THE NERVOUS SYSTEM. 567 



(ii.) Direct cerelellar tract (fig. 352, 6). This tract is situated on the 

 outer part of the cord between the crossed pyramidal tract and the mar- 

 gin. Jt is found in the cervical, thoracic and upper lumbar regions of 

 the cord, and increases in size from below upward. It degenerates on 

 injury or section of the cord itself, but not on section of the posterior 

 nerve-roots. As its name implies it is believed to pass up into the cere- 

 bellum. Its fibres are coarse. 



(iii.) Antero-lateral ascending tract (Tract of Gowers and Tooth) (fig. 

 352, 8). This tract has been shown on injury to the spinal cord; it is 

 situated at the margin of the cord outside of the corresponding descend- 

 ing tract. It is traceable throughout the whole length of the cord. Its 

 fibres are composed of mixed, fine and coarse, elements. 



(iv.) Tract of Lissauer^ or posterior marginal zone (fig. 352, 4). A 

 small tract of fine white fibres, situated at the apex of the posterior 

 horn, is made up of fibres from the posterior nerve-roots which enter the 

 column and pass up and down for a short distance, finally entering the 

 posterior horn where they terminate in fine end-brushes around the cells 

 of the posterior horn. 



It will thus be seen that the white matter of the spinal-cord has three 

 general divisions, into the anterior, the lateral, and posterior columns. 

 These columns are subdivided into columns in which the fibres degener- 

 ate upward, those in which the fibres degenerate downward, and other 

 columns in which the fibres do not degenerate either way when the cord is 

 cut across. These parts of the cord are composed of commissural fibres 

 which connect different levels of the cord. These commissural columns 

 are the antero-lateral columns, the lateral limiting layer, and the column 

 of Burdach. The arrangement of these columns is shown well in the 

 figure (fig. 352). 



Spinal Nerves. The spinal nerves consist of thirty-one pairs, issuing 

 from the sides of the whole length of the cord, their number correspond- 

 ing with the intervertebral foramina through which they pass. Eacli 

 nerve arises by two roots, an anterior and posterior, the latter being the 

 larger. The roots emerge through separate apertures of the sheath of 

 dura mater surrounding the cord ; and directly after their emergence, 

 where the roots lie in the intervertebral foramen, a ganglion is found 

 on the posterior root. The anterior root lies in contact with the anterior 

 surface of the ganglion, but none of its fibres intermingle with those in 

 the ganglion (fig. 350, 4). But immediately beyond the ganglion the 

 two roots coalesce, and by the mingling of their fibres form a compound 

 or mixed spinal nerve, which, after issuing from the intervertebral 

 canal, gives off anterior and posterior or ventral and dorsal branches, 

 each containing fibres from both the roots (fig. 350), as well as a third 

 or visceral branch, ramus communicans, to the sympathetic. 



The anterior root of each spinal nerve arises by numerous separate 



