DIGESTION 331 



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 destroy the 

 tissues of the frog's leg. The alkalinity of the blood has 

 nothing to do with the explanation, for the frog's blood is 

 also alkaline, and the cells that line the pancreatic ducts are 

 preserved from the pancreatic juice, which is intensely active 

 in an alkaline medium. 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, however 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, possibly by turning any 

 of the ferment which may invade it into an inert substance 

 like the zymogen, or by opposing its entrance as the epithe- 

 lium of the bladder opposes the absorption of urea. 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 the 

 reason, or at least one of the reasons, for the existence of 

 the mother-substance. But this is not the whole explana- 

 tion, for the living frog's leg is not harmed by a weakly 

 alkaline pancreatic extract, which does not digest the epi- 

 thelium, because it cannot kill it. On the other hand 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 

 intestine 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. Indeed, it is impossible to escape the 



