The largest of the paranasal sinuses occupying the body of the maxilla; if large, it may extend into the zygomatic bone. On its roof it is separated from the orbital cavity by a thin orbital plate through which run the infra-orbital vessels and nerve. The sinus floor is closely related to the apices of the permanent molar teeth and is a potential hazard during tooth extraction, as a tooth may be inadvertently pushed into the sinus. The anterior wall of the sinus is bounded by the facial surface of the maxilla and the posterior wall is related to the pterygo-palatine fossa. The middle and anterior alveolar (dental) nerves run in canals in the lateral and anterior walls of the sinus. The opening into the sinus is high up on the nasal wall close to the roof and communicates with the middle meatus of the nasal cavity; this location makes the gravitational drainage of pus and fluids from the sinus difficult. The medial (nasal) wall of the sinus, composed mainly of cartilage, is bounded by the nasal surface of the body of the maxilla and by parts of the palatine, lacrimal, ethmoid, and inferior turbinate bones. The sinus is lined with ciliated mucoperiosteum.