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Human skull. Coloured X-ray of a child's healthy skull. The rounded cranium (upper centre) houses and protects the brain. The eye sockets (orbits) and nasal cavity (centre) are also seen. Some of the permanent (secondary) teeth have displaced the milk (deciduous) teeth in the jaws (lower centre), while others are seen below the teeth they will replace. All the bones of the skull, apart from the hinged lower jaw, are fused together. (KEYSTONE/SCIENCE PHOTO LIBRARY/D. ROBERTS)