CH. XLII.] DEGENERATION TRACTS 615 



of the spinal cord, after the division of a number of nerve-roots" on 

 one side. The microscopic section is taken high up, so that all the 

 degenerated fibres have passed into the column of Goll on the same 

 side ; the inner set (1) are shaded differently from the outer set (2), 

 indicating that those nearest the middle line come from tho lowest 

 nerve-roots. 



We may pass from this > to consider the tracts of degeneration- 

 that occur when the spinal cord is cut right across in the thoracic 

 region. Some tracts will be found degenerated in the piece of cord 

 below the lesion; these consist of nerve-fibres that are connected 

 with the nerve-cells in the brain ; the principal ones are the pyramidal 

 tracts. Other tracts are found degenerated in the piece of cord 

 above the lesion; these consist of nerve-fibres that are connected 

 with the nerve-cells of the spinal ganglia, or with the cells of the 

 spinal cord itself below the lesion and are passing upwards. 



In general terms we may say that the tracts which degenerate 

 downwards are the motor tracts, and those which degenerate upwards 

 are the afferent or sensory channels. We must also take into 

 account groups of association fibres which unite together different 

 regions of the cord ; these are generally short tracts in which, there- 

 fore, degeneration can only be traced a short distance up or down. 

 The long tracts are those which connect cord or spinal nerves with 

 brain, like those of Goll and Burdach just mentioned, or the pyramidal 

 tracts the main efferent pathways. 



Tracts of Descending Degeneration (fig. 450). 



(1.) The crossed pyramidal tract. This is situated in the lateral 

 column on the outer side of the posterior cornu of grey matter. At 

 the lower part of the spinal cord it extends to the margin, but higher 

 up it becomes displaced from this position by the interpolation of 

 another tract of fibres, to be presently described, viz., the direct 

 cerebellar tract. The crossed pyramidal tract is large, and may 

 touch the grey matter at the tip of the posterior cornu, but is 

 separated from it elsewhere. Its shape on cross section is somewhat 

 like a lens, but varies in different regions of the cord, and diminishes 

 in size from the cervical region downwards, its fibres passing off as 

 they descend, to arborise around the nerve-cells and their branchings 

 in the grey matter of the cord. The fibres of which this tract is 

 composed are moderately large, but are mixed with some that are 

 smaller. 



(2.) The direct or uncrossed pyramidal tract, or column of Tilrck. 

 This tract is situated in the anterior column by the side of the 

 anterior fissure. It ends in the mid or lower thoracic region of the 

 cord. 



