160 



LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



into the spinal cord. Here it occupies two regions, first, the 



innermost portion of the anterior column on 

 the side of the brain-lesion, and, secondly, a 

 large area, in the lateral column of the oppo- 

 site side. High up, at the commencement 

 of the oblongata, we see that the crossed de- 

 generation passes hack to the side of the un- 

 crossed degeneration ; that is, the degenerated 

 tract decussates with the normal pyramidal 

 tract. The tract which, in this instance, is 

 involved in the descending degeneration is 

 called, as in the brain, the pyramidal tract. 

 In the spinal cord it is divided into the 

 anterior (columnar) pyramidal tract (most 

 median part of anterior column) and the 

 lateral (columnar) pyramidal tract (in the 

 posterior portion of the lateral column). 

 There are grounds for believing that these 

 pyramidal tracts contain most of those fibres 

 from the brain to the spinal cord which con- 

 vey the impulses for conscious movement. 

 Thev degenerate onlv downward ; their nerve- 



J %> 



fibres always disappear if the cross-section of 

 the tract is destroyed at any point of its course 

 in the brain or spinal cord. In human beings 

 all the tracts in the spinal cord have their 

 medullary sheaths at the time of birth. The 

 pyramidal tract is the only exception to this 

 rule. On a section taken from the spinal 

 cord of a newborn child, therefore, the pyra- 

 midal tracts appear gray in the midst of the 



Secondary descending white lateral Columns. 



The cross-section of the pyramidal tract 

 diminishes gradually toward the lumbar 

 region. In the lower dorsal region the pyramidal tract of the 



FIG. 97 



degeneration following a 

 lesion in the left cerebral 

 hemisphere. (Alter Erb.) 



