ON THE MAMMALIAN NERVOUS SYSTEM. 417 



or come into relation with cells shortly after entering the cord, the degeneration in 

 consequence ceasing. This cessation of degeneration is indicated by the numerical 

 superiority of the degenerated fibres in the posterior roots over those in the- posterior 

 column, and above all the postero-median column. As far as it goes the evidence 

 itself afforded by this method is fairly conclusive ; whatever other channels there may 

 be, undoubtedly the above-named do exist, and thus the posterior column of the same 

 side offers a direct tract up which some nervous impulses undoubtedly not only 

 can, but do, pass. 



(6.) A similar investigation applied to the cord, since it determines all the fibres 

 in which degeneration is seen above the level of a hemisection, carries us a step 

 further than the above method. In this not only the fibres just indicated in 

 the posterior column degenerate, but all the through fibres from every root below 

 on the side of the hemisection. Further, it marks out all fibres which have their 

 nutritive or developmental centre, not in cells in the ganglion on the root, but in cells 

 situated in the cord below the section. Such fibres now seen as degenerated above 

 the lesion are grouped into two parts of the lateral column, the antero-lateral, and the 

 direct cerebellar. Except that the former is much more diffuse, the degenerated fibres 

 being scattered about among other sound ones, there is little to distinguish the 

 character of these two groups except their situation. They both appear to have 

 similar terminal connections, at any rate, on the cerebral side. They are both 

 characterised by containing fibres of very variable length, many of which appear after 

 a shorter or longer distance to again come into relation with nerve cells, so that in 

 these the degenerative change ceases. The characteristics of these tracts are, there- 

 fore, twofold ; they appear to afford not only a direct connection between certain cells 

 in the cord (CLARKE'S column, &c.) and the peduncle of the cerebellum of the same 

 side, but also a series of interriuncial connections, by means of which cells at variable 

 distances are brought into relation with each other. 



The tracts which degenerate on the peripheral side of a hemisection are foreign to 

 the purpose of the present chapter ; one, however, may be mentioned, the so-called 

 " comma-shaped " patch in the ventral part of the postero-external column, since it 

 is in all probability a tract of ascending fibres which are looped downwards, the 

 degeneration corresponding with the loop.* 



As far as this evidence under (6) goes it is not so -direct as that in (a). It is 

 conclusive as showing that, if there be crossing of any of these tracts from one side 

 of the cord to the other, such crossing must occur by means of cells, since the 

 degenerative change does not cross. It is further conclusive in showing that there 

 are continuous nerve fibres in these two situations. It is not conclusive as regards 

 the physiological characteristics of the fibres, since in anatomical evidence, as in cir- 

 cumstantial evidence; every link of the chain must be present for the evidence to be 



* Possibly these fibres are the descending branches of the bifurcating posterior root fibres, of. espe- 

 cially KOLLIKEB. 



MDCCCXCI. R. 3 H 



