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 
