THE CENTRAL NERVOUS SYSTEM 763 



posterior horns. It is best marked in the thoracic region, up to 

 about the second thoracic segment, although in the correspond- 

 ing situation there are scattered cells in the lumbar swelling and 

 the cervical cord. There is reason to believe that the axons of 

 cells of the intermedio-lateral tract, which pass out as small 

 medullated fibres in the anterior roots, form the preganglionic 

 segments of the efferent vascular and visceral nerves (p. 169). 

 (4) The cells of the posterior horn, which, although numerous, are 

 smaller than those of the anterior horn. Throughout the whole 

 cord, however, two small groups of cells may be distinguished, 

 one on the lateral side of the horn, about its middle, and the 

 other on the mesial side, a little in front of i.e., ventral to the 

 edges of the substance of Rolando. Both of these groups are 

 broken up by the passage through them of bundles of fibres 

 which form a network, and they are therefore called respectively 

 the group of the lateral and the group of the posterior reticular 

 formation. 



The white matter of the cord is anatomically divided by the 

 position of the nerve-roots and the anterior and posterior fissures 

 into three columns on each side : the anterior, lateral, and 

 posterior columns. The first two, since they are not separated 

 by a perfectly definite boundary, are often grouped together 

 as the antero-lateral column. In the cervical region it may be 

 seen with the microscope that the posterior white column is 

 almost bisected by a septum running in from the pia mater 

 towards the grey commissure. The inner half is called the 

 postero-median column, or column of Goll ; the outer half the 

 postero-external column, or column of Burdach (Fig. 314). 

 No localization of any of the other conducting paths in the cord 

 is possible by gross anatomical examination ; but by means of the 

 developmental method and the method of degeneration the 

 columns of Goll and Burdach can be followed throughout the 

 cord, and several similar areas can be mapped out. We shall 

 only mention those that are physiologically the most important. 



When the spinal cord is divided, and the animal allowed to 

 survive for a time, certain tracts are picked out by the degenera- 

 tion of their fibres, although in every degenerated tract some 

 fibres remain unaffected. We may distinguish the tracts that 

 degenerate above the lesion (ascending degeneration) from those 

 that degenerate below the lesion (descending degeneration). 



Ascending Tracts. Above the lesion degeneration is found 

 both in the posterior and the antero-lateral columns. Imme- 

 diately above the section nearly the whole of the posterior 

 column is involved. Higher up the degeneration clears away 

 from Burdach's tract, and, shifting inwards, comes to occupy 

 a position in the column of Goll. In the antero-lateral column 



