Skeleton: Movement and Locomotion

The 206 bones of the body are grouped into two divisions. The axial skeleton forms the central axis: the skull, the vertebral column, the sternum (breastbone) and the ribs. It protects the brain, spinal cord, heart and lungs. The appendicular skeleton is made of the appendages - the bones of the arms and legs - together with the two girdles that anchor them: the pectoral (shoulder) girdle and the pelvic (hip) girdle. Axial gives protection and support; appendicular gives movement.

Bone is a living tissue, hard because it is rich in calcium and phosphorus salts, yet supplied with blood and nerves. The vertebral column, or backbone, is a chain of 33 small bones called vertebrae arranged in five regions: cervical, thoracic, lumbar, sacral and coccygeal. It holds the body upright, bears the weight of the head and trunk, and forms a bony tunnel that protects the delicate spinal cord running through it.

A joint is any point where bones meet. Immovable (fixed) joints, like the sutures between skull bones, allow no movement. Slightly movable joints, like those between adjacent vertebrae, allow only a little movement. Freely movable (synovial) joints allow large, smooth movements and come in several kinds: the ball-and-socket joint at the shoulder and hip turns in all directions, the hinge joint at the elbow and knee swings in one plane, the pivot joint between the first two neck vertebrae lets the head rotate, and the gliding joint between wrist bones lets surfaces slide.

Bones cannot move themselves; muscles move them. A skeletal muscle is joined to bone by tough cords called tendons, and a muscle can only pull, never push. So muscles are arranged in antagonistic pairs that pull in opposite directions. At the elbow, the biceps is the flexor: when it contracts it bends (flexes) the arm. The triceps is the extensor: when it contracts it straightens (extends) the arm. As one contracts the other relaxes, and this coordinated pairing produces controlled movement and, over the whole body, locomotion.