Skull, computer artwork. An adult human skull is normally made up of 22 bones and can be subdivided into two parts: the cranium and the mandible (lower jaw bone). The bones of the cranium are joined together by sutures to form a solid structure that encloses and protects the brain. The very tiny amount of movement permitted at these joints contributes to the compliance and elasticity of the skull. The skull contains the sinus cavities, which are air-filled cavities lined with respiratory epithelium, and sockets for the eyes (orbits).