A natural hollow or cavity. The maxillary antrum, in the upper jaw, is now known as the maxillary SINUS. The mastoid antrum is situated in the mastoid process, the mass of bone felt behind the ear. It may become the seat of an ABSCESS in cases of suppuration of the middle ear (see EAR, DISEASES OF). The pyloric antrum is the part of the stomach immediately preceding the PYLORUS.