656 



STRUCTURE OF THE SPINAL CORD 



[CH. XLV. 



fibres goes straight on, on the same side of the cord, and these cross 

 at different levels in the anterior commissure of the cord lower down ; 

 hence the disappearance of the direct pyramidal tract in the lower 

 part of the cord. The fact that the crossed pyramidal tract of one 

 side is the fellow of the direct pyramidal tract of the other side, is 

 indicated in the diagram by the direction of shading (see fig. 403). 



Comma tract Septo^-marginal 



Crossed 

 idal 



Prepyramidal 

 ^tract " 



Antero-lateral 

 descending 

 tract 



I bundle 



Bundle of 

 Helweg 



Direct pyramidal 

 tract 



Fio. 403. Tracts of descending degeneration. For the sake of clearness each is shown on only one 



side. (After Schafer.) 



Mingled with the fibres of the crossed pyramidal tract are a few 

 fibres of the pyramid which have not crossed in the medulla 

 oblongata, and are therefore derived from the same side of the 

 cerebrum (uncrossed lateral pyramidal fibres). 



The pyramidal fibres are not found at all in vertebrates below 

 the mammals. In the lower mammals they are very few, and in 

 some rodents (rat, mouse, guinea-pig) they are placed in the posterior 

 columns. The direct pyramidal tract is found only in man and 

 the higher apes. 



Paralysis resulting from section of the pyramidal tracts is not 

 complete and persistent. There is, therefore, some other path by 

 which volitional impulses arising in the cortex can reach the motor 

 cells of the cord. This alternative path is furnished by the descend- 

 ing fibres of the anterior column, and of the ventral parts of the 

 lateral column, especially in the tract next to be described. 

 Section of this part of the cord produces quite as complete a 

 paralysis as does section of the pyramidal tracts. In vertebrates 

 below mammals this is probably the only path between the higher 

 brain centres and the lower motor centres of the cord (Schafer). 



(3.) Antero-lateral descending tract, or tract of Loewenthal, lies by 

 the side of the anterior median fissure, and extends along the margin 

 of the cord towards the lateral column. These fibres originate from 



