592 A MANUAL OF PHYSIOLOGY 



or from peripheral organs to nerve-centres, or from one 

 nerve-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, how- 

 ever, 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 system), by sensa- 

 tion, or by reflex muscular or glandular effects. 



Whether the wave which passes along the nerve is a wave 

 of chemical change (such, for 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, there is no definite experi- 

 mental evidence to decide, although the former is the most 

 probable view. 



That chemical changes go on in living nerve, we need not 

 hesitate to assume ; and, indeed, if the circulation through 

 a limb of a warm-blooded animal be stopped for a short 

 time, the nerves lose their excitability. But the metabolism 

 appears to be very slight compared with that in muscle or 

 gland. Even in active nerve no measurable production of 

 carbon dioxide has ever been observed, nor, in fact, has any 

 chemical or physical difference between the excited and the 

 resting state ever been unequivocally made out. Neither in 

 cold-blooded nor in mammalian nerves does there seem to 

 be any sensible rise of temperature during stimulation. 



Stimulation of Nerve. With some differences, the same 

 stimuli are effective for nerve as for muscle (p. 549) ; but 

 chemical stimulation is not in general so easily obtained. 



