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 



