Bone: Bone is the substance that forms the skeleton of the body. It is composed chiefly of calcium phosphate and calcium carbonate. It also serves as a storage area for calcium, playing a large role in calcium balance in the blood.
The 206 bones in the body serve several other purposes. They support and protect internal organs (for example, the skull protects the brain and the ribs protect the lungs). Muscles pull against bones to make the body move. Bone marrow, the soft, spongy tissue in the center of many bones, makes and stores blood cells.