THE SPINAL NERVES 



549 



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

 shown well in figures 362 and 364. 



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 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 



Entering posterior 

 root 



Lissauer's tract 



ng anterior root 



FIG. 364. Diagrammatic Transverse Section of the Spinal Cord, Showing the Conduction 

 Paths and Groups of Cells. (Cunningham.) 



of its fibers intermingle with those in the gangl'on, figure 361. But imme- 

 diately beyond the ganglion the two roots coalesce, and by the mingling of 

 their fibers form a mixed spinal nerve; the spinal nerve, after issuing from 

 the intervertebral canal, gives off anterior and posterior (or ventral and dorsal) 

 branches, each containing fibers from both the roots. A third or visceral 

 branch of the spinal nerve, ramus communicans, joins the sympathetic chain. 

 The anterior root of each spinal nerve arises by numerous separate and 

 converging bundles from the anterior columns of the cord; the posterior root 

 by more numerous parallel bundles, from the posterior column. The 

 anterior roots of each spinal nerve consist chiefly of efferent fibers; the pos- 

 terior exclusively of afferent fibers. 



