66 THE ANATOMY OF VEKTEBRATED ANIMALS. 



fibres also abound in the gray matter ; but the so-calleil " white 

 matter," which constitutes the external substance of the cord, 

 contains only the fibrous nervous matter, and has no gangli- 

 onic corpuscles. 



The spinal nerves arise in opposite pairs from the two 

 halves of the cord, and usually correspond in numl^er with the 

 vertebrfe through, or between, which they pass out (Fig. 23). 

 Each nervo has two roots, one from the dorsal, and one from 

 the ventral, region of its half of the cord. Tlie former root 

 has a ganglionic enlargement, and only contains sensory 

 fibres ; the latter has no ganglion, and exclusively contains 

 motor fibres.* After leaving the vertebral canal, each spinal 

 nerve usually divides into a dorsal and a ventral branch ; but, 

 in the Ganoid fishes, each of these branches is a distinct nerve, 

 arising by its own proper roots. 



The Cerebral Nerves. — The greatest number of pairs of 

 nerves ever given off from the vertebrate brain is twelve, in- 

 cluding the so-called olfactory nerves, and the optic nerves, 

 which, as has been seen, are more properly diverticula of the 

 brain, than nerves in the proper sense of the word. 



The olfactory " nerves " (olfactorii) constitute the first 

 pair of cerebral nerves. They always retain their primary 

 connection with the cerebral hemispheres, and frequently con- 

 tain, throughout life, a cavity, the olfactory vciUridc, which 

 communicates with the lateral ventricle. 



The optic " nerves " [optici) are the second pr/ir of cere- 

 bral nerves. In the Lampreys and Hags (Marsipoh'anchii) 

 tliese nerves retain their embryonic origin from the thulani- 

 encephalon, and each goes to the eye of its own side. In 

 other VerteirratK, the nerves cross one another at the base of 

 the brain (Teleostei), or are fused together into a chiasma 

 [Ganoidei, Ef.asmobrdiiclui, and all the highetYertebraia). 

 In the higher Vertebrata, again, the fibres of the optic nerves 

 become connected chiefly with the mesencephalon. 



All the other cerel)ral nerves differ from these in arising, 

 not as diverticula of any of the cerebral vesicles, but by histo- 

 logical differentiation of the primitive brain-case, or lamuice 

 dorsnle^ of the skull. 



The third {motores oculorurn) w\^ fourth. [patJtctici) paiis 

 of nerves are distributed to the muscles of the eve ; the third 

 l;o the majority of these muscles, the fourth to the superior 



• ATnphioxufi appears to ho an exception to tliis, as to most o^her, rules oS 

 Vertebrate anatomy. 



