EXCITATION AND CONDUCTION 81 



vety slow rate. 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 a narcotic. 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 Limulns, 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. 



