Sinuses. Coloured X-ray image of a person's skull with the healthy paranasal sinuses highlighted, as seen from the front. The two large maxillary sinuses (orange/purple) are located under the eye sockets on either side of the nose. The frontal sinuses (orange) are located higher in the skull. The sinuses are air-filled cavities which are lined with mucus membranes that drain into the nasal cavity. Their function is not clearly understood, but they may help to moisten and warm inhaled air. Sinuses may become inflamed (sinusitis) due to an infection or allergy, leading to severe pain.