60 A CHEMICAL SIGN OF LIFE 
ishes as the nerve approaches death. The point of 
minimum production of the gas corresponds approxi- 
mately to the point where an electrical response ceases 
(see p. 28). 
b) Although the nerve remains active for some time 
without oxygen, it is known that the absence of oxygen 
diminishes the excitability of the nerve. This diminu- 
tion of the excitability when in hydrogen is accompanied 
with a lowering of carbon dioxide production in the nerve. 
c) Further facts showing the relation between 
excitability and metabolic activity are brought out by 
the study of the effects of narcotics on the nervous 
metabolism. There are several compounds which alter 
the state of excitability of nerves to a considerable 
degree. The discovery of just what happens to respira- 
tion during anesthesia will throw much light on the 
nature of irritability. It is this:which we shall now 
study in detail. 
In recent years many experiments have been per- 
formed which are supposed to prove that oxygen: con- 
sumption can go on uninterruptedly during narcosis, and 
the consequent conclusion has been that narcosis is not 
produced by asphyxiation. This is not the place for us 
to weigh the merit of these arguments, nor are we con- 
cerned here with the question of how narcotics act on 
protoplasm, but it is very important to know whether or 
not metabolic activity in nervous tissue can go on undis- 
turbed while the tissue is unable to perform its own 
function. Are respiration and irritability independent 
processes? To show that they are dependent we shall 
cite in detail experiments on the effects of anesthetics on 
respiration. 
