One of a series of slender curved bones that form a cage to enclose, support, and protect the heart and lungs (see thorax). Ribs occur in pairs, articulating with the thoracic vertebrae of the spinal column at the back and (in reptiles, birds, and mammals) with the sternum (breastbone) in front. Movements of the rib cage, controlled by intercostal muscles between the ribs, are important in breathing (see respiratory movement).