THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 161 



the sixth and fifth in these instances by no means proves that the 

 sixth belongs to the fifth, but is more probably due to the same cause 

 — whatever it may be — that determines the approximation or fusion 

 of the seventh and fifth nerves. 



Concerning the actual value of the sixth nerve, I see no reason to 

 alter the opinion I have previously expressed, that the sixth nerve 

 may be regarded as having the same relation to the seventh that the 

 anterior root of a spinal nerve has to its posterior root. I shall return 

 to this point when considering the seventh nerve. 



VII. The Seventh, or Facial Nerve. — As to the segmental value 

 of the seventh nerve there can be no doubt whatever ; for, like the 

 fifth, it completely and indisputably fulfils all the conditions of a seg- 

 mental nerve laid down on page 133. 



It develops 1 very early as an outgrowth from the neural crest on 

 the dorsal surface of the hind-brain (fig. 2). At an early stage the 

 nerve acquires a new or secondary attachment to the side of the brain 

 (fig. 3) ; but, unlike all the other nerves, cranial or spinal, the original 

 or primary root is retained as well as the secondary root, whereas 

 in all the other nerves the primary attachment appears to be lost. 

 The general course of the nerve is at right angles to the axis of 

 the head at its point of origin. The relations of its branches to the 

 hyo-mandibular cleft, first pointed out by Stannius, and afterwards 

 insisted on by Gegenbaur, are those of a typical segmental nerve, as are 

 also its relations to the head-cavities ; whilst, finally, it is ganglionic at 

 its division into the two main ventral branches. 



As to the independent rank of the seventh nerve, 1 have already 

 discussed fully the theory that the seventh and fifth nerves are con- 

 nected together primarily, and have stated the arguments leading to 

 the conclusion, that although in many vertebrates — fish and am- 

 phibians — the two nerves are more or less closely fused together, yet 

 that embryology shows that this fusion is a secondarily acquired 

 character. 



The relation between the sixth and seventh nerves is of still greater 

 importance, from its bearing on the disputed question of whether there 



1 For an account of the development of the seventh nerve in Elasmobranchs, vide Balfour, 

 Elasmobranch Fishes, 1878, pp. 198—202 ; and Marshall and Spencer, Quart. Joum. Micros. 

 Science, July 1881, pp. 679—691 ; in the Chick, vide Marshall, Quart. Joum. Micros, Science, 

 Jan. 1878, pp. 34—36 ; and in Mammals, Kolliker, Entwicklungsgeschichte. 



M 



