THE STRUCTURE OF THE SPINAL CORD. 557 



We have as yet no very clear evidence as to the origin of the fibres which 

 compose the tract. Unlike the case of the median posterior tract of which 

 we have next to speak, no degeneration, at least in the lumbar and thoracic 

 regions, appears in the tract after section merely of the roots of the nerves ; 

 to produce the degeneration the cord itself must be injured. From this we 

 may infer that the tract is not fed directly by the fibres of the posterior roots. 

 Some observers maintain that the tract is fed by fibres coming from the 

 vesicular cylinder and point out that both the tract and the column begin at 

 the same level somewhat suddenly ; but the want of parallelism between the 

 course of the tract and that of the cylinder along the length of the cord, the 

 latter being as we said conspicuous in the thoracic region while the tract 

 steadily increases upward, is distinctly opposed to such a view. From the 

 fact that the degeneration taking place in it is an ascending one, it is sup- 

 posed that the tract is the channel for ascending, that is to say, in a broad 

 sense, afferent impulses. And considerable interest attaches to the fact that 

 these impulses should be carried, not to the cerebrum but to the cerebellum. 

 Our knowledge on this point, however, is very imperfect, and what can be 

 said in the matter had better be said later on. 



490. The median posterior tract is the other conspicuous tract of ascend- 

 ing degeneration ; it also is supposed to be a channel for ascending afferent 

 impulses; and this view is rendered almost certain by the intimate relations 

 of the tract to the fibres of the posterior roots. 



In dealing so far with the tracts of degeneration in the spinal cord we 

 have always spoken of the degeneration as being the result of lesions of the 

 spinal cord itself. Experiments on animals, however, and clinical experience 

 have shown that division or injury of the fibres of the posterior roots is fol- 

 lowed by tracts of degeneration in the spinal cord, though no damage what- 

 ever may have been done to the substance of the cord itself. These tracts 

 make their appearance in the median posterior columns, the exact path and 

 limits of the degeneration differing with the different spinal nerves. The 

 results of the division of different groups of nerves are so instructive that we 

 may dwell upon them in detail. 



If the posterior roots of two or three lumbar nerves (on one side) be 

 divided, an examination of the cord, after an interval long enough to allow 

 degeneration to be well established, will bring to light the following features : 

 The divided roots will be found to have degenerated right up to their entrance 

 into the cord. A section of the cord opposite the entrance of the lowest 

 divided root will show no degeneration of the cord beyond that of the bundles 

 of fibres passing in. A little higher up degeneration will be observed in the 

 external posterior column close to the posterior horn ; and as we ascend we 

 find that this degeneration first spreads over a large portion of the external 

 posterior column, and then invades the median posterior column ; the de- 

 generation does not affect the whole of the median posterior column but 

 leaves intact a small dorsal portion, roughly triangular in shape, at the angle 

 between the fissure and the dorsal surface of the cord, as well as some portion 

 of the more ventral part of the column nearest the gray commissure. Still 

 a little higher up we should find that degenerated fibres had disappeared 

 from the external portion of the external posterior column close to the gray 

 matter, though still existing in the more median part of that column as well 

 as in the median posterior column to the extent just indicated. Still a little 

 higher up the whole of the degeneration would have disappeared from the 

 external posterior column, but the tract of degeneration in the median pos- 

 terior column would remain, the extent of degeneration being dependent on 

 the number of roots which had been divided. Lastly, by carrying the sec- 

 tions still higher up the cord we should be able to trace this tract in the 



