352 . PHYSIOLOGY 



or dorsal columns. On account of the scattered distribution of the anterior 

 root fibres over a considerable area of the surface of the cord, the division 

 between the anterior and the lateral columns is ill defined, and the whole 

 region is often called as the antero-lateral column. In the cervical and 

 upper dorsal region of the cord, slight grooves on the surface of the cord 

 indicate a division of the anterior column into the antero-median and antero- 

 lateral columns, and of the posterior column into the postero-median and 

 postero-lateral columns. These two posterior columns are often designated 

 as the columns of Goll and Burdach. In order to determine the origin, 

 course, and destination of the fibres which make up these white columns, we 

 must have recourse to the indirect nlethods of development and of degenera- 

 tion which were described on p. 319. By these means we may divide 

 the white matter into ascending and descending tracts. An ' ascending ' 

 tract means, not that the direction of conduction of the impulse is necessarily 

 in the upward direction, i.e. from spinal cord to brain, but that the nerve cell 

 which gives off the fibres sends its axons towards the brain, while a descending 

 fibre in the cord is the axon of a nerve-cell situated in the upper part of the 

 cord or in some part of the brain. If the assumption which we have made 

 as to the normal direction of conduction in axons and dendrites be correct, 

 an ascending fibre will also conduct impulses in an ascending direction. 

 After section of the cord, say in the mid-dorsal region, transverse sections 

 of the cervical and lumbar regions of the cord, taken at the appropriate 

 period after the lesion has been inflicted, show patches of degenerated fibres 

 in the white matter. The fibres which are degenerated above the section 

 represent the ascending tracts, whereas those which degenerate below the 

 section, i.e. in the lumbar region, are the descending tracts of the cord (cp. 

 Figs. 174 and 175). 



In this way the following tracts have been distinguished : 



A. DESCENDING TRACTS 



(1) PYRAMIDAL TRACTS. If the spinal cord be divided in the upper cervical 

 region, degeneration of two distinct tracts on each side, in the anterior and postero-lateral 

 columns, is produced. These are the anterior or direct and the crossed pyramidal 

 tracts. The fibres composing these tracts are derived from large nerve-cells in the 

 motor area of the cerebral cortex, and therefore degenerate if the motor area of the cortex 

 is destroyed. The pyramidal tracts are derived from the cerebral cortex of the opposite 

 side, having crossed the middle line at. the lower level of the medulla oblongata in the 

 pyramidal decussation. The anterior pyramids represent a certain number of fibres 

 which have not crossed with the others, but continue the course of medullary pyramids 

 fora time, crossing gradually by the anterior commissure on their way down the cord, 

 it as a rule they come to an end in the mid-dorsal region, all tin- fibres having 

 passed into the lateral columns of the opposite side. A few fibres of the pyramids on 

 their way from the cerebral cortex pass into the lateral columns of the same side ; these 

 are the uncrossed pyramidal' fibres. The greater number of the fibres however finally 

 reach the crossed pyramidal tracte, in which they can be traced as far as the lower end 

 <>f the cord. They end iu the spinal cord by turning into the grey matter where they 

 l.i.-.ik up into a fine bunch of fibrils in close connection with the motor cells of the 

 anterior horn or, according t<> Schnfer, with the cells of the posterior horn. 



