THE CENTRAL NERVOUS SYSTEM. 53 



the grey matter at the base of the posterior horn of grey matter, where 

 they form arborisations around nerve cells. By means of intermediate 

 neurons these cells are brought into relationship with the cells of the 

 anterior horn of grey matter, the axons of the latter forming the nerve 

 fibres of the anterior nerve roots. 



The fibres of the direct pyramidal tract cross in the spinal cord itself 

 in the anterior white commissure to terminate in relation with the nerve 

 cells of the grey matter of the opposite side. 



Both pyramidal tracts convey motor impulses from one cerebral 

 hemisphere to the opposite side of the body, a number of fibres 

 terminating in each segment of the spinal cord, so that the tracts 

 become progressively smaller as they descend. The fibres of the 

 direct pyramidal tract all cross in the cervical and thoracic regions. 



Less is known of the function of the other tracts and fibres which 

 undergo descending degeneration. The best marked of these is the 

 rubro-spinal tract, which lies in the lateral column, immediately in 

 front of the crossed pyramidal tract, and consists of the axons of cells 

 forming the red nucleus of the mid-brain. These fibres cross in the 

 mid-brain close to their place of origin* 



The olivo-spinal tract lies close to the surface opposite the anterior 

 horn of the grey matter. As its name indicates, it is made up chiefly 

 of fibres which are the axons of cells in the olivary nucleus of the 

 medulla oblorigata. 



Vestibulo-spinal fibres, probably derived from the cells of Deiters' 

 nucleus, are found in the antero-lateral column. 



The cerebello-spinal tract of Lowenthal, also lying in the antero-lateral 

 column, consists of scattered fibres derived from cells in the cerebellum. 



The comma tract, lying in the posterior column, consists of the short 

 descending branches of fibres which enter the spinal cord from the 

 posterior nerve roots. 



THE ASCENDING TRACTS. 



The ascending tracts may be classified as exogenous or endogenous, 

 according as they originate from cells in the ganglia of the posterior 

 roots of the spinal nerves, or from cells in the grey matter of the 

 spinal cord itself. 



The exogenous tracts are the funiculus (fasciculus) gracilis (Goll's 

 column), the funiculus (fasciculus) cuneatus (Burdach's column), and 

 the bundle of Lissauer. The first and second of these together form 

 the greater part of the posterior column, while the third lies close to 

 the tip of the posterior horn of grey matter. 



The funiculus gracilis occupies the mesial portion of the posterior 



