n. the hard extremely dense connective tissue that forms the skeleton of the body. It is composed of a matrix of collagen fibres impregnated with bone salts (chiefly calcium carbonate and calcium phosphate; see hydroxyapatite), in which are embedded bone cells (see osteocyte). Compact (or cortical) bone forms the outer shell of bones; it consists of a hard virtually solid mass made up of bony tissue arranged in concentric layers (Haversian systems). Spongy (or cancellous) bone, found beneath compact bone, consists of a meshwork of bony bars (trabeculae) with many interconnecting spaces containing marrow. (See illustration overleaf.)
Individual bones may be classed as long, short, flat, or irregular. The outer layer of a bone is called the *periosteum. The medullary cavity is lined with *endosteum and contains the marrow. Bones not only form the skeleton but also act as stores for mineral salts and play an important part in the formation of blood cells.... bone
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