The bones, 12 on each side, which enclose the cavity of the chest. The upper seven are joined to the breast-bone by their costal cartilages and are therefore known as true ribs. The lower five do not reach the breast-bone, and are therefore known as false ribs. Of the latter, the eighth, ninth and tenth are joined by their costal cartilages, each one to the rib immediately above it, while the 11th and 12th are free from any such connection and are therefore known as floating ribs. Each rib has a head, by which it is joined to the upper part of the body of the vertebra with which it corresponds, as well as to the vertebra immediately above. The greater part of the bone is made up of the shaft, which runs at first outwards and at the angle turns sharply forwards. On the lower margin of the shaft is a groove, which lodges the corresponding intercostal artery and nerve.