Section 281: The stapedius muscle (sm) can be seen in the posterior wall of the middle ear space. The endolymphatic sac (es) is present in the posterior cranial fossa. The osseous opening of the canal which transmits the endolymphatic duct is called the external aperture of the vestibular aqueduct. The medial lip of that aperture is known as the "operculum" (o) (i.e., like the gill flap in fish). Nearly the entire course of the foramen singulare (fs) is visible. The internal carotid artery (ICA) is still seen in the carotid canal. VIIf - Mastoid segment of facial nerve.