52 



THE STRUCTURE OF 



mals we have a still farther concentration of the nervous 

 system. In lower terms of the series, such as Fishes 

 and Amphibia (fig. 56), this concentration is seen to the 

 most marked extent in the chain of ganglia pertaining 

 to the thorax and abdomen. In these animals and in 

 all other vertebrates they are most completely fused into 



12 



FIG. 19. Transverse section throxigh Human Spinal Cord in cervical region, 

 showing the organ to be composed of two symmetrical halves. (Sappey after Stilling.) 

 The black portions correspond to regions containing longitudinal fibres ; the lighter 

 portions represent the central Grey Matter and the horizontal roots of nerves; 

 5, 6, commissures connecting the symmetrical halves of the grey matter ; 11, 11, 

 11, anterior or motor roots of spinal nerves, coming from anterior Horns or Comua 

 of Grey Matter, in which are numerous groups of large ganglion cells ; 13, pos- 

 terior or sensory roots of spinal nerves, entering the posterior Horns of Grey Matter. 

 Magnified about eight diameters. 



a more or less cylindrical column known as the ' spin-al 

 cord.' This cord constitutes a double and fused series 

 of nerve centres in relation with the superficial as well as 

 with the deeper structures of the greater part of the body, 

 including all the great nerves of the limbs. 



