A, The insert shows the site of the scalp incision and mastoid tip. The scalp flap has been reflected forward. The mastoidectomy exposes the dense cortical bone housing the semicircular canals. The bone flap is outlined. The occipital artery courses backward between the digastric and superior oblique. B, Enlarged view. The tympanic segment of the facial nerve courses below the lateral canal. The chorda tympani arises from the mastoid segment of the facial nerve. The mastoid antrum, which has been drilled away, opens through the aditus into the epitympanic part of the tympanic cavity. C, The presigmoid and temporal dural incisions have been outlined. D, The temporal and presigmoid dura has been opened. One goal of the procedure is to preserve the vein of Labbé, which empties into the transverse sinus. (Images courtesy of AL Rhoton, Jr.)