Paranasal Sinuses, air-filled, mucus-lined cavities in the head and cheekbones that drain into the nasal cavity. The largest are the two maxillary sinuses found directly under the eye sockets. Others are the two frontal sinuses, found in the frontal bone just above and between the eye sockets, and the sphenoid and ethmoid sinuses, located deeper in the skull over the nasal cavity. The sinuses produce mucus, which is propelled by small, hairlike processes called cilia into the nasal cavity. Their function is not clearly understood, but they are believed to help warm and moisten inhaled air.