THE SPINAL CORD AS A CONDUCTOR 397 



On inspection the white matter is seen to be divided by the anterior 

 and posterior fissures of the cord into two symmetrical halves, and 

 the nerve-roots divide each half into anterior or ventral, lateral, and 

 posterior or dorsal columns. On account of the scattered distribu- 

 tion 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 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 degeneration which were described on p. 359. By these 

 means we may divide the white matter into ascending and descending 

 tracts. An ' ascending ' tract means, not that the direction of con- 

 duction 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 ascend- 

 ing 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. 175 and 

 176). 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 



