50 A CHEMICAL SIGN OF LIFE 



is no heat produced in excited nerves. How shall we 

 explain the fact that this relatively tremendous chemical 

 transformation can occur without heat formation? 

 There are several explanations which might be given 

 of this fact, but before considering them we may see 

 first what the evidence is that there is no heat produced. 



Although there have been in the literature many 

 contradictory statements as to heat formation in the 

 active nerve, the original negative results of Helmholz, 

 Stewart, and Rolleston have been confirmed recently 

 by A. V. Hill's work, which shows that there is no meas- 

 urable liberation of heat when the nerve is stimulated. 

 Since his apparatus is exceedingly sensitive, being sus- 

 ceptible to the change of 1/1,000,000 of a degree Centi- 

 grade, the lack of observed heat production is not ap- 

 parently to be explained by any lack of a proper method 

 of measuring temperatures. His work is remarkably 

 significant in that according to his calculation not more 

 than one single oxygen molecule in every cube of a 

 nerve containing 3 . 7 cubic /* can be used up by a single 

 propagated nerve impulse, since more than this amount 

 would produce a measurable amount of heat. Thus he 

 is convinced that a nerve impulse is not of an irreversible 

 chemical nature, but must be of a purely physical nature. 



Negative evidence of this kind cannot be taken at 

 its face value without considering the limitations of the 

 method. Stewart calls attention to the fact that we 

 should not forget that if the axis cylinder is the only 

 portion which is conducting a nerve impulse, as we 

 believe, the measurement in medullated nerves with 

 which most experiments were made does not express 

 the true state in the axis cylinder. We should consider, 



