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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.[1] They are outgrowths from the nasal cavity. All of them drain into the superior or lateral aspect of the nose.[2] The sinuses' lining mucosa is continuous with the nasal cavity; therefore, any infections from the nasal mucosa can easily spread to the sinuses.[3] See Image. Nasal Cavity.