EXCITATION AND CONDUCTION 81 
very slowrate. And, being foolish, this does not happen. 
We can use the speed of contraction, therefore, as a rough 
index of the relative velocities of the nerve impulses in 
nerves supplying the muscles. In this way the rates of the 
nerve impulse of the ambulacral nerves of the king crab, 
the spider crab, and the lobster are in the ratio of 1:2:4. 
This ratio, as far as the king crab and the spider crab are 
concerned, is very nearly the same as that found for the 
carbon dioxide output of these nerves, as may be seen 
in Table IV (p. 32). Low respiration, low irritability, 
and low speed of conduction appear to go together. 
We can also test the rate of metabolism in these 
nerves indirectly by Child’s method, and this gives us 
just the same result. This method consists in determin- 
ing the speed with which the excitability of the nerves 
is abolished when they are treated with the same con- 
centration of anarcotic. The greater the rate of metabo- 
lism the more susceptible is the tissue to narcotics. We 
found in the case of these three nerves that the excita- 
bility of the lobster nerve is most quickly abolished, 
then that of the spider crab, and finally that of the king 
crab. This is the same order in which the nerves con- 
duct the impulse, the lobster conducting fastest. This 
is therefore additional proof of the fact that the speed 
of the impulse and the degree of metabolism in the 
resting nerve are correlated, at least in these nerves. It 
is remarkable to note that the carbon dioxide output 
of the nerves of Limulus, the king crab, is very low in 
comparison with that of other animals. This may be 
correlated with the very sluggish behavior of this animal 
and its power of living for a long time without food and 
with very little air. 
