[Anatomical study on the facial nerve innervating the floor of the mouth in chondrichthyes. Homology of the chorda tympani].

Abstract

This paper deals with the results of the investigation of the facial nerves of chondrichthyes in order to consider the phylogenetic origin of the Chorda tympani in human. Six species of elasmobranchs (Chlamydoselachus anguineus, Cephaloscyllium umbratile, Squalus acanthias, Dasyatis akajei, Raja kwangtungensis and Mobura diabolus) were dissected under a stereoscopic microscope for this purpose, and the following results were obtained. Ramus palatinus and R. pre-spiracularis were observed as pre-trematic branches, while R. mandibularis externus, R. mandibularis internus and R. hyoideus originating from R. hyomandibularis were observed as post-trematicus of the facial proper in chondrichtyes. The rami intermedii indicated by Tanaka and Nakao (1979) were observed only in Dasyatis akajei. The R. hyomandibularis of Squalus acanthias had cutaneous branches, and the same branches were described in Chimaera by Takahashi and Kobayashi (1988). R. pre-spiracularis and R. mandibularis internus supply the floor of mouth in Squalus acanthias. As for the other chondrichthyes, R. mandibularis internus was only the one that could be found at the floor of mouth cavity under a stereoscopic microscope. From the observations described above and from previous studies, it may be concluded that the problem of whether the Chorda tympani is homologous with whether the pre- or post-trematicus of branchial nerves seems to depend on the animal species.