GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 199 



when centripetally travelling impulses reach them, or to 

 transmit these to other efferent fibres and so cause reflex 

 movements; and therefore when a motor fibre is stimulated 

 in the middle of its course the out ward -going impulse causes 

 a movement, while the centrally travelling impulse, starting 

 at the same time, gives no direct sign of its existence. Simi- 

 larly for a sensory nerve such as the ulnar, already referred 

 to: if it be stimulated at the elbow the centrally travelling 

 impulse will cause a sensation of pain by exciting the brain- 

 centre connected with it, but the outward travelling impulse 

 not reaching muscular fibres or other parts which it can 

 arouse to activity, remains concealed from our notice. In 

 other words, the so-called specific energy of a nerve-fibre de- 

 pends upon the terminal organs on which it can act, and not 

 on any peculiarity of the nerve-fibre itself. 



proofs that all Nerve-Fibres are Physiologically Alike. 

 (1) Thelnost marked difference between nerve-fibres is obvi- 

 ously that between efferent and afferent, and the microscope 

 fails entirely to show any differences between the two. Some 

 motor and some se!nsory fibres may be bigger or less than 

 others, some may be white and others may be gray; but such 

 differences are secondary, and have no direct relation to the 

 function of a fibre as afferent or efferent. We can recognize 

 no constant difference in structure between the two. (2) 

 The physical properties and chemical composition of motor 

 and sensory nerves agree in all known points. (3) When a 

 nerve, say a motor one, is stimulated half-way between the 

 centre and a muscle, a nervous impulse, as we call it, is 

 propagated to the muscle, which impulse starts at the point 

 stimulated and travels at a definite rate to the muscle, the 

 contraction of which latter gives proof of its arrival. Now 

 starting at the same moment from the same point, and 

 travelling at the same rate, is that change in the elec- 

 trical condition of the nerve which can be detected by a 

 galvanometer, the so-called negative variation or action cur- 

 rent. When a nerve is excited from its end-organ, as for 

 example the optic nerve by light falling into the eyeball, 

 or a motor nerve by a stimulus arising in a centre, an action 

 current exactly like that observed with artificial stimulation 

 travels along it. Since this negative variation always accom- 

 panies a nervous impulse, appearing when it appears and dis- 

 appearing when it disappears, we conclude that it is a change 



