

STRUCTURE OF THE WHITE COLUMNS OF THE SPINAL CORD. 241 



side, in order to ascend in them towards the cerebrum, proceed 

 from the grey substance of the anterior cornua in such a 

 manner that the fibres derived from the right anterior cornu 

 ascend in the left anterior column, and those from the left 

 anterior cornu in the right anterior column (fig. 226).- In 

 order to follow this course, the fibres must necessarily traverse 

 the median line, and the place at which this occurs is the 

 anterior white commissure. The fact that the white com- 

 missure is wider in those regions of the spinal cord in which 

 the grey substance is most abundant, than, in those where 

 the grey matter is smaller, tells strongly in favour of this 

 explanation. 



2. The second place where horizontal fibres occur is in the 

 median portions of the lateral columns close to the grey 

 substance (fig. 220, c). These fibres emerge from the sides 

 of the grey substance of the anterior cornua, and ascend in the 

 lateral columns ; at the point where they change their direction 

 they are for a very short distance horizontal. Certain fibres 

 of the posterior columns which are continuous with fibres of 

 the posterior cornua lying in front of the substantia gelatinosa 

 pursue a similar course (fig. 221, g). These, as sections 

 treated with chloride of gold very distinctly show, run perfectly 

 horizontally for considerable distances in the posterior columns. 

 But whilst it is pretty well ascertained that the horizontal 

 fibres of the lateral columns enter these from the grey sub- 

 stance, and then ascend in them- it cannot be so satisfactorily 

 determined in the case of the horizontal fibres of the posterior 

 columns, whether they proceed from the grey substance of the 

 posterior cornua, and continue their course in the posterior 

 columns, or whether they belong to that portion of the 

 posterior roots of the spinal nerves which, before they enter 

 the posterior cornua, ascend or descend for a certain distance 

 in the posterior columns, and then form curves in the latter in 

 order to penetrate into the grey substance of the posterior 

 cornua. 



3. The fibres of the posterior roots are also to be regarded as 

 tolerably horizontal fibres of the white substance of the cord, 

 as they run from without towards the middle line (fig. 221, e), 

 and therefore appear transversely divided in sagittal longi- 



