ANTRUM, a chamber or recess. When applied to the human body, the unqualified term usually refers to the antrum of High more or maxillary sinus. This is an air space situated in the cheek bone on either side of the nasal orifice. It is the largest of the bony sinuses which communicate with the nasal cavity. It is lined with mucous membrane, and opens into the nasal cavity by a small hole at the highest point on its inner wall. During the course of a cold its lining membrane is usually inflamed like that of the rest of the nose, and if its opening becomes closed by a plug of mucus or by swelling of the mucous membrane, pus may accumulate in the cavity and give rise to fever and pain. On account of the position of its opening, natural drainage is poor, and sometimes it is necessary to make an artificial opening at a lower level, to facilitate irrigation and drainage. Other antra are : the pyloric antrum, the lower third of the stomach between the large fundus and the narrow pyloric canal; the duodenal antrum or duodenal bulb, a dilatation in that part of the duodenum con tinuous with the stomach; the tympanic or mastoid antrum, a small air space in the mastoid bone leading from the middle ear to other air spaces in the mastoid.