Anatomy of the Thorax

The thorax — the region between the neck and abdomen — is bounded superiorly by the thoracic inlet (superior thoracic aperture), inferiorly by the diaphragm, and laterally and posteriorly by the thoracic cage. It houses the vital cardiopulmonary organs and the major vessels connecting them to the rest of the body. Thoracic anatomy is foundational for cardiothoracic surgery, pulmonology, emergency medicine, and intensive care. This guide is for educational purposes only.

## Thoracic Wall

The thoracic wall consists of the sternum, 12 pairs of ribs with their costal cartilages, and 12 thoracic vertebrae, along with the muscles spanning them. The sternum has three parts: the manubrium (articulating with the clavicles and first ribs), the body (articulating with ribs 2–7), and the xiphoid process. The sternal angle (angle of Louis) — the junction between manubrium and body — is a reliable clinical landmark at the level of the second costal cartilage, T4–T5 intervertebral disc, and tracheal bifurcation.

Ribs 1–7 are "true" ribs (directly articulating with the sternum via their costal cartilages); ribs 8–10 are "false" ribs (articulating via the seventh costal cartilage); ribs 11–12 are "floating" ribs (no anterior attachment). Each rib articulates posteriorly with the thoracic vertebral column at the costovertebral and costotransverse joints. The intercostal neurovascular bundle — intercostal vein, artery, and nerve — runs in the costal groove along the inferior margin of each rib, protected by it; this explains why thoracic procedures (chest tubes, thoracentesis, intercostal nerve blocks) are performed immediately above the upper border of the rib below, avoiding the bundle.

The three layers of intercostal muscles (external, internal, innermost intercostal) fill the intercostal spaces. The external intercostals elevate the ribs during inspiration; the internal and innermost intercostals assist during forced expiration. The diaphragm, the primary muscle of inspiration, separates the thorax from the abdomen and contains three major apertures: the caval hiatus at T8 (IVC), the esophageal hiatus at T10 (esophagus and vagal trunks), and the aortic hiatus at T12 (aorta, thoracic duct).

## Mediastinum

The mediastinum is the central partition of the thoracic cavity between the two pleural sacs, extending from the thoracic inlet to the diaphragm. It is divided into superior (above the sternal angle/T4–T5 level) and inferior mediastinum; the inferior is further divided into anterior (between sternum and pericardium), middle (the pericardial sac and its contents), and posterior (between pericardium and vertebral column) mediastina.

The superior mediastinum contains the aortic arch and its branches (brachiocephalic trunk, left common carotid, left subclavian arteries), the superior vena cava, brachiocephalic veins, trachea, esophagus, thoracic duct, vagus nerves (and the left recurrent laryngeal nerve looping under the aortic arch), and the phrenic nerves. The posterior mediastinum contains the descending thoracic aorta, azygos venous system, thoracic duct, esophagus, and sympathetic chains.

## Heart In Situ

The heart lies obliquely in the middle mediastinum, with approximately two-thirds of its mass to the left of the midline. Its borders on chest X-ray are: right border (right atrium), left border (left ventricle, with the aortic knuckle and pulmonary trunk superiorly), superior border (great vessels), and inferior border (right ventricle predominates). The cardiac notch of the left lung accommodates the heart's position. Surface projection of the heart valves is important for auscultation: the mitral valve is best heard at the cardiac apex (fifth intercostal space, midclavicular line); the aortic valve at the right second intercostal space; the pulmonary valve at the left second intercostal space; and the tricuspid valve at the left fourth to fifth intercostal space parasternally.

## Lungs and Pleura

The parietal pleura lines the inner surface of the thoracic wall (costal pleura), the diaphragm (diaphragmatic pleura), and the mediastinum (mediastinal pleura), and reflects at the lung hilum to become the visceral pleura covering the lung surface. The pleural recesses are potential spaces where parietal pleura extends beyond the lung margin: the costodiaphragmatic recess (where fluid accumulates in pleural effusion) and the costomediastinal recess. Percussion detects fluid in the costodiaphragmatic recess.

The lung hila contain the principal bronchi, pulmonary arteries and veins, bronchial arteries, lymphatic vessels, and autonomic nerve plexuses. The right pulmonary artery is longer and passes anterior to the right bronchus; the left pulmonary artery arches over the left bronchus ("eparterial" position of the right upper lobe bronchus).

## Great Vessels

The aorta ascends from the left ventricle as the ascending aorta (giving off the coronary arteries), arches over the pulmonary trunk and left main bronchus as the aortic arch, and descends as the thoracic aorta to the left of the vertebral column. The superior vena cava (SVC) is formed by the union of the two brachiocephalic veins posterior to the right first costal cartilage and drains into the right atrium at T3. The thoracic duct ascends from the cisterna chyli in the abdomen, entering the thorax through the aortic hiatus and ultimately draining into the left venous angle (junction of left subclavian and left internal jugular veins).

## Thoracic Nerves

The phrenic nerves (C3, C4, C5 — "C3, 4, 5 keeps the diaphragm alive") descend from the neck, passing anterior to the lung hila (right phrenic anterior to SVC, left phrenic anterior to left ventricle) to supply the diaphragm. They also provide sensory supply to the pericardium and diaphragmatic pleura and peritoneum, explaining referred pain to the shoulder tip in diaphragmatic irritation (e.g., blood from liver rupture irritating the right hemidiaphragm causes right shoulder tip pain). The vagus nerves pass through the posterior mediastinum, giving off the recurrent laryngeal nerves (right recurrent loops under the right subclavian artery; left recurrent loops under the aortic arch) before forming the esophageal plexuses.

## Clinical Procedures

Chest tube insertion (tube thoracostomy) drains pleural fluid, blood, or air. The tube is placed in the "safe triangle" — bounded by the anterior border of the latissimus dorsi, the lateral border of the pectoralis major, and a line above the nipple — typically in the 4th or 5th intercostal space, mid-axillary line, inserted just above the upper border of the rib. Thoracentesis (needle aspiration of pleural fluid for diagnosis or drainage) is performed in the costodiaphragmatic recess, usually at the 8th–9th intercostal space in the posterior axillary line under ultrasound guidance.