424 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Finally, the junction of the two rami ophthalmici, most re- 

 markable if they are branches of a single nerve, would present 

 nothing astonishing when they are regarded as branches of two 

 separate nerves. 



If this view be adopted, certain modifications of the more 

 generally accepted views of the morphology of the cranial 

 nerves will be necessitated ; but this subject is treated of at the 

 end of this section. 



Some doubt hangs over the fate of the other branches of 

 the seventh nerve, but their destination is not so obscure as that 

 of the anterior branch. The branch to the roof of the mouth 

 can be at once identified as the ' palatine nerve ', and it only 

 remains to speak of the mandibular branch. 



It may be noticed first of all with reference to this branch, 

 that the seventh behaves precisely like the less modified succeed- 

 ing cranial nerves. It forks in fact over a visceral cleft (the 

 hyomandibular) the two sides of which it supplies ; the branch 

 at the anterior side of the cleft is the later developed and smaller 

 of the two. There cannot be much doubt that the mandibular 

 branch must be identified with the spiracular nerve (prae-spira- 

 cular branch Jackson and Clarke) of the adult, and if the chorda 

 tympani of Mammals is correctly regarded as the mandibular 

 branch of the seventh nerve, then the spiracular nerve must 

 represent it. Jackson and Clarke 1 take a different view of the 

 homology of the chorda tympani, and regard it as equivalent to 

 the ramus mandibularis internus (one of the two branches into 

 which the seventh eventually divides), because this nerve takes 

 its course over the ligament connecting the mandible with the 

 hyoid. This view I cannot accept so long as it is admitted that 

 the chorda tympani is the branch of a cranial nerve supplying 

 the anterior side of a cleft. The ramus mandibularis internus, 

 instead of forming with the main branch of the seventh a fork 

 over the spiracle, passes to its destination completely behind 

 and below the spiracle, and therefore fails to fulfil the conditions 

 requisite for regarding it as a branch to the anterior wall of 

 a visceral cleft. It is indeed clear that the ramus mandibularis 

 internus cannot be identified with the embryonic mandibular 

 branch of the seventh (which passes above the spiracle or 



1 Loc. tit. 



