PEHIPHEEAL-NBHVE ROOTS, SPINAL GANGLIA, SPINAL COBD. 



345 



is ground for the assumption that these pyramidal tracts contain the 

 majority of the fibers extending from the brain to the cord, which subtend 

 acquired movement. They degenerate only downward; their fibers dis- 

 appear regularly, if the tract be destroyed anywhere in the brain or the cord. 

 In the area of the lateral columns, occupied partly by the tradus cortico- 

 spinalis, there are to be found other fibers, which, belonging to the associa- 

 tion-bundles of the cord, Join its different levels together. The longest fibers 



Fig. 221. — Scheme of the descending degeneration in the tractua cortieo-spinalis 

 in a case of focal lesion in the left internal capsule. 



of this category are in this territory. So it happens, in case of lesion of the 

 lateral column of the cord, that a larger area degenerates downward than 

 accords with the spread of the pyramids in the medulla oblongata. From 

 this circumstance has arisen the common error of supposing that the pyra- 

 mids receive fibers from the cord itself. 



At birth all the tracts of the cord, in man, have received their medul- 

 lary sheaths. Only the tradus cortico-spinales form exceptions. With the 



