Mediastinum - Is an interpleural space (area between the pleural cavities) in the thorax and is bounded laterally by the pleural cavities, anteriorly by the sternum and the transversus thoracis muscles, and posteriorly by the vertebral column (does not contain the lungs).
- Consists of the superior mediastinum above the pericardium and the three lower divisions: anterior, middle, and posterior.
Superior mediastinum- Is bounded superiorly by the oblique plane of the first rib and inferiorly by the imaginary line running from the sternal angle to the intervertebral disk between the fourth and fifth thoracic vertebrae.
- Contains the superior vena cava (SVC), brachiocephalic veins, arch of the aorta , thoracic duct, trachea, esophagus, vagus nerve, left recurrent laryngeal nerve, and phrenic nerve.
- Also contains the thymus, which is a lymphoid organ; is the site at which immature lymphocytes develop into T lymphocytes; and secretes thymic hormones, which cause T lymphocytes to gain immunocompetence.
Anterior mediastinum- Lies anterior to the pericardium and posterior to the sternum and the transverse thoracic muscles.
- Contains the remnants of the thymus gland, lymph nodes, fat, and connective tissue.
Middle mediastinum- Lies between the right and left pleural cavities.
- Contains the heart, pericardium, phrenic nerves, roots of the great vessels (aorta, pulmonary arteries and veins, and vena cavae), arch of the azygos vein , and main bronchi.
Posterior mediastinum - Lies posterior to the pericardium between the mediastinal pleurae.
- Contains the esophagus, thoracic aorta, azygos and hemiazygos veins, thoracic duct, vagus nerves, sympathetic trunk, and splanchnic nerves.
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