The Internal Acoustic Meatus, Semicircular Canals, and Cochlea. The posterior surface of the right temporal bone has been drilled while preserving the bone around the internal acoustic meatus, semicircular canals, and cochlea. The superior canal protrudes upward toward the arcuate eminence in the floor of the middle fossa. The posterior canal is exposed on the lateral side of the superior canal. The posterior end of the superior canal and the upper end of the posterior canal join to form the common crus, which opens into the vestibule. The mastoid air cells have been removed from the petrous apex. The cochlea bulges upward anteromedial to the fundus of the internal acoustic meatus. Some of the mastoid air cells medial to the sigmoid sulcus have been removed. The cochlear aqueduct, which ends just above the petrosal part of the jugular foramen, has been preserved. The endolymphatic sac sits on the posterior surface of the temporal bone below the superior and lateral canals. The intrajugular processes of the temporal and occipital bones separates the petrosal and sigmoid parts of the jugular foramen. The glossopharyngeal, vagus, and accessory nerves exit the cranium through the intrajugular part of the jugular foramen located between the petrosal and sigmoid parts. (Image courtesy of AL Rhoton, Jr.)