THE STRUCTURE OF THE SPINAL CORD. 563 



fibres of an anterior root. We do not at present know what is the exact 

 manner by which the fibre in the pyramidal tract is connected with the fibre 

 of the anterior root. It seems certain, however, that the connection is not 

 in the form of a fibre isolated from the rest of the gray matter, containing, 

 so to speak, the pyramidal fibre into a cell of the anterior horn whence the 

 fibre of the anterior root issues. Most probably the pyramidal fibre makes 

 connections with the segmental groundwork spoken of above, whether with 

 or without the intervention of a cell we cannot at present tell. The direct 

 pyramidal tract is a like tract of less extent downward, and the less known 

 antero-lateral descending tract is probably of a similar nature. 



The cerebellar and antero-lateral ascending tracts are in like manner to 

 be regarded as longitudinal commissures between the successive spinal seg- 

 ment below and some part of the brain above. We have reason to think 

 that these tracts convey upward impulses of a nature which may be called 

 afferent, and are, therefore, in some way probably connected with the pos- 

 terior roots. We do not know as yet the exact nature of the connection ; 

 but probably in those cases also the commissural fibres are united not directly 

 to the posterior fibres, but indirectly by means of the segmental 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 upward, 

 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. 



494. All the evidence which we possess goes to show 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 gray matter are so 

 fused together as to form what appears to be a continuity of gray 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 na- 

 ture 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 determined 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 fre- 

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



