Air cavity in the body of the maxilla that is lined by the Schneiderian membrane consisting a pseudostratified ciliated columnar epithelium. It normally lies superior to the roots of the premolars and molars and generally extends from the canine or premolar region posterior to the molar or tuberosity region. Anatomically it is a pyramidal cavity, with thin bony walls corresponding to the orbital, alveolar (floor), facial, and infratemporal aspects of the maxilla. Its apex extends into the zygomatic process. Its base is medial, forming the lateral wall of the nasal cavity. It communicates with the nasal cavity through an opening in the middle meatus called ostium. The floor is formed by the maxillary alveolar process and partly by the hard palate. The floor exhibits recesses and depressions in the premolar and molar regions. Each sinus has a volume of about 15ml.