1104 CUTANEOUS SENSATIONS. [BOOK in. 



ticular kind, and more particularly those of muscular sense. 

 There is less a priori objection to this view, since the tract 

 starts from the grey 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 know- 

 ledge 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 

 continuous 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 ( 581), 

 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 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 we 

 cannot ascertain, but which may be short. That such inter- 

 nuncial 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. 

 It is of course, as we have already urged, possible that the effect 

 of a section of a tract of fibres may be not the mere block due to 

 loss of continuity, but some action on the grey matter with which 

 the fibres are connected, whereby that grey matter fails of its 

 usual functions and ceases to carry onward the sensory impulses 

 reaching it from below ; it is also possible that this or that lesion 

 of disease may, directly or indirectly, affect particular parts of 

 the grey matter or affect the grey matter in a particular way, so 

 that a certain kind of sensory impulse, and none other is blocked. 

 On the other hand we have reason to think that the rate at which 

 impulses travel along the grey matter is very slow compared with 

 that along nerve fibres; and in the struggle for life, rapidity of 

 transmission of nervous impulses is of great importance. Hence 

 the view that internuncial fibres intervene has more to commend 

 it; it is moreover to a certain extent supported by clinical 

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

 time admit that, in animals at least, the lines provided by the 

 internuncial tracts are not rigid, that within limits and under 

 circumstances alternative routes are possible. 



