CHAP. L] THE SPINAL CORD. 893 



degeneration of a part of the median posterior column, reaching 

 from a little distance above the entrance of the divided nerve-roots 

 right up to the bulb. This latter tract of degeneration would 

 however not occupy the same position as that consequent upon 

 division of the lumbar nerves ; its position would be more ventral, 

 nearer the grey commissure, and rather more lateral. Compare 

 Fig. 104, D 2 , where IT indicates the degeneration due to section 

 of the lumbar nerves, and dr that of the thoracic nerves. If we 

 divided some of the cervical posterior roots we should get similar 

 results, with the difference that the tract of degeneration in the 

 median posterior columns would occupy a position still more 

 ventral and still more lateral (Fig. 104, C 5 c.r.) ; while if we 

 divided the sacral nerves the tract of degeneration would be dorsal 

 and median to the tract belonging to the lumbar nerves, and 

 would occupy more or less of the triangle left below that tract 

 (Fig. 104, D 2 s.r.). The degeneration it will be understood is in all 

 cases confined to the same side of the cord as that of the divided 

 roots. We may add, in order to complete the story of the effects 

 of division of the posterior roots, that the section leads to 

 degeneration of the marginal zone (Lissauer's tract), but this 

 degeneration reaches for a certain distance only up the cord and 

 then disappears. It will be remembered that this zone is fed by 

 fibres (of fine calibre) belonging to the external or lateral bundle 

 of the posterior roots. 



These results may be interpreted as follows. The (great 

 majority of the) fibres of the posterior root, cut off from their 

 ganglion by the division, degenerate centripetally towards the 

 spinal cord. We have previously seen that many of the fibres 

 of the root pass into the external posterior column and run up in 

 that column for some distance. The degeneration observed in 

 this column for some distance above the entrance of the divided 

 roots shews that the fibres run lengthways for some distance in 

 this column, while the disappearance of the degeneration a little 

 higher up similarly shews that the fibres eventually leave the 

 column. The appearance of degeneration in the median posterior 

 column shews that some of these fibres have passed into that 

 column from the external posterior column, and the continuation 

 of that degeneration right up to the bulb indicates that these 

 fibres pursue an unbroken course in that column along the 

 whole length of the cord. The area of degeneration, or more 

 exactly the number of degenerated fibres in the continued tract 

 of degeneration in the median posterior column is much less 

 than that in the temporary or short tract of degeneration in the 

 external posterior column. This shews that some only of the 

 fibres passing into the external posterior column go on to join the 

 median posterior column and so reach the bulb ; the rest obviously 

 take another path, and we have already seen reason to think that 

 many of these end in the grey matter of the cord. Hence of all 



