184 BULLETIN OF THE 



Figure 10 be examined, it will be seen that the fifth nerve draws its fibres 

 from both the posterior part of the first neuromere and the anterior 

 part of the second. Succeeding sections show the connection between 

 the nerve and the neuromeres, both anteriorly and posteriorly, to be 

 more extensive than in the section drawn. Beraneck says, apropos of 

 the connection between the fifth nerve and second neuromere : " J'avais 

 toujours ete frappe de ce que la seconde paire des replis medullaires 

 etait la seule ne paraissant avoir aucune relation avec les nerfs craniens. 

 En etudiant la question d'un peu plus pres je reconnus que cette deuxi- 

 eme paire emet uu tronc nerveux qui vient se fondre avec celui partant 

 de la premiere. Ainsi les elements qui constituent le trijumeau sont 

 fournis par les deux premieres paires des replis medullaires. . . . Le 

 trijumeau correspondant a deux paires de replis, ne serait done pas un 

 nerf simple, mais resulterait de la fusion de deux troncs nervaux primi- 

 tivement independants." * 



From the fact that the second neuromere, as well as the first, con- 

 tributes fibres to the formation of the fifth nerve, I cannot argue, as 

 Beraneck does, that these two neuromeres correspond to the two primi- 

 tive segments to which, from its peripheral distribution, the fifth nerve 

 is supposed to be related. 



Of the two segmental nerves united in the fifth, one is surely repre- 

 sented by the main branch of the fifth with its Gasserian ganglion, the 

 other segmental nerve is usually supposed to be represented, at least 

 in part, by the ramus ophthalmicus profundus, with its ciliary ganglion. 

 In fact, Van Wijhe claims to have traced in the elasmobranch the back- 

 ward growth of the ramus ophthalmicus, from its connection with the 

 brain in the immediate neighborhood of the ciliary ganglion to its final 

 fusion with the main body of the fifth nerve. This being the case, one 

 would hardly look for the neuromere of that anterior segmental nerve, 

 which has thus lost its independence, back of the neuromere to which 

 the fifth nerve is itself attached. It is possible, indeed, to suppose that 

 the whole fifth nerve has transferred its connection from the posterior 

 to the anterior neuromere, but such a supposition takes away any seg- 

 mental value which may attach to the neuromeres from their present 

 relation to the cranial nerves. 



Aside from any theory of segmentation, the fact remains that the fifth 

 nerve, at the time of its origin, arises from the depression between the 

 first and second neuromeres, while the cells composing its root are inti- 

 mately connected with the cells forming the mass of the ridge which 



1 Loc. cit., p. 337. 



