THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 157 



5. That the fourth nerve is itself not an entire segmental nerve is 

 rendered probable by the fact, noticed by Schwalbe, that it has no 

 ganglion, and is strongly supported by the further fact that the third 

 nerve almost certainly arises at first from the dorsal surface of the 

 brain, and beyond all doubt is, dming its early stages, attached much 

 higher up the side of the brain than it is at a later stage, i.e. that 

 the third nerve behaves like a posterior spinal root, 



Since, as vre have seen, there is no room for a separate segmental 

 nerve between the third and the fifth, I am inclined to view the third 

 and fourth nerves as together equivalent to a segmental nerve, which 

 has divided into two portions, whereof one — the fourth — has remained 

 in its primitive position on the top of the brain, while the other — the 

 third — has, like the other cranial nerves and the posterior spinal roots, 

 shifted downwards, the extent of the shifting being greater than that 

 of any of the other nerves, but the several steps of the process pro- 

 bably the same as in these. This view will be found to be very closely 

 in accordance with that advocated by Schwalbe. 1 



V. The Fifth, or Trigeminal Nerve. — It will be convenient to 

 continue the consideration of the cranial nerves in the usual sequence, 

 and to take the remaining eye-muscle nerve — the sixth — after the tri- 

 geminal. 



The fifth nerve completely fulfils all the conditions of a true seg- 

 mental nerve. 2 It appears very early as an outgrowth from the neural 

 crest. The root of origin from the brain shifts down at an early 

 period, acquires a secondary attachment to the side of the brain, and 

 loses its primary attachment completely. The direction of the stem 

 is at right angles to the axis of the head at the point of origin of the 

 nerve. The maxillary and mandibular branches are related to the 

 maxillo-mandibular or buccal cleft in the manner characteristic of the 

 posterior segmental nerves, as was first pointed out by Stannius. The 

 relations of the fifth nerve to the second and third head cavities are of 

 a perfectly typical nature ; and finally a ganglionic enlargement — the 

 Gasserian ganglion — is present in the nerve a short distance above 

 its division into the two main branches. 



1 Schwalbe, Das Ganglion Oculomotorii, pp. 77, 73. 



2 For the development of the fifth nerve in Elasmobranchs, vide Balfour, Elasmobranch 

 Fishes, 1S7S, pp. 196 — 19S ; also Marshall and Spencer, " Observations on the Cranial Nerves 

 of Scyllium," Quart. Journ. of Micros. Science, July 1881, pp. 471—479; in the Chick, vide 

 Marshall, Quart. Journ. of Micros. Science, Jan. 187S, pp. 28—32 ; and in the Rabbit, K61* 

 liter, Entmckdv.ngsgeschichte, 1879, pp. 610—712. 



