6l4 STRUCTURE OF THE SPINAL CORD. [CH. XLU. 



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. 



(ii.) The direct or uncrossed pyramidal tract. This tract is 

 situated in the anterior column by the side of the anterior fissure. 

 It is smaller than (i.), and is not present in all animals, though 

 conspicuous in the human cord. It can be traced upwards to the 

 brain, and downwards as far us the mid or lower thoracic region, 

 where it ends. 



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 fibres goes straight on, on the same side of the cord, and 

 these cross at different levels in the anterior commissure of the 

 cord lower down; hence the disappearance of the direct pyramidal 

 tract in the lower part of the cord. The fact that the crossed 

 pyramidal tract of one side is the fellow of the direct pyramidal 

 tract of the other side, is indicated in the diagram by the direc- 

 tion of shading 



(iii.) Antero-lateral descending tract. An extensive tract, 

 elongated but narrow, and reaching from the crossed to the direct 

 pyramidal tract. It is a mixed tract, since not all of its fibres 

 degenerate below the lesion. 



(iv.) Comma tract is a small tract of fibres which degenerate 

 below section or injury of the cord. It is only found for a few 

 millimetres below the actual lesion ; though it degenerates down- 

 wards it is in reality a sensory tract, being composed, as we 

 have already seen, of the branches of the entering posterior 

 root-fibres which pass downwards on entering the cord. 



Tracts of ascending degeneration (fig. 456). 



(i.) Poster o-median column. This tract degenerates upwards on 

 injury or on section of the cord, as well as on section of the 

 posterior nerve-roots. It exists throughout the whole of the cord 



