THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 153 



But little criticism is called for by the above account. As was 

 urged in the case of Lepidosiren, the presence of a distinct root of 

 origin in the normal position must be held to prove that the sixth 

 nerve is in the cases quoted above really an independent nerve, in 

 spite of its apparent fusion with the fifth at the Gasserian ganglion. 

 The fact that the sixth in an allied genus (Bufo) retains its indepen- 

 dence, is an additional argument in favour of the fusion being secon- 

 darily acquired ; and this view must be considered to be established 

 by the statement made by Stannius, 3 on Fischer's authority, that the 

 sixth nerve is independent of the fifth in the larval stages of those 

 forms which, when adult, have the two nerves fused. 



This concludes the list of recorded instances of exceptional inner- 

 vation of the eye-muscles. Leaving out, as we are fairly entitled to, 

 the cases of Amphioxus and of those forms in which, as in Amblyopsis, 

 the eyes are rudimentary and functionless ; the results of an examina- 

 tion of the remaining instances may be stated thus : — 



1. That in no single instance has it been established that any one 

 of the eye-muscle nerves is replaced by a branch of the fifth, or of 

 any other nerve — the cases in which this is alleged to occur being far 

 more naturally explained by supposing partial fusion between the 

 nerves concerned to have occurred. 



2. That in the alleged cases of replacement of one or more of the 

 eye-muscle nerves by a branch of the fifth nerve, the " branch of the 

 fifth " in question is very probably the ramus ophthalmicus profundus, 

 which is really a communicating nerve between the third and fifth, 

 belonging as much to one as to the other in its posterior portion, and 

 in its anterior part belonging exclusively to the third. 



3. That the instances in which the absence of one or other of the 

 eye-muscle nerves has been alleged are either, as in Petromyzon, Lepi- 

 dosteus, Pipa, Hyla, <&c, cases in which the nerves in question arise 

 from the brain in a perfectly normal manner, and after running a 

 certain distance within the skull become connected more or less inti- 

 mately with the fifth nerve ; or else cases in which, as in Lepidosiren, 

 the eyes are small, the eye-muscles imperfectly developed, and the 

 descriptions of their anatomy incomplete and unsatisfactory. 



4. That such cases do not in any way invalidate the proposition 

 that the third, fourth and sixth are independent nerves throughout 

 the vertebrate sub-kingdom. 



1 Stannius, Handbuch der Zootomie, Zweites Bucb, Die AmphiUen, 1856, p. 150, note 3. 



