Anatomy of Mediastinum

Definition of the Mediastinum

• The mediastinum is the space between the lungs, within the thoracic cavity, bounded by the sternum and vertebral column.

Divisions of the Mediastinum

• The mediastinum can be divided into superior and inferior mediastinum by an imaginary plane, the plane of the sternal angle.
• The inferior mediastinum can be further divided into three parts: anterior, middle, and posterior mediastinum.

Superior Mediastinum

• The superior mediastinum lies behind the manubrium of the sternum, making it difficult to visualize from the front.
• The superior mediastinum contains several structures, including the thymus gland, great veins, branches of the arch of the aorta, trachea, and oesophagus, and the thoracic duct.

Cross-section of Superior Mediastinum

• The section passes through the T3 vertebra, and anteriorly passes through the sternum and 1st costal cartilages.
• The image is presented as though the viewer is standing at the patient's feet, looking upwards.

Cross-section of Inferior Mediastinum

• The cross-section shows the positions of the chambers of the heart, the aorta, oesophagus, and azygos vein.

Relations of the Oesophagus

• The oesophagus is a major organ of the posterior mediastinum, lying behind the trachea in the superior mediastinum.
• The oesophagus moves to the left as it approaches the plane of the sternal angle, passing behind the left main bronchus, and then moves anterior to the aorta.
• The oesophagus ultimately passes through the left muscular hemi-diaphragm to reach the abdomen, where it enters the stomach.

Relationship of the Trachea and Oesophagus

• The trachea and oesophagus are in the midline, but the oesophagus deviates to the left.

Veins of the Posterior Mediastinum

• The azygos vein receives blood from the posterior chest wall, and drains to the superior vena cava.
• The hemiazygos vein drains the lower left side of the thoracic cage, and the upper left side drains to the accessory hemiazygos vein.

Arteries of Posterior Mediastinum

• The aorta supplies blood to the posterior chest wall via the posterior intercostal arteries, and gives visceral branches to the lungs and oesophagus.

Lymphatics of Posterior Mediastinum

• The thoracic duct is the most posterior structure in the mediastinum, starting at the upper end of the cisterna chyli at the upper border of T12, and terminating in the root of the neck.

Nerves of the Chest

• The phrenic nerves are the motor control for the diaphragm, arising from C3-5 and passing down in front of the subclavian artery at the thoracic inlet.
• The vagus nerves (Xth cranial nerves) are complex nerves with both somatic and autonomic elements, providing a wide variety of functions in both the head and neck, thorax, and abdomen.

Autonomic Supply to the Heart

• The parasympathetic supply to the heart is via the vagus nerves, which reduce heart rate.

Autonomic Supply to Lungs

• The pulmonary plexus supplies the lungs, with sympathetics from T1-T5, and parasympathetics from the vagus.

Autonomic Supply to the Oesophagus

• The oesophageal plexus supplies the oesophagus, with sympathetics from T2-T6, and parasympathetics from the vagus.