The Mediastinum

Sources: Gray's Anatomy for Students; Sabiston Textbook of Surgery; Grainger & Allison's Diagnostic Radiology

Definition and Overview

The mediastinum is a thick midline partition occupying the central compartment of the thoracic cavity. It extends:

Anteriorly - from the sternum
Posteriorly - to the thoracic vertebrae
Superiorly - from the superior thoracic aperture (thoracic inlet)
Inferiorly - to the diaphragm

The two pleural cavities lie on either side of it. All thoracic viscera except the lungs are contained within it.

Subdivisions

A key horizontal plane - drawn from the sternal angle (angle of Louis) to the intervertebral disc between T4 and T5 - divides the mediastinum into two major parts:

Fig. - Lateral view of the mediastinum showing the T4/T5 dividing plane (dashed line) and all four compartments. (Gray's Anatomy for Students)

1. Superior Mediastinum

Feature	Detail
Boundaries	Superior: oblique plane from jugular notch to T1; Inferior: sternal angle/T4-T5 plane; Lateral: mediastinal pleura
Continuous with	Neck above; inferior mediastinum below

Major contents:

Thymus (most anterior structure)
Right and left brachiocephalic veins
Superior vena cava (SVC)
Arch of the aorta (with 3 branches: brachiocephalic trunk, left common carotid, left subclavian)
Trachea
Esophagus
Phrenic nerves
Vagus nerves
Left recurrent laryngeal nerve (loops under arch of aorta)
Thoracic duct
Left superior intercostal vein

Structures in the Superior Mediastinum - showing trachea, esophagus, aortic arch branches, SVC, brachiocephalic veins

Fig. 3.89 - Structures in the Superior Mediastinum. (Gray's Anatomy for Students)

2. Inferior Mediastinum

The inferior mediastinum lies below the T4/T5 plane and is further divided by the pericardium into three compartments:

Subdivisions of the mediastinum - lateral view showing all compartments with vertebral levels

Fig. 3.59 - Subdivisions of the Mediastinum. (Gray's Anatomy for Students)

a) Anterior Mediastinum

Location: Between the sternum and the anterior surface of the pericardial sac
Contents: Inferior extension of the thymus gland, fat, connective tissue, lymph nodes, mediastinal branches of the internal thoracic vessels, and sternopericardial ligaments
Note: the smallest of the three inferior compartments

b) Middle Mediastinum

Location: Centrally located - defined by the fibrous pericardium
Contents:
- Pericardium (fibrous + serous layers)
- Heart and its four chambers
- Proximal roots of the great vessels (ascending aorta, pulmonary trunk, SVC, pulmonary veins)
- Phrenic nerves (run along the pericardium)
- Pericardiacophrenic vessels

c) Posterior Mediastinum

Location: Posterior to the pericardial sac and diaphragm; anterior to the mid and lower thoracic vertebrae (T5-T12)
Contents:
- Esophagus and esophageal nerve plexus (vagal trunks)
- Descending thoracic aorta
- Azygos system of veins (azygos, hemiazygos, accessory hemiazygos)
- Thoracic duct and associated lymph nodes
- Sympathetic trunks
- Thoracic splanchnic nerves (greater, lesser, least)

Cross-Sectional Orientation

Cross-section of thorax showing mediastinum flanked by right and left pleural cavities

Fig. 3.57/3.58 - Cross-section of the thorax showing the mediastinum flanked by the pleural cavities. (Gray's Anatomy for Students)

Clinical Relevance by Compartment

The compartmental classification guides clinical diagnosis because different pathologies tend to cluster in specific compartments. The classic "4 T's" mnemonic for anterior mediastinal masses captures this:

Compartment	Common Pathologies
Anterior	Thymoma, Teratoma/Germ cell tumor, Thyroid mass, Terrible lymphoma (4 T's)
Middle	Lymphoma, pericardial cyst, bronchogenic cyst, aortic aneurysm
Posterior	Neurogenic tumors (most common posterior mass - e.g., schwannoma, neurofibroma), esophageal lesions, aortic aneurysm
Superior	Retrosternal goiter, lymphoma, SVC syndrome

Note on nomenclature: Radiologists sometimes use a three-compartment model (anterior, middle/visceral, posterior) without a separate "superior" division. The boundaries are not absolute anatomical planes - they vary between sources and do not form barriers to disease spread. (Grainger & Allison's Diagnostic Radiology)

Summary Table

Compartment	Anterior boundary	Posterior boundary	Key contents
Superior	Manubrium	T1-T4 vertebral bodies	Aortic arch, SVC, trachea, esophagus, thymus, thoracic duct
Anterior (inferior)	Sternum body	Anterior pericardium	Thymus (inferior part), fat, lymph nodes
Middle (inferior)	Anterior pericardium	Posterior pericardium	Heart, pericardium, great vessel roots
Posterior (inferior)	Posterior pericardium	T5-T12 vertebral bodies	Aorta, esophagus, azygos veins, thoracic duct, sympathetic trunk

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