NATURE OF THE NERVE IMPULSE. 121 



an iron wire in what is called the passive state. An iron wire 

 exposed temporarily to strong nitric acid becomes covered with a 

 thin film, probably an oxide of some kind, which makes the iron 

 very resistant to chemical action. If now this wire in the passive 

 condition is touched at any point with a piece of ordinary iron, or 

 if it is bent or scratched, or if a current is passed into it the film at 

 the point acted upon is destroyed by reduction and a bit of ordinary 

 or active iron is exposed. The electric potential of the active iron 

 is different from that of the passive iron, so that a local electric 

 current passes from one to the other through the surrounding 

 electrolyte. The process of activation propagates itself along the 

 wire with a considerable velocity and its progress can be followed 

 by the eye in consequence of the evolution of bubbles of gas. The 

 propagation is explained by the fact that the local current between 

 the active iron and the passive iron serves as a stimulus, causing a 

 reduction in the film of the passive area affected and the exposure 

 of a new active spot. Between this and the passive area beyond a 

 new current develops and so the activation spreads. Moreover, 

 the activated portions are brought back to the passive state by a 

 reaction with the nitric acid, so that as the ^tate of activation is 

 propagated down the wire it is followed by a process of restoration. 

 After a certain period, which may be compared with the refractory 

 period in the nerve, the restoration is complete and a new stimulus 

 will start a new process of activation along the wire. In this kind 

 of transmission we have a self-propagated disturbance which on 

 the chemical side consists of a reduction followed l^y an oxidation 

 that restores the original condition, and on the electrical side ex- 

 presses itself as a moving change in potential. Lillie thinks it 

 probable that in the nerve an analogous condition exists. In the 

 fiber (axon) there is a surface film of unstable reducible material 

 which is readily acted upon by stimuh with the destruction at that 

 point of the surface film and the exposure of an area in which the 

 electric potential is negative as regards the rest of the fiber. Be- 

 tween the two a local electric current passes which causes the 

 destruction of a new portion of the surface film, and so the dis- 

 turbance is propagated by a series of these local currents, and 

 wherever it is present it can be detected by a difference in electric 

 potential. When the disturbance reaches the muscle it is trans- 

 mitted to the latter through the end-plate and spreads over the 

 muscle fibers, as what we may call the muscle impulse. Here it 

 acts as a hberating stimulus to cause a sudden almost explosive 

 chemical reaction, from which in turn the energy for contraction is 

 obtained. In the nerve fiber as in the wire of passive iron the 

 reduction of the film is followed by a return to normal resting con- 

 ditions, and the suggestion naturally arises that the role of oxygen, 



