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The outer layer of a bone is covered with a tought, tight-fitting membrane called periosteum. Nutrients are carried by small blood vessels to the bone through the periosteum. Under the periosteum is a hard strong layer of bone called compact bone. The compact bone contains blood vessels, blood cells, and a protein with deposits of calcium and phosphorus. The hard compact bone is what keeps your bones from breaking easily. Spongy bone is found in the ends of long bones such as the femur. Spongy bone is much less compact and has pockets that keep your bones light-weight. If it weren't for the light-weight spongy bone, your body would have a much larger mass and weigh much more. In the very middle of the bone is the marrow. New red blood cells, and some white blood cells, are made here.
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