Sternum, or breastbone, is flat, dagger-shaped bone that forms the front, middle part of the chest wall. It is about 7 inches long in men, and 1.5 inches long in women, and 1.5 inches across at its widest part.

The sternum is made up of three segments: the widest, the top part (the manubrium), also known as the handle;; the body of the sternum; and the tip (the xiphoid process or xiphisternum). In children, the three parts of the sternum are joined. In adults they are fused to form one continuous bone.

Each side of the sternum is known as a hemisternum and is curved in two directions: boat-shaped along the ventral (front) surface of the embryo, and curved away from the midline. As the ventral surface of the embryo closes, so the two hemisterna move closer together and fuse, at least at the cranial end. There is considerable species difference in the degree of fusion. Anomalies of closure of the two hemisterna, and of fusion of the last two true ribs are relatively common occurences.

As the sternum grows there is considerable variation in shape and size between species. In man, the sternum expands to form a flat plate of the bone that might be considered important in protection of the thorax. The sternum joins with the collarbones (clavicles) at the top, and seven pairs of ribs are attached to it. The sternum is extremely strong to protect the heart from injury. The same structure is elongated, thin, and clearly reflects its segmental origin in the dog, while in the horse and cow the sternum retains its original boat-shaped appearance and even grows to form a ventral projection akin to a keel that serves for muscle attachment.

In flying birds this bone is keel-shaped to which are attached the pectoral muscles associated with flight. It is connected to the shoulder girdle, and the ventral ends of the ribs are attached along its length.

Sternum Anomalies: Pectus excavatum

Anomalies of the sternum may produce a protruberant, pigeon-chested appearance or a sunken, funnel-chested appearance. These relatively common anomalies do not usually cause any major problems.

Pectus excavatum (congenital chondrosternal depression or funnel chest) is a concave deformity of the sternum and associated bentral thoracic wall (front chest). It has been reported in humans, dogs, cats, cattle and various exotic animals. It may occur with various lysosomal storage diseases both in humans and animals. In most cases, the cause of this anomaly remains unclear.

Cleft Sternum
Failure in the normal midline fusion of cartilage in the early embryo can result in cleft sternum. This is a rare congenital chest wall defect. A cleft may leave the heart and great vessels unprotected, which particularly is serious in the very rare occurence of a complete cleft. Sternal cleft is often associated with other malformations such as cardiovascular malformations. However, cleft sternum can occur without other abnormalities.