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 7.9X1077 g. and the latter 3.7 1077 g. per 
centigram per ten minutes. 
TABLE X 
Carson DioxipE PropucTion FROM Two DIFFERENT 
Portions oF CLaw NERVE oF SPIDER CRAB, 
Libinia canaliculata 
* Gin ae bas CO, 
. ‘emperature| Given y 10 mg. 
Portion of Nerve Degrees C. of the Nerve in 
to Minutes 
15-16 6.7X107—7 g. 
Whole sedsee eee { 368 7.9X1077 g. 
Proximal.......... 2I 8.0X107 7g. 
Distal............ 23.2 3.7X1077 g. 
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: (1) 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 
