The Mediastinum Subdivisions and contents
- The thoracic mediastinum (L. middle septum) is the space between the two pleura sacs.
- It contains all the structures of the thorax except the lungs and the pleura.
- It extends from the superior thoracic aperture to the diaphragm inferiorly; from the sternum and costal cartilages anteriorly to the bodies of the thoracic vertebrae posteriorly.
- The structures in the mediastinum are surrounded by loose connective tissue, nerves, blood and lymphatic vessels, and fat.
- In a living person, the looseness of the connective tissue and fat, and the elasticity of the lungs and pleura enable the mediastinum to accommodate movement and changes in volume in the thoracic cavity.
Subdivisions of the Mediastinum
- The mediastinum is descriptively divided into inferior and superior parts by a plane passing though the sternal angle and the inferior border of T4 vertebra, the manubriosternal plane.
- The superior mediastinum is from the superior thoracic aperture to this plane.
- The inferior mediastinum is from the plane to the diaphragm and is further subdivided into three compartments by the pericardium (anterior, middle and posterior).
The Superior Mediastinum
- This subdivision is superior to the manubriosternal plane.
- Structures in the superior mediastinum are: the thymus (or its remains) anteriorly; the great vessels related to the heart in the middle; and the oesophagus, trachea and thoracic duct posteriorly.
The Anterior Mediastinum
- This is the smallest subdivision of the mediastinum.
- It is located anterior to the pericardium and posterior to the sternum and transversus thoracis muscles.
- Although this mediastinum is small in adults, it is relatively large in childhood as the thymus extends into it from the superior mediastinum.
The Middle Mediastinum
- This subdivision is of the highest clinical importance as it contains the pericardium and the heart and the immediately adjacent parts of the great arteries, phrenic nerves, main bronchi, and other structures in the root of the lungs.
The Posterior Mediastinum
- This subdivision is located posterior to the pericardium and diaphragm, and anterior to the bodies of the inferior eight thoracic vertebrae.
- Its main contents are the oesophagus and the descending thoracic aorta, which have descended into it from the superior mediastinum.