The Thoracic Cage: Ribs

The thoracic cage comprises the sternum, twelve pairs of ribs and their costal cartilages.

The first seven pairs, the true or vertebrosternal ribs, directly attach to the sternum through their anterior cartilaginous extensions or the costal cartilages.

The remaining five pairs, the false or vertebrochondral ribs, either join the costal cartilages above them or are free-floating with their costal cartilages embedded in the chest wall. 

A typical rib has a head, neck, and body. A Tubercle arising from the short neck joins with the transverse costal facet of the same-numbered vertebra.

In each rib, the articular facet in the head joins the costal facets of two thoracic vertebrae, the same numbered and the one above it.

The body has an internal costal groove for the blood vessels and nerves to pass. Its external surface attaches to the muscles of the pelvic girdle and trunk.

The intercostal muscles between the ribs contract and relax during respiration, changing the width and depth of the thoracic cage. This helps to increase the volume of the thoracic cavity while breathing in, and decrease it while breathing out.