Left presigmoid approach with dissection of the facial nerve (CN VII) . Mastoid and petrous segments of the temporal bone have been drilled. The facial nerve courses through the internal acoustic meatus (meatal segment) and at its lateral end (fundus of internal acoustic canal) passes between the cochlea anteriorly and the labyrinth posteriorly (as the labyrinthine segment) arriving at the geniculate ganglion where the greater superficial petrosal nerve (GSPN, not visible) branches anteriorly. The facial nerve then bends sharply posteriorly coursing within the facial canal along the medial wall of the middle ear cavity immediately inferior to the lateral semicircular canal as the tympanic segment. Just distal to the pyramidal eminence in the posterior wall, it continues inferiorly to the stylomastoid foramen as the mastoid segment. This segment gives rise to branches, including the nerve to the stapedius and the chorda tympani. The facial nerve ultimately exits the skull base via the stylomastoid foramen just deep (medial) to the mastoid tip. Also visible is the cortical segment of the left posterior inferior cerebellar artery, supplying the posterior inferior cerebellum. (Image courtesy of AL Rhoton, Jr.)