice saturated with alimentary matter, it must be 

 artificially digested with food in test-tubes, where this constant ab- 

 sorption and renovation cannot take place. 



An unnecessary difficulty has sometimes been felt in understand- 

 ing how it is that the gastric juice, which digests so readily all albu- 

 minous substances, should not destroy the walls of the stomach 

 itself, which are composed of similar materials. This, in fact, was 

 brought forward at an early day, as an insuperable objection to the 

 -doctrine of Eeaumur and Spallanzani, that digestion was a process 

 of chemical solution performed by a digestive fluid. It was said 

 to be impossible that a fluid capable of dissolving animal matters 

 should be secreted by the walls of the stomach without attacking 

 them also, and thus destroying the organ by which it was itself 

 produced. Since that time, various complicated hypotheses have 

 been framed, in order to reconcile these apparently contradictory 

 facts. The true explanation, however, as we believe, lies in this 

 that the process of digestion is not a simple solution, but a catalytic 

 transformation of the alimentary substances, produced by contact 

 with the pepsine of the gastric juice. We know that all the or- 

 ganic substances in the living tissues are constantly undergoing, in 



