CHEMICAL SIGNS OF IRRITABILITY 27 
we should expect to find that less carbon dioxide was 
produced by nerves in an atmosphere of hydrogen than 
in normal air. On the other hand, if the carbon dioxide 
was due to some fermentation, or non-vital process, then 
it should not be influenced by the absence of oxygen. 
When, with Dr. Adams, we determined the rate of 
carbon dioxide production in nerves placed in an atmos- 
phere of hydrogen gas, care having been taken to insure 
the gas being perfectly pure, we found that the rate was 
only about half that of the normal nerve. It appears 
from this determination that in a medium deficient in 
oxygen the claw nerve of the spider crab gives off less 
carbon dioxide than in an ordinary atmosphere. The 
effect cannot be due to the hydrogen, since that gas has 
no physiological action, but is quite inert, and we may 
conclude that the lowering of the carbon dioxide is due 
to the lack or absence of oxygen. This is additional 
evidence that the lowering of the gaseous output is a 
physiological phenomenon, and that the carbon dioxide 
measured in normal isolated nerves is a product of normal 
metabolism, and is not the mere diffusion outward of the 
gas which is present in the tissue, being produced there 
by other than living processes. 
Carbon dioxide production of the isolated nerve at suc- 
cessive time intervals.—If the carbon dioxide production 
is due to a vital process, it might be expected to diminish 
gradually in the isolated nerve as its vitality diminishes. 
On the other hand, there was a possibility that the iso- 
lated nerve had become infected with bacteria and that 
the carbon dioxide might be due to their action. If 
this were the case, it would be expected that the carbon 
dioxide would gradually increase. Accordingly, experi- 
