Thorax

                                                                      
                                                            Thorax
Thorax forms the upper part of the trunk of the body. It permits boarding and lodging of not only the thoracic viscera, but also provides necessary shelter to some of the abdominal viscera. The trunk of the body is divided by the diaphragm into an upper part, called the thorax, and a lower part, called the abdomen. The thorax is supported by a skeletal framework, thoracic cage. The thoracic cavity contains the principal organs of respiration - the lungs and of circulation - the heart, both of which are vital for life.
THE SKELETON OF THE THORAX
The skeleton of the thorax is also known as the thoracic cage. It is an osseocartilaginous, elastic cage which is primarily designed for increasing and decreasing the intrathoracic pressure, so that air is sucked into the lungs during inspiration and ex-pelled during expiration.
FORMATION
Anteriorly by the sternum . Posteriorly by the 12 thoracic vertebrae and the intervening intervertebral discs . On each side by 12 ribs with their cartilages. Each rib articulates posteriorly with the vertebral column. Anteriorly, only the upper seven ribs articu-late with the sternum through their cartilages and these are called true or vertebrosternal ribs. The costal cartilages of the next three ribs, i.e. the eighth, ninth and tenth, end by joining the next higher costal cartilage. These ribs are, therefore, known as vertebrochondral ribs. The costal cartilages of the seventh, eighth, ninth and tenth ribs form the costal margin. The anterior ends of the eleventh and twelfthribs 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 chon-drosternal joints permit movements of the thoracic cage during breathing.
SHAPE
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 supra-pleural membrane or Sibson's fascia. The broad or 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 shoul-ders, 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 reniform (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 quadrupeds, the anteroposterior diameter is greater than the transverse, . In infants, the ribs are horizontal and as a result the respiration is purely abdominal, by action of the diaphragm. In adults, the thorax is oval. The ribs are oblique and their movements alternately increase and de-crease the diameters of the thorax. This results in the drawing in of air into the thorax called inspiration and its expulsion is called expiration. This is called thoracic respiration. In the adult, we, therefore, have both abdominal and thoracic respiration. 
 
CLINICAL ANATOMY
 

1. The chest wall of the child is highly elastic, and fractures of the ribs are, therefore, rare. In adults, the ribs may be fractured by direct or indirect violence. In indirect violence, like crushing injury, the rib fractures at its weakest point located at the angle. The upper two ribs which are protected the clavicle, and the lower two ribs which are to swing are least commonly injured. 

2. A cervical rib is a rib attached to vertebra C7. It occurs in about 0.5% of subjects. Such a rib may exert pressure on the lower trunk of the brachial plexus which arches over a cervical rib. Such a person complains paraesthesiae or abnormal sensations along the ulnar border of the forearm, and wasting of the small muscles of the hand supplied by segment T1. Vascular changes may also occur. 3. In coarctation or narrowing of the aorta, the posterior intercostal arteries get enlarged greatly to provide a collateral circulation. Pressure of the enlarged arteries produces characteristic notching on the ribs.