Bone

Bone is a structural element found in many animal bodies. Made up of hard [calcium carbonate]?, it acts a frame from which to suspend muscles. An evolutionary alternative to bone is a shell.

Bone is relatively hard? for its density, although it is brittle and may snap on impact, creating what is medically called a fracture?. The reason for the good material characteristics in bone is that it is not solid calcium, but instead a mesh?, the density of which may vary at different points in the bone. Thus bird bones are generally less dense than mammalian bones.

Bones are generally tubular in structure, and the hollow space in the middle is filled with marrow?. The marrow is where [blood cell]?s are produced and thus critically important to healthy blood. At the ends are connections to other bones at joint?s which are lubricated and sometimes padded by softer tissue called cartilige?.

The science of the interaction of bone and muscle is called biomechanics?. The science of bones is called osteology?.

Some illnesses afflict human bones, for example osteoporosis and cancer. The joints can be affected by arthritus.