Difference Between Sternum And Ribs
The sternum, or breastbone, looks like a dagger pointing downward. To the “handle” of the dagger is hinged the narrower “blade,” consisting of four fused segments. The tip, or point, of the dagger consists of cartilage in youth, but becomes bony in middle life.
The cartilages of the first seven ribs are attached to the sides of the sternum. The two clavicles, or collarbones, are joined to its upper part. The wide “handle” of the sternum protects the blood vessels that diverge from the heart. The “blade” and adjacent rib cartilages guard the heart.
The ribs consist of 12 paired bows of bone that enclose the chest and upper abdomen. Each pair is attached to one of the 12 thoracic vertebrae. To their ends are fastened elastic bars known as rib cartilages.
The first seven pairs of rib cartilages connect to the breastbone, at the front of the chest; the ribs to which they are attached are known as the true ribs. Of the next five pairs of ribs, called the false ribs, the cartilages of each of the first three are attached to the ribs immediately above them. In the last two pairs of ribs—also known as floating ribs—the cartilages are reduced to short, pointed tips that are not fastened to anything. The floating ribs extend only halfway around the body.
When air is inhaled during breathing, the ribs are lifted upward and swing out; the lower part of the breastbone swings forward, too. These motions increase the girth of the chest. At the same time, the diaphragm, the domed muscular partition that separates the thorax above from the abdomen below, contracts. This movement lessens its curvature and lengthens the up-and-down dimension of the chest. The resulting increase in chest size allows air to be drawn into the lungs through the trachea, or windpipe.
Expiration (breathing out) occurs automatically. Normally the muscles that move the ribs relax, and the elastic recoil of the rib cartilages brings the ribs back to their former position. At the same time, the diaphragm also relaxes and again arches upward. Together, these movements diminish the chest size. The lungs contract, air is forced out through the windpipe, and the breathing cycle is complete.
Not only do the ribs play an important part in breathing, but they also shield the heart and lungs. Some of the abdominal organs, including the stomach, liver, and kidneys, are protected by the lower ribs.