M, A retrosigmoid craniotomy has been completed and the nerves in the cerebellopontine angle exposed. The vestibulocochlear nerve has been depressed to expose the facial nerve. N, The facial nerve has been reflected forward out of the facial canal. The promontory has been drilled to expose the cochlea and the vestibule. Both ends of the semicircular canals open into the vestibule, as does the basal turn of the cochlea. The jugular bulb has been removed to expose the jugular fossa in which the bulb resides. The jugular bulb is located below the vestibule. The nerves exiting the jugular foramen have been reflected backward to expose the hypoglossal nerve exiting the hypoglossal canal. The nerves passing through the jugular foramen and hypoglossal canal exit the skull on the medial side of the internal jugular vein and descend between the internal carotid artery and internal jugular vein. O, The bone above the occipital condyle has been drilled to expose the hypoglossal nerve in the hypoglossal canal. P, The posterior wall of the internal acoustic meatus has been removed to provide this presigmoid inferolateral view of the nerves in the internal meatus. The cochlear nerve separates off the main bundle of the vestibulocochlear nerve and penetrates the modiolus. The inferior vestibular nerve divides into the singular nerve to the posterior ampullae and a branch to the saccule. The superior vestibular nerve innervates the superior and lateral ampullae and sends a branch to the utricle. Q, The medial wall of the jugular fossa has been removed and the nerves passing through the jugular foramen have been exposed. The glossopharyngeal nerve passes through the foramen anterior to the vagus and accessory nerves. A large superior petrosal vein ascends to the superior petrosal sinus. R, The glossopharyngeal, vagus, and accessory rootlets arise behind and the hypoglossal rootlets arise anterior to the inferior olive. (Images courtesy of AL Rhoton, Jr.)