The mediastinum (from Medieval Latin: mediastinus, lit.'midway'[2]) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esophagus, the trachea, the phrenic and cardiac nerves, the thoracic duct, the thymus and the lymph nodes of the central chest.

3D rendering of a high resolution computed tomography of the thorax, with mediastinum marked in blue.
Mediastinum. The division between superior and inferior is at the sternal angle.
Anatomical terminology


The mediastinum can be seen from a frontal view in this illustration, with the superior mediastinum labeled a, and the pericardial cavity, which is part of the inferior mediastinum, labeled d.

The mediastinum lies within the thorax and is enclosed on the right and left by pleurae. It is surrounded by the chest wall in front, the lungs to the sides and the spine at the back. It extends from the sternum in front to the vertebral column behind.[3][4] It contains all the organs of the thorax except the lungs.[4][5] It is continuous with the loose connective tissue of the neck.

The mediastinum can be divided into an upper (or superior) and lower (or inferior) part:

  • The superior mediastinum starts at the superior thoracic aperture and ends at the thoracic plane.
  • The inferior mediastinum from this level to the diaphragm. This lower part is subdivided into three regions, all relative to the pericardium – the anterior mediastinum being in front of the pericardium, the middle mediastinum contains the pericardium and its contents, and the posterior mediastinum being behind the pericardium.[6]

Anatomists, surgeons, and clinical radiologists compartmentalize the mediastinum differently. For instance, in the radiological scheme of Felson, there are only three compartments (anterior, middle, and posterior), and the heart is part of the middle (inferior) mediastinum.[7]

Thoracic plane

The transverse thoracic plane, thoracic plane, plane of Louis or plane of Ludwig is an important anatomical plane at the level of the sternal angle and the T4/T5 intervertebral disc.[8][9][10] It serves as an imaginary boundary that separates the superior and inferior mediastinum. [8][9][10]

A number of important anatomical structures and transitions occur at the level of the thoracic plane, including:

Superior mediastinum

The superior mediastinum is bounded:

  • superiorly by the thoracic inlet, the upper opening of the thorax;
  • inferiorly by the transverse thoracic plane. which is an imaginary plane passing from the sternal angle anteriorly to the lower border of the body of the 4th thoracic vertebra posteriorly;
  • laterally by the pleurae;
  • anteriorly by the manubrium of the sternum;
  • posteriorly by the first four thoracic vertebral bodies.
Mediastinum anatomy.
Some mediastinal structures on a chest radiograph.

Inferior mediastinum

Anterior mediastinum

Is bounded:

  • laterally by the pleurae;
  • posteriorly by the pericardium;[4]
  • anteriorly by the sternum ,[4] the left transversus thoracis and the fifth, sixth, and seventh left costal cartilages.
  • A quantity of loose areolar tissue
  • Some lymphatic vessels which ascend from the convex surface of the liver
  • Two or three anterior mediastinal lymph nodes
  • The small mediastinal branches of the internal thoracic artery
  • Thymus (involuted in adults)
  • superior and inferior sternopericardial ligaments
Middle mediastinum

Bounded: pericardial sac – It contains the vital organs and is classified into the serous and fibrous pericardium.[4]

  • the heart enclosed in the pericardium
  • the ascending aorta
  • the lower half of the superior vena cava with the azygos vein opening into it
  • the bifurcation of the trachea and the two bronchi
  • the pulmonary trunk dividing into its two branches
  • the right and left pulmonary veins
  • the phrenic nerves
  • some bronchial lymphatic glands
  • pericardiocophrenic vessels
Posterior mediastinum

Is bounded:

  • Anteriorly by (from above downwards): bifurcation of trachea; pulmonary vessels; fibrous pericardium and posterior sloping surface of diaphragm
  • Inferiorly by the thoracic surface of the diaphragm (below);
  • Superiorly by the transverse thoracic plane;
  • Posteriorly by the bodies of the vertebral column from the lower border of the fifth to the twelfth thoracic vertebra (behind);[4]
  • Laterally by the mediastinal pleura (on either side).

Clinical significance

Mediastinal adenopathy

The mediastinum is frequently the site of involvement of various tumors:

  • Anterior mediastinum: substernal thyroid goiters, lymphoma, thymoma, and teratoma.
  • Middle mediastinum: lymphadenopathy, metastatic disease such as from small cell carcinoma from the lung.
  • Posterior mediastinum: Neurogenic tumors, either from the nerve sheath (mostly benign) or elsewhere (mostly malignant).

Mediastinitis is inflammation of the tissues in the mediastinum, usually bacterial and due to rupture of organs in the mediastinum. As the infection can progress very quickly, this is a serious condition.

Pneumomediastinum is the presence of air in the mediastinum, which in some cases can lead to pneumothorax, pneumoperitoneum, and pneumopericardium if left untreated. However, that does not always occur and sometimes those conditions are actually the cause, not the result, of pneumomediastinum. These conditions frequently accompany Boerhaave syndrome, or spontaneous esophageal rupture.

Widened mediastinum

Widened mediastinum
Other namesMediastinal widening
Widened mediastinum in a patient with achalasia

Widened mediastinum/mediastinal widening is where the mediastinum has a width greater than 6 cm on an upright PA chest X-ray or 8 cm on supine AP chest film.[11]

A widened mediastinum can be indicative of several pathologies:[12][13]

See also

  • Mediastinum testis (unrelated structure in the scrotum)
  • Mediastinal germ cell tumor
  • Mediastinitis
  • Mediastinal tumor


This article incorporates text in the public domain from page 1090 of the 20th edition of Gray's Anatomy (1918)

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  • Anatomy figure: 21:01-03 at Human Anatomy Online, SUNY Downstate Medical Center – "Divisions of the mediastinum."
  • Anatomy figure: 21:02-03 at Human Anatomy Online, SUNY Downstate Medical Center – "The anatomical divisions of the inferior mediastinum."
  • thoraxlesson3 at The Anatomy Lesson by Wesley Norman (Georgetown University) – "Subdivisions of the Thoracic Cavity"
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