56 A CHEMICAL SIGN OF LIFE 



carbon dioxide two or three times above that of their 

 resting state. This increase comes whatever the method 

 of stimulation, provided only that the nerve is alive 

 and irritable. Dead or anesthetized nerves show no 

 such an increase. The state of excitation in a nerve, 

 which we call a nerve impulse when it spreads from one 

 place to another, is not a purely physical change of 

 state, as it has been represented hitherto as being, but it 

 undoubtedly involves a corresponding chemical change. 

 Perhaps the excitation is this chemical change itself. 

 Furthermore, the facts that nerves do not increase 

 their heat output on stimulation and that they are nearly 

 free from fatigue effects are evidently not incompatible 

 with the vigorous metabolism discovered to exist in them. 



