EXCITATION AND CONDUCTION 63 
in the actual determination of carbon dioxide were 
reproduced. 
From the results so obtained the minimum concen- 
tration which produced a reversible loss of irritability was 
chosen for our experiments on the carbon dioxide pro- 
duction, and we are thus assured that the nerve has been 
narcotized, but that, since its excitability returns, no 
permanent injury has been caused. The carbon dioxide 
production of the nerve thus treated has been determined 
and compared with that of a normal nerve. These 
results are tabulated with the physiological data and 
given in Table VII. 
ETHYL URETHANE 
As shown by physiological tests, a freshly isolated 
claw nerve on immersion in a 4 per cent solution of ethyl 
urethane loses its excitability within ten minutes. Such 
a nerve, however, if left in a moist chamber for ten or 
fifteen minutes and then returned to sea-water, comes 
back to a normal condition of excitability with appar- 
ently no injurious effects. That the nerve so narcotized 
gives off less carbon dioxide than a normal one can be 
demonstrated qualitatively as follows: 
Two nerves of approximately the same weight are 
isolated, and one is immersed in sea-water while the 
other is treated with a 4 per cent urethane solution for 
ten minutes. At the end of this time their rates of 
carbon dioxide production are compared simultaneously 
in the biometer by placing the normal nerve, for example, 
in the right chamber and the other in the left. Within 
ten minutes the difference in carbon dioxide output 
will become evident, for not only does the precipitate 
