PHYLUM CHORDATA 



93 



outwards. Each root arises from one of the horns of the grey 

 matter, and the two mingle to form the trunk (sp. 1-3) of the 

 nerve, which emerges from the spinal canal usually between the 

 arches of adjacent vertebrae. Soon after its emergence it divides 

 into two chief divisions, the dorsal (d.) and ventral (sp. 1, &c.) 

 nerves. The spinal nerves supply the muscles and skin of the 

 trunk and limbs, and are therefore spoken of as somatic nerves. 



FIG. 732. Transverse section of spinal cord. 1, ventral fissure ; 2, dorsal fissure ; 3, central canal ; 

 4, 5, bridges connecting grey matter of right and left sides ; 6, 7, 8, white matter ; 9, dorsal 

 root of spinal nerve ; 10, ventral root, a, 6, dorsal horn of grey matter ; c, Clarke's column ; 

 e. ventral horn. (From Huxley's Physiology.) 



Frequently groups of nerves unite with one another to form 

 more or less complex networks called plexuses. 



Closely associated with the spinal are the sympathetic nerves 

 (Fig. 734, sym). They take the form of paired longitudinal cords, 

 with ganglia (sym. gn.) at intervals, lying one on each side of the 

 aorta in the dorsal wall of the coelome. They contain both 

 afferent and efferent fibres, the afferent derived from the dorsal, 

 the efferent from the ventral roots of the spinal nerves, and both 

 traceable, through those roots, into the grey matter of the cord. 

 The sympathatic nerves supply the enteric canal and its glands, 

 the heart, blood-vessels, &c., and are therefore denominated 

 splanchnic nerves. 



