Skull. Coloured computed tomography (CT) scan of a normal skull, seen in a coronal section. The bony tissue has been coloured red. The black areas at centre represent the nasal cavity. The two black, triangular-shaped areas at centre left and right are the maxillary sinuses. These are air-filled cavities, lined with a mucous membrane, which are connected to the nasal cavity. At top centre is the brain which is encased by the cranium.