The skeleton has an axial and a perpendicular part. The perpendicular part includes bones of fins, wings or limbs and the axial skeleton consists of the vertebral column, the skull and the ribs as well as the sternum. According to a hypothesis the skull is a modified vertebra in that it encloses neural tissue with entry and exit points for blood vessels and nerves. By way of evolutional economics in higher animals, the important sensory organs are located very near the cranial cavity and are in fact appendages or extensions of the brain. By common usage the word skull includes the mandible, the word cranium is used for the rest of the bones, the word calvarium is given to that bony part which encloses the brain and the vault is what you see of the calvarium from the top. The cranial base is that part on which the brain rests. The visible part of the calvarium is formed by two unpaired and two paired bones. The frontal bone is in the front, the occipital bone is at the back and they are unpaired. The parietal bones superiolaterally and the temporal bones inferio-laterally are paired. Theoretically the frontal bone is also in two parts with the inter-frontal suture between them fusing very early (before two years).