Thorax is the anatomical region of the trunk between the neck and abdomen. Its chest wall surrounds a cavity containing the heart, each lung and major vessels. The diaphragm forms its lower boundary, while the mediastinum, the central compartment between the lungs, separates the two sides.
What forms the thoracic cage?
The thoracic cage is the framework of bone and cartilage supporting the chest wall. It consists of the sternum at the front, 12 pairs of ribs at the sides, and the thoracic vertebrae behind. Costal cartilages are the bars of cartilage connecting most ribs to the sternum directly or indirectly.
Each rib curves around the chest from its attachment to the vertebral column, or spine. Each thoracic vertebra belongs to the thoracic portion of that column and has joint surfaces for ribs. Most ribs also meet the transverse processes, the sideways projections of vertebrae.
The sternum has an upper manubrium, a central body and a lower xiphoid process. The sternal angle is the ridge at the junction of the manubrium and body; it marks the attachment of the second costal cartilage and helps orient rib numbering.
Ribs are grouped by their anterior attachments:
- True ribs, pairs 1–7, attach directly to the sternum through their own costal cartilages.
- False ribs, pairs 8–12, lack a direct cartilage connection to the sternum. The cartilages of pairs 8–10 usually join the cartilage above.
- Floating ribs, pairs 11–12, are also false ribs, but their anterior ends have no attachment to the sternum or other costal cartilages.
Movement at the rib joints raises or lowers the ribs during breathing, changing the dimensions of the thoracic cavity.
Where are the boundaries and openings of the thorax?
The superior thoracic aperture is the upper opening of the thoracic cage. Its boundaries are the first thoracic vertebra posteriorly, the first ribs and their cartilages laterally, and the upper border of the manubrium anteriorly. Structures pass through it between the neck and thorax.
The inferior thoracic aperture is the larger lower opening. It is bounded by the twelfth thoracic vertebra, the lower ribs, the costal margins and the junction between the sternal body and xiphoid process. The costal margins are the lower anterior borders formed by joined costal cartilages.
The diaphragm closes this lower opening but contains passages for vessels, nerves and the digestive tract. The thoracic cavity therefore communicates with neighbouring regions through defined routes rather than being a completely sealed space.
What lies in the intercostal spaces?
Intercostal spaces are the intervals between neighbouring ribs. Each space takes its name from the rib above it. The external, internal and innermost intercostal muscles form layers within these spaces and contribute to chest wall movement and support.
The main intercostal vein, artery and nerve run between the internal and innermost muscle layers. They lie chiefly in the costal groove, a channel along the lower internal border of the rib above, arranged from top to bottom as vein, artery and nerve. Smaller collateral branches run near the upper border of the rib below.
Intercostal nerves are the anterior rami, or anterior branches, of thoracic spinal nerves T1–T11 that supply the spaces. Posterior intercostal arteries supply all eleven spaces from behind, while anterior intercostal arteries supply the first nine spaces from the front. The internal thoracic artery descends beside the sternum and supplies anterior intercostal branches directly and through its musculophrenic branch, which follows the costal margin.
Where is the diaphragm, and what passes through it?
The diaphragm is a dome-shaped muscle separating the thoracic and abdominal cavities. Its muscular fibres attach to the lower sternum, lower ribs and upper lumbar vertebrae, the vertebrae of the lower back. These fibres converge on the central tendon, a strong, non-muscular sheet near its centre.
The right dome usually stands higher than the left because the liver lies beneath it. During inspiration, or breathing in, contraction lowers the diaphragm and increases the vertical dimension of the thoracic cavity. Its motor supply, which activates the muscle, comes from the phrenic nerves arising chiefly from cervical spinal nerves C3–C5 in the neck.
The principal passages are:
- Caval opening: lies in the central tendon at approximately T8, the eighth thoracic vertebral level, and transmits the inferior vena cava, the large vein returning blood from below the diaphragm.
