paranasal sinuses

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paranasal sinuses (pa-ră-nay-zăl) pl. n. the air-filled spaces, lined with mucous membrane, that occur in some bones of the skull and open into the nasal cavity. They comprise the frontal sinuses and the maxillary sinuses (one pair of each), the ethmoid sinuses (consisting of many spaces inside the ethmoid bone), and the two sphenoid sinuses.