CRANIAL NERVES 



dorsal roots of the gill-region join the epibranchial nerve. Meta- 

 meric epibranchial placodes, or proliferations of the epidermis 

 (Beard [32], Koltzoff [272]), are found above each gill-slit ; from 

 these are derived the ganglia on the branchial nerves which become 

 joined together by the longitudinal connection. Similar longi- 

 tudinal nerves also unite the trigeminal with the facial nerve, and 



Fio. 4. 



A'-i'iith'n's i-i'iijm-i.t, Risso. A, outer view of muscles and nerves of right orbit, from which 

 the eye has been removed. B, dorsal view of right eye. ar, anterior, ir, inferior, pr, posterior, 

 and sr, superior rectus muscle ; ob.i, inferior, and ob.s, superior oblique muscle'; os, cartila- 

 ginous optic stalk ; 2, optic, 3, oculomotor, 4, pathetic, and 6, abducens nerve. 



the latter with the glossopharyngeal in Petromyzon and most 

 Gnathostomes. The exact composition and mode of formation of 

 the vagus root is still far from thoroughly understood, and it is 

 possible that it may have been formed by a gathering together not of 

 whole posterior dorsal roots as Gegenbaur suggested, but of only those 

 components of each root which supplied the gills and alimentary 

 canal ; this supposition (of a slipping forward, so to speak, of 



