CHAP. L] THE SPINAL CORD. 891 



this we infer that the fibres composing the tract must start 

 successively from other parts of the cord along its length, that is 

 to say, the tract must be fed by fibres coming from other structures 

 in the cord. On the other hand, it is found that the degenerated 

 area following upon a section or injury diminishes as it is traced 

 upward ; when, for instance, a section is made in the mid thoracic 

 region the area of degeneration in the tract is greater immediately 

 above the section than it is higher up, say in the cervical region. 

 From this we are led to infer that though the tract is successively 

 fed along its course by fibres coming from other parts of the 

 cord, some of the fibres entering the tract, though like their 

 companions undergoing an ascending degeneration, do not like 

 them continue in the tract right up to the cerebellum, but pass off 

 to other parts of the cord on their way upward. This, however, is 

 equivalent to saying that the tract is not a pure or homogene- 

 ous one, but consists of at least two sets of fibres, only one of 

 which is continued on to the cerebellum and strictly deserves the 

 name of ' cerebellar.' It may perhaps here be mentioned that 

 while the fibres composing the tract are as a whole conspicuously 

 coarse, large fibres, with these there are mingled, especially in 

 the thoracic region, a number of much finer fibres ; but these 

 apparently undergo a descending not an ascending degeneration 

 and do not therefore really belong to the tract ; they may be 

 fibres which have strayed from the pyramidal tract. 



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 supposed 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. 



577. The median posterior tract is the other conspicuous 

 tract of ascending degeneration; it also is supposed to be a 

 channel for ascending, afferent impulses ; and this view is rendered 



