The thoracic cage includes the thoracic vertebrae, sternum, ribs, and costal cartilage (see Figure 1). The sternum (breastbone) consists of three fused bones: the manubrium, body, and xiphoid process. There are 12 pairs of ribs. All ribs articulate posteriorly with a corresponding thoracic vertebra. At their anterior ends, they differ as to how they attach, as follows:
- Seven pairs of true ribs (vertebrosternal ribs) attach directly to the sternum with hyaline cartilage called costal cartilage.
- Three pairs of false ribs (vertebrochondral ribs) do not attach to the sternum. Rather, they connect (with costal cartilage) to the rib directly above them.
- Two pairs of false ribs (floating ribs or vertebral ribs) do not attach to anything at their anterior ends.
Figure 1. The thoracic cage.
Here are important features of a rib:
- The head is the end of the rib that articulates with the vertebral column.
- The superior and inferior facets on the head articulate with the facets of the thoracic vertebrae.
- The neck, just beyond the head, bears a tubercle (rounded process) that articulates with the facet of the vertebral transverse process. Part of the tubercle also presents a place of attachment for ligaments.
- The costal angle designates the sharp turn of the rib.
- The costal groove, a passageway on the inside of the bending rib, provides for blood vessels and intercostal nerves.
- The body (shaft) is the major part of the rib—that part beyond the costal angle.
- Intercostal spaces, the areas between the ribs, are occupied by the intercostal muscles.