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A sinus is a pouch or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue. The term is applied to a large number and variety of structures in the human body, such as a dilated portion of a blood vein, a chronically infected tract such as a fistula, and the air cavities within the cranial bones, especially those located near the nose and connecting to it. These nose-associated sinuses are called paranasal sinuses and are usually what is meant when a person's "sinuses" are referred to.

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