The sternum, commonly known as the breastbone, is a long, flat bone that forms the central structure of the anterior chest wall. It is a fundamental component of the skeletal system, supporting the ribs and providing a protective barrier. This flat bone is often likened to a sword or a necktie in shape, situated vertically in the midline of the body, establishing the framework of the thoracic cage.
Placement in the Body
The breastbone is centrally located in the anterior part of the chest, spanning vertically from the neck down toward the upper abdomen. This position places it directly in front of the delicate organs of the mediastinum, the central compartment of the chest cavity. In adults, the sternum is approximately six to seven inches long and serves as the single anchor point where the ribs meet in the front. Its midline position makes it an important anatomical reference point for medical procedures.
Structure: The Three Distinct Parts
The sternum is not a single, continuous bone but rather a fusion of three distinct segments: the manubrium, the body, and the xiphoid process. These parts develop separately and gradually fuse together with age.
The manubrium is the superior, widest, and thickest part of the sternum, resembling the handle of a sword. It features a concave depression at its top border called the jugular notch, which is often visible and palpable at the base of the neck. Laterally, the manubrium contains notches where the clavicles, or collarbones, and the cartilages of the first pair of ribs attach.
Inferior to the manubrium is the body, which is the longest and flattest section. The junction where the manubrium and the body meet forms a slight forward angle, known as the sternal angle or the Angle of Louis. This ridge is an easily identifiable landmark that corresponds precisely to the attachment point of the second pair of ribs. The body of the sternum provides articulation points for the costal cartilages of the third through seventh pairs of ribs.
The final, most inferior segment is the xiphoid process, a small, pointed projection that is primarily cartilaginous in youth. It gradually ossifies, or turns into bone, typically beginning around the age of 40. The articulation between the body and the xiphoid process is called the xiphisternal joint, which is another useful anatomical reference.
Essential Functions of the Sternum
The sternum performs two primary roles: serving as a shield for internal organs and acting as a central anchor for the thoracic skeleton. Its rigid yet slightly flexible structure allows it to withstand external forces directed at the chest. The sternum, along with the ribs and the vertebral column, forms the bony cage that protects the heart, lungs, and the great blood vessels. The flat, broad surface helps disperse impact energy, safeguarding the vital structures immediately behind it.
The sternum also functions as the central attachment site for the entire anterior portion of the rib cage and the shoulder girdle. The first seven pairs of ribs connect to the sternum either directly or indirectly via strips of hyaline cartilage, known as costal cartilages. This attachment provides necessary stability to the chest wall while still allowing for the slight movement required during breathing.
Beyond its role in the skeletal framework, the sternum provides attachment points for several muscles, including the pectoralis major and certain neck and abdominal muscles. The xiphoid process specifically serves as an insertion point for the diaphragm, the dome-shaped muscle that is the main driver of respiration.