bone
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Hard connective tissue comprising the skeleton of most vertebrate animals. Bone is composed of a network of collagen fibres impregnated with mineral salts (largely calcium phosphate and calcium carbonate), a combination that gives it great density and strength, comparable in some cases with that of reinforced concrete. Enclosed within this solid matrix are bone cells, blood vessels, and nerves. The interior of the long bones of the limbs consists of a spongy matrix filled with a soft marrow that produces blood cells.
There are two types of bone: those that develop by replacing cartilage and those that form directly from connective tissue. The latter, which includes the bones of the cranium, are usually platelike in shape and form in the skin of the developing embryo. Humans have 206 distinct bones in the skeleton (see human body), of which the smallest are the three ossicles in the middle ear. However, a different total is sometimes given, because of a number of fused pairs of bones in the skull that may be counted as one or two bones.
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