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:
- supporting the thorax,
- protecting internal thoracic organs from injury,
- resisting the negative pressures that are generated by the elastic recoil of the lungs and movements induced by respiration,
- providing attachment for the upper limbs,
- supporting the weight of the upper limbs,
- providing the attachment sites for many muscles that participate in moving and positioning the upper limbs in relation to the trunk.
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:
- manubrium of the sternum - the most superior part that articulates with clavicles and the first pair of ribs;
- body of the sternum - the middle part that articulates with 3rd to 7th ribs;
- xiphoid process - a small cartilaginous extension of the lower sternum.
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:
- ribs 1 to 7 are considered the true or vertebrosternal ribs, which attach directly to the sternum via their own costal cartilage;
- ribs 8 to 10 are the false or vertebrochondral ribs, which are connected to the sternum indirectly via the cartilage of the rib above them;
- floating or free - ribs 11 and 12, which do not articulate with other ribs, costal cartilages or sternum; their cartilages tend to end within the abdominal musculature.
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:
- intervertebral joints - between adjacent vertebrae;
- sternoclavicular joints - between the manubrium of sternum and the clavicles;
- sternochondral joints - between the sternum and costal cartilages of ribs 1 to 7;
- manubriosternal joint - between the manubrium and body of sternum;
- xiphisternal joint - between the xiphoid process and the body of sternum;
- costochondral joints - between costal cartilages and ribs;
- costovertebral joints - formed by ribs and the bodies of corresponding vertebrae;
- interchondral joints - joining the costal cartilages with each other.