A, The internal meatus is located near the center and the jugular foramen at the lower edge of the posterior surface. The sigmoid sinus descends along the posterior surface of the mastoid and turns forward on the occipital bone to pass through the sigmoid part of the jugular foramen. The inferior petrosal sinus descends along the petroclival fissure and passes through the petrosal part of the jugular foramen. The subarcuate fossa is located superolateral and the ostium for the vestibular aqueduct lateral to the internal acoustic meatus. The trigeminal impression is a shallow trough on the upper surface of the temporal bone behind the foramen ovale. The arcuate eminence overlies the superior semicircular canals. B, Temporal bone with the nerves preserved. The abducens nerve ascends to enter Dorello’s canal. The trigeminal nerve passes above the petrous apex to enter the porus of Meckel’s cave. The facial and vestibulocochlear nerves enter the internal acoustic meatus, and the glossopharyngeal, vagus, and accessory nerves enter the jugular foramen. The posterior and superior semicircular canals have been exposed. C, Enlarged view. The upper end of the posterior canal and the posterior end of the superior canal share the common crus. The endolymphatic duct extends down-ward from the vestibule and opens into the endolymphatic sac located beneath the dura inferolateral to the meatus. The endolymphatic ridge, the bridge of bone forming the posterior lip of the vestibular aqueduct, has been preserved. The jugular bulb can be seen through the thin bone below the internal meatus. D, Enlarged view of the fundus of the meatus after removal of the posterior wall. The upper edge of the porus has been preserved. The subarcuate artery enters the subarcuate fossa. The inferior vestibular nerve gives rise to the singular branch to the posterior ampullae, plus utricular and saccular branches. The superior vestibular nerve innervates the ampullae of the superior and lateral semicircular canals and commonly gives rise to a utricular branch. (Images courtesy of AL Rhoton, Jr.)