Ribs. The ribs, twelve in number on each side, enter largely into the constitution of the bony framework of the thorax. They articulate behind with the dorsal vertebrae, the head or posterior extremity presenting two facets for articulation with the bodies of two contiguous vertebrae, while each rib also articulates with the transverse process of the lower of the two vertebrae with which the head is connected. Anteriorly the first seven ribs on each side are joined by their cartilages to the sternum (true ribs). The cartilages of the ribs below these are not inserted directly into the sternum (false ribs), and the eleventh and twelfth ribs are quite free anteriorly, whence their name of floating ribs. The spaces between the ribs are termed the intercostal spaces, and across these pass the two sets of muscular fibres called the external and internal intercostal muscles. The inferior border of each rib presents internally a groove, the subcostal groove, in which the intercostal vessels and nerve lie. Fracture of the ribs is not an uncommon result of violence, and this accident assumes a serious character if the underlying pleura and lung are involved in the injury.