SELF-PRESERVATION AND THE MENTAL LIFE 149 



quiet and will, in a few minutes, cease to move, 

 although the heart still continues to beat. On 

 transferring the animal to plain water, the alcohol 

 which has passed into the brain by diffusion finds 

 its way by diffusion into the water, and the activities 

 of the tadpole are soon resumed. 



There are a great many forms of physical disease 

 in which substances are formed which disturb the 

 functions of the brain. The pus-forming bacteria, 

 the bacteria of influenza, and many others make 

 poisons that irritate or depress certain brain centers. 

 In diseases of the kidney, of the liver, and of the 

 intestine there are formed poisons which may prove 

 very injurious to the brain and induce a variety of 

 mental and nervous derangements. I shall limit 

 myself to one example of this sort, the effect of 

 chronic intestinal infections. 



It has already been pointed out that, in health, the 

 human intestine contains bacteria possessing cer- 

 tain protective functions, and that in disease the 

 normal bacteria are in part replaced by others. 

 These substituting bacteria form poisons which, 

 after absorption from the intestine, are carried to the 

 liver and rendered harmless there by means of special 

 protective methods. It may readily come to pass 

 that the intestinal poisons, finding their way to the 

 liver, are not wholly neutralized there, but in part 

 slip by this guard and find an entry, by the blood, 

 into the helpless brain. A variety of poisonous 

 effects may in this way arise. Our knowledge of the 



