808 CUTANEOUS SENSATIONS. [Book in. 



visional conclusion, that sensory impulses pass either by the 

 grey matter alone, or by a series of steps as it were, by relays 

 of grey matter connected by internuncial tracts of fibres, whose 

 length Ave do not know, but which may be short. That such 

 internuncial tracts intervene is rendered probable, on the one 

 hand by the fact that section of the white matter, leaving the 

 grey untouched, does affect sensations, and on the other hand 

 by the fact that the several kinds of sensation appear to travel 

 along the cord by separate paths, or at least may be separately 

 blocked. But, if we accept this view, we must at the same time 

 admit that, in animals at least, the lines provided by the inter- 

 nuncial tracts and their relays are not rigid, that within limits 

 and under circumstances alternative routes are possible. 



§ 510. We may here perhaps raise once more, and this time 

 more pointedly than before, the doubt whether we are justified 

 in assuming, as we generally do assume, that the events which 

 take place in the fibres connecting relays of grey matter within 

 the central nervous system, are exactly the same as those which 

 take place in the fibres of nerves outside the central system, 

 during the passage of what we call a nervous impulse. Most of 

 our knowledge of a nervous impulse has been gained by the study 

 of the motor nerve of a muscle-nerve preparation. Our know- 

 ledge of the processes in afferent nerves is much more imperfect. 

 And, with regard to the processes taking place in fibres within 

 the central nervous system we have hardly any exact experimental 

 knowledge at all. It has been maintained by many observers that 

 not only the grey matter but also the tracts of white matter in 

 the spinal cord, while they are capable of conveying impulses in 

 one direction or the other, are incapable of being so excited by 

 artificial stimuli as to generate new impulses. These observers 

 maintain that, when movements or signs of sensation follow the 

 direct stimulation of various parts of the cord, the effects are due 

 to issuing motor fibres or entering sensory fibres having been 

 stimulated, and not to a stimulation of the intrinsic substance of 

 the parts themselves ; they propose accordingly to call these parts 

 'kinesodic' and ' assthesodic ' respectively, that is to say, serv- 

 ing as paths for motor or sensory impulses without being them- 

 selves either motor or sensory. The evidence on the whole goes 

 to shew that this view is a mistaken one, that the various tracts of 

 the spinal cord, like the pyramidal tract and indeed other parts of 

 the brain, are excitable towards artificial stimuli. The question 

 cannot, however, be considered as definitely closed ; and the very 

 fact that it has been raised illustrates the point on which we are 

 now dwelling. We may further quote, in similar illustration of 

 the same point, the following remarkable fact which was observed 

 in the series of experiments referred to in § 491 on the effects of 

 repeated hemisection of the spinal cord in dogs. The animal had 

 partially recovered voluntary movements in his hind limbs after 



