An air-filled cavity in the skull connected to the nose; these sinuses can become infected or inflamed causing congestion.
From Latin 'sinus' meaning 'curve' or 'fold,' originally referring to any curved recess. Medical usage named the nasal cavities because of their curved, recessed shape in the skull.
Your sinuses are basically built-in air pockets in your skull that make your head lighter and give your voice resonance—they're why your voice changes when you have a cold!
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