The thorax is anatomical structure supported by a skeletal framework (thoracic cage) and contains the principal organs of respiration (lungs) and circulation (heart), both of which are vital for life.
It 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.
It is formed anteriorly by the sternum, posteriorly by the 12 thoracic vertebrae and the intervening intervertebral discs and on each side: by ribs with their cartilages.
Each rib articulates posteriorly with the vertebral column.
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 form the costal margin.
The anterior ends of the 11th and 12th ribs are free: these are called floating or vertebral ribs.
The vertebrochondral and vertebral ribs i.e., the last five ribs, are also called false ribs because they do not articulate with the sternum.
The costovertebral, manubriosternal and chondrosternal joints movements of the thoracic cage during breathing.
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 which is deeply concave downwards.
The thoracic cavity is actually much smaller than what it appears to be because the narrow upper part appears broad due to the shoulders, and the lower part is greatly encroached upon by the abdominal cavity due to the upward convexity of the diaphragm.
In transverse section the thorax is bean shaped or kidney shaped. The transverse diameter is greater than the anteroposterior diameter. However, in infants below the age of two years it is circular.
In infants the ribs are horizontal and as result the respiration is purely 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.
In the adults, both abdominal and thoracic respiration are present.
Thorax has following joints.
This is a secondary cartilaginous joint. It permits slight movements of the body of the sternum on the manubrium during respiration. In about 10% of subjects, the joint may be obliterated by fusion of the two bones.
The head of a typical rib articulates with its own vertebra, and also with the body of the next higher vertebra, to form two plane synovial joints separated by an intra-articular ligament. This ligament is attached to the ridge on the head of the rib and to the intervertebral disc.
The tubercle of a typical rib articulates with the transverse process of the corresponding vertebra to form a synovial joint. The capsular ligament is strengthened by three costotransverse ligaments. The superior costotransverse ligament has two laminae which extend from the crest on the neck of the rib to the transverse process of the vertebra above.
The inferior costotransverse ligament passes from the posterior surface of the neck to the transverse process of its own vertebra. The lateral costotransverse ligament connects the lateral nonarticular part of the tubercle to the tip of the transverse process.
The articular facets on the tubercles of the upper 6 ribs are convex, and permit rotation of the rib neck for pump-handle movements of these ribs. The articular surfaces of the 7th to 10th ribs are flat, permitting up and down gliding movements for the bucket-handle movements of the lower 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 in two parts. The joints of 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 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.
Text adapted from: Human Anatomy by BD Chaurasia
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