900 THE COMMISSURAL TRACTS. [BOOK in. 



groundwork. And since these tracts do not degenerate after 

 section of the posterior roots, but only after section or other lesion 

 of the cord itself, we may infer that their junction with the 

 groundwork is effected by means of trophic cells, by means of 

 some or other of the cells spoken of a little while before. 



The median posterior tract seems to be a commissural tract of 

 a nature different from any of the above. Through it a certain 

 part of each posterior root is brought into connection, not with its 

 own spinal segment but with the bulb above, and so with the brain, 

 which thus receives direct representatives of each afferent spinal 

 nerve. If however, as some maintain, the bundle in this tract 

 starting from a spinal nerve below, diminishes as it proceeds 

 upwards, throwing off fibres to pass elsewhere, though always 

 carrying some fibres right up to the bulb, we must add to the 

 above the further view that this tract connects also each posterior 

 root, not with its own segment but with other more or less 

 distant segments. 



581. All the evidence which we possess goes to shew that 

 each strand of each of these tracts runs isolated, that is to say, 

 makes no connections with adjoining structures at any part of 

 its course, from its beginning or end in the brain and its end or 

 beginning in its appropriate spinal segment, or in the case of the 

 median posterior tract from its beginning in the ganglion of a 

 posterior root and its end in the bulb or in some distant spinal 

 segment. In the crossed pyramidal tract, for instance, we have 

 reason to think that one or more fibres run a quite unbroken and 

 isolated course from the cortex of the cerebrum through various 

 parts of the brain, along the whole length of the cord until they 

 reach the lowermost spinal segmental mechanism. These tracts 

 serve in no way to connect one segmental mechanism with another. 

 The segmental mechanisms are however connected together ; and 

 the connections between them seem to be of two kinds. In the 

 first place, as we have already suggested, the segmental pieces of 

 grey matter are so fused together as to form what appears to be 

 a continuity of grey matter from one end of the cord to the other. 

 Though we cannot actually track our way histologically through, 

 and are still less aware of the physiological nature of the labyrinth 

 of nerve cells, fibres and fibrils which make up what we have 

 called the groundwork, we may with considerable probability 

 assume that the passage of nervous impulses along it is de- 

 termined as much by the condition of the material as by its 

 anatomical disposition ; that, for instance, the restrictions to the 

 flow of an impulse are brought about much more frequently 

 by the refusal of the molecules of nervous matter to take up the 

 molecular disturbance which is the essence of the impulse, that 

 is to say, by molecular resistance, than by actual breaks of con- 

 tinuity in the nervous matter. Indeed we have some reasons 

 for thinking that actual structural continuity of nervous material 



