46 A CHEMICAL SIGN OF LIFE 



heat produced by the exciting current itself, to the 

 escape of carbon dioxide which had been dissolved in 

 the living cells in the connective tissue around the nerve 

 fiber. We have cited several experiments the results of 

 which exclude this possibility. In addition to these, 

 the apparent lack of any increase of this gas on applica- 

 tion of induction shocks to a nerve in an oxygen-free 

 medium like hydrogen should be taken as conclusive evi- 

 dence that the increased gas production by a nerve when 

 stimulated in the air is due to physiological processes, and 

 not to experimental errors. 



Lack of fatigue. If the chemical change of the nerve 

 tissue is as active as the observations just cited indicate, 

 one naturally asks how we can explain the fact that the 

 nerve impulse can pass continuously through the fiber 

 without any measurable sign of fatigue. There is no 

 doubt that this apparent lack of fatigue of medullated 

 nerves is a very remarkable and striking phenomenon. 

 Nerves can be stimulated for many hours continuously 

 without marked fatigue. But it does not at all mean 

 that there is no chemical change in the nerve, for, in 

 the first place, it must not be forgotten that medullated 

 nerves have in the medullary sheath a very large supply 

 of raw material, or food, which is more than sufficient for 

 their nutrition during the longest experiments which have 

 been tried. The only surprising feature of the physiology 

 of the nerve is that in the isolated nerve, where there is 

 no opportunity for getting rid of the products of decom- 

 position, accompanying functional activity, by way of 

 the blood, nevertheless these products do not seem to 

 act deleteriously on the nerve function. But, after all, 

 we have only to assume, in order to understand this, that 



