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NCI Dictionary of Cancer Terms


paranasal sinus  listen  (PAYR-uh-NAY-zul SY-nus)

  One of many small hollow spaces in the bones around the nose. Paranasal sinuses are named after the bones that contain them: frontal (the lower forehead), maxillary (cheekbones), ethmoid (beside the upper nose), and sphenoid (behind the nose). The paranasal sinuses open into the nasal cavity (space inside the nose) and are lined with cells that make mucus to keep the nose from drying out during breathing.

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Anatomy of the paranasal sinuses; drawing shows front and side views of the frontal sinus, ethmoid sinus, maxillary sinus, and sphenoid sinus. The nasal cavity and pharynx (throat) are also shown.