The Mediastinum
The mediastinum lies in the midline between the two pleural cavities. It is divided into
superior,anterior, middle and posterior compartments. In the superior mediastinum lie the
brachiocephalic veins, the branches from the aortic arch, the trachea and esophagus. In the thin
anterior mediastinum lies the thymus gland. The middle mediastinum is occupied by the heart.
The posterior mediastinum is continuous with the superior mediastinum and contains the
esophagus, primary bronchi and the thoracic aorta.
The esophagus
The esophagus continues on from the pharynx in the neck. It passes into the superior
mediastinum through the thoracic inlet lying anterior to the vertebral bodies and posterior to the
trachea. As the esophagus enters the posterior mediastinum it is crossed by the arch of the aorta
on its left side. The arch of the azygos vein lies on its right side. The course of the esophagus
through the mediastinum is from midline to left so that it lies anterior and left of the aorta as it
pierces the diaphragm level with the T10 vertebra. The thoracic duct lies posterior to the
esophagus. The esophagus is constricted at its formation from the pharynx, where the left
bronchus crosses in front and the aortic arch passes to the left, and where the esophagus passes
into the stomach. The esophagus for part of its course lies behind the left atrium. Enlargement of
the left atrium can compress the esophagus. Arterial supply to the esophagus is from the inferior
thyroid artery in the neck, the thoracic aorta, and the left gastric artery which supplies the
stomach. Venous drainage follows the arterial supply. Nerves to the esophagus are supplied by
the esophageal plexus formed by both vagus nerves.
The trachea and bronchi
These structures are dealt with in the respiratory system
Applied anatomy
The construction of the mediastinum, and the fascial planes, allow endoscopic examination. The
endoscope can be introduced behind the manubrium and passed inferiorly to examine the trachea
and bronchi and to sample mediastinal lymph nodes.
The position of the mediastinum depends on the relative pressures in the two pleural cavities. A
pneumothorax on one side, or removal of one lung will cause a shift away from the midline.
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