Bones may be classified as long (femur, humerus, radius, ulna), tubular (small bones of hands and feet) or flat (scapula, pelvis, rib, vertebrae). The shell of the bone is called the cortex and the interior is known as the medulla, which consists of interconnecting bars of bone called trabec-ulae. This trabecular bone is also referred to as cancellous bone. The thickness of the cortex varies considerably along the length of a given bone and especially between different bones. The proportion of a bone occupied by cortical and cancellous bone also varies between bones and with age. The trabecular bone is set in a fatty marrow containing haemopoietic tissue. The cortex is covered by a thin, tough mesenchymal layer known as periosteum. The ends of a bone are known as the epiphysis. The metaphysis is the region immediately adjacent to the epiphysis and the diaphysis is the shaft (area between the two metaphyses) (Figure 43.2).

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