Mediastinum

The central compartment of the thoracic cavity, between the two pleural cavities, extending from the sternum anteriorly to the thoracic vertebral column posteriorly, and from the thoracic inlet superiorly to the diaphragm inferiorly.

• Divided by an imaginary transverse plane at the sternal angle (T4/T5 level) into superior and inferior mediastinum.
• The inferior mediastinum is further subdivided (relative to the pericardium) into anterior, middle, and posterior parts.

Boundaries: From the thoracic inlet superiorly to the sternal angle/T4-T5 plane inferiorly; sternum anteriorly, upper four thoracic vertebrae posteriorly.

Major contents (roughly anterior to posterior layering):

• Thymus: Anterior-most structure, overlying the great vessels; large and prominent in children, progressively involutes and is replaced by fat with age (though some functional tissue persists into adulthood).
• Great veins: Left and right brachiocephalic veins, uniting to form the superior vena cava: lie anterior to the arterial structures.
• Aortic arch: Arches posteriorly and to the left, from the ascending aorta (beginning at the sternal angle level) to the descending aorta (ending at the same level, on the left side of T4); gives off three branches (brachiocephalic trunk, left common carotid, left subclavian).
• Trachea: Posterior to the great vessels, in the midline, descending to the bifurcation at the sternal angle level.
• Oesophagus: Posterior to the trachea, continuing into the posterior mediastinum below.
• Thoracic duct: Ascends on the left side of the oesophagus, posterior to the aortic arch, en route to draining into the venous system near the left subclavian/internal jugular vein junction.
• Vagus nerves: Descend on either side, related to the trachea and oesophagus; the left vagus crosses the aortic arch anteriorly (giving off the left recurrent laryngeal nerve, which loops under the arch).
• Phrenic nerves: Descend anterior to the lung roots, between the mediastinal pleura and the pericardium, continuing toward the diaphragm.
• Recurrent laryngeal nerve (left): Loops under the aortic arch (posterior to the ligamentum arteriosum), then ascends in the tracheo-oesophageal groove back to the larynx, this long, hooking course around the aortic arch is a key anatomical vulnerability.

Subdivided relative to the pericardium into three parts.

ANTERIOR MEDIASTINUM

Boundaries: Narrow space between the sternum (anteriorly) and the pericardium (posteriorly).

Contents:

• Fat and loose connective tissue (the predominant content in adults).
• Lymph nodes (parasternal/internal thoracic nodes).
• Thymic remnants: the inferior continuation of the thymus gland extends into this space; in children the thymus itself occupies much of this region.
• Relatively sparse in significant anatomical structures compared to the other two compartments, but clinically relevant as a site for thymomas, lymphoma, and germ cell tumours (mediastinal masses arising here).

MIDDLE MEDIASTINUM

Boundaries: Contains the pericardium and its contents; bounded by the anterior and posterior mediastinum.

Contents:

• Pericardium (fibrous and serous layers).
• Heart: enclosed within the pericardium.
• Roots of the great vessels: ascending aorta, pulmonary trunk, superior vena cava (lower part), and the roots of the pulmonary veins (as they enter the pericardium/left atrium).
• Phrenic nerves: pass through this compartment (between the mediastinal pleura and the fibrous pericardium) en route to the diaphragm, accompanied by the pericardiacophrenic vessels.
• Main bronchi also pass through this region as they enter the lung roots (at the junction with the middle mediastinum).

POSTERIOR MEDIASTINUM

Boundaries: Between the pericardium/diaphragm (anteriorly) and the lower eight thoracic vertebrae (posteriorly), extending from the sternal angle plane to the diaphragm.

Contents:

• Descending thoracic aorta: continues from the aortic arch (from T4 to approximately T12, where it passes through the aortic hiatus).
• Oesophagus: continues from the superior mediastinum, passing anterior to the descending aorta before crossing to lie anterior and slightly left of it more inferiorly, en route to the oesophageal hiatus (T10).
• Thoracic duct: ascends between the azygos vein (right) and the descending aorta (left), posterior to the oesophagus.
• Azygos system: azygos vein (right side, ascending along the right side of the vertebral bodies before arching over the right lung root to join the SVC), hemiazygos and accessory hemiazygos veins (left side, draining across the midline to join the azygos vein).
• Vagus nerves: continue their descent, forming the oesophageal plexus around the lower oesophagus (with fibres reorganising into anterior and posterior vagal trunks by the time the oesophagus reaches the diaphragm).
• Sympathetic trunks: paravertebral chains, running along either side of the vertebral column throughout the posterior mediastinum.
• Thoracic splanchnic nerves (greater, lesser, least), arise from the sympathetic trunks, pass through the diaphragmatic crura to reach abdominal target organs.