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. 
