CHEMICAL SIGNS OF IRRITABILITY 29 
parallelism between the decrease in metabolism and 
decrease of irritability in the nerve. The gas produc- 
tion slows up as the nerve approaches death. This 
indicates, also, that the carbon dioxide is formed by a 
vital process. 
Comparison between the metabolism of resting nerves 
and other tissues—While a comparison of the rate of the 
metabolism of the nerve with that of other tissues is 
subject to a good many limitations, since there are so 
many and great variations in conditions which do not 
affect all tissues similarly, it is nevertheless interesting 
to note whether the nerve respires relatively more or less 
than most other tissues. In order to give a better 
numerical picture of the amount of metabolism in the 
resting nerve, as compared with other tissues, we have 
set down in Table III the figures for carbon dioxide 
production in various animals. Since there are no exact 
determinations made of the carbon dioxide production of 
the spider crab as a whole, or of its tissues, we have used 
for comparison various other crustacea where these 
data have been determined. It will be noticed from an 
inspection of this table that the spider crab nerve pro- 
duces, weight for weight, carbon dioxide at a rate three 
to four times that of the whole body of crabs, and almost 
as much in proportion to weight as a human being at 
rest. Recently Bayliss, in his admirable book entitled 
Principles of General Physiology, expressed a doubt of our 
figures. He thinks that the gas we measured must be 
due to some cause other than the metabolic activity of 
the nerve, because, he says, the data show that it is 
greater than that of an equal weight of muscle. It is 
rather difficult for us to understand the force of this 
