XIII 



PHYLUM CHORDATA 



99 



dorsal and a ventral. The dorsal root (Fig 779 d r) is dis 

 tmguished by the presence of a ganglion {gn. d'.r.) containing 

 nerve-cells and its fibres are usually wholly afferent, conveying 

 impulses from the various parts and organs of the body to the 

 central nervous system ; the ventral root (v. r.) is not ganglionated 

 and Its fibres are efferent, conveying impulses from the neuron 

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

 matter, and the two mingle to form the trunJe {sp. 1-3) of the 

 nerve, which emerges from the spinal canal usually between the 



Fig. 77T. — Transverse section of spinal cord. 1, ventral fissure ; 2, dorsal fissure ; 3, central canal ; 

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

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

 e, ventral horn. (Fi'om Huxley's Physiology.) 



arches of adjacent vertebrae. Soon after its emergence it divides 

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

 The spinal nerves supply the muscles and skin of the 

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

 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. 779, 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 



