THE SEGMENTAL VALUE OP THE CRANIAL NERVES. 151 



in close contact with it as it does so ; and that it supplies only three 

 muscles — the rectus interims, rectus inferior, and obliquus inferior-^— 

 the rectus superior receiving its branch from the " nasal nerve," and 

 this branch coming off beyond the point of crossing of the third and nasal 

 nerves ; and that this condition of things is interpreted by both the 

 writers who have investigated it directly — Fischer and Schwalbe — as 

 merely implying that the third nerve has become partially fused with 

 the fifth. 



Concerning this " nasal nerve," from which, in the two genera in 

 question, the branch to the rectus superior always, and that to the 

 obliquus superior usually, arises, there is a further point of importance. 

 Schwalbe 1 has attempted to prove that this " nasal nerve " really cor- 

 responds, in part at least, to the ramus ophthalmicus profundus of 

 Selachians. The point could only be decided by a study of the de- 

 velopment of this nerve in Urodela, of which at present we know 

 nothing ; but should Schwalbe prove to be correct, the very slight 

 amount of deviation from the normal condition which we have found 

 to be all that really occurs in Salamandra and Triton would be still 

 further reduced ; for embryology teaches us that the ramus ophthal- 

 micus profundus of Selachians is really a connecting branch between 

 the third and fifth nerves, which cannot be said to belong distinctly 

 to either the one or the other, and that the portion of this nerve 

 beyond the point at which it crosses the third nerve, from which por- 

 tion we have seen that the branch to the rectus superior arises, has 

 nothing whatever to do with the fifth, but belongs really to the third 

 nerve. 2 



From what has been said above, I think that no other conclusion 

 can be drawn than that the cases of Salamandra and Triton do not 

 afford any reason for regarding the eye-muscle nerves as other than 

 independent and constant nerves. 



(c) Menobranchus. — Gegenbaur 3 states, on Fischer's authority, that 

 in Menobranchus, as in Salamandra and Triton, the fourth nerve is 

 replaced by a branch of the fifth. I have been unable to refer to 

 Fischer's account, so that any discussion of the case would be unprofit- 

 able. It is, however, very possible that the condition is really what 

 Schwalbe has shown to occur in Salamandra. 



1 Schwalbe, Das Ganglion Oculomotorii, p. 26. 



a Marshall, «* Head Cavities and Associated Nerves of Elasmobranchs," Quart. Journ. of 

 Micros. Science, January 1881, p. 89 ; and Marshall and Spencer, " Cranial Nerves of Scyl- 

 lium," Quart. Journ. Micros. Science, July 1881, pp. 494 seq. 



3 Gegenbaur, Hexanchus, p. 549, note 1. 