- Oesophageal hiatus: a muscular opening at approximately T10, mainly within the right crus, a muscular attachment to the lumbar vertebrae. It transmits the esophagus, the tube carrying swallowed material to the stomach, and the vagal trunks, bundles of fibres from the vagus nerves.
- Aortic hiatus: lies behind the diaphragm at approximately T12. It transmits the aorta, the main artery supplying the body, and the thoracic duct, the main vessel returning lymphatic fluid to the venous circulation.
How are the lungs and pleural cavities arranged?
Each lung is covered by visceral pleura, a thin membrane closely applied to its surface. Parietal pleura lines the inner chest wall, diaphragm and lateral surface of the mediastinum. The pleural cavity is the potential space between these layers: normally, they are separated only by a thin film of fluid.
The right and left pleural cavities are separate. Each extends above the first rib into the root of the neck. Inferiorly, the pleura extends beyond the lung to form recesses, spaces into which the lung can expand during inspiration.
The right lung has upper, middle and lower lobes, separated by horizontal and oblique fissures, which are clefts in the lung surface. The left lung has upper and lower lobes separated by an oblique fissure. Its anterior border has a cardiac notch, an indentation accommodating the heart.
The hilum is the area on the medial surface, facing the mediastinum, where structures enter or leave a lung. The lung root is the bundle passing through this area, including a main bronchus, blood vessels, lymphatic vessels and nerves. The trachea, the central airway, divides into the main bronchi, the air passages leading to the lungs.
What are the divisions and contents of the mediastinum?
The mediastinum extends from the sternum to the vertebral column and from the superior thoracic aperture to the diaphragm. A transverse plane, an imaginary horizontal surface, through the sternal angle and the disc between T4 and T5 separates its superior and inferior portions. The inferior mediastinum is subdivided by its relationship to the pericardium, the sac enclosing the heart.
| Division | Position | Principal contents |
|---|---|---|
| Superior mediastinum | Above the sternal-angle plane | Thymus, brachiocephalic veins, upper superior vena cava, aortic arch and branches, trachea, esophagus, thoracic duct, vagus and phrenic nerves |
| Anterior mediastinum | Between sternum and pericardium, below the plane | Fat, connective tissue, lymph nodes and variable thymic remnants |
| Middle mediastinum | Region occupied by the pericardium, below the plane | Heart, pericardium, ascending aorta, pulmonary trunk, lower superior vena cava, terminal inferior vena cava, pulmonary veins, main bronchi and phrenic nerves |
| Posterior mediastinum | Behind the pericardium, below the plane | Descending thoracic aorta, esophagus, thoracic duct, azygos and hemiazygos veins, vagus nerves and thoracic splanchnic nerves |
The thymus is a lymphatic organ prominent in childhood, lying behind the sternum and extending across the superior and anterior divisions. Brachiocephalic veins unite to form the superior vena cava, which returns blood from the upper body. The azygos and hemiazygos veins drain much of the posterior chest wall; splanchnic nerves carry autonomic fibres involved in involuntary functions towards abdominal structures.
Where are the heart and great vessels positioned?
The heart lies obliquely in the middle mediastinum, behind the sternum and between the lungs. Its apex, the pointed tip formed by the left ventricle, faces forwards, downwards and leftwards. Its base, the posterior surface formed mainly by the left atrium, faces towards the esophagus.
The right ventricle forms most of the anterior surface, while the left ventricle forms most of the left border. The pulmonary trunk leaves the right ventricle and divides into arteries carrying blood to the lungs. Pulmonary veins return blood to the left atrium.
The ascending aorta leaves the left ventricle and curves into the aortic arch. The arch usually gives rise to the brachiocephalic trunk, the left common carotid artery and the left subclavian artery, supplying the head, neck and upper limbs. It continues as the descending thoracic aorta, initially along the left side of the vertebral column and approaching the midline as it descends towards the diaphragm.
In the 3D model beside the article, rotate the thorax and fade the chest wall to explore these spatial relationships.