CHEMICAL SIGNS OF IRRITABILITY 23 



tissues, which produce carbon dioxide either by direct 

 oxidation, by fermentation, or by the decomposition of 

 carbonates by acids, the possibility that this carbon 

 dioxide which we have detected is not a product of 

 vital activity cannot be so easily disproved. Inasmuch 

 as our apparatus detects such a small amount of the gas 

 as that which is contained in one-sixth of a cubic centi- 

 meter of the purest air, we cannot accept the results just 

 cited as certain proof that the normal nerve undergoes 

 metabolic changes. We must inquire, therefore, whether 

 this carbon dioxide is produced by living processes. In 

 the first place, as the biometer in its present form cannot 

 examine the carbon dioxide production of a nerve in its 

 normal position and with its muscle attached to it, we 

 have to use an isolated nerve. Certain experimental 

 factors are thus introduced which must be carefully 

 considered before we interpret our observations. It 

 is first necessary to be sure that this isolated nerve lives 

 and remains excitable for a considerable period after it 

 has been removed from the body. We can be quite 

 certain that this is the case because of the fortunate 

 circumstance that the passage of the nerve impulse 

 through such an isolated nerve produces a characteristic 

 electrical disturbance, which we may detect by a sensi- 

 tive galvanometer. As long as this electrical dis- 

 turbance occurs and the nerve is excited, we may be 

 perfectly sure that the nerve is living. It is as certain 

 a sign of the passage of the nerve impulse, and conse- 

 quently as sure an evidence of the vitality of the nerve, 

 as would be the contraction of the muscle which the 

 nerve supplies, had this remained attached, to it. By 

 thus testing with a galvanometer isolated nerves, such 



