266 ANALOGY OF NERVES AND ELECTRIC CONDUCTORS. 



The experiments of Helmlioltz give, for the rate of propagation, from 83 

 to 88 feet per second in the frog, and in man 200 feet, the velocity ris- 

 ing with the mean animal heat. At one time it was thought that there 

 are perceptible differences in this rate in the same nerve of different indi- 

 viduals, or in different nerves in the same individual, but these conclu- 

 sions are admitted to be erroneous, or to be explained upon another prin- 

 ciple. It can not be denied that there is a general resemblance between 

 the manner in which a nervous fibril transmits its influences and that in 

 which a conducting medium conveys an electric current, though the ve- 

 Resembiance locity may be very different. There is a resemblance be- 

 between the tween the arrangement of the axis cylinder surrounded by 

 trical conduct-" ^s white substance and membranous tube, and that of a met- 

 ors - alline wire wrapped round with silk, or other non-conduct- 



ing material in many electrical arrangements. An electric current arti- 

 ficially transmitted along a nerve trunk will, as the nature of that trunk 

 may be, give rise to muscular contraction, or produce general or special 

 sensations, or originate reflex action. For these reasons, it has long been 

 supposed, by many physiologists, that the influence which passes along 

 nervous fibres is analogous to electricity, if it be not identical therewith ; 

 but all attempts to prove the existence of an electric current, either in the 

 centripetal or centrifugal fibres, have thus far been abortive. It may, 

 however, be remarked, that the arguments which are commonly present- 

 ed against the hypothesis of the identity of the nervous agent and elec- 

 tricity are but of little weight when critically examined. Thus it is said 

 that an electric current, passing along a nerve fibre, spreads laterally, 

 whereas the nervous agent never does ; but this is all dependent upon 

 that quality formerly known among electricians as intensity. There is 

 no reason to suppose that a thermo-electric current, the intensity of which 

 is very low, would exhibit such a lateral propagation ; whereas a voltaic 

 current, whose intensity is high, does it without difficulty. Moreover, 

 though it has been stated that the electric conductibility of a nervous 

 trunk is indefinitely worse than that of a metal, even lower than that of 

 a bundle of muscular fibre, it should be remembered in these discussions 

 that the conducting power is in the axis cylinder, and no attempt has 

 ever yet been made by any experimenter to isolate that structure and sub- 

 mit it to proper examination. It is just the same as though we should 

 take a bundle of copper wires, each one of which is separated from its 

 neighbors by a layer of non-conducting fat ; that we should cut out a sec- 

 tion of such a construction with a pair of scissors, and then attempt to 

 determine its conductibility. That, under any circumstances, would be 

 low enough ; and the chances are that the non-conducting material would 

 be smeared over the ends in the act of making the section, and the speci- 

 men refuse to conduct at all. In a similar manner, we may dispose of 



