Sinuses. Coloured Magnetic Resonance Imaging (MRI) scan of a person's head with the healthy paranasal sinuses marked in yellow. This is a transverse view, with the nose at top centre. The two large maxillary sinuses are located under the eye sockets on either side of the nose. The sphenoidal sinuses are located deeper in the skull over the nasal cavity. 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.