214 SEGMENTATION OF THE HEAD. 



tlie seventh nerve are the same as those present in all the 

 posterior nerves, viz. the branches to the two sides of a branchial 

 cleft, in the present instance the spiracle ; the seventh nerve 

 being clearly the nerve of the hyoid arch. 



The fifth nerve presents in the arrangement of its branches 

 a similarity to the seventh nerve so striking that it cannot be 

 overlooked. This similarity is at once obvious from an inspec- 

 tion of the diagram of the nerves on PL xvi. fig. 1, v., or from an 

 examination of the sections representing these nerves (PI. xvi. 

 figs. 3 and 4). It divides like the seventh nerve into three 

 main branches : (1) an anterior and dorsal branch {r. ophthal- 

 micus profundus), whose course lies parallel to but ventral to 

 that of the dorsal branch of the seventh nerve; (2) a main 

 branch to the mandibular arch (r. maxillsG inferioris); and (3) an 

 anterior branch to the palatine arcade (?'. maxillae superioris). 

 I was at first inclined to regard the anterior branch of the fifth 

 (ophthalmic) as representing a separate nerve, and was supported 

 in this view by its relation to the most anterior of the head- 

 cavities ; but the unexpected discovery of an exactly similar 

 branch in the seventh nerve has induced me to modify this 

 view, and I am now constrained to view the fifth as a single 

 nerve, whose branches exactly correspond with those of the 

 seventh. The anterior branch of the fifth is, like the corre- 

 sponding branch of the seventh, the ramus doi^salis, and the two 

 other branches are the equivalent of the branches of the 

 seventh, which fork over the spiracle, though in the case of 

 the fifth nerve no distinct cleft is present unless we regard the 

 mouth as such. Embryology thus appears to teach us that 

 the fifth nerve is a single nerve supplying the mandibular arch, 

 and not, as has been usually thought, a complex nerve result- 

 ing from the coalescence of two or three distinct nerves. My 

 observations do not embrace the origin or history of the third, 

 fourth, and sixth nerves, but it is hardly possible to help sus- 

 pecting that in these we have the nerve of one or more segments 

 in front of that supplied by the fifth nerve; a view which well 

 accords with the most recent morphological speculations of 

 Professor Huxley \ 



1 Preliminary note upon the brain and skull of Ampliioxus, Froc. of the Boyal 

 Society, Vol. xxii. 



