360 A MANUAL OF PHYSIOLOGY 



It is true that it has first been killed and then digested, but 

 the question is, why the stomach-wall is not first killed and then 

 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 pancreatic 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, 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, 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 hydrochloric acid 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 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 conclusion that each membrane becomes accustomed, and, 

 so to speak, ' immune,' to the secretion normally in contact with 



