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The Mediastinum
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The Mediastinum

The Mediastinum
Overview
  • The mediastinum is the area centrally located between the right and left pleural cavities.
  • The mediastinum can be broadly divided into superior and inferior compartments.
The Superior Mediastinum* is a space for structures to pass between the head/neck and the thorax. The Inferior Mediastinum* comprises the anterior, middle, and posterior sub-compartments. – Anterior mediastinum comprises connective and fatty tissue that protect the deeper structures. – Middle mediastinum comprises the heart and the roots of the great vessels; – Posterior mediastinum provides a passageway for structures in the thorax.
Cross Section
We first show the mediastinum in cross section to view its location between the pleural cavities.
  • We draw the external body, and indicate anterior and posterior; we draw the sternum and a vertebra, for context.
  • Then, indicate the right and left pleural cavities.
  • Between them, show the mediastinum.
Sagittal Section
Now, we draw the mediastinum in sagittal section so we can see the four compartments.
Posteriorly, show the spinal column; we'll omit the details, but indicate vertebrae TIV and TV.*
  • Inferiorly, show the diaphragm, which separates the thoracic and abdominal cavities.
Middle compartment has a sac-like shape; it houses the pericardium, heart, and roots of the great vessels. Anterior compartment lies anterior to this, and extends from the sternal angle, superiorly, to the diaphragm, inferiorly. Posterior compartment lies posterior to the middle mediastinum, and, like the anterior mediastinum, extends from the sternal angle to the diaphragm. Superior mediastinum fills the space between the superior thoracic opening to the sternal angle.
Key anatomical structures
  • The thymus lies within the superior and anterior regions; recall that this structure regresses after childhood.
  • Then, return to where the root and ascending portion of the aorta arise in the middle mediastinum, and show that the aorta continues through the superior and posterior compartments. We've also shown the branches of the aortic arch as they extend through the superior mediastinum.
  • Next, posterior to the heart, show the opening of the left pulmonary artery as it passes to the left lung;
  • The opening of the left bronchus; indicate that the trachea extends through the superior and posterior compartments of the mediastinum.
  • The esophagus also passes through these compartments.
Pathology
  • Let's indicate some key masses that can develop in the mediastinum; we'll broadly categorize these by region of the mediastinum, but beware of overlap.
  • Anterior/superior mediastinum: thymoma, germ cell neoplasm, and lymphoma.
  • Middle mediastinum: pericardial cysts, bronchogenic cysts, lymph node enlargement, carcinoma, and lymphoma.
  • Posterior compartment: watch for neurogenic tumors and diaphragmatic hernias.
Summary Table
Superior compartment:
Anterior mediastinum:
  • Thymus.
  • Branches of the internal thoracic arteries and veins, and, the parasternal lymph nodes.
  • Middle mediastinum:
  • The heart and the roots of the great vessels.
  • The ascending aorta, pulmonary trunk, and pericardiacophrenic arteries.
  • The superior vena cava, pulmonary veins, and pericardiacophrenic veins.
  • And, the vagus nerves, phrenic nerves, and sympathetic nerves.
Posterior mediastinum:
  • The esophagus.
  • Thoracic aorta.
  • Azygos, hemiazygos, and accessory hemiazygos veins.
  • The thoracic duct.
  • The vagus nerves and sympathetic nerves.
References
  • Moore, K.L., Dalley, A.F., & Agur, A.M.R. Moore Clinically Oriented Anatomy, 7th ed. (Lippencott Williams & Wilkins, 2014).
  • Drake, R.L., Vogl, W., & Mitchell A.W.M. Gray’s Anatomy for Students (Elsevier, 2005).
  • Netter, F.H. Atlas of Human Anatomy, 4th ed. (Saunders, 2006).
  • Chung, K.W. & Chung, H.M. BRS Gross Anatomy, 7th ed. (Lippincott Williams & Wilkins, 2012).
  • Standring, S. & Gray, H. Gray's anatomy : the anatomical basis of clinical practice. 40th edn, (Churchill Livingstone/Elsevier, 2008).
  • Stoddard, Nathan, and David R. Lowery. “Anatomy, Thorax, Mediastinum.” In StatPearls. Treasure Island (FL): StatPearls Publishing, 2019. http://www.ncbi.nlm.nih.gov/books/NBK539819/.

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