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 
I 
