CUTANEOUS AND SOME OTHER SENSATIONS. 713 



immediately connected with the anterior root. But no anatomical evi- 

 dence of such a tract is forthcoming ; and, as we have before remarked, 

 along all the tracts which seem to be sensory in nature, in contrast to what 

 takes place in the motor tracts, relays of gray matter are continually being 

 interpolated. 



The median posterior tract, since it gathers up representatives of succes- 

 sive nerves, presents itself as the nearest approach to such a sensory homo- 

 logue of the pyramidal tract, though it ends in the bulb and is not con- 

 tinued directly on to the cortex. And possibly it does play a somewhat 

 analogous part, in so far as it serves as a special connection between the 

 brain and the whole series of spinal nerves. But we are wholly ignorant as 

 to what it really does ; and whatever be the exact nature of the part which 

 it plays, it probably has relations not to one kind of sensation only, but to 

 all the different kinds of sensation. It has indeed been supposed by some 

 to be especially a tract for the impulses of the muscular sense ; but neither 

 experiment nor clinical study affords adequate proof of this view. The con- 

 dition known as locomotor ataxia, the salient feature of which is loss or 

 impairment of muscular sense, is associated with disease of the posterior 

 root and of its entrance into the cord, not with disease confined exclusively 

 to the median posterior column. Moreover, the tract cannot carry all the 

 impulses of muscular sense, since some of them must pass at once into the 

 gray matter to take part in the coordination of reflex movements, and must 

 therefore travel by fibres which do not form this tract. Similarly there is 

 no adequate proof of the tract being an exclusive channel for tactile or for 

 painful sensations. 



We may also, perhaps, urge similar considerations with regard to the 

 cerebellar tract, which, though starting from a relay of gray matter, is thence 

 onward to the cerebellum a continuous tract. This tract also has been sup- 

 posed to carry impulses of a particular kind, and more particularly those of 

 muscular sense. There is \essa priori objection to this view, since the tract 

 starts from the gray matter, where the impulses of muscular sense may have 

 already done their, so to speak, local work, and ends in the cerebellum, 

 which, as we have seen, seems especially connected with the coordination of 

 movements. But with respect to this tract also neither experiment nor 

 clinical study affords any clear and decisive proof that it is solely or even 

 especially concerned with the muscular sense. 



With regard to the antero-lateral ascending tract our knowledge is too 

 imperfect to justify us in supposing that it is the special or exclusive channel 

 for any one kind of sensation, or indeed in drawing any conclusions at all 

 concerning it. 



But when we subtract from the white matter of the cord these con- 

 tinuous tracts of ascending degeneration of presumably sensory or afferent 

 function, and the continuous tracts of descending degeneration, which we 

 may confidently speak of as motor or at least efferent, there are left only the 

 fibres which we have ( 494) supposed to be longitudinal commissural or 

 internuncial fibres between successive segments. We are thus driven back 

 to our former conclusion, that sensory impulses pass either by the gray matter 

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

 by iuternuncial tracts of fibres, whose length we cannot ascertain, 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 

 gray matter 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. It is of course, as we 

 have already urged, possible that the effect of a section of a tract of fibres 



