66 THE AXATOMY OF VERTEBRATED ANIMALS. 



fibres also abound in the gray matter ; but the so-called " 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 number with the 

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

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

 the rentral, reo^ion of its half of the cord. The former root 

 has a ganglionic enlargement, and only contains sensory 

 fibres ; tlie 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 owm 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 Jirst 

 pair of cerebral -nerves. They always retain their primary 

 connection with the cerebral hemispheres, and frequently con- 

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

 communicates with the lateral ventricle. 



The optic " nerves " {pptici) are the second pair of cere- 

 bral nerves. In the Lampreys and Hags [Marsip>ohranchli) 

 these nerves retain their embryonic origin from the thalam- 

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

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

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

 [Ganoidei, Elasmobranchii, and all the higher Verteb7X(ta). 

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

 become connected chiefly with the mesencephalon. 



All the other cerebral nerves difi'er from these in arising, 

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

 logical differentiation of the primitive brain-case, or laniincB 

 dorsales of the skull. 



The third (motores oculorum) and foui'th [2K(fheti':'i) pairs 

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

 to the majority of these muscles, the fourth to the superior 



♦ Amphioxm appears to be an exception to this, as to most ot.her, rules of 

 Vertebrate anatomv. 



