202 THE GLOSSOPHARYNGEAL AND VAGUS NERVES. 



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 wliich 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. Scy Ilium, but at other times 

 arises independently from the main stem of the seventh \ 

 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 sur- 

 prising to find an atrophy of the spiracular nerve accompanying 

 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 enlarg-ements are 

 present at an early period of development. 



The Glossopharyngeal 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 (PL xiv. 

 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 glosso- 

 pharyngeal. The next four are, as has been shewn by Gegen- 

 baur ^, equivalent to four independent nerves, but form, together 

 with the glossopharyngeal, a compound nerve, which we may 

 briefly call the vagus. 



1 Hexanchns, Gegenbaur, Jenaische Zeitsclirift, Vol. vi. ^ Lqc. cit. 



