The thoracic cage (rib cage) is the skeletal framework of the thoracic wall, which encloses the thoracic cavity. The thoracic cage consists of the 12 thoracic vertebrae, the associated intervertebral discs, 12 pairs of ribs with their costal cartilages, and the sternum.
The shape of the thoracic cage is like a domed bird cage with the horizontal bars formed by ribs and their associated costal cartilages. The thoracic cage is supported anteriorly by the sternum, and posteriorly by the thoracic part of the vertebral column.
Because cartilaginous structures such as the intervertebral discs (between the thoracic vertebrae) and costal cartilages participate in forming the thoracic cage, it can be also described as an osteocartilaginous cage.
The main functions of the thoracic cage include:
The sternum is a bone that is located in the anterior midline of the thorax, supporting the thoracic cage anteriorly. The three parts of the sternum are:
There are twelve vertebrae forming the thoracic part of the vertebral column. This part of the vertebral column supports the thoracic cage posteriorly.
A special characteristic of the thoracic vertebrae is that they articulate with ribs. The typical thoracic vertebra has a somewhat heart-shaped vertebral body when viewed from above and a circular vertebral foramen. The vertebral body of a typical thoracic vertebra has two partial facets (superior and inferior costal facets) on each side for articulation with the head of its corresponding rib and the head of the rib below. Each transverse process of these vertebrae has a transverse costal facet that articulates with the tubercle of its corresponding rib.
The ribs are long, flat, curved bones that form most of the thoracic cage. There are twelve pairs of ribs, all of which articulate with the vertebral column, while only the first seven ribs directly articulate with the sternum. The ribs also provide attachment sites for thoracic muscles.
The ribs can be classified based on their articulations:
The thoracic cage presents with spaces between adjacent ribs, which are called intercostal spaces. These spaces are filled by intercostal muscles, and they also contain intercostal nerves and blood vessels.
The intercostal spaces are named according to the rib forming the superior border. For example, the space below the 2nd rib (between the 2nd rib and 3rd rib) is called the 2nd intercostal space. Therefore, there are 11 intercostal spaces in the thoracic cage. The space below the 12th rib is called the subcostal space.
Eleven intercostal nerves and two sets (main and collateral) of intercostal blood vessels are also named by the same number assigned to the according intercostal space.
There are several joints that participate in forming the thoracic cage, including: