CHEMICAL SIGNS OF IRRITABILITY 37 
if Ehrlich’s method of staining tissues with methylene 
blue is used, a spot of the surface of the brain loses its 
blue color when it is stimulated, owing to the consump- 
tion of oxygen and the resulting decolorization of the 
blue. 
We may now consider the carbon dioxide output of 
nerves on stimulation. © 
Non-medullated nerves.—The biometer is so delicate 
that in trying these experiments many precautions had 
to be taken to make certain that the experimental con- 
ditions themselves did not produce an increase of 
carbon dioxide independent of the change in the metabo- 
lism of the nerve. If we stimulate with an electrical 
current, we have to be on our guard lest there should be 
direct decomposition of some substances at the elec- 
trodes, resulting in more production of carbon dioxide. 
But by trying various kinds of stimulation, mechanical 
and chemical as well as electrical, we can throw out 
these possible sources of error. We found, in the first 
place, that there was no appreciable increase of carbon 
dioxide due to any direct electrical decomposition by 
stimulating a dead nerve. In all the quantitative 
experiments which follow, the current for stimulating 
was so small as to be barely perceptible to the tongue. 
The heating effect was, therefore, practically negligible. 
A nerve of the claw of the spider crab was isolated 
as before. A comparative estimate was first made. 
Two pieces of the nerve of equal weights and lengths 
were placed separately on the glass plates, each nerve 
being laid across the electrodes of the plate, in the man- 
ner shown in Fig. 2. In this way either nerve can be 
stimulated at will. These glass plates are hung upon the 
