The human skeleton is a strong, flexible framework of 206 bones that supports the body and protects internal organs. In addition, the bones of the skeleton store calcium, a mineral essential for the activity of nerve and muscle cells. The soft core of bone, the bone marrow, is where red blood cells, certain white blood cells, and blood platelets form.
Bones come in different shapes and sizes, each adapted to perform specific functions. The breastbone, for example, is a flat plate of bone that helps to protect the heart and lungs in the chest. The fused bones of the skull safely encase the brain. The short, delicate bones in the wrist and hand enhance dexterity, providing flexibility for small, precise motions. The long, heavy femur bone in the leg acts as a strong lever for powerful or speedy movement. Cartilage is flexible connective tissue that provides support to skeletal bones and allows joints to move without rubbing against each other.