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-meduUated 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 



