Left lateral transmastoid view of bony labyrinth, sigmoid sinus, epitympanic recess, and facial canal. Mastoidotomy reveals the posterior and lateral semicircular canals and the position of the sigmoid sinus in the posterior aspect of this field.  Beneath the lateral semicircular canal is the mastoid segment of the facial nerve. It gives rise to the chorda tympani branch prior to its exit through the stylomastoid foramen. Bony resection has opened the fossa incudis, a small depression in the posterior aspect of the epitympanic recess for the short crus of the incus.  (Image courtesy of AL Rhoton, Jr.)