558 THE SPINAL COED. 



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 toward 

 the median line, and wholly disappearing at a certain height above, accom- 

 panied by a 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 gray commissure, and rather more 

 lateral. Compare Fig. 127, Z) 2 , where Ir. 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. 127, C 5 c. r.), while if we divided the sacral nerves the tract of degen- 

 eration 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. 127, 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 toward 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 shows 

 that the fibres run lengthways for some distance is this column, while the 

 disappearance of the degeneration a little higher up similarly shows that the 

 fibres eventually leave the column. The appearance of degeneration in the 

 median posterior column shows 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 ex- 

 ternal posterior column. This shows 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 gray matter of 

 the cord. Hence of all the fibres joining the cord in a posterior root, while 

 some, and these we may add are chiefly fine fibres, entering the gray matter 

 directly or passing into the posterior marginal zone, soon make such con- 

 nections that the degeneration due to the section of the roots spreads no 

 further, a large number, and these chiefly coarse fibres, before they make any 

 such connection pass into and occupy for some length of the cord the external 

 posterior column. We may here remark that though these fibres are spread 

 over the greater part of this column, they do not form the whole of the 



