Transmastoid view of left lateral labyrinth and endolymphatic sac. Removal of mastoid and petrous bone reveals the bony labyrinth and the posterior, superior, and lateral semicircular canals. Emerging through an external aperture of the vestibular aqueduct is the endolymphatic sac, which terminates in the epidural space in the posterior fossa in close proximity to the sigmoid sinus (located here just beneath the retractor). In this dissection, the mastoid segment of the facial nerve begins near the lateral semicircular canal and is seen descending in the facial canal along the posterior wall of the middle ear cavity. (Image courtesy of AL Rhoton, Jr.)