Sinus is a sack or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue. In common usage, "sinus" usually refers to the paranasal sinuses, which are air cavities in the cranial bones, especially those near the nose and connecting to it. Each individual has four paired cavities located in the cranial bone or skull. These cavities are vital for a number of reasons [Wikipedia: http://en.wikipedia.org/wiki/Sinus_(anatomy)].
Hierarchy: 1
Parent: respiratory system
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