The thoracic cage is the skeletal framework that encloses the contents of the thorax. The thorax contains organs of respiration and circulation. Functioning of both of these is very vital for life.
Structure of Thoracic Cage
Thoracic cage is formed by bones and cartilage [osseocartilaginous]. It is an elastic cage which is primarily designed for increasing and decreasing the intrathoracic pressure so that air is sucked into the lungs during inspiration and expelled during expiration.
Thoracic cage is formed anteriorly by the sternum, posteriorly by the 12 thoracic vertebrae and the intervening intervertebral discs and on each side. The ribs on both the sides complete the cage.
Each rib articulates posteriorly with the vertebral column.
The thorax resembles a truncated cone which is narrow above and broad below. The narrow upper end is continuous with the root of the neck from which it is partly separated by the suprapleural membrane or Sibson’s fascia [a covering on the lung].
The broad lower end is almost completely separated from the abdomen by the diaphragm.
In transverse section, the thorax is bean shaped or kidney-shaped except in in infants below the age of two years, where it is circular.
Anteriorly, only the upper seven ribs articulate with the sternum through their cartilages. These are called true or vertebrosternal ribs.
The costal cartilages of the next three ribs (i.e., 8th, 9th, and 10th) end by joining the next higher costal cartilage. These ribs are, therefore, known as vertebrochondral ribs.
The costal cartilages of the 7th, 8th, 9th and 10th, ribs forms the costal margin.
The anterior ends of the 11th and 12th ribs are free and these ribs are termed as floating ribs. Some authors also call them vertebral ribs.
The last five ribs are collectively called false ribs. This includes both the vertebrochondral and vertebral ribs. It is so because they do not articulate with the sternum.
The costovertebral, manubriosternal and chondrosternal joints movements of the thoracic cage during breathing.
In infants the ribs are horizontal. Therefore the respiration is only by action of the diaphragm.
In adults the thorax is oval. The ribs are oblique and their movements alternately increase and decrease the diameters of the thorax. This results in the drawing in of air into the thorax (inspiration) and its expulsion (expiration). This is called thoracic respiration.
Adults have both diaphragmatic and thoracic respiration
Joints of Thorax
Thoracic cage has following joint
- Between manubrium and sternum
- Secondary cartilaginous joint.
- Only slight movements of the body of the sternum on the manubrium during respiration.
- Fused in 10%
- Between the head of a typical rib and two vertebrae to form two-plane synovial joints
- One vertebra is of the same level as that of rib
- Other is the body of the next higher vertebra
- Separated by an intra-articular ligament
- Ligament attached to the ridge on the head of the rib and to the intervertebral disc.
- Synovial joint
- Between the tubercle of a typical rib and the transverse process of the corresponding vertebra
- Capsular ligament
- three costotransverse ligaments
- Superior costotransverse ligament
- Extends from the crest on the neck of the rib to the transverse process of the vertebra above
- has two laminae
- Inferior costotransverse ligament
- From the posterior surface of the neck to the transverse process of its own vertebra.
- Lateral costotransverse ligament
- Connects the lateral nonarticular part of the tubercle to the tip of the transverse process.
- Upper six ribs
- Articular facets on the tubercles of the upper 6 ribs are convex
- Permit rotation of the rib neck for pump-handle movements of these ribs.
- 7th to 10th ribs are
- Flat facets
- Permitting up and down gliding movements for the bucket-handle movements of the lower ribs.
- Upper six ribs
The first chondrosternal joint is a primary cartilaginous joint. It does not permit any movement. This helps in the stability of the shoulder girdle and of the upper limb.
The 2nd to 7th costal cartilages articulate with the sternum by synovial joints. Each joint has a single cavity except in the second joint where the cavity is divided into two parts. The joints of the thorax are held together by the capsular and radiate ligaments.
The cartilaginous joints between the sternal end of ribs and the lateral ends of costal cartilages.
Each rib has a depression shaped like a cup that the costal cartilage articulates with.
There is normally no movement at these joints of the thorax.
The 5th to 9th costal cartilages articulate with one another by synovial joints. The tenth cartilage is united to the ninth by fibrous tissue.