82 A CHEMICAL SIGN OF LIFE 
While there seems, then, to exist a very close corre- 
lation between the rate of respiration of resting nerves 
and the velocity with which they conduct a nerve 
impulse, the data for the establishment of this generaliza- 
tion must necessarily be cumulative, and we are not yet 
able to state positively that all nerves which give off 
much carbon dioxide in the resting state will be found 
to conduct the impulse more rapidly than those which 
give off less. There are, however, several conditions 
which influence the rate of the nerve impulse, and we 
have investigated the effect of these conditions on the 
production of carbon dioxide. Two of these con- 
ditions are: changes in the salts in the nerves, and 
temperature. 
a) Changes in salts: Mayer found that the rate of 
nervous conduction in the sub-umbrella regions of the 
subtropical jelly fish, Medusa cassiopea, increases about 
5 per cent in sea-water diluted with distilled water in the 
proportion of 9:1, while it decreases 50 per cent in sea- 
water diluted to 50 per cent with fresh water. By sub- 
stituting o.g M dextrose for the distilled water he 
demonstrated that the change in rate of the impulse in 
diluted sea-water was not due to the decrease of osmotic 
pressure, but was due to the change in concentration of 
the salt. If under those conditions which decrease 
the rate of the conduction a measurement is made of the 
amount of carbon dioxide produced from the thin layer 
of the regenerating ectoderm tissue, it is found that a 
change in the rate of carbon dioxide production goes 
parallel with the decrease in the rate of conduction. As 
a result of using the regenerating tissue just mentioned, 
the nervous tissue regenerates before the muscular, so 
