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. 



