388 THE SENSORIAL FUNCTIONS. 



senses, (such as the *^plysia, of which the nervous system is 

 exhihited in Fig. 446,) there is generally a special cephalic 

 ganglion (c,) which may be supposed to serve the office of 

 brain.* In others, again, as in the Patella (Fig. 447,) the 

 cephalic ganglion is scarcely discernible, and its place is sup- 

 plied by two lateral ganglia (l, l;) and there is besides a 

 a transverse ganglion (t,) below the oesophagus. The ce- 

 phalic ganglion, on the other hand, attains a considerable 

 size in the Cephalopoda (c, Fig. 448,) where it has extensive 

 connexions with all the parts of the head: the optic ganglia 

 (o, o,) in particular, are of very great size, each of them, 

 singly, being larger than the brain itself.t 



§ 2. Nervous System of Vertebrated Jinimals. 



The characteristic type of the nervous system of verte- 

 brated animals is that of an elongated cylinder of nervous 

 matter, (m, z. Fig. 449,) extending down the back, and 

 lodged in the canal formed by the grooves and arches of the 

 vertebrce. 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 to- 

 gether, but leaving frequently a central canal, which is filled 

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

 all the adjacent vertebras, there proceed two sets of nervous 

 filaments, those which are continuous with the posterior co- 

 lumns (p,) being appropriated to the function of sensation; 

 and those arising from the anterior columns (a,) being sub- 



* This figure also shows ag-ang-lion (a,) which is placed higher, and com- 

 municates by lateral filaments with the cephalic gang-lion (c;) two lateral 

 gunglia (l, l,) of great size; and a large abdominal ganghon (g.) 



f Some peculiarities in the structure of the cephalic ganghon of the Sepia 

 have been supposed to indicate an approach to the vertebrated structure; for 

 this ganglion, together with the labyrinth of the ear, is enclosed in a cartila- 

 ginous ring, perforated at the centre to allow of the passage of the CESopha- 

 giis, and imagined to be analogous to a cranium. 



