DEGENERATIONS IN THE SPINAL CORD. 29 



in fact a succession of lamellar tracts, which lie in each case at first next to the 

 posterior cornu, and become gradually shifted medianwards by those which enter 

 the cord with the higher nerve-roots. 



It is further to be noted that in all cases the degeneration falls off 

 markedly in amount as we trace it up the cord, and that what remains is 

 eventually confined to a part of the posterior column which contains normally 

 fine or medium-sized fibres only. From this it may be inferred that the larger 

 fibres of the posterior roots which in fact form the bulk of those roots have 

 a relatively limited course after entering the cord. They probably end by their 

 collateral branches, and ultimately by their main ascending branches, turning into 

 the grey matter and breaking up into terminal ramifications in the fine interlacements 

 of nerve-fibrils which occur in the neighbourhood of the nerve-cells and cell-groups. 

 These terminal branches and the nerve-fibrils which result from their ramifications, 

 should also, of course, from the Wallerian law, degenerate after section of the roots, and 

 there is no doubt that they actually do so, although from their scattered course in 

 the grey matter it is difficult to prove the fact. It has, however, been shown by 

 Mott that after section of the lower posterior roots, the fine nervous interlacement 

 which surrounds the cells of Clarke's column disappears for some little distance 

 above the entrance of the cut nerve-roots, and it may therefore be fairly inferred 

 that some of the fibres of the posterior roots give origin directly and by collaterals 

 to this interlacement. Others, probably, are similarly related to other cells, both in 

 the posterior and anterior horns of the same side, and even on the other side of 

 the cord, for it will be remembered that ramifying collaterals can be traced from the 

 posterior root-fibres to all these parts (see figs. 16 and 20). Thus all the larger and 

 some of the smaller * fibres of the posterior roots gradually end as they are prolonged 

 up the cord, until finally only those remain which pass up the postero-mesial 

 column towards the medulla oblongata. Even these become gradually diminished 

 in number, no doubt from the fact that some of them terminate in the grey matter 

 as they proceed. 



It will further be remembered that the fibres of the posterior roots divide on 

 their entrance into the cord into two main longitudinal branches, ascending and 

 descending. Of these the ascending only has been traced by the above degenerations, 

 but the descending branch must also undergo degeneration. It is, however, not 

 easy to trace out its course. The only known "descending" degeneration in the 

 posterior columns is along the narrow curved tract in the postero-lateral column 

 which is known as the "comma," but would be better termed the posterior 

 descending trad. Mott has found that this degeneration results not only in sections 

 of the cord, but also after section of posterior nerve-roots, and that it has a 

 limited extent, one or two centimeters only. It is therefore not improbable that 

 this degeneration may represent the descending branches of the cut posterior root- 

 fibres (cf. p. 26, and fig. 20, 8 to 9). 



It will be convenient here to sum up what is known or may be inferred as to the course 

 of the fibres of the posterior roots within the cord as determined by anatomical, embryological 

 and experimental (degenerative) methods. (1.) Each fibre on entering the cord divides into 

 an ascending and a descending branch, which form the longitudinal fibres of the posterior 

 column. Both from the root-fibre before division and from the branches, collaterals come off 

 which lose themselves in terminal ramifications enveloping nerve-cells in the grey matter. 

 Probably also after a longer or shorter course the main branches terminate by passing in like 

 manner into the grey matter. (2.) From each root on entering the cord a lateral bundle of 

 small fibres (Lissauer's bundle) is given off, which lies partly lateral and partly mesial to 

 the apex cornu posterioris, while the remainder of the root forms a large lamellar bundle of 

 mixed large and small fibres which run longitudinally in the postero-lateral column close 

 to the cornu of grey matter. (3.) From this lamellar bundle many fibres pass upward 



1 Those of Lissauer's bundle. 



