The maxillary sinus is the largest of the paranasal sinuses (air sinuses). The maxillary sinus (of which there is one in the cheekbone on each side of the nose) occupies the whole of the body of the maxilla and may extend into its zygomatic process. It is below the orbit of the eye, in front of the pterygopalatine and infratemporal fossae, lateral to the lower part of the cavity of the nose, and above the molar and premolar teeth.

The lowest part of its floor is opposite the second premolar and first molar, and is nearly half an inch (1.2 centmeters) below the level of the floor of the nose. The opening out of the maxillary sinus leads into the middle meatus of the nasal cavity. The opening is in the upper part of the medial wall of the sinus – a position highly unfavorable for the escape of liquids that may collect in the sinus. The infraorbital groove and, containing the infraorbital vessels and nerve, run forward in the roof of the sinus, and as the canal bends downward toward the infraorbital foramen it produces a marked ridge in the angle between the roof and anterior wall of the sinus. The posterior superior dental nerve and vessels descend in the lower part of the posterior and lateral walls of the sinus; the anterior superior dental nerve and vessels are in the roof and anterior wall. Occasionally some of the canals in which the nerves run are partly defective; the nerves then lie between the bones and the mucous lining, and the sensation of toothache may arise from affection of the nerves during inflammation of the lining of the sinus. As in the other air sinuses, the mucous membrane is covered with ciliated epithelium; the action of the cilia promotes the movement of the mucus toward the opening into the nose.