Thorax
The thorax is the upper part of the trunk. It consists of an external musculoskeletal cage, the thoracic wall, and an internal cavity that contains the heart, lungs, oesophagus, trachea, thymus, the vagus and phrenic nerves and the right and left sympathetic trunks.
Inferiorly the thorax is separated from the abdominal cavity by the diaphragm, superiorly it communicates with the neck and the upper limbs.
The thoracic wall also offers protection to some of the abdominal viscera: the greater part of the liver lies under the right dome of the diaphragm; the stomach and spleen lie under the left dome of the diaphragm;
the posterior aspects of the superior poles of the kidneys lie on the diaphragm and are anterior to the twelfth rib on the right, and to the eleventh and twelfth ribs on the left.
Thoracic capacity is less in females than it is in males, both absolutely and proportionately: the female sternum is shorter, the thoracic inlet more oblique, and the suprasternal notch is level with the third thoracic vertebra (whereas it is level with the second in males).
The thoracic skeleton consists of twelve thoracic vertebrae and their intervening intervertebral discs (midline, posterior), twelve pairs of ribs and their costal cartilages (predominantly lateral) and the sternum (midline, anterior).
Laterally the thoracic cage is formed by the ribs, and anteriorly it is formed by the sternum and the distal parts of the ribs and their costal cartilages.
The first seven pairs of ribs are connected to the sternum by costal cartilages, the costal cartilages of the eighth to tenth ribs usually join the superjacent cartilage, and the eleventh and twelfth ribs are free (floating) at their anterior ends.
The ribs and costal cartilages are separated by intercostal spaces, which are deeper anteriorly and between the upper ribs. Each space is occupied by three layers of flat muscles and their aponeuroses and neurovascular bundles
The narrow thoracic inlet (superior
thoracic aperture) typically measures 5 cm
antero posteriorly and 10 cm transversely.
It is bounded by the first thoracic vertebral
body posteriorly, the medial border of the
first ribs on each side, and the superior
border of the manubrium sterni anteriorly.
It slopes down and forwards, so that the
apex of the lung extends upwards into the neck behind the anterior end of the first rib.
The broad thoracic outlet (inferior thoracic aperture) is limited posteriorly by the twelfth thoracic vertebral body, postero laterally by the twelfth rib and the distal end of the eleventh rib, antero laterally by the distal cartilaginous ends of the seventh to tenth ribs (which unite and ascend to form the costal margin), and anteriorly by the xiphoid process. It is wider in the transverse plane
than in the sagittal plane and slopes obliquely downward and backward, so that the thoracic cavity is deeper posteriorly than anteriorly.
The horizontal plane that passes through the intervertebral disc between the fourth and fifth thoracic vertebrae posteriorly and the manubriosternal joint (sternal angle or angle of Louis) at the level of the second costal cartilage anteriorly is a useful aid to orientation in the thorax. It is the level at which the trachea bifurcates into right and left main bronchi.
MEDIASTINUM
The mediastinum lies between the right and left pleural sacs in and near the median sagittal plane of the chest. It extends
from the sternum anteriorly to the vertebral column behind. A horizontal plane passing through the manubriosternal joint and the intervertebral disc between the fourth and fifth thoracic vertebrae separates the mediastinum into superior and inferior portions.
