Thorax

The thorax is an irregularly shaped cylinder with a superior thoracic aperture opening to the neck and an inferior thoracic aperture closed by the diaphragm. Its wall is composed of vertebrae, ribs, and muscles, with the sternum anteriorly. The cavity is divided into left and right pleural cavities (surrounding lungs) and the mediastinum.

The mediastinum is a median partition containing the heart, esophagus, trachea, major nerves, and major systemic blood vessels. It separates the left and right pleural cavities.

Each pleural cavity surrounds a lung and is lined by the pleura. They are completely separated by the mediastinum, meaning issues in one cavity don't necessarily affect the other. They extend above rib I into the root of the neck.

The thorax facilitates breathing by housing the lungs and providing the necessary machinery: the diaphragm and thoracic wall. Movements of the diaphragm and ribs alter the thoracic cavity volume, enabling air movement.

The thorax protects vital organs like the heart, lungs, and great vessels. The diaphragm's dome shape also protects abdominal viscera such as the liver, stomach, and spleen.

The mediastinum serves as a conduit for structures passing between the thorax, neck, and abdomen, including the esophagus, vagus nerves, thoracic duct, and phrenic nerves.

The thoracic wall consists of: posteriorly, twelve thoracic vertebrae; laterally, twelve ribs and intercostal muscles; and anteriorly, the sternum (manubrium, body, xiphoid process).

Ribs articulate with thoracic vertebrae posteriorly. Ribs II-IX have three articulations: with their own vertebra, the vertebra above, and the transverse process. Ribs I-VII articulate with the sternum via costal cartilage. Ribs VIII-X articulate with the cartilage above. Ribs XI and XII are floating ribs.

The superior thoracic aperture is formed by the body of vertebra TI posteriorly, the medial margin of rib I laterally, and the manubrium of the sternum anteriorly. It is obliquely angled, facing anteriorly.

The inferior thoracic aperture is formed by vertebra TXII posteriorly, ribs XI and XII posterolaterally, the costal margin (ribs VII-X) anterolaterally, and the xiphoid process anteriorly. It is closed by the diaphragm.

The musculotendinous diaphragm seals the inferior thoracic aperture. Its contraction decreases dome height, increasing thoracic volume for breathing. It has openings for the esophagus, inferior vena cava, and aorta.

A plane through the sternal angle (TIV/V) divides the mediastinum into superior and inferior parts. The inferior part is further divided by the pericardium into anterior, middle (containing the heart), and posterior mediastinum.

The pleura consists of parietal pleura (lining the cavity wall) and visceral pleura (covering the lung surface). A potential space exists between them, containing pleural fluid.

The pleural cavities have recesses, spaces not filled by lung, accommodating volume changes. The costodiaphragmatic recess, between the thoracic wall and diaphragm, is the largest and most clinically significant.

The superior thoracic aperture directly connects to the root of the neck. The pleural cavities extend into the neck above rib I, and major visceral structures pass between the neck and mediastinum.

Axillary inlets on each side of the superior thoracic aperture provide a gateway to the upper limb. They are formed by the scapula, clavicle, and rib I, and communicate with the root of the neck.

The diaphragm separates the thorax and abdomen. Structures like the inferior vena cava, esophagus, and aorta pass through or posterior to the diaphragm to connect these cavities.

Breasts are located on the anterior thoracic wall. Their blood supply comes from internal thoracic arteries, and lymphatic drainage goes to parasternal and axillary nodes. Innervation is via intercostal nerves.

The plane at vertebral level TIV/V (transthoracic plane) is significant as it marks the sternal angle, the separation of mediastinal compartments, the beginning/end of the aortic arch, the tracheal bifurcation, and the superior limit of the pulmonary trunk.

Veins like the left brachiocephalic vein, hemiazygos, and accessory hemiazygos veins cross the midline within the thorax to deliver blood from the left side to the right atrium, demonstrating left-to-right venous shunting.

The thoracic wall is segmentally supplied by posterior and anterior intercostal arteries (from thoracic aorta and internal thoracic arteries) and intercostal nerves. These run along the inferior margin of the ribs.

