I, Lateral view of mastoid and tympanic cavity before removing the tympanic ring. The tympanic segment of the facial nerve passes below the lateral semicircular canal and turns downward as the mastoid segment to exit the stylomastoid foramen. The stylomastoid foramen and the mastoid segment are located lateral to the jugular bulb. The semicircular canals are located above the jugular bulb. J, A probe has been placed in the eustachian tube, which passes downward, forward, and medially from the tympanic cavity and across the front of the petrous carotid. The third trigeminal division passes through the foramen ovale on the lateral side of the eustachian tube. K, Enlarged view of the tympanic ring with the tympanic membrane removed. The tensor tympany muscle passes backward above the eustachian tube and gives rise to a tendon that turns sharply lateral around the trochleiform process to attach to the malleus. The chorda tympani crosses the inner surface of the tympanic membrane and neck of the malleus. The round window opens into the vestibule. The stapes sit in the oval window. The promontory is located lateral to the basal turn of the cochlea. L, The floor of the middle fossa and the tympanic sulcus have been removed to expose the jugular bulb and petrous carotid. The greater petrosal nerve courses along the floor of the middle fossa on the upper surface of the petrous carotid. The deep petrosal nerve arises from the sympathetic bundles on the internal carotid artery. The deep and greater petrosal nerves join to form the vidian nerve, which passes forward through the vidian canal to join the maxillary nerve and pterygopalatine ganglion in the pterygopalatine fossa. The pharyngobasilar fascia and upper part of the longus capitis have been reflected downward to expose the lower margin of the clivus. M, The jugular bulb has been removed from the jugular fossa located below the vestibule and semicircular canals. The vertical segment of the petrous carotid has been removed. The cochlea, which has been opened, is located above the lateral genu of the petrous carotid. The tympanic segment of the facial nerve passes posteriorly below the lateral semicircular canal. N, The retrosigmoid and presigmoid dura have been opened. The lateral wall of the vestibule and cochlea have been removed. The vestibule, semicircular canals, and cochlea are exposed above the jugular bulb and lateral genu of the petrous carotid. (Images courtesy of AL Rhoton, Jr.)