THE SECRETION OF THE DIGESTIVE JUICES 383 



digested ? When the wall has been injured by caustics or by an 

 embolus, the gastric juice acts on it. But the living epithelium 

 that covers it is able to resist the action of the acid and pepsin, 

 which destroys the tissues of the frog's leg. The explanation is not 

 to be found in the alkalinity of the blood, for the frog's blood is also 

 alkaline, and the cells that line the intestine are preserved from the 

 pancreatic juice, which is intensely active in an alkaline medium, 

 while the living frog's leg is not harmed by a weakly alkaline pan- 

 creatic extract, which does not digest the epithelium because it 

 cannot kill it. (A certain amount of protection may be afforded to 

 the walls of the stomach by the thin layer of mucus which covers the 

 whole cavity, for mucin is not affected by peptic digestion/ And 

 a mucous secretion seems in some other cases to act as a protective 

 covering to the walls of hollow viscera, whose contents are such as 

 would certainly be harmful to more delicate membranes, e.g., in 

 the urinary bladder, large intestine, and gall-bladder. Still, how- 

 ever important such a mechanical protection may be, it does not 

 explain the whole matter, and it is necessary to suppose that the 

 gastric epithelium has some special power of resisting the gastric 

 juice, either by turning any of the ferment which may invade it 

 into an inert substance and neutralizing any intrusive acid, or by 

 opposing their entrance as the epithelium of the bladder opposes the 

 absorption of urea. ^There is reason to believe that, as a matter of 

 fact, free hydrochloncjicid cannot penetrate, the living cells, and 

 it is to be noted that both active pepsin and free acid must be 

 present at the same point within the cells before digestion of them 

 can take place. In the gland-cells of the pancreas the protoplasm 

 is, no doubt, shielded from digestion by the existence of the ferment 

 in an inert form as zymogen ; and it is possible that this is one of the 

 reasons for the existence of the mother-substance. But no such 

 explanation is, of course, available for the intestinal epithelium. 

 Trypsin when injected below the skin causes the tissue to break 

 down and ulcerate. And while an active solution of trypsin can 

 be allowed to remain a long time in an isolated loop of small intes- 

 tine without producing any ill effect, damage is soon caused not 

 only to the intestinal wall, but also to the liver, when the mucous 

 membrane of the loop has been injured before the introduction of 

 the trypsin. We must suppose, then, that the normal mucous 

 membrane of the intestine prevents the absorption of trypsin, or, 

 if it absorbs any of it, renders it harmless. On the other hand, the 

 intestinal mucosa is injured by the natural gastric juice when intro- 

 duced directly into it unless the animal takes food simultaneously 

 or a little earlier. But for reasons already given (p. 364) injury to 

 the intestine cannot be produced in this way in normal digestion. 

 It is impossible to escape the conclusion that each membrane becomes 

 accustomed, and, so to speak, ' immune,' to the secretion normally 



