The Sternum or breastbone is a blade-shaped, elongated bone that closes the front of the thorax and is part of the axial skeleton.  The sternum is formed from two fused sections of bone and, at the lowest tip, the xiphoid process which is made of cartilage. 

The top of the sternum articulates with the two clavicles, or collarbones, which are part of the pectoral (shoulder) girdle. This bony attachment links the axial skeleton to the appendicular skeleton.

The first seven pairs of ribs, which have costal cartilage at their ends, also articulate with the sternum.  The cartilage creates a degree of elasticity in the structure of the ribcage, which expands and contracts during breathing. The thoracic cavity can expand in three directions during breathing but the amount that it is able to do so is dependent on the person's poise and the degree of freedom available in the intercostal muscles and ribcage.

A number of muscles from the neck, thorax and abdomen originate from the somewhat wrinkled anterior surface of the sternum. Upper arm muscles originate from the top and abdominal muscles originate from the lower section of the smooth posterior surface of the sternum.

The fact that so many bones and muscles are attached to the sternum, is a good example of how the different parts of the body are linked and interdependent, so that how we use one part, effects the rest of the body. The angle at which the sternum is held is altered by the way we each use our body so, for instance, if we sit in a very slumped manner or have a spinal condition such as kyphosis, the sternum will be angled a long way forwards. This will impact on the poise of the neck and back, plus the functioning of the arms, internal organs and ribcage, and therefore our breathing.