136 PHYSIOLOGY OF ALIMENTATION. 



tapeworm may exist for months, even years, in the intestine 

 of a man or other animal. 



Similarly mysterious has appeared to us the immunity 

 which both the stomach and small intestine possess against 

 their own secretions. The stomach is not digested by the 

 gastric juice, nor the duodenum when the pylorus opens. 

 Neither is any portion of the small or large intestine, under 

 normal circumstances, acted upon by the pancreatic juice 

 which passes through it. 



The hypotheses which have been proposed from time to time 

 to explain this immunity of intestinal parasites and alimen- 

 tary tract have been many and, for the most part, of a vital- 

 istic nature. CLAUDE BERNARD believed that the epithelial 

 covering of the intestinal tract protected the underlying tissues 

 from digestion, but this idea, besides explaining nothing, 

 stands in contradiction to the facts of pathology, which show 

 that the absence of epithelial covering (for instance, in ulcers 

 of the gastro-intestinal tract from any cause whatsoever) is 

 by no means always, in fact only at times, accompanied by a 

 loss of the underlying muscular or other tissues. These sub- 

 epithelial structures are therefore just as immune against 

 digestion as the epithelium itself. 



Nor can PAVY'S theory, which assumes that the stomach is 

 not digested because it is protected through the alkalinity 

 of the blood, be looked upon as any more serviceable. The 

 blood is, first of all, not alkaline, but neutral in reaction, and, 

 secondly, PAVY'S explanation is of no value when we deal 

 with the intestine, the contents of which at no time possess 

 the decided acid properties of those of the stomach and are 

 at times perhaps even alkaline. The remaining theories 

 which have been proposed from time to time need not be dis- 

 cussed, for they have almost without exception covered up 

 the problem at hand by attributing to the intact mucosa 

 "living" properties which we could never hope to find in 

 "dead" matter. As is well known, the "dead" mucosa 

 of the stomach undergoes partial digestion, as seen in 



