PHENOMENA AND CONDITIONS OF LIFE 47 



preclude every possibility of living. For do not numerous para- 

 sitic bacteria, protozoa and vermes live in the digestive juices of 

 the intestinal canal of the higher animals, which are fatal to all 

 other organic life? The question has often been asked, How 

 is it possible that the intestinal parasites are not digested in the 

 same manner as all other organic substances introduced into the 

 intestines? But this fact is no more remarkable than the fact 

 that the intestinal cells are not themselves decomposed by the 

 juices produced by them. This does actually occur in cases of intes- 

 tinal affections, for it is frequently observed, especially during 

 the summer season, in a human corpse examined soon after death, 

 that certain larger or smaller portions of the walls of the stomach 

 and intestines have been digested. 



Formerly it was believed, on the ground of these observations, 

 that each living organism possessed a protective agent in a special 

 vital force (vis vitalis), and that digestion of the stomach-wall 

 set in after the death of this protective force. As for a long time 

 no other reasonable explanation was forthcoming, this belief was 

 held even in expert circles. It is a common weakness of the 

 human mind to substitute for the mysterious an equally mys- 

 terious phrase, pretentious of being an explanation, and thus 

 to deceive ourselves into believing that we know, rather than 

 frankly admit our ignorance of that subject. When it was 

 proved by various experiments first by Pavy, who introduced 

 the ear of a living rabbit into the gastric fistula of a dog that 

 parts of an animal still alive are digested by the juices of the 

 stomach, the convenient assumption of the effects of a mys- 

 terious vital force had to be abandoned. To-day it is generally 

 believed that the impregnability of the healthy intestinal mucous 

 membrane, as well as the power of resistance of intestinal para- 

 sites, rests in their ability to secrete a substance which acts 

 as an antidote to the gastric juices and renders them harmless. 



This assumption is in perfect accord with our knowledge of 

 the effect of poisons. It is well known that every animal and 

 human body is able to become accustomed to poisons as for 

 instance nicotine, alcohol, morphia, opium, arsenic by gradually 

 increasing the dose to such a degree that a quantity of poison, 

 fatal under natural conditions, will cause very little or no harm. 

 It is said of Mithridates, the great ruler of Pontus, that having 



