44 A CHEMICAL SIGN OF LIFE 
state of excitability. On the other hand, if this carbon 
dioxide increment were not correlated with the vitality 
of the nerve, and if nerve activity did not involve respira- 
tion, we expected to find that putting the nerve in 
hydrogen gas would not affect the output on stimulation. 
For this experiment the claw nerve of a spider crab was 
used, and stimulation was effected in the usual manner 
by a tetanizing induced current of the same strength as 
that which had been used before and found to increase 
the output in normal nerves. The results are given in 
Table VI. It will be seen in this table that the pro- 
duction of carbon dioxide by this nerve was reduced 
TABLE VI 
CoMPARATIVE RATES OF CO: PRODUCTION IN THE NERVE WITH AND WITHOUT OXYGEN 
Amount of CO: Amount of CO. 
Produced by Produced by 
Nerve Medium ro mg. of Resting to mg. of Stimu- 
Nerve in ro lated Nerve in 
Minutes ro Minutes 
Claw nerve of spider crab. .| CO: free air|6.7 X10 7 g.(15°-16°)| 16 X107 7 g.(14°-16°) 
Claw nerve of spider crab. .| CO: free air|7.9 X10 7 g.(2022) 
Claw nerve of spider crab. .}| COs free 
hydrogen|3.4X10—7 g.(23°0) |3.6X1077 g.(21°0) 
almost exactly 50 per cent when the nerve was in hydro- 
gen, as compared with when the nerve was in the air; 
and still more remarkable is the fact that stimulation in 
the hydrogen atmosphere produced practically no 
change in the carbon dioxide output. We interpret this 
to mean that the excitability of the nerve had been so 
reduced by the lack of oxygen that this strength of 
stimulus was unable to cause any excitation in the nerve. 
We base this conclusion on the known fact that lack of 
oxygen lowers considerably the irritability of the nerve 
