NERVOUS SYSTEM OF THE VERTEBRATA. 457 



nerve probably also originates from this crest. It will however 

 be convenient to deal separately with this nerve, after treating of 

 the other nerves which undoubtedly arise from the neural cTcst. 



The cranial nerves just enumerated present in their further 

 development many points of similarity ; and the glossopha- 

 ryngeal nerve, as it develops in Elasmobranchii, may perhaps be 

 taken as typical. This nerve is connected by a commissure with 

 those behind, but this fact may for the moment be left out of 

 consideration. Springing at first from the dorsal line of the 

 hind-brain immediately behind the level of the auditory capsule, 

 it apparently loses this primitive attachment and acquires a 

 secondary attachment about half-way down the side of the 

 hind-brain. The primitive undifferentiated rudiment soon be- 

 comes divided, exactly like a true posterior root of a spinal 

 nerve, into a root, a ganglion and a nerve. The main branch of 

 the nerve passes ventralwards, and supplies .the first branchial 

 arch (fig. 271 gl). Shortly afterwards it sends forwards a 

 smaller branch, which passes to the hyoid arch in front ; so that 

 the nerve forks over the hyobranchial cleft. A typical cranial 

 nerve appears therefore, except as concerns its relations to the 

 clefts, to develop precisely like the posterior root of the spinal 

 nerve. 



Most of the cranial nerves of the above group, in correlation 

 with the highly differentiated character of the head, acquire 

 secondary differentiations, and render necessary a brief descrip- 

 tion of what is known with reference to their individual develop- 

 ment. 



The Glossopharyngeal and Vagus Nerves. Behind the ear 

 there are formed, in Scyllium, a series of five nerves which pass down to 

 respectively the first, second, third, fourth and fifth branchial arches. 



For each arch there is thus one nerve, whose course lies close to the 

 posterior margin of the preceding cleft ; a second anterior branch, forking 

 over the cleft and passing to the arch in front, being developed later. These 

 nerves are connected with the brain by roots at first attached to the dorsal 

 summit, but eventually situated about half-way down the sides. The 

 foremost of them is the glossopharyngeal. The next four are, as has been 

 shewn by Gegenbaur 1 , equivalent to four independent nerves, but form 

 together a compound nerve, which we may briefly call the vagus. 



1 "Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeilschrtft, Vol. vi. 

 1871. 



