The thoracic cage
The thoracic cage is formed by the sternum and costal cartilages in front, the vertebral column behind and the ribs and intercostal spaces laterally.
It is separated from the abdominal cavity by the diaphragm and communicates superiorly with the root of the neck through the thoracic inlet (Fig. 1.1).
The ribs (Fig. 1.1)
- Of the 12 pairs of ribs the first seven articulate with the vertebrae posteriorly and with the sternum anteriorly by way of the costal cartilages (true ribs).
- The cartilages of the 8th, 9th and 10th ribs articulate with the cartilages of the ribs above ( false ribs).
- The 11th and 12th ribs are termed ‘floating’ because they do not articulate anteriorly ( false ribs).
Typical ribs (3rd–9th)
These comprise the following features (Fig. 1.2):
- A head which bears two demifacets for articulation with the bodies of: the numerically corresponding vertebra, and the vertebra above (Fig. 1.4).
- A tubercle which comprises a rough non-articulating lateral facet as well as a smooth medial facet. The latter articulates with the transverse process of the corresponding vertebra (Fig. 1.4).
- A subcostal groove: the hollow on the inferior inner aspect of the shaft which accommodates the intercostal neurovascular structures.
Atypical ribs (1st, 2nd, 10th, 11th, 12th)
- The 1st rib (see Fig. 63.2) is short, flat and sharply curved. The head bears a single facet for articulation. A prominent tubercle (scalene tubercle) on the inner border of the upper surface represents the insertion site for scalenus anterior. The subclavian vein passes over the 1st rib anterior to this tubercle whereas the subclavian artery and lowest trunk of the brachial plexus pass posteriorly.
A cervical rib is a rare ‘extra’ rib which articulates with C7 posteriorly and the 1st rib anteriorly. A neurological deficit as well as vascular insufficiency arise as a result of pressure from the rib on the lowest trunk of the brachial plexus (T1) and subclavian artery, respectively (Fig. 1.3).
- The 2nd rib is less curved and longer than the 1st rib.
- The 10th rib has only one articular facet on the head.
- The 11th and 12th ribs are short and do not articulate anteriorly.
They articulate posteriorly with the vertebrae by way of a single facet on the head. They are devoid of both a tubercle and a subcostal groove.
The sternum (Fig. 1.1)
- The sternum comprises a manubrium, body and xiphoid process.
- The manubrium has facets for articulation with the clavicles, 1st costal cartilage and upper part of the 2nd costal cartilage. It articulates inferiorly with the body of the sternum at the manubriosternal joint.
- The body is composed of four parts or sternebrae which fuse between 15 and 25 years of age. It has facets for articulation with the lower part of the 2nd and the 3rd to 7th costal cartilages.
- The xiphoid articulates above with the body at the xiphisternal joint. The xiphoid usually remains cartilaginous well into adult life.
These are bars of hyaline cartilage which connect the upper seven ribs directly to the sternum and the 8th, 9th and 10th ribs to the cartilage immediately above.
Joints of the thoracic cage (Figs 1.1 and 1.4)
- The manubriosternal joint is a symphysis. It usually ossifies after the age of 30.
- The xiphisternal joint is also a symphysis.
- The 1st sternocostal joint is a primary cartilaginous joint. The rest (2nd to 7th) are synovial joints. All have a single synovial joint except for the 2nd which is double.
- The costochondral joints (between ribs and costal cartilages) are primary cartilaginous joints.
- The interchondral joints (between the costal cartilages of the 8th, 9th and 10th ribs) are synovial joints.
- The costovertebral joints comprise two synovial joints formed by the articulations of the demifacets on the head of each rib with the bodies of its corresponding vertebra together with that of the vertebra above. The 1st and 10th–12th ribs have a single synovial joint with their corresponding vertebral bodies.
- The costotransverse joints are synovial joints formed by the articulations between the facets on the rib tubercle and the transverse process of its corresponding vertebra.
(Source: Anatomy At A Glance)