The xiphoid process of the sternum is the lower portion of the sternum, or breastbone that projects down. The xiphoid process begins as a piece of cartilage. It slowly hardens into bone until, by middle life, it is usually fused to the body of the sternum. The sides of the manubrium and the body are notched where they unite with costal cartilages. It also joins the clavicles (shoulder blades) on its upper border. It usually remains a separate bone until middle age or later, when it fuses to the body of the sternum.