rib

Long, usually curved bone that extends laterally from the spine in vertebrates. Most fishes and many reptiles have ribs along most of the spine, but in mammals they are found only in the chest area. In humans, there are 12 pairs of ribs. The ribs protect the lungs and heart, and allow the chest to expand and contract easily.

At the rear, each pair is joined to one of the vertebrae of the spine. The upper seven (‘true’ or vertebro-sternal ribs) are joined by cartilage directly to the breast bone (sternum). The next three (‘false’ or vertebro-costal ribs) are joined by cartilage to the end of the rib above. The last two (‘floating ribs’) are not attached at the front. The diaphragm and muscles between adjacent ribs are responsible for the respiratory movements which fill the lungs with air.