Function 1. As a lever. The bones of the upper and lower limbs pull and push, with the help of muscles. 2. As a calcium store. 97% of the body's calcium is stored in bone. Here it is easily available and turns over fast. In pregnancy the demands of the fetus for calcium require a suitable diet and after menopause hormonal control of calcium levels may be impaired: calcium leaches out leaving brittle osteoporotic bones. 3. Protective? This is often quoted in books: in fact protection against outside forces is rarely needed, and if it is we usually wear a cycling helmet, or a crash hat, or a hard hat. Or sit in a very strong structure like a formula 1 carbon fibre tub or a Volvo. So the bone can't be that good. In practice these are exceeded by the almost continuous large forces exerted by our own muscles. Respiratory movements need ribs. If a thigh bone or a humerus fractures the pull exerted by the muscles, even though not in active use, will be enough to overlap or otherwise displace the broken ends and we need considerable force,
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