Left lateral transmastoid view of bony labyrinth, sigmoid sinus, and mastoid antrum. Mastoidectomy reveals the posterior and lateral semicircular canals, the sigmoid sinus in the posterior aspect of the mastoid process, and the mastoid antrum superiorly, which communicates with the middle ear cavity. The sinodural angle is formed at the junction of the sigmoid sinus with the superior petrosal sinus, which can be seen in the upper border of this dissection. Anterior to the mastoid process is the external auditory canal, the end at which is the tympanic membrane. The Spine of Henle, or suprameatal spine, is a small projection near the upper posterior margin of the external auditory canal just inferior to the posterior root of the zygomatic process. The tegmen is the bony roof of the tympanic cavity. (Image courtesy of AL Rhoton, Jr.)