Anatomy of the cranial nerve foramina in the Risso's dolphin (Grampus griseus)

In the present study, we examined the cranial nerve foramina of Risso's dolphin (Grampus griseus). There were two distinguishable characters in the cranial nerve foramina compared with terrestrial mammals. One was that the foramen infraorbitale was composed of several holes, but not a single hole. They should therefore be termed foramina infraorbitales. The infraorbital nerves ran through these foramina, went into the 'melon' and then branched in a complicated fashion. The facial nerve innervated the muscles surrounding the melon. A well-developed infraorbital and facial nerve complex may control the melon. Another was the presence of a porus acusticus internus and independent tympano-periotic bone. The separate ear bone forced the vestibulocochlear and facial nerves to exit from the cranial cavity through the porus acusticus internus. The independent ear bone structure may shut off the noise from the cranial bone to the periotic bone with a true receptor of hearing. It may be an adaptation for an acute sense of hearing. Compared with other dolphins, the cranial foramina of Risso's dolphin are definitely separate. The structure of the foramina is similar to that of pilot whales, but not to dolphins, so that Grampus may be closely allied to pilot whales.