SECTION VI 



STRUCTURE OF THE SPINAL CORD 



IN the higher representatives of the invertebrate class the central 

 nervous system consists, as we have seen, of a chain of ganglia, each 

 ganglion or pair of ganglia presiding over the reactions of its own seg- 

 ment, but connected by long paths with the other ganglia and with the 

 head ganglia. The latter, being especially developed in connection 

 with the organs of special sense which are projicient in function, acquire 

 a control over the rest of the ganglia (Fig. 154). The vertebrate spinal 



DORSAL 



Spin al canal 



Neui-erttc 



;i_ .. 



Spinal Cord * Segtm.en.tal Nerves 



/n/undctulum VENTRAL 



DORSAL 



Ventral Chain. f (Jan. yli a 



An, 



VENTRAL 



FIG. 154 Vertebrate central nervous system compared with that of the arthropod. 

 (GASKELL.) (Note that according to Gaskell the ventricles of the brain and the 

 primitive neural canal correspond to the invertebrate stomach and intestine.) 



cord may be looked upon as a chain of ganglia which have become fused 

 concurrently with a diminution in the importance of the local seg- 

 mental reactions and with a growth in the solidarity of the whole 

 system ; so that in the higher vertebrates, at any rate, little trace of the 

 primitive segmental arrangement is evident in the internal structure 

 of the cord. Some remains of this arrangement still persist, however, 

 in the origin from the cord of nerve-roots, which are distributed roughly 

 within the area of the corresponding segment of the body. 



In man the spinal cord is an elongated cylindrical structure slightly 

 flattened from before backwards and about eighteen inches long. It 



355 



