THE GLOSSOPHARYNGEAL AND VAGUS NERVES. 425 



hyomandibular cleft) when there is present in the adult another 

 nerve (the spiracular nerve), which exactly corresponds in 

 distribution with the embryonic nerve in question. My view 

 accords precisely with that already expressed by Gegenbaur 

 in his masterly paper on the nerves of Hexanchus, in which 

 he distinctly states that he looks upon the spiracular nerve as 

 the homologue of an anterior branchial branch of a division 

 of the vagus. In the adult the spiracular nerve is sometimes 

 represented by one or two branches of the palatine, e.g. Scyllium, 

 but at other times arises independently from the main stem 

 of the seventh 1 . The only difficulty in my identification of the 

 embryonic mandibular branch with the adult spiracular nerve, 

 is the extremely small size of the latter in the adult, compared 

 with the size of mandibular in the embryo ; but it is hardly 

 surprising to find an atrophy of the spiracular nerve accompany- 

 ing an atrophy of the spiracle itself. The palatine appears to 

 me to have been rightly regarded by Jackson and Clarke as the 

 great superficial petrosal of Mammals. 



On the common root of the branches of the seventh nerve, 

 as well as on its hyoid branch, ganglionic enlargements are 

 present at an early period of development. 



The GlossopJiaryngcal and Vagus Nerves. Behind the ear 

 there are formed a series of five nerves which pass down to 

 respectively the first, second, third, fourth and fifth visceral 

 arches. 



For each arch there is thus one nerve, whose course lies 

 close to the posterior margin of the preceding cleft, a second 

 anterior branch being developed later. These nerves are con- 

 nected with the brain (as I have determined by transverse 

 sections) by roots at first attached to the dorsal summit, but 

 eventually situated about half-way down the sides (PI. 15, 

 fig. 6,) nearly opposite the level of the process which divides 

 the ventricle of the hind-brain into a dorsal and a ventral moiety. 

 The foremost of these nerves is the glossopharyngeal. The 

 next four ' are, as has been shewn by Gegenbaur 2 , equivalent 

 to four independent nerves, but form, together with the glosso- 

 pharyngeal, a compound nerve, which we may briefly call the 



vagus. 



1 Hexanchus, Gegenbaur, Jenaische Zeitscbrift, Vol. vi. - Loc. cit. 



B. 28 



