There are more than two hundred separate bones in the adult human skeleton (Figure 1.1, Table 1.1). They may be divided into three classes according to their basic shape. Long-bones are found in the limbs and take the form of hollow tubes closed at both ends. Flat bones tend to take the form of broad bony plates (for example, the bones which make up the skull vault), and irregular bones, as their name suggests, fit into neither of the above categories on account of their irregular shape. Examples of irregular bones are the vertebrae and the bones of the skull base.