The paranasal sinuses (the hollow spaces in the skull and facial bones around the nose) are air-filled cavities within the frontal, ethmoidal, sphenoidal, and maxillary bones. They are outgrowths from the nasal cavity. All of them drain into the superior or lateral aspect of the nose. The sinuses' lining mucosa is continuous with the nasal cavity; therefore, any infections from the nasal mucosa can easily spread to the sinuses.
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