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 segment, 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. 153). 

 The vertebrate spinal cord may be looked upon as a chain of ganglia which 



DORSAL 



Spinaf canal 



eanaf 





Spinal Cord + Se^mealal Nerves 



VENTRAL 



Aitu.. 



DORSAL 



Ve n. I T* a I Caaiiv f G a. n gli a, 



Anus 



VENTRAL 



'* ton. 



FIG. 153. 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.) 



have become fused concurrently with a diminution in the importance of the 

 local segmental 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. 

 gives of? a series of nerve-roots, which are arranged in thirty-one pairs 

 and are distributed symmetrically to the two sides of the body. Each 



315 



