The sternum is the medical name for the breastbone, a long, narrow, flat plate that forms the center of the front of the chest. It develops in three parts: an upper portion, or manubrium, a middle body, and a lower xiphoid process 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....

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    Full Sternum Description

    [Continued from above] . . . 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. The sternum is very strong and requires great force to fracture. The main danger in this type of injury is not the fracture itself, but the chance that the broken bone may be driven into the heart, which lies just behind it.