Bone

Bone, hard tissue that forms the skeleton. Bone supports the body, protects its organs, acts as an anchor for muscles and as a lever for the movement of limbs, and is the main reserve of calcium and phosphate in the body. Bones have a compact, hard shell surrounding a porous, spongy bone inner layer. Human bones are classified as long bones (mostly in the arms and legs) and short bones (e.g., the skull, pelvis, and vertebrae). A hollow cavity in long bones contains marrow, which helps to form blood cells. Broken bones (fractures) mend by the production of new cartilage and its gradual replacement by new bone. Insufficient calcium, phosphate, or vitamin D causes soft bones (rickets). Other disorders include osteoporosis (loss of bone density, common in old age) and osteoarthritis.

See also: Skeleton.