287 



the different segments of the cord with the brain, or the different 



regions of the brain with 



the cord. The bundles in 



the second group, on the 



other hand, are the short 



intraspinal ascending or 



descending paths which 



connect together different 



elements of the cord at 



different levels. 



We must glance at 

 the principal facts in 

 regard to the nature and 

 relations of the main 

 tracts in the two 

 groups : 



(a) The pyramidal 

 tracts are composed of 

 the axis-cylinders of cells 

 in the Rolandic area of 

 the cerebral cortex. They 

 pass through the internal 

 capsule,. cerebral ped- 

 uncle, and pyramids of 

 the medulla oblongata, 

 where most of the fibres 

 decussate before descend- 

 ing in the form of a 

 compact bundle in the 

 dorsal portion of the 

 lateral column (crossed 

 lateral pyramidal tract). 

 A small number of these 

 fibres which do not cross 

 in the bulb descend on 

 both sides of the ventral 

 median (fissure as the 

 direct pyramidal tract 

 described by Tiirck ; this 

 usually ends about half- 

 way down the thoracic 



COrd. The fibres Of the FIG. 174. Secondary 



pyramidal tracts give off 



collaterals along their 



course, which arborise 



around the cells of the ventral roots, so that each fibre is able 



to convey excitations to a series of cells which are distributed 



scending degeneration due 

 to a lesion in left cerebral 

 hemisphere. (After Brb.) 



FIG. 175. Secondary ascend- 

 ing and descending de- 

 generations due to lesions 

 of upper thoracic cord. 

 (After Striimpell.) 



