rib

A rib is one of the bones that form the thoracic cage in humans. There are 24 ribs, arranged in 12 pairs on the left and right sides. The upper seven pairs are true ribs, each with its own costal cartilage that attaches directly to the sternum. Pairs 8–10 are false ribs, attaching indirectly through cartilage of the rib above. Pairs 11–12 are floating ribs, which do not connect to the sternum.

A typical rib has a head, neck, tubercle, and a curved shaft. The head articulates with the bodies of two adjacent thoracic vertebrae at the costovertebral joints. The tubercle articulates with the transverse process of the same-numbered vertebra at the costotransverse joint. The shaft provides structural support and serves as an attachment for intercostal muscles and connective tissues. Costal cartilage connects most ribs to the sternum: true ribs attach directly via cartilage, false ribs attach to the cartilage of the rib above, and floating ribs have no anterior attachment.

Functionally, the rib cage protects thoracic organs such as the heart and lungs and supports respiration by allowing chest expansion during breathing. Ribs are involved in the mechanical action of the intercostal muscles and diaphragm. Clinically, rib injuries, particularly fractures, are common after trauma or in osteoporosis. Anatomical variations can occur, including cervical ribs or other supernumerary ribs in rare cases.

Developmentally, ribs arise from mesenchymal tissue and skeletal precursors that form part of the evolving thoracic skeleton. Evolutionarily, the rib cage contributes to the protection of viscera and the mechanics of breathing across mammals.