GENERAL LAW OF NERVE EXCITEMENT. 147 



intelligible. It is a well-known fact that a magnetic 

 needle suspended by a tliread assumes such a position, 

 in consequence of the magnetic attraction of the earth, 

 that one of its ends points to the north, the other to 

 the south. Now, supposing a series of many magnetic 

 needles, all suspended one behind the other in the same 

 meridian line, as in fig. 34, then each of these needles 



NS NS NS NS NS 



12 3 4 5 



Fig. 34. A seuies of jMagxeiic needles ahranged as a diagkaji 



OF THE PAUTICLES OF A NERVE. 



will be yet more firmly retained in its position by its 

 neighbours, for the adjacent north and south poles of 

 the needles mutually attract each other. If, for ex- 

 ample, we wish to move the middle needle, No. 3, more 

 force must be used to do this than would be necessary 

 if the needle were alone. But when the centre needle 

 is turned, the immediately adjacent needles cannot re- 

 main at rest, but are similarly deflected ; these exercise 

 a similar deviating influence on their neighbours ; and 

 so on. So that the disturbance created at one point 

 in this series of magnetic needles passes like a wave 

 through the whole series. 



This evidently bears much resemblance to that 

 which takes place in nerves. It explains not only 

 how a disturbance commencing at any point in the 

 nerve propagates itself, but also how each separate part 

 of the nerve is able to influence the other parts. We 

 have already found that the excitability of any point of 

 the nerve increases if the immediately superior portion 

 of the nerve is cut away. The magnetic needles show 

 that just in the same way each is more readily move- 



