166 PROFESSOR MARSHALL, 



sence of anterior motor roots to these nerves ; and on this point I 

 have no additional evidence beyond what I have already stated else- 

 where. 1 



VIII. The Eighth, or Auditory Nerve. — In all the forms in 

 which the development of the auditory nerve has been ascertained, it 

 arises as part of the seventh nerve. Neither its development nor its 

 anatomical relations afford the slightest ground for thinking it to be 

 of segmental rank. 2 



IX. The Ninth, or Glossopharyngeal Nerve. — Like the audi- 

 tory, the ninth nerve can be disposed of very briefly, but for a directly 

 opposite reason. Since Gegenbaur confirmed Stannius' account of its 

 relations to the first branchial cleft, the claim of the glossopharyngeal 

 to rank as an independent segmental nerve has been very generally 

 admitted ; and as the history of its development 3 shows that it con- 

 forms in all respects to the characters of a segmental nerve as defined 

 on page 133, it woxild be superfluous to discuss in detail its now uni- 

 versally recognised claims to segmental value. 



X. The Tenth, or Vagus Nerve. — The tenth nerve stands in much 

 the same position as the ninth, with the exception that while the 

 glossopharyngeal is a single segmental nerve, the vagus, from its 

 relations to a number of visceral clefts, must be considered as equi- 

 valent to an equal number of segmental nerves fused together. This 

 was first pointed out by Stannius, and subsequently developed in 

 much more detail by Gegenbaur ; and since the publication by the 

 latter of his memorable essay on the cranial nerves of Hexanchus, has 

 been accepted almost universally as the true theory of the morpho- 

 logical value of the tenth nerve. It is only necessary to add here that 

 the study of its development shows that it completely fulfils all the 

 conditions required of segmental nerves. 



Concerning the number of primitively separate segmental nerves 

 fused together to form the vagus, we cannot speak positively. The 



1 Marshall, " Head Cavities and Associated Nerves of Elasmobranchs," Quart. Joum. of 

 Micros. Science, Jan. 1881, pp. 91—93. 



3 For the development of this nerve in Elasmobranchs, vide Balfour, Elasmobranch Fishes, 

 p. 198 ; in the Chick, Marshall, " Development of Cranial Nerves in the Chick," Quart. Joum, 

 of Micros. Science, Jan. 1878, pp. 34—36. 



3 For the development of the glossopharyngeal and vagus nerves in Elasmobranchs, vide 

 Balfour, Elasmobranch Fishes, pp. 202, seq.; in the Chick, Marshall, loc. cit., pp. 36—39. 



