NERVOUS SYSTEM OF VERTEBRATA. 553 



§ 2. Nervous System of Vertehrated Animals. 



The characteristic type of the nervous system of 

 vertebrated 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 5/j/w«/ 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 farther 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- 



