Rib: One of the 12 paired arches of bone that form the skeletal structure of the chest wall (the rib cage). The ribs attach to the vertebrae of the spine in the back. The 12 pairs of ribs consist of 7 pairs of ribs that attach to the sternum in the front and are known as true, or sternal, ribs; and 5 pairs of lower ribs that do not connect directly to the sternum and are known as false ribs. The upper 3 false ribs connect to the costal cartilages of the ribs just above them. The last 2 false ribs usually have no anchor in front and are known as floating, fluctuating, or vertebral ribs.