96 A CHEMICAL SIGN OF LIFE 
this could be; the change in state, which was the essence 
of irritability, was pictured as a change in the state of 
the colloids or the structure of the protoplasm, or, more 
recently, in the state of its permeability. But how on 
this basis irritability was to be understood was by no 
means clear. On examining the irritability of nerves— 
the apparent exception which had led to the conclusion 
that irritability had a physical and not a chemical 
basis—we found that this apparent exception was 
really due to the fact that our methods had not hitherto 
been sufficiently delicate to detect the chemical changes 
which accompanied the process. By devising a new 
method for the study of carbon dioxide—one of the 
terminal products of metabolism everywhere—we found 
that living nerve fibers in reality were undergoing 
chemical change at quite a remarkable rate and were pro- 
ducing carbon dioxide faster than any other tissue of the 
body, if equal weights were compared. And we found, 
further, that reagents or physical methods which change 
the state of excitability of the nerve changed also the 
rate at which it was producing carbon dioxide, so that 
the gas production was evidently correlated with its 
vitality and not with adventitious processes. 
In chapter iii we found that although the chemical 
activity is a necessary condition for all living nerves, yet 
by itself it is not a demonstrative sign of life; i.e., it is not 
a sufficient criterion of living. An additional criterion is 
needed in order to be sure that any tissue is living. 
In the case of the nerve, we demonstrated that this 
additional sign was also present. ‘This sign is the fact 
that all living matter, including the nerve, responds to 
a stimulus by the production of more carbon dioxide. 
