THE CRANIAL NERVES. 269 



relations characteristic of the nerve in the adult bird. Its 

 development in the chick is unknown. 



V. The trigeminal, or fifth cranial nerve. The trigeminal 

 nerve arises from the neural ridge on the first or most anterior of 

 the vesicles of the hind -brain, and its development accords exactly 

 with that of a typical cranial nerve as described above. The 

 ganglion of the trigeminal nerve, or Gasserian ganglion, is 

 formed mainly from a portion of the neural ridge, reinforced 

 from an independently arising knob of the surface epiblast. The 

 permanent attachment of the nerve to the side of the hind-brain 

 is acquired at the commencement of the third day ; and about 

 the same time the nerve divides distally into ophthalmic and 

 mandibular branches, of which the former (cf. Fig. 115, v') runs 

 forwards along the inner side of the eyeball to the front of the 

 head, while the latter, V, runs downwards and backwards in the 

 mandibular arch. From the mandibular nerve, the maxillary 

 nerve arises on the third day as a branch (cf. Fig. 115), which 

 runs forwards in the maxillary arch or upper jaw. 



The development of the motor root of the trigeminal nerve 

 in the chick has not been determined satisfactorily, and it is not 

 yet certain whether this is a part of the original nerve, or whether, 

 as seems more probable, it arises independently as an outgrowth 

 from the brain itself. 



VI. The sixth cranial nerve. The sixth nerve is of a very 

 different nature to the trigeminal or facial nerves, and in its 

 mode of origin and relations agrees more closely than any of the 

 other cranial nerves with the ventral or motor root of a spinal 

 nerve. 



It appears during the fourth day, arising from the base of the 

 hind-brain, near the median plane, by a number of very slender 

 rootlets, the most anterior of which is on a level with the hinder 

 part of the root of the trigeminal nerve, and the most posterior 

 one opposite the root of the facial nerve. The rootlets unite 

 together to form a slender nerve, which runs forwards below the 

 base of the brain to the external rectus muscle of the eyeball, in 

 which it ends. 



VII. The facial, or seventh cranial nerve arises from the 

 neural crest on the top of the second vesicle of the hind-brain ; 



