A, Posterior view of a right temporal bone. The squamosal part forms part of the floor and lateral wall of the middle fossa. The sigmoid sulcus descends along the posterior surface of the mastoid portion. The internal acoustic meatus enters the central portion of the petrous part of the bone. The trigeminal impression and arcuate eminence are located on the upper surface of the petrous part. The vestibular aqueduct connects the vestibule in the petrous part with the endolymphatic sac, which sits on the posterior petrous surface inferolateral to the internal acoustic meatus. B, Enlarged view. The transverse crest separates the meatal fundus into a superior part where the facial canal and superior vestibular areas are situated, and an inferior part where the cochlear and inferior vestibular areas are located. The vertical crest separates the facial and superior vestibular areas. C, Enlarged view of another internal acoustic meatus. The transverse crest divides the meatal fundus into superior and inferior parts. The anterior part above the transverse crest is the site of the facial canal and the posterior part is the site of the superior vestibular area. Below the transverse crest, the cochlear area is anterior and the inferior vestibular area is posterior. D, Another internal acoustic meatus. The view is directed to expose the singular foramen, for the singular branch of the inferior vestibular nerve that innervates the posterior ampullae. The inferior vestibular nerve also has a saccular and, occasionally, a utricular branch. (Images courtesy of AL Rhoton, Jr.)