: a cavity or hollow in the body: as a: a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus <a tuberculous sinus>b (1): a cavity in the substance of a bone of the skull that usually communicates with the nostrils and contains air (2): a channel for venous blood (3): a dilatation in a bodily canal or vessel
Body cavity or hollow. The paranasal sinuses, which are known commonly simply as the sinuses, are any of four sets of cavities in the bones adjoining the nose: maxillary, the largest, between the eye socket and the palate and upper jaw; frontal, just above and between the eye sockets; ethmoid, consisting of 3–18 thin-walled cavities between the nasal cavities and the eye sockets; and sphenoid, behind the nasal cavity. All are absent or small at birth, enlarge gradually until puberty, and then grow rapidly. They affect the sound of the voice and may help to warm inhaled air. Their lining produces mucus, which drains into the nasal cavity. Blockage of their outlets by swelling (from allergy or infection; seesinusitis), polyps, or structural problems hampers breathing through the nose and can lead to serious infection. Severe obstruction may require surgery, which must be done with extreme care to avoid harm to nearby brain structures or the eyes.