CHEMICAL SIGNS OF IRRITABILITY 53 
irrespective of the stimulus, and accordingly excitation 
appears to generate no heat and there appears to be no 
fatigue, unless we deprive the nerve of oxygen for some 
time, and yet we have a copious production of carbon 
dioxide which is increased on stimulation. The fact, 
therefore, that there is no increased heat production in a 
stimulated nerve is by no means contrary to our results, 
although it is certainly surprising. It indicates, perhaps, 
that the act of excitation is not primarily an oxidation, 
but that the oxidation is concerned in the processes of 
repair. There are several facts which might be cited, 
were the space at our command, which would lead to the 
same conclusion. There are also other suggestions which 
might be made to account for this seeming incompatibil- 
ity, but it would be useless to do so without experiments. 
We may therefore close this brief discussion with the 
statement that the failure to detect heat production in 
nerves during excitation is no evidence of value against 
the occurrence there of chemical changes resulting in 
carbon dioxide production and correlated with the irri- 
tability. The conclusion drawn from it by some authors 
that the nerve impulse does not, on this account, involve 
any chemical processes is entirely unwarranted. 
We may in this connection stop for a moment to 
consider what is known of the oxygen consumption of 
nerves, for while we have ourselves as yet carried out 
‘no experiments in this line, yet there have been some 
observations made which can be correlated with the 
carbon dioxide production. In the first place, it may 
be noted that there is no immediate dependence of some 
nerves, at least, on atmospheric oxygen for their activity. 
In this respect the carbon dioxide production and the 
