I, A mastoidectomy has been completed to expose the semicircular canals and the mastoid segment of the facial canal. The endolymphatic sac sits under the presigmoid dura. J, The external canal has been resected to expose the structures in the tympanic cavity. The tympanic segment of the facial nerve courses between the lateral semicircular canal and the stapes sitting in the oval window. The chorda tympani arises from the mastoid segment of the facial nerve, passes forward along the inner surface of the tympanic membrane and the neck of the malleus to enter its anterior canaliculus, exits the skull along the petrotympanic suture, and joins the lingual nerve in the infratemporal fossa. The promontory overlies the basal turn of the cochlea. The tendon of the tensor tympani muscle makes a right-angle turn around the trochleiform process to insert on the malleus. K, The incus and malleus have been removed while preserving the stapes and the tensor tympani muscle. The petrous carotid has been exposed. The nerves exiting the jugular foramen have been retracted forward to expose the hypoglossal nerve exiting the hypoglossal canal. L, A frontotemporal craniotomy has been completed and the floor of the middle cranial fossa removed. The semicircular canals have been exposed above the jugular bulb and the stapes has been removed from the oval window. The maxillary nerve has been exposed in the pterygopalatine fossa. The membranous wall of the eustachian tube has been opened to expose the tube’s opening into the nasopharynx. (Images courtesy of AL Rhoton, Jr.)