But their internal structure is essentially the same. The hard outer part consists largely of proteins, such as collagen, and a substance called hydroxyapatite. Composed mainly of calcium and other minerals, hydroxyapatite stores much of the body's calcium and is largely responsible for the strength of bones. The marrow in the center of each bone is softer and less dense than the rest of the bone and contains specialized cells that produce blood cells. Blood vessels run through a bone, and nerves surround it.
Bones come together to form joints.
The configuration of a joint determines the degree and direction of possible motion. Some joints, such as those between the plates of the skull, called sutures, don't move in adults. Others allow a range of motion. Other components of joints provide stability and reduce the risk of damage from constant use. Muscles are bundles of fibers that can contract.