Sternum (breastbone): structure, function, and comparative anatomy

The sternum, or breastbone, is a central flat bone of the chest that protects thoracic organs and anchors ribs and muscles; this article covers its parts, development, variations across vertebrates, and clinical relevance.

The sternum, commonly called the breastbone, is a flattened bone forming the central front of the thorax in many vertebrates. In human and other mammalian anatomy it provides a midline attachment for the ribs and several chest muscles, and helps protect the heart and lungs. For an overview of related skeletal structures see vertebrate and anatomy resources.

Structure and parts

The adult sternum typically consists of three regions: the superior manubrium, the elongated body (or gladiolus), and the inferior xiphoid process. The anterior surface shows costal notches where the true ribs connect by cartilage. Internally it may show grooves for major blood vessels and muscle attachments. These parts often fuse over time through ossification and growth.

Key components include:

Manubrium: articulates with the clavicles and first ribs;
Body: central segment with multiple costal articulations;
Xiphoid process: a small, variable cartilaginous or bony tip.

Ossification patterns vary among species and individuals. In many vertebrates the sternum begins as cartilage before mineralizing. In some groups it remains cartilaginous for most of life, while in others it forms distinct bony plates.

Comparative anatomy shows wide diversity: in amphibians the sternum is often a shield-like cartilage (amphibians), and in many reptiles it follows a similar form (reptiles) with a characteristic shield-shaped element. Cartilaginous composition is common (cartilage). Turtles lack a conventional sternum because their shell incorporates the rib elements (turtles), and snakes also do not have a separate breastbone (snakes). Birds possess an enlarged sternum with a keel for flight muscle attachment (birds) that supports powerful flight muscles. Mammals show the elongated sternum form seen in humans (mammals).

Functionally, the sternum contributes to thoracic stability, protects vital organs, and serves as an anchor for respiration-related muscles. Clinically it is significant in trauma (sternal fractures), surgical access (median sternotomy), and developmental variants (sternal clefts or accessory ossicles). Understanding its variation across species helps in fields from comparative anatomy to veterinary medicine and paleontology.