EXCITATION AND CONDUCTION 85 
other, generally toward the central nervous system 
in sensory nerves and toward the periphery in motor 
nerves. The nerve impulse generally travels in the 
direction of this gradient. When we stimulate a nerve 
at any point, the stimulation consists in the local in- 
crease of metabolic activity at the point of irritation. 
The irritability is raised and the carbon dioxide output 
is increased at that point above the production on either 
side of it. This causes a local metabolic gradient in 
.the nerve in both directions from the point of excitation, 
but the difference between this point and its surroundings 
will be greater on one side than on the other, owing 
to the gradient in the nerve just mentioned. If this 
state of excitation is sufficiently great, it upsets the 
equilibrium and the impulse will be propagated in each 
direction from the point of excitation. The possi- 
bility exists that it ought to travel more easily in the 
direction which the nerve impulse normally takes, and it 
ought to be possible, with a proper amount of stimulus, 
to start a propagation in only one direction from the 
point of stimulus, but we have not yet tested this possi- 
bility experimentally. The excitation always travels 
from the point where the excitation is greatest to that 
where it is less. The repair process, or the anabolic 
process, is also propagated. 
The conditions which affect the rate of nervous 
metabolism not only alter the state of excitability of the 
nerve, but also change the speed of the conduction of the 
state of that excitation. Although we have no evi- 
dence to show that the chemical change itself constitutes 
the nerve impulse, the conclusion is almost inevitable 
that the nerve impulse is brought about by, or is itself 
