Sternum

The sternum, or breastbone, is a long, flat bone running down the center of the chest. It forms the front of the rib cage by connecting to the ribs through cartilage, shielding the heart, lungs, and major blood vessels. Its shape is somewhat like a necktie, making it one of the body's largest and longest flat bones. The sternum has three main sections: the manubrium at the top, the body in the middle, and the xiphoid process at the bottom. The word "sternum" comes from the Ancient Greek *stérnon*, meaning "chest."

The bone is narrow and flat, sitting in the middle of the chest's front. Its top supports the collarbones (clavicles), and its edges join with the costal cartilages of the first two rib pairs. The inner surface anchors the sternopericardial ligaments, while the top connects to the sternocleidomastoid muscle. In its natural position, the sternum angles obliquely downward and forward. It is slightly convex in front and concave behind, broad at the top like a "T," then narrows where the manubrium meets the body, widens a bit below the middle of the body, and finally narrows toward the bottom. It is usually longer in males than in females.

The manubrium, Latin for "handle," is the broad upper part of the sternum. It is quadrangular, narrowing from the top, with four borders. In the middle of its widest upper part is the suprasternal notch (jugular notch), which can be felt between the two clavicles. On either side of this notch are the right and left clavicular notches. The manubrium joins with the body of the sternum, the clavicles, and the cartilages of the first one and a half pairs of ribs. The joint between the manubrium and the body, called the manubriosternal joint, is where the sternal angle is located. This angle can often be felt on the body's surface. The manubrium's lower border is oval and rough, covered with a thin cartilage layer for articulation with the body. Its lateral borders each have a depression above for the first costal cartilage and a small facet below that, together with a similar facet on the body's upper angle, forms a notch for the second rib's costal cartilage. Between these points is a narrow, curved edge sloping downward and inward. The superior sternopericardial ligament attaches the pericardium to the manubrium's back side.

The body, or gladiolus, is the longest part of the sternum. It is flat, with only a front and back surface. The front surface faces upward and forward and is marked by three transverse ridges opposite the third, fourth, and fifth articular depressions. The pectoralis major muscle attaches to it on both sides. Occasionally, a sternal foramen of varying size appears at the junction of the body's third and fourth parts. The back surface, slightly concave, also has three less distinct transverse lines; from its lower part on each side, the transversus thoracis muscle originates. The sternal angle, where the body meets the manubrium, can be felt where the sternum projects farthest forward, though in some people it is concave or rounded. This angle is a useful landmark during physical exams because the second rib attaches here. Each outer border has a small facet at its upper angle that, with a similar facet on the manubrium, forms a cavity for the second rib's cartilage. Below this are four angular depressions for the cartilages of the third through sixth ribs. The lower angle has a small facet that, with one on the xiphoid process, forms a notch for the seventh rib's cartilage. These depressions are separated by curved intervals that shorten from top to bottom, matching the intercostal spaces. Most true rib cartilages articulate with the sternum at the junctions of its primitive segments, which is clearer in some other vertebrates where the bone parts stay separated longer. The body's upper border is oval and articulates with the manubrium at the sternal angle, while its narrow lower border articulates with the xiphoid process.

At the sternum's lower end is the pointed xiphoid process. Improper chest compressions during CPR can snap it off, driving it into the liver and causing fatal hemorrhage. The sternum is made of highly vascular tissue covered by a thin layer of compact bone, thickest in the manubrium between the clavicular articular facets. The inferior sternopericardial ligament attaches the pericardium to the back of the xiphoid process.

The cartilages of the top five ribs join the sternum at the sternocostal joints. The right and left clavicular notches articulate with the corresponding clavicles. The second rib's costal cartilage articulates at the sternal angle, making it easy to locate. The transversus thoracis muscle, innervated by an intercostal nerve, attaches superiorly to the lower sternum's back surface and inferiorly to the internal surface of costal cartilages two through six, working to depress the ribs.