38 THE NERVOUS SYSTEM AND ITS < < >\SKKV ATI" >\ 



must utterly discard them. The nerves are not lube-, 

 but cables, and even the axons give no evidence of any 

 progressive movement of their substance. We have to 

 do with conductors in which the material is stationary; 

 this is an important limitation as we strive to conceive of 

 the propagated impulse. 



If we think of instances of conduction outside the 

 nervous system we shall be attracted by the figure of a 

 fuse. The spark which runs along the train of powder is 

 the visible sign of a destructive chemical change. A 

 protoplasmic strand might be imagined through which a 

 similar chemical change should make its way, leaving 

 behind it a track barren of further possibilities in the line 

 of conduction. Of course, this does not correspond closely 

 with what we know to be true of the nerves; they are 

 adapted to carry impulse after impulse without showing 

 impairment. The extreme resistance of white matter to 

 fatigue has made it hard to believe that its axis-cylinders 

 are in any sense "burnt out" in the performance of their 

 function. We cannot think that there is any extensive 

 waste of material as the unseen impulses travel along 

 the paths laid for them. 



Conductors of physical changes are not necessarily 

 altered in the least by their service. An iron wire is not 

 worn out by the passage through it of the electric cm-rent. 

 It has not contributed energy from its own stores, as has 

 the fuse, to reinforce that which ran from one end of it to 

 the other. All the energy has come from outside the 

 conductor, and when that energy is gone the wire is left 

 sensibly unaffected. In the same way the harp-siring, 

 which is plucked near one end and is swept throughout 

 its length by 1 he resulting vibrations, come- to re->t un- 

 changed except for a trilling wear and tear. Here again 

 the energy came from outside and the conductor gave up 

 none of its own. The nerve-liber seems much more 

 like the wire that carries the current or the musical siring 

 set in vibration than like 1 he fuse continued in doing its 

 duty. Yet we must not hastily condud" thai a nerve- 



