THE NERVOUS SYSTEM. 809 



These short fibres make up the so-called ground bundles or fundamental columns of the 

 cord. They attain their greatest development in those regions of the cord where the 

 reflex centres are most extensive, i.e., in the lumbar and cervical enlargements. 



Secondary Degenerations. 



Secondary degenerations are dependent upon the fact already noted 

 that the cell body, being the trophic centre of the neurone, the axone 

 when separated from its cell body dies. 



DESCENDING DEGENERATION. Any cortical lesion, such as embolic 

 softening and apoplectic clots, which destroys the nerve cells, or any 

 lesion of the brain or cord which interrupts the axoue tracts of the 

 cortico-spinal system of neurones, determines a complete degeneration of 

 the affected axoues (Fig. 529). The course of these axones has been 

 described. Thus a lesion of the brain affecting motor neurones is fol- 



FIG. 529. SECONDARY DESCENDING DEGENERATION. 



From haemorrhage into the internal capsule, almost complete degeneration of the direct pyramidal tract 

 on the same side as the lesion and of the crossed pyramidal tract on the opposite side. There were a few 

 degenerated fibres in the crossed tract on the same side as the lesion. 



lowed by a degeneration in the motor tract of the same side down to the 

 pyramidal decussation, and below that point, in the direct motor tract 

 on the same side and in the crossed tract on the opposite side, the num- 

 ber of degenerated fibres being proportionate to the number of cells de- 

 stroyed or axoues interrupted. In a number of cases of descending de- 

 generation due to cerebral lesions, degenerated fibres were found in the 

 crossed pyramidal tract on the same side as the lesion. This degenera- 

 tion was not present in all cases, and in no case was it as marked as the 

 degeneration in the opposite lateral tract. The most probable explana- 

 tion is that a small number of fibres instead of decussating pass down 

 into the cord in the lateral tract of the same side. 



In descending degeneration due to a lesioii below the pyramidal de- 



