The word skeleton comes from the Greek word skeletos, meaning "dried up." The parts of the skeletal system—the bones and other structures that make up the joints of the skeleton—are anything but dried up. Strong yet light, the skeletal system is made of living material, with networks of blood vessels running throughout. The system protects body organs, supports the body, and provides attachment points for muscles to enable body movement. All bones act as storage sites for minerals such as calcium and phosphorus, and certain bones also produce blood cells.

Design: Parts of the Skeletal System

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton (endo means "within"). In animals that have an external skeleton, such as the crab, the skeleton is called an exoskeleton (exo means "outside"). Exoskeletons restrict the movement of...