NERVE 679 



centre to another. And in the normal body these impulses 

 never, or only very rarely, originate in the course of the nerve- 

 fibres ; they are set up either at their peripheral or at their central 

 endings. By artificial stimulation, however, a nerve-impulse 

 may be started at any part of a fibre, just as a telegram may 

 be despatched by tapping any part of a telegraph wire, although 

 it is usually sent from one fixed station to another. 



The Nerve-impulse : its Initiation and Conduction. 



What the nerve-impulse actually consists in we do not know. 

 All we know is that a change of some kind, of which the only 

 external token is an electrical change, passes over the nerve 

 with a measurable velocity, and gives tidings of itself, if it is 

 travelling along efferent fibres that is, out from the central 

 nervous system by the contraction or inhibition of muscle or 

 by secretion ; if it is travelling along afferent fibres that is, up 

 to the central nervous systemby sensation, or by reflex mus- 

 cular or glandular effects. 



Whether the wave which passes along the nerve is a wave of 

 chemical change (such, to take a very crude example, as runs 

 along a train of gunpowder when it is fired at one end), or a wave 

 of mechanical change, a peculiar and most delicate molecular 

 shiver, if we may so phrase it, or a shear in a definite direction 

 along the colloidal substance of the axis-cylinder (Sutherland), 

 there is no absolutely definite experimental evidence to decide. 

 An electrical change accompanies the nerve-impulse travelling 

 at the same rate, and although this is to be distinguished from 

 the impulse itself, there is little doubt that the latter is essentially 

 connected with a disturbance of the electrical equilibrium of the 

 nerve-substance. 



An attempt has been made to settle the question by determining 

 the temperature coefficient of the velocity of conduction of the 

 impulse i.e., the quantity which measures the change of velocity 

 for a given change of temperature. For most physical processes 



the quotient velocity -^.I5?, where Tn is any given tempera- 

 ture, is not greater than i'2, while for frog's sciatic nerve the tem- 

 perature coefficient for the most part lies between 2 and 3 (Snyder) . 

 The mean value of a large number of observations is i'jg, with 

 Tn = S to 9 C. (Lucas). For the pedal nerve of the giant slug the 

 mean value of the temperature coefficient is ry8 (Maxwell). In 

 other words, while for most physical processes an increase of 10 C. 

 increases the velocity of the process by at most one-fifth, the same 

 increase of temperature increases the velocity of conduction of the 

 nerve-impulse by four-fifths, or even more. While it is true that it 

 may not be entirely safe to apply such a criterion to a biological 



