Paranasal sinuses. Coloured X-ray of a sagittal section through a human skull. The skull has been sliced in half down the centre and the regions of the paranasal sinuses coloured. The frontal sinus (purple, upper left), the ethmoidal sinus (green), the sphenoidal sinus (red) and the maxillary sinus (yellow) are all shown. The paranasal sinuses are spaces in the bones surrounding the eyes and nose, and the cheek bones of the upper jaw. They lighten the bones of the skull and reduce the weight of the head. They are connected to the nasal passages and are lined with mucous membranes. They can fill with fluid and secrete mucus during an infection. The sinus membranes are inflamed in sinusitis.