Sternum

This article is about the vertebrate sternum in general. For other uses, see Human sternum and Sternum (arthropod anatomy).

The sternum or breastbone, in vertebrate anatomy, is a flat bone that lies in in the middle front part of the rib cage. It is endochondral in origin . It probably first evolved in early tetrapods as an extension of the pectoral girdle; it is not found in fish. In amphibians and reptiles it is typically a shield-shaped structure, often composed entirely of cartilage. It is absent in both turtles and snakes. In birds it is a relatively large bone and typically bears an enormous projecting keel to which the flight muscles are attached. Only in mammals does the sternum take on the elongated, segmented form seen in humans. In some mammals, such as opossums, the individual segments never fuse and remain separated by cartilagenous plates throughout life.

In arachnids, the sternum is the ventral (lower) portion of the cephalothorax. It consists of a single sclerite situated between the coxa, opposite the carapace.