CHAPTER I 

 IRRITABILITY AS A SIGN OF LIFE 



In the pages which follow we shall consider par- 

 ticularly the question of the chemical processes which 

 take place in nerves when nerve impulses pass over 

 them. There is scarcely a subject in the world more 

 interesting than this one, for the question of what is 

 the nature of that disturbance in nerve tissue which 

 shows itself in our thoughts has attracted men from the 

 earliest days. We must first find out the changes 

 of a material kind if there are any such changes- 

 which occur in the brain when we think before we can 

 form any probable idea of the relation of these changes 

 to the psychical changes which accompany them. 

 Obviously, we must first try to solve the simplest prob- 

 lem in this field and discover what are the changes 

 of a chemical or physical kind when a nerve impulse 

 flashes over a nerve before we can form any conception 

 of the relation of the material to the psychic world. 

 The following pages do not contain, of course, the solu- 

 tion of this problem of such absorbing interest, but 

 they do present the first accurate information we have 

 had of the chemical changes which accompany the 

 nerve impulse; they have in them, therefore, the 

 foundation upon which a solid structure of fact can be 

 based. 



The observations which we have to present are not 

 confined to nerves, however, for psychic phenomena are 



