80 A CHEMICAL SIGN OF LIFE 
fishes, mammals, crabs, and arachnids. We feel certain, 
therefore, that the existence of a metabolic gradient 
in nerve fibers correlated with the functional activity, and 
related to the direction the nerve impulse takes, must be 
a very general phenomenon. From these facts we come 
to the very simple conclusion that is summarized in the 
following statement: The normal nerve impulse passes 
from a point of higher toward a point of lower carbon 
dioxide production—from the more irritable to the less 
irritable parts. There is also a decrement in the nerve 
impulse. It cannot proceed indefinitely along a nerve; 
it will ultimately die out. 
Velocity of the nerve impulse and its relation to res pira- 
tion.—If the nerve impulse cannot pass through the 
fiber without consuming substance, it is reasonable to 
expect that there may be some relation between the 
rate of production of carbon dioxide in the resting nerve 
and the velocity of the nerve impulse. The reason for 
such a supposition is clear. It has already been indi- 
cated that the more irritable a nerve is the more carbon 
dioxide does it produce in the resting state. Hence, 
the more it respires, the more irritable it is, the faster 
should it conduct the impulse. This is actually found, 
within limits, to be the case. If one compares corre- 
sponding nerves of different animals, or different nerves 
of the same animal, it is found, other things being equal, 
that there is a relation between the speed of contraction 
of the muscles supplied by the nerve and the velocity 
with which the nerve supplying the muscle conducts 
the impulse. Obviously it would be foolish to have 
a very rapidly contracting muscle, or limb, supplied with 
a nerve which conducted the impulse to the muscle at a 
