THE SIMILARITY OF NERVE-FIBRES. 



traveling impulse starting at the same time, gives- 

 no direct sign of its existence. Similarly for a sensory 

 nerve such as the optic; if it be stimulated somewhere be- 

 tween the brain and the eyeball the centrally traveling 

 impulse will cause a sensation of light, by exciting the 

 brain-centre connected with it, but the outward traveling 

 impulse not reaching muscular fibres or other parts which 

 it can arouse to activity, remains concealed from our no- 

 tice. In other words the so-called specific energy of a 

 nerve-fibre depends 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) The most marked difference between nerve-fibres is* 

 obviously that between efferent and afferent, and the mi- 

 croscope fails entirely to show any differences between the- 

 two. Some motor and some sensory 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 cor- 

 relation with the function of the nerve. 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 mus- 

 cle, the contraction of which latter evidences its arrival. 

 Now starting at the same moment from the same point, and 

 traveling at the same rate, is a change in the electrical 

 properties of the nerve which can be detected by a good 

 galvanometer and which is called the "negative variation." 

 Since this negative variation and the nervous impulse co- 

 exist at any given moment in a particular point of the- 

 nerve and disappear from it together, we conclude that the 

 negative variation is a change in the electrical properties 

 of the nerve dependent on that internal movement of its 

 molecules which constitutes a nervous impulse. It is an 

 externally recognizable physical sign, and the only known 

 one, of the existence of the nervous impulse as it travel* 



