io8 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? 



