892 THE FEATURES OF DIFFERENT REGIONS. [BOOK in. 



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 shewn that division or 

 injury of the fibres of the posterior roots is followed by tracts 

 of degeneration in the spinal cord, though no damage whatever 

 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 shew 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 ; 

 thejlegeneration does not affect the whole of the median posterior 

 column but leaves intact a small dorsal portion, roughly tri- 

 angular 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 grey 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 grey 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 posterior column would remain, the extent of degeneration 

 being dependent on the number of roots which had been divided. 

 Lastly, by carrying the sections still higher up the cord we should 

 be able to trace this tract in the median posterior column right 

 up to the bulb, where it would come to an end. 



If we divided some of the thoracic nerves instead of the. 

 lumbar we should obtain very similar results: a degeneration of 

 the external posterior columns a little above the entrance of the 

 roots, spreading across the column towards the median line, and 

 wholly disappearing at a certain height above, accompanied by a 



