516 THE NERVOUS SYSTEM 



Antero-lateral Descending Tract. This is an extensive tract, elongated 

 but narrow, and reaching from the crossed to the direct pyramidal tract. It is 

 a mixed tract, since not all of its fibers degenerate below the lesions. 



Comma Tract. This is a small tract of fibers which degenerate below 

 the point of section or injury of the cord. Its presence has been demonstrated 

 in the cervical and thoracic regions. It is supposed to consist of the descending 

 collaterals of the posterior nerve roots as they pass into the postero-external 

 columns. 



Tracts of Ascending Degeneration. Postero-median Column and 

 Poster o-lateral Column. These tracts degenerate upward on injury or 

 on section of the cord, also on section of the posterior nerve roots, figure 360, 

 i. They exist throughout the whole of the cord from below up , and can be 

 traced into the bulb. They consist of fine fibers. 



Direct Cerebellar Tract. This tract is situated on the outer part of the cord 

 between the crossed pyramidal tract and the margin. It is found in the cervi- 

 cal, 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, since its fibers arise from the 

 cells of Clarke's column. As its name implies, it is believed to pass up into 

 the cerebellum. 



Antero-lateral Ascending Tract, Tract of Gowers, figure 360, 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 descending tract. It is traceable 

 throughout the whole length of the cord. 



Tract of Lissauer, or Posterior Marginal Zone. A small tract of fine white 

 fibers, situated at the apex of the posterior horn, is made up of fibers 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 gen- 

 eral divisions into the anterior, the lateral, and posterior columns. These 

 columns are subdivided into tracts in which the fibers degenerate upward, 

 those in which the fibers degenerate downward, and others in which the fibers 

 degenerate neither way except for short distances when the cord is cut across. 

 These parts cf the cord are composed of commissural fibers which connect 

 different levels of the cord. The commissural tracts form the antero-lateral 

 columns and the lateral limiting layer. The arrangement of these tracts is 

 shown well in figure 360. 



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

 from the sides of the whole length of the cord, their number corresponding 

 with the intervertebral foramina through which they pass. Each nerve arises 

 by two roots, an anterior and a posterior, the latter being the larger. The roots 

 emerge through separate apertures of the sheath of dura mater surrounding 



