Left lateral transmastoid view of bony labyrinth and sigmoid sinus after mastoidectomy. Removal of soft tissue and periosteum overlying the posterolateral skull reveals portions of the temporal, parietal, and occipital bones and associated sutures. The asterion is a craniometric point and surgical landmark where three cranial sutures meet: lambdoid, parietomastoid, and occipitomastoid. Medial to the mastoid process is the mastoid notch for attachment of the posterior digastric muscle. Mastoidectomy reveals the sigmoid sinus, which continues as the jugular bulb, and the superior petrosal sinus. The latter drains the cavernous sinus to the transverse sinus. Trautmann's triangle is shown bounded by the bony labyrinth anteriorly, the sigmoid sinus posteriorly, and the superior petrosal sinus superiorly. The labyrinth has been opened to visualize the posterior, superior, and lateral semicircular canals. Just inferior to the lateral semicircular canal is the mastoid portion of the facial nerve.  (Image courtesy of AL Rhoton, Jr.)