Transmastoid view of left lateral labyrinth and endolymphatic sac. Removal of mastoid and petrous bone reveals the bony labyrinth and the posterior, superior, and lateral semicircular canals. Emerging through an external aperture of the vestibular aqueduct is the endolymphatic sac, which terminates in the epidural space in the posterior fossa in close proximity to the sigmoid sinus. Trautmann's triangle is bounded superiorly by the superior petrosal sinus, posteriorly by the sigmoid sinus, and anteriorly by the bony labyrinth. It serves as a surgical pathway to the posterior fossa from the mastoid cavity. (Image courtesy of AL Rhoton, Jr.)