870 THE TRACTS OF WHITE MATTER. [BOOK in. 



column of Burdock (Fig. 96, e. p.\ the lateral part of which, nearer 

 the grey matter, has, for reasons which we shall see later on, 

 been called the posterior root-zone. But beyond this neither the 

 irregular septa nor other features will enable us to distinguish 

 one part of the white matter as different in nature from another. 

 Nor have we better success when with the scalpel we attempt to 

 unravel out the white matter into separate strands. Nevertheless 

 we have convincing evidence that the white matter is arranged in 

 strands, or tracts, or columns, which have different connections 

 at their respective ends, which behave differently under different 

 circumstances, which we have every reason to believe carry out 

 different functions, but which cannot be separated by the scalpel 

 because each of them is more or less mixed with fibres of a 

 different nature and origin. The evidence for the existence of 

 these tracts is twofold. 



One kind of evidence is embryological in nature. When a 

 nerve fibre is being formed in the embryo, either in the spinal 

 cord or elsewhere, the essential axis cylinder is formed first and 

 the less essential medulla is formed later. Now when the develop- 

 mental history of the spinal cord is studied it is found that, in 

 the several regions of the cord, all the fibres of the white matter 

 do not put on the medulla at the same time. On the contrary, 

 in certain tracts, the medulla of the fibres makes its appearance 

 early, in others later. By this method it becomes possible to 

 distinguish certain tracts from others. 



Another kind of evidence is supplied by facts relating to the 

 degeneration of the fibres of the white matter. We have seen 

 ( 561) that the degeneration of a nerve fibre is the result of the 

 separation of the fibre from its trophic centre, and that while 

 the trophic centre of the afferent fibres is in the ganglion on the 

 posterior root, that of the efferent fibres is in some part of the 

 spinal cord. In the case of the efferent fibres the degeneration 

 might be spoken of as descending from the spinal cord to the 

 muscles or other peripheral organs. In the case of the afferent 

 fibres of the trunk of the nerve, the degeneration is also one 

 descending from the ganglion down to the skin or other peri- 

 pheral organ. When however the section is carried through the 

 posterior root of a spinal nerve, the degeneration takes place in 

 the part of the nerve between the section and the spinal cord, it 

 runs up from the section to and into the spinal cord, and may 

 therefore be called an ascending degeneration. Thus we may say 

 that when a nerve trunk or when a nerve root is cut completely 

 across, all the fibres which are thereby separated from their trophic 

 centres, degenerate. When the nerve trunk is divided all the 

 fibres below the section undergo descending degeneration. If 

 the anterior root be cut across, all the fibres of the root below the 

 section undergo descending degeneration. If the posterior root 

 be cut across, all the fibres of the root above the section undergo 



