NERVOUS SYSTEM OF VERTEBRATA. 497 



section, Fig. 450), is composed of six parallel 

 4^0 -A columns, two posterior (f), two 



middle (m), and two anterior (a) ; 



which last are continuous with one 

 'p another, and are joined in front 



transversely by a broad commissure. On each side 

 of the spinal cord, and between all the adjacent ver- 

 tebrae, there proceed two sets of nervous filaments; 

 those which are continuous with the posterior 

 columns, being appropriated to sensation ; and 

 those arising from the anterior columns, being motor 

 nerves. The former, soon after their exit from the 

 spine, pass through a small ganglion, and then unite 

 with the nerves from the anterior column ; com- 

 posing, by the intermixture of their fibres, a single 

 nervous trunk, which is afterwards divided and 

 subdivided in the course of its further distri- 

 bution, 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- 

 thetic nerve, which, as was formerly described, 

 passes down on each side, parallel and near to the 

 spine. 



Enlargements of the spinal cord are observed in 

 those parts (w and l, Fig. 449), which supply the 

 nerves of the extremities ; the increase of diameter 

 being proportional to the size of the limbs requiring 

 these nerves. In Serpents, which are wholly des- 

 titute of limbs, the spinal cord is not enlarged in 

 any part, but is a cylindrical column of uniform 

 diameter. In Fishes, these enlargements appear 

 to have a relation to the size of the organs of motion 

 or sensation, and correspond to them in their situa- 

 tion. Thus in the Trigla lyra (the Red or Piper 



VOL. II. K K 



