The bones support the soft or fleshy parts of the limbs, and form a framework that gives them shape. They form, in some cases, levers upon which the muscles act, and give origin to the various motions of the leg and foot. When dead bone is examined it is a hard-looking, whitish-yellow, tough substance. It is light in weight when compared with its strength. In its living state it has a pinky colour, due to the blood circulating through its minute channels. The exterior of the bone is cased - except when covered by cartilage - with a thin, firm membrane. If this membrane is injured, local death takes place in that part of the bone, because of its being deprived of its nutrition. This membrane also affords means whereby the muscles, tendons, and ligaments may be attached to the bone. It forms a smooth surface to the bone, and so reduces friction; so that its use is threefold : (1) to act as a medium to convey nutriment to the bone it covers; (2) to lessen friction; and (3) to provide a means of attachment for muscles, etc. If a piece of bone be examined under a microscope it is seen to be filled with an infinite number of minute canals containing blood-vessels, and it is through these little tunnels that the bone is built up and nourished.

The bones at the commencement of their formation are composed of cartilage, or gristle, and are gradually made into bone by the earthy salts being deposited through the blood-vessels, thereby imparting rigidity to the cartilage. In childhood bones are made up of parts which do not unite until maturity is reached, so that it is easy to bend or misshape them.