THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 143 



for viewing both the third and fifth nerves as segmental, and there- 

 fore primitively independent of one another. 



(d) If Wiedersheim's view were correct, we should certainly expect 

 the third nerve of higher vertebrates in its eai'ly stages of develop- 

 ment to show some indication of its supposed primitive connection 

 with the fifth. So far, however, is this from being the case, that in 

 all cases where the development of the third nerve has yet been 

 traced, it is a perfectly independent nerve from the start. 1 



(e) A crucial test is afforded by the fact that other nerves — e.g. 

 the fifth and seventh — though, as a rule, separate from one another 

 throughout the vertebrate series, may in some forms become more or 

 less closely united together, so that it is impossible by mere ana- 

 tomical evidence to distinguish branches of the one from those of the 

 other ; the forms in which this fusion of the fifth and seventh nerves 

 occurs being, as we shall see more fully later on, in many cases the 

 same as those in which the fifth and the eye-muscle nerves tend to 

 fuse. In the case of the dog-fish, in which this fusion of the fifth and 

 seventh nerves is a marked feature of the adult state, all the stages 

 of development are now known, 2 and it is found that, so far from the 

 state of fusion being a primitive one, the two nerves are in their early 

 stages quite independent and some distance apart, as in other verte- 

 brates, and that their subsequent gradual approximation and fusion 

 are purely secondary characters. 



The above arguments appear to me to establish the proposition that 

 the third nerve is primitively an independent one, 8 and that its partial 

 fusion with the fifth, when it occurs, is a purely secondary and not a 

 primary character. 



If they prove the case for the third nerve, so also for the fourth 

 and sixth nerves. The presence of independent roots of origin from 

 the brain must be held to establish that, however close may be the 

 connection of their trunks with the fifth nerve, they are really in- 

 dependent nerves, and are not to be described as being " replaced by 

 branches of the fifth nerve." 



1 Marshall, "On the Development of the Cranial Nerves in the Chick," Quart. Journ. of 

 Micros. Science, Jan. 1878, pp. 23 — 27 ; and "On the Head Cavities and Associated Nerves of 

 Elasmobranchs," Quart. Journ. of Micros. Science, Jan. 1881, pp. 78— 88. 



a Marshall and Spencer, "Observations on the Cranial Nerves of Scyllium," Quart. Journ. 

 of Micros. Science, July 1881, pp. 482—486. 



3 The independence of the third nerve has recently been upheld on anatomical grounds 

 by Schwalbe — Das Ganglion Oculomotorii ; and by Balfour, on embryological ones— Com* 

 parative Embryology, vol, ii. 



