NERVOUS SYSTEM OP VEHTEBRATA. 553 



§ 2. Nervous System of Vertehrated Animals. 



The characteristic type of the nervous system of 

 vertehrated animals is that of an elongated cy- 

 linder of nervous matter (m z, Fig. 449), ex- 

 tending down the back, and lodged in the canal 

 formed by the grooves and arches of the verte- 

 brae. It has received the name of spinal marrow, 

 or more properly spinal cord; and, (as is seen in 

 the transverse section, Fig. 450), is composed of 

 six parallel columns, two posterior, two middle, 

 and two anterior, closely joined together, but 

 leaving frequently a central canal, which is filled 

 with fluid. On each side of the spinal cord, and 

 between all the adjacent vertebrae, there proceed 

 two sets of nervous filaments ; those which are 

 continuous with the posterior columns (p), being 

 appropriated to the function of sensation ; and 

 those arising from the anterior columns (a), being 

 subservient to voluntary motion. The former, 

 soon after their exit from the spine, pass through 

 a small ganglion (g), and then unite with the 

 nerves from the anterior column ; composing, by 

 the intermixture of their fibres, a single nerv- 

 ous trunk (n), which is afterwards divided and 

 subdivided in the course of its further distribu- 

 tion, both to the muscular and the sentient 

 organs of the body. Each of these spinal nerves 

 also sends branches to the ganglia of the sympa- 



