158 PROFESSOR MARSHALL, 



The only possible doubt as to the independent segmental value of 

 the fifth nerve hinges on the fact that in the two lower classes of 

 vertebrates — Pisces and Amphibia — the fifth is very generally fused 

 more or less completely with the seventh in the adult condition ; the 

 fusion sometimes, as in most fishes, involving the roots to a greater 

 or less extent, sometimes, as usually in Amphibians, occurring a short 

 distance beyond the roots and close to the Gasserian ganglion. 



This approximation or fusion of the fifth and seventh nerves has, as 

 mentioned above, been employed by J. Muller, Stieda, and others, as 

 an argument against the two nerves being of independent segmental 

 value. 



A crucial test of the force of this argument is afforded by a study 

 of the development of the roots of the two nerves in Elasmobranchs, 

 in which the fusion of the roots in the adult is so complete that what 

 is really one of the roots of the seventh has hitherto been almost in- 

 variably described by anatomists as a root of the fifth. 1 In the dog- 

 fish it has been shown that the two nerves, though so intimately 

 connected in the adult, are in the early embryonic stages perfectly 

 distinct from one another, and some distance apart, as far from one 

 another, indeed, as they are in corresponding stages of such forms as 

 the chick or lizard in which they remain completely separate through- 

 out life ; and that the gradual approximation and fusion of the two 

 nerves, which occur during the later developmental stages, and all the 

 steps of which have been traced, must, like the partial fusion which 

 we have seen may occur in some forms between the third and fifth 

 nerves, be viewed as purely secondary features. 



In early stages of both Teleosteans and Amphibians, I have also 

 noticed that the roots of the fifth and seventh nerves are perfectly 

 distinct from one another, and some distance apart, and that their sub- 

 sequent approximation must accordingly be, as in Elasmobranchs, of 

 a purely secondary nature. 



The claim of the fifth nerve to rank as an independent segmental 

 nerve must, I think, from what has been said above, be considered as 

 definitely established. 



VI. The Sixth, or Abducent Nerve. — The proper morphological 

 position of this nerve is by no means easy to determine with any 



1 A full account of the development of the roots of the fifth and seventh nerves in the 

 dog-fish, and of the relation of the embryonic to the adult roots, will be found in the paper 

 by Mr. Spencer and myself quoted above, Qvmt . Joum, of Micros, Science, July 1881, pp. 

 482—486. 



