12 A CHEMICAL SIGN OF LIFE 



is that by chemical transformations it sets free energy 

 and moves itself. 



It is much better, too, to take the carbon dioxide pro- 

 duced, rather than the oxygen consumed, as the measure 

 of the metabolism associated with irritability, for the 

 reason that sometimes organisms get their oxygen from 

 sources other than the air, whereas their carbon dioxide 

 production is always something positive and universal. 



Indirect evidence of the presence of metabolic activity 

 in the nerve fiber. The search for some kind of metab- 

 olism, such as the production . of carbon dioxide, in 

 nerves had been made by many physiologists on many 

 occasions, but it was impossible for them to discover 

 this substance because their methods were not sufficiently 

 delicate. No carbon dioxide could be found, and for 

 this and other reasons the conclusion was incorrectly 

 drawn that there was none produced, or that, if it were 

 produced, it had no connection with the vital functions 

 of the nerve. Most physiologists were accordingly of the 

 opinion that the conduction of the nerve impulse was 

 a physical process and involved no transformation of 

 energy and no consumption of material. There was 

 one exception to this rule. Professor A. D. Waller, 

 the eminent English physiologist, maintained that, 

 because of their electrical behavior, nerves certainly 

 produced carbon dioxide. In 1896 he showed that 

 carbon dioxide when applied to a nerve produced a very 

 characteristic change in the electrical response which a 

 nerve exhibits when it is irritated. It will be remem- 

 bered that when a nerve or, in fact, any kind of proto- 

 plasm is irritated in any way, if one applies two electrodes 

 to the living tissue in such a way that one electrode is on 



