CH. XLV.] DEGENERATION TRACTS 655 



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

 indicating that those nearest the middle line come from the lowest 

 nerve-roots. Those which cross to the opposite side soon after 

 entrance into the cord, are not shown ; they will be found forming a 

 scattered degeneration in the ascending tracts of the other side. 



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 

 bracts. 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, such as those of Goll and Burdach just mentioned, or the 

 pyramidal tracts the main efferent pathways. 



Tracts of Descending Degeneration (fig. 403). 



(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 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 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. 



The two pyramidal tracts come down from the brain ; in the 

 medulla oblongata, the greater number of the pyramidal fibres cross 

 over to the other side of the cord, which they descend ; hence the 

 term crossed pyramidal tract ; a smaller collection of the pyramidal 



