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 flat, and irregular. Those whose chief use is to protect are broad and flat. The bones which furnish support are thick and solid; those designed to aid in motion are long and straight. Including six small bones in the ear, there are two hundred and six bones in the adult skeleton.

Gross Structure of Bones.—The structure of a long bone is shown in Fig. 29. It has a long, hollow shaft of hard, compact bone, and enlarged ends composed of spongy bone. The hollow in the shaft is filled with yellow marrow, which is composed of blood vessels and fat, and aids in nourishing the bone. The long bones are found in the limbs (Fig. 28). The ribs and other flat bones and the irregular bones contain no yellow marrow; they are spongy inside, and hard and compact near the surface. There is a red marrow in the cavities in the spongy parts of bones (Fig. 29). New red blood cells are formed in this marrow. The bones have a close-clinging, fibrous covering composed of connective tissue and blood vessels. It is called periosteum.

, sawed lengthwise. The red blood cells are formed in the red marrow of the spongy part.