THE NERVOUS SYSTEM 147 



chorda tympani occurs in lower vertebrates at all, it is to be looked 

 for as a branch of the pre- or post-trematic nerve. In 1 88 i Balfour 

 claimed that it was a />r<?-trematic branch of the Vllth nerve. In 

 1887 Froriep came to the conclusion that the chorda tympani is to 

 be regarded as a ^oj/-trematic nerve, and although others have since 

 supported Balfour's alternative suggestion, Froriep's conclusion re- 

 presents the view now generally held. 



This discussion with regard to the morphological position of the 

 chorda tympani has considerably complicated the second point at 

 issue, namely, whether the R. alveolaris VII of the Salamander, and 

 other Urodeles, is pre- or post-trematic, and is therefore to be re- 

 garded as the true homologue of the chorda tympani or not. 



In 1895 Strong came to the conclusion that the chorda tympani of 

 mammals, the R. mandibularis internus VII of Anura, and the R. 

 alveolaris VII of Urodeles were all homologous structures. 



In 1 90 1 Driiner, who apparently accepted Froriep's view that the 

 chorda tympani is post-trematic, denied its homology with the R. 

 alveolaris VII of Salamandra on the ground that he believed the 

 latter nerve to be pre-trematic. In both larval and adult Salamander 

 the floor of the mouth is somewhat depressed ventrally between the 

 mandibular and hyoid arch so as to form a sort of pocket — the plica 

 hyomandibularis (plc.hy.m.; Figs. 37 and 38) — and this has been 

 supposed to represent a vestige of the gill-slit between these two 

 arches. On this supposition therefore Driiner argued that, since the 

 R. alveolaris VII is a sensory branch of the hyomandibular nerve 

 lying anterior to this supposed hyoid cleft, it must be pre-trematic. 

 He accordingly thought the homologue of the chorda tympani was 

 wanting in Urodeles. 



In 1902 Coghill, while accepting the view that R. alveolaris VII 

 is a pre-trematic nerve, apparently regarded the chorda tympani also 

 as a pre-trematic nerve, and, maintaining at the same time a some- 

 what cautious attitude, held them to be homologous. He could not, 

 however, agree with Strong in regarding these two structures as 

 homologous with the R. mandibularis internus of Anura, since he 

 believed this nerve to be post-trematic. 



In the same year C. K. Hoffmann (i 902) regarded both the chorda 

 tympani and the R, alveolaris VII of Salamandra as pre-trematic 

 branches of the Vllth nerve, and held that they were homologous 

 structures. 



By 1 903 Driiner's views had undergone a profound change. As the 

 result of the investigation of a developmental series of Siredon he 

 found that the R. alveolaris VII was morphologically post-trematic 