Dermatomes of the thoracic wall generally follow the segmental organization of thoracic spinal nerves (T1-T12). T1 dermatome is mostly in the upper limb, and the anterosuperior trunk is supplied by cervical plexus branches.

Preganglionic sympathetic nerve fibers originate from spinal nerves T1 to L2. Fibers destined for the head, for example, emerge from the spinal cord in spinal nerve T1.

The thoracic wall's flexibility, due to rib articulation and orientation, allows for changes in anterior and lateral dimensions during breathing. The diaphragm's muscular action changes the vertical dimension.

The diaphragm is innervated by the phrenic nerves, originating from cervical nerves C3, C4, and C5. Their cervical origin is due to the embryological development of the diaphragm.

The sternal angle, formed by the manubrium and body of the sternum, is a key surface landmark. It corresponds to the articulation of the second costal cartilage and is used to count ribs.

The costal cartilages of ribs I-VII articulate directly with the sternum. Those of ribs VIII-X articulate with the cartilage above, contributing to the mobility and elasticity of the thoracic wall.

Ribs XI and XII are known as floating ribs because they do not articulate with the sternum or other ribs anteriorly. Their small costal cartilages only cover their tips.

The mediastinum can be surgically accessed without opening the pleural cavities because the pleural cavities are completely separated by the mediastinum.

The apex of each lung extends into the root of the neck, above the level of rib I. This means that conditions in the root of the neck can affect the pleura and lung, and vice versa.

The aorta passes posterior to the diaphragm at the midline at vertebral level TXII, through an opening called the aortic hiatus.

The esophagus penetrates the muscular part of the diaphragm at vertebral level TX to enter the abdomen, passing through the esophageal hiatus.

The inferior vena cava pierces the central tendon of the diaphragm at vertebral level TVIII to enter the right side of the mediastinum.

These vessels run along the deep aspect of the anterior thoracic wall on either side of the sternum and supply branches to the anterior chest wall and the anteromedial parts of the breast.

Posterior and anterior intercostal vessels and nerves run segmentally around the thoracic wall, mainly along the inferior margin of each rib, supplying the wall, pleura, and skin.

When inserting chest tubes, the position of intercostal nerves and vessels relative to the ribs must be considered to avoid injury. They are typically found along the inferior margin of the rib.

The phrenic nerves, which innervate the diaphragm, arise from the anterior rami of cervical nerves C3, C4, and C5, with C4 providing the major contribution.

The superior thoracic aperture and axillary inlet are critical passageways for neurovascular structures. Compression or irritation in this region can lead to thoracic outlet syndrome.

The rib cage, formed by vertebrae, ribs, and sternum, provides a strong, yet flexible, protective enclosure for the heart, lungs, and great vessels within the thoracic cavity.

The diaphragm forms two domes that arch superiorly into the thoracic cavity. The right dome is typically higher than the left, reaching as far as the level of the fifth rib.

The mediastinum's central position and its complete separation of the pleural cavities are crucial for independent lung function and surgical access to thoracic organs.

This recess is a clinically important space where excess pleural fluid can accumulate. Its presence allows for full lung expansion during deep inspiration.

The axillary inlet, connecting the thorax to the upper limb, is defined by the superior margin of the scapula posteriorly, the clavicle anteriorly, and the lateral margin of rib I medially.

The thoracic wall exhibits clear segmental organization, reflected in the arrangement of vertebrae, ribs, intercostal spaces, and the segmental supply of nerves and blood vessels.

The sternal angle is a vital landmark for physical examination, helping to locate the second rib, the bifurcation of the trachea, and the superior limit of the pericardium.

The costal cartilages provide flexibility and elasticity to the anterior thoracic wall, allowing for expansion during breathing and absorbing impact.

The oblique angle of the superior thoracic aperture, sloping inferiorly from posterior to anterior, influences the passage of structures entering and leaving the thorax.

The inferior thoracic aperture is tilted superiorly when viewed anteriorly, with its posterior margin being inferior to its anterior margin, reflecting the dome shape of the diaphragm.