ELECTROPHYSIOLOGY 335 



Nerve Conduction. — Recent studies on nervous impulses show 

 a very definite relationship between electric and vital reactions. 

 Early speculations on nerve action assigned to the nerve fiber 

 a purely passive role. It was supposed to carry a stimulus from 

 one end to the other, as would a wire or pipe. Later, the dis- 

 covery was made that an electric potential is set up by the nerve 

 itself during activity, the intensity of the charge being independ- 

 ent of the nature of the stimulus applied. This indicates that 

 electricity is associated with the transmission of an impulse in 

 nerves, but it does not distinguish between cause and effect. 



Fig. 149. — The circuit of an excitation wave along a nerve fiber. The shaded 

 portion is the momentarily activated region (its length is 6 cm.); the maximum 

 potential reached is 40 millivolts. 



Among the first hypotheses of nerve conduction was that of 

 Ralph Lillie, who postulated that there is an essential similarity 

 between a nerve impulse and the conductance of an electric 

 current by the surface film of certain metals. The reaction of 

 iron wire and nitric acid exhibits an automatic rhythm of often 

 remarkable regularity, consisting of an alternation of active and 

 passive periods. Waves of activation travel along both wire and 

 nerve at intervals followed by passive, or nonreactive, periods (Fig. 

 149). The wave impulse sets up an electric eddy, or a succession 

 of such eddies, as it travels forward. The number of cycles per 

 minute in iron wire varies between 40 and 100 or more, and is 

 determined by several factors including the constitution of the 

 wire, the concentration of the acid, and temperature. A wire 

 freely suspended is nonrhythmical. Contact with glass or 

 another surface appears to be necessary. The wave is apparently 

 one of oxidation and reduction of the iron on the surface of the 

 wire (see page 341). It does not take place on copper wire, and 

 it is not a current comparable to a flow of electricity. There is no 

 constant fall of potential along the wire as in an electric circuit, 

 but instead a succession of waves or impulses of chemical reac- 

 tions and consequent electric potentials. This leaves the wire 

 positively charged over a short section when oxidation is taking 

 place and negatively charged when reduction is going on. In 

 time, the wire spontaneously ceases activity, the potential drops, 



