108 A CHEMICAL SIGN OF LIFE 
ful, keenly metabolic nervous system which is most 
responsive to its environment. 
Whatever may be the nature of that activity going on 
in our minds, we have at least discovered something 
about its simplest chemical accompaniment. Perhaps 
the nerve impulse is something in the nature of a prop- 
agated explosive wave in a continuous substance. 
Whether that wave is in the nature of a hydrolysis or an 
oxidation we cannot say, but at any rate it results in the 
liberation, in some manner, of carbon dioxide. This 
substance tells us whether the nerve impulse has passed 
this way or not. The change which liberates it may be 
the impulse itself. Three kinds of changes occur, then, 
in our brains when the nerve‘impulses are passing—an 
electrical change, a chemical change, and a psychical 
change. Which is the fundamental change ? 
