Aditus to mastoid antrum

The entrance to the antrum (Aditus to mastoid antrum) is a large irregular aperture, which leads backward from the epitympanic recess into a considerable air space, named the tympanic or mastoid antrum. The antrum communicates behind and below with the mastoid air cells, which vary considerably in number, size, and form; the antrum and mastoid air cells are lined by mucous membrane, continuous with that lining the tympanic cavity. On the medial wall of the entrance to the antrum is a rounded eminence (Prominence of lateral semicircular canal), situated above and behind the prominence of the facial canal; it corresponds with the position of the ampullated ends of the superior and lateral semicircular canals.