EXCITATION AND CONDUCTION 



73 



biometer and measure its rate of carbon dioxide produc- 

 tion, we find that the part of the nerve nearer the body, 

 the proximal portion, gives off more gas than the distal 

 end of the nerve. This nerve is mainly efferent and 

 normally conducts the impulse down the claw from 

 proximal to distal direction. A quantitative estimation 

 shows that the proximal end gives more than twice as 

 much carbon dioxide as the distal, the former giving 

 at least y.pXio" 7 g. and the latter 3.7 io~ 7 g. per 

 centigram per ten minutes. 



TABLE X 



CARBON DIOXIDE PRODUCTION FROM Two DIFFERENT 

 PORTIONS OF CLAW NERVE OF SPIDER CRAB, 

 Libinia canaliculate 



From a study of the various conditions which mod- 

 ify this difference in carbon dioxide production in the 

 various portions of the nerve we are probably safe in 

 stating that we are here dealing with a physiological 

 gradient, experimental error playing no part. There 

 are three physiological causes which might account for a 

 different rate of carbon dioxide production: (i) different 

 degrees of injury; (2) different rapidity of death; or 

 (3) unequal rates of metabolism. The first alternative 

 is possible only on the assumption that the proximal 

 region must be more excitable (greater susceptibility to 

 an injury), which always causes the production of 



