DEVELOPMENT OF ELASMOBRAXCH FISHES. 201 



nothing astonishing when they are regarded as branches of 

 two separate nerves. 



If this view be adopted, certain modifications of the more 

 generally accepted view^s 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 wdth reference to this branch, 

 that the seventh behaves precisely like the less modified 

 succeeding 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 tw^o. There cannot be much 

 doubt that the mandibular branch must be identified with 

 the spiracular nerve (prse-spiracular branch Jackson and Clarke) 

 of the adult, and if the chorda tympani of Mammals is cor- 

 rectly regarded as the mandibular branch of the seventh 

 nerve, then the spiracular nerve must represent it. Jackson 

 and Clarke^ 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 wdth 

 the hyoid. This view I cannot accept so long as it is ad- 

 mitted that the chorda tympani is the branch of a cranial 

 nerve supplying the anterior side of a cleft. The ramus man- 

 dibularis 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 hyomandibular cleft) when there is 



1 Loc, cit. 

 B. 14 



