SEGMENTATION OF THE HEAD. 437 



formed by the junction of three conspicuous branches, (i) an 

 anterior dorsal branch which takes a more or less horizontal 

 course above the eye (vil. a) ; (2) a main branch to the hyoid 

 arch (VII. Jiy) ; (3) a smaller branch to the posterior edge of the 

 mandibular arch (VII. ;//;/). The first of these branches can 

 clearly be nothing else but the typical "ramus dorsalis," of which 

 however the auditory may perhaps be a specialized part. The 

 fact that this branch pursues an anterior and not a directly 

 dorsal course is probably to be explained as a consequence of 

 the cranial flexure. The two other branches of the 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 PI. 17, fig. i, v., or from an 

 examination of the sections representing these nerves (PI. 17, 

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

 branches: (i) an anterior and dorsal branch (r. ophthalmicus 

 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. maxillae inferioris) ; and (3) an anterior 

 branch to the palatine arcade (r. maxilla; superioris). I was at 

 first inclined to regard the anterior branch of the fifth (ophthal- 

 mic) 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 corresponding branch of the 

 seventh, the ramus dorsalis, 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 supply- 

 ing the mandibular arch, and not, as has been usually thought, a 



