30 DR. A. MILNES MARSHALL. 



regarded as a single segmental nerve, of which the superior and 

 inferior maxillary nerves are the primary branches — the branches 

 which stand in relation with the corresponding cleft. 



Since the ophthalmic branch of the fifth nerve crosses the 

 third at right angles, it is at once obvious that these two nerves 

 cannot be morphologically equivalent, and that if one of them is 

 a segmental nerve the other is something of a totally different 

 nature. The claims of the third nerve to be considered a segmental 

 nerve have already been fully discussed. What, then, is the 

 ophthalmic nerve ? 



The answer that first suggested itself to me is that it is the 

 ramus dorsal is of the fifth nerve, whose primitive direction has 

 been altered by cranial flexure. This is a somewhat tempting 

 view ; it accords well with the fact of the ophthalmic nerve lying 

 superficially to the third, and is greatly strengthened by the fact 

 of its having been already brought forward, on totally (liferent 

 grounds, by Gegenbaur^ and by Balfour.~ 



However, there appear to be very serious difficulties in the 

 way of accepting this explanation. In the first place, the only 

 cause that we are acquainted with to which we could ascribe 

 such a change in the direction of the ramus dorsalis of the fifth 

 nerve is cranial flexure. Now, a glance at figures 21 and 22 

 shows that cranial flexure does not affect the j^^irt of the brain 

 from which the flfth nerve arises to any appreciable extent, and 

 is totally inadequate to produce a change of direction of about 

 130°; and yet such a change must have occurred if this view be 

 true. Secondly, no amount of cranial flexure, or of any other 

 process that I can conceive of, could possibly caiise the raimis 

 dorsalis of one segmental nerve to pass right across the segment 

 next in front of that to vjhich it belongs ; and if the third nerve 

 is a segmental nerve, the ophthalmic not only does this, but 

 actually crosses over into the next segment (the olfactory), and 

 reaches almost to the extreme anterior end of the head. More- 

 over, no explanation is offered of the remarkably close connection 

 between the ophthalmic and olfactory nerves. These considera- 

 tions seem to me to render the ramus dorsalis hypothesis 

 untenable. 



There is one other view which 1 would venture to suggest 

 here, though I do so with great diffidence, inasmuch as I have 

 no direct embryological evidence lo offer on its behalf. We have 

 seen above that the nerves do not arise separately from the 



* " Ueber die Kopfnervcn von Hexanchus," 'Jeiiaische Zeitschrift,' vi, 

 1871, pp. 508,510 seq. 



■ ' Jourii;il of Anat. and I'liys.,' vol. xi, pari; iii, p. 480. The comparisou 

 suggested itself to me quite iiidepciidently, simply from a consideration of 

 the relations of the ophthalmic and third nerves at an early stage. 



