Aditus to mastoid antrum
The aditus to mastoid antrum is a large irregular cavity that 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, 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.
