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SYSTEMATIC ZOOLOGY. 



FIG. 108. Diagrammatic section of 

 spinal cord. d, dorsal nerve-root; 

 g, gray matter; v, ventral nerve- 

 root; w, white matter. 



The central system consists of an anterior brain, passing 

 behind into the spinal cord. The brain is contained in the 

 cranium; the spinal cord passes through the tube formed 

 by the neural arches of the vertebrae. 



The spinal cord (fig. 108) is somewhat cylindrical, ta- 

 pering behind, and contains in its centre a small canal. 



Nerves arise from the cord 

 in pairs in regular sequence, 

 and pass out between the 

 vertebrae to all parts of the 

 body and to the limbs. 

 Each of these spinal nerves 

 has two places of origin 

 (roots) from the cord one 

 near the dorsal, the other near the ventral surface, but 

 after a short course these roots unite into a common 

 trunk. These roots differ greatly in structure and func- 

 tion. The dorsal root bears a nervous enlargement or 

 ganglion; the ventral has no such structure. Experiment 

 shows that the dorsal root is concerned in bringing sensa- 

 tions to the central nervous system, and, if it be cut, the 

 parts to which it goes will be without feeling. The ven- 

 tral root, on the other hand, is motor; i.e., it controls the 

 action of muscles, glands, etc. If this root be cut, the 

 parts which it supplies are paralyzed. Hence we may 

 speak of the dorsal roots as afferent, since they bring sensa- 

 tions to the central nervous system; while the ventral 

 roots are efferent, because they carry nervous impulses in 

 the opposite direction. 



The brain must be recognized as an enlarged and special- 

 ized portion of the central nervous system. The canal of 

 the spinal cord continues into the brain, enlarging there 

 into four or more cavities or ventricles, connected by 



