200 THE HUMAN BODY. 



in the electrical properties of the nerve dependent on that 

 internal movement of its molecules which constitutes a ner- 

 vous impulse. It is an externally recognizable physical sign, 

 and the only known one, of the existence of the nervous im- 

 pulse while it is travelling along the fibre. If the muscle were 

 cut away from the end of the nerve we could still detect that 

 a nervous impulse had travelled from the point of stimulation 

 to that where the fibres were divided, by tracking the nega- 

 tive variation. If, while stimulating a motor nerve half-way 

 in its course, we examine galvanometrically the portion lying 

 central to the stimulated point we find a well-marked centripe- 

 tally travelling action current; it starts at the same moment as 

 the efferent negative variation and travels in the same manner, 

 but the nervous impulse of which it is a sign produces no more 

 effect than the efferent impulse would after the muscle had been 

 cut away; for it does not reach any muscular fibre, or sen- 

 sory or reflex centre, which it can arouse to activity. Hence 

 it is clear that the motor nerve can conduct impulses equally 

 well in either direction; and similar experiment proves the 

 same thing for pure sensory nerves. 



While, however, by chemical or electrical stimulation of 

 a motor or a secretory nerve we can get a muscular con- 

 traction or a secretion apparently quite identical with that 

 produced by natural stimulation, we cannot make the same 

 assertion with regard to afferent nerves. It is possible by 

 gentle stimulation of a cutaneous afferent nerve through its 

 end-organs in the skin to excite the centres, so that they in 

 turn give rise to definitely combined reflex muscular con- 

 tractions, producing, even in the absence of all consciousness, 

 an useful movement. But if the skin be removed and the 

 outer end of its afferent nerve stimulated directly, though 

 the centres can be thus excited and caused to send out im- 

 pulses to muscles, the movements which result are random 

 kicks and jerks, very different from the definite, orderly 

 movements which follow suitable stimulation through the 

 skin. And as regards certain nerves of special sense some- 

 thing similar seems to be true. It has indeed been stated 

 that mechanical injury of the optic nerve, as by cutting it 

 during a surgical operation, causes a sensation of light in 

 patients not anaesthized, but this has been denied; and 

 though one positive observation counts for more in such a 

 case than many negative, we must take into account the 



