The Sternum, or Breast-Bone, is in the middle of the upper part of the anterior wall of the thorax. It articulates on each side with the upper seven ribs and the clavicle. It consists of three parts-the manubrium or presternum, the body, and the ensiform or xiphoid cartilage.

Manubrium

The Manubrium is separate from the body, although occasionally it is found to be fused. It is a flattened, four-sided bone, the upper end being wider than the lower. At the superior angles it articulates with the first rib on each side, and immediately in front of this articulation on the upper margin are the facets for articulation with the clavicles, between which is the suprasternal notch. At its junction with the body is found the articulation for the second rib.

The Body of the Sternum is a long-shaped flat bone, with its sides notched for articulation with the second to the seventh ribs inclusive. At its upper end it articulates with the manubrium, and at the lower end is attached the ensiform cartilage, a pointed process which is partly ossified.

The Sternum lies obliquely downwards and forwards in the front of the chest. At rest its upper end is opposite the lower border of the second dorsal vertebra, and its lower end opposite that of the tenth.

Fig. 31. - Sternum.

Ossification

Several centres are developed before birth, the whole becoming fused about the fortieth year.