maxillary sinus, one of a pair of large air cells forming a pyramidal cavity in the body of the maxilla. The apex of each sinus extends into the zygomatic arch, and its floor, formed by the alveolar process, is usually 1 to 10 mm below the floor of the nose. In the adult the volume of the sinus averages 14.75 cc. The mucous membrane of the sinus is continuous with that of the nasal cavity. In the fourth month of gestation the embryonic sinus appears as a shallow groove on the medial surface of the bone. It does not reach full size until after the second teething. Also called antrum of Highmore. Compare ethmoidal air cell, frontal sinus, sphenoidal sinus.
