THE ELEMENTS OF NERVE PHYSIOLOGY 39 



impulse is an electric current, nor that it can be called a 

 vibratory disturbance. We must not hastily abandon the 

 idea that it has chemical features. 



A chemical change occurring in white matter need not 

 be destructive in character. It may take place in one 

 instant and be reversed in the next with the restoration 

 of the initial condition. Something like this must be 

 true of the retinal cells in which the light-waves are 

 momentarily causing photochemical reactions which are 

 almost, though not perfectly, neutralized by the recupera- 

 tive changes which accompany them. It may be that when 

 a nerve-impulse passes there is a disruption of molecules 

 and an almost instantaneous recovery. It has been sug- 

 gested that upon the passage of each impulse a slight 

 contribution is made to the axon by the enclosing sheaths 

 and that in this way the conductivity is maintained almost 

 indefinitely. If there is any consumption of substance 

 attendant on the transfer, we should expect to obtain 

 evidence of the formation of waste-products in nerve- 

 trunks which have been functioning for a long time. The 

 fact that no conclusive evidence of this sort has been 

 secured shows that the conduction process is one of extra- 

 ordinary economy of material. 1 



One is constantly tempted to present the plan and 

 working of the nervous system in terms of an electric 

 plant like that of a telephone exchange. Nothing made 

 by man is so like the nervous mechanism, with its channels 

 leading in and out, and shifting connections to mediate 

 between them. Moreover, as we have hinted, there is an 

 electric change slight, but definite and characteristic 

 which betrays the passage of each nervous impulse. The 

 flying energy is, nevertheless, not to be regarded as an 



1 Even while these statements stand in manuscript comes the 

 statement of the Japanese, Tashiro, that he has demonstrated an 

 augmented discharge of carbon dioxid from a nerve-trunk convey- 

 ing impulses. His method is one of remarkable delicacy (American 

 Journal of Physiology, Proceedings of the American Physiological 

 Society, 1913, xxxi, 22). 



