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 denned, and the whole 

 region is often defined 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 methods 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 an- 

 terior 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 for a time, crossing gradually by the anterior 

 commissure on their way down the cord, so that, as a rule, they come to an 

 end in the mid-dorsal region, all the 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 tracts, in which they can be traced as far 



