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 
