The Nervous System. 2G5 



generated either in an upward or downward direction. 

 By this and other means, it was ascertained that certain 

 paths or tracts exist in the white matter of the cord, con- 

 necting the brain with the cord and vice versa. It must 

 not be supposed that the function of a downward or 

 upward tract is entirely exerted in the direction given to it 

 by its name ; they are called upward or downward depend- 

 ing upon the direction of the degeneration resulting from 

 experimental injury. 



The following are the tracts in man according to Foster : 



Descending Tract*. Ascending Tract*. 



Pyramidal tract. Cerebellar tract. 



Direct pyramidal tract (not found Median posterior tract. 



in animals). Anterolateral ascending tract. 



Antero-lateral descending tract. 

 Comma tract (limited to cervical 



and upper thoracic regions). 



These paths, known to various observers by somewhat 

 different names, are distributed between the anterior, lateral, 

 and posterior columns ; an examination of the diagram 

 (Fig. 25) will render their position in the cord more 

 clearly understood. These tracts are not found throughout 

 the entire length of the cord, and in all cases they 

 diminish in size from the head to the tail. 



The pyramidal tract is large in man but small in the 

 monkey and dog ; it is in connection with the motor region 

 of the brain, and its great size in man appears to bear a 

 distinct relation to the complexity of the motor region: 

 the more intricate the changes in the central nervous 

 system the larger the pyramidal tract. (Foster.) 



Some of the tracts run from the white and terminate in 

 the grey matter. The tracts differ microscopically, some 

 being composed of coarse fibres, others of line, some of 

 mixed fibres. 



It has not been found possible to divide the whole of the 

 white matter into tracts : even after all the above have 

 been defined, there is still much left unaccounted for. 



