CENTRAL NERVOUS SYSTEM. 



669 



number of fibres among those degenerating after section of the dorsal roots 

 may run the longer course, the larger portion run a short or an intermediate 

 course, and are therefore distributed at different points between the termini. 

 Injury to the dorsal roots at different levels shows, moreover, that the fibres 



ABC 



FIG. 175. Sections showing the degeneration in the dorsal columns of the dog's spinal cord when the 

 dorsal roots from the sixth lumbar to the second sacral have been cut on the left side (Singer) : A, level 

 of the sixth lumbar; B, level of the fourth lumbar ; C, level of the sixth thoracic. Degenerated area in 

 black. 



from a given level which run the length of the dorsal columns do not mingle 

 indiscriminately with those from other levels, but form a bundle, and that 

 this bundle in the cephalic part of the cord tends to lie nearer the middle line 

 the more caudad the level from which it arises. 



From these relations it is evident that comparatively few of the dorsal 

 root-fibres run the entire length of the dorsal columns. If, then, it is remem- 

 bered that in describing the arrangements of the cord emphasis is usually 

 placed on the very short pathways formed in part by collaterals and con- 

 cerned in the simpler reflexes, and on the longest pathways concerned in the 

 voluntary reactions, as two extremes between which are to be found a more or 

 less complete series of intermediate arrangements, the unevenness of the pre- 

 sentation can be corrected. 



OP 



Since these fibres in the dorsal columns of the cord degenerate on destruc- 

 tion of the dorsal roots, it is inferred that they must be morphologically con- 

 tinuous with certain fibres in the roots, and, since the dorsal roots are afferent 

 pathways, they too must form part of the afferent pathway in the cord. 



It is of course a portion only of the afferent pathway that is thus formed, 

 for both the intermediate and lateral groups of root-fibres enter the gray 

 matter of the dorsal horn, and must there come into physiological connection 

 with other nerve-cells both central and efferent. The fact that the connection 

 is only physiological accounts for the arrest of the Wallerian degeneration at 

 these points after section of the dorsal roots. 



The continuation of the paths for the afferent impulses must therefore be 

 formed by the neurons of the central cells with which the dorsal root-fibres 

 connect. 



Degeneration after Hemisection of Cord. Upon hemisection of the 

 cord involving one lateral half the ascending fibres which degenerate appear 

 in the dorsal columns, in the dorso-lateral ascending tract, and in the ventro- 

 lateral ascending tract. The number of degenerated fibres is large on the side 

 of the lesion, but on the opposite side there are also degenerated fibres in all 



