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The mastoid is a projection of the inferior-posterior part of the temporal lobe, an apophysis. It is located close to the facial nerve, temporal lobe, and venous sinuses. In its position behind the external ear, the mastoid consists of air-filled cavities. These cavities vary in number and size depending on the anatomy of each individual. The largest cavity is the antrum. It is via the antrum that the mastoid can communicate with the air of the tympanic cavity, thus acting as a resonator.