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DIVISIONS OF THE MEDIASTINUM

BIG PICTURE

The mediastinum is the anatomic region medial to the pleural sacs, between the sternum, vertebral column, rib 1, and the diaphragm. The mediastinum is further divided into inferior and superior regions by the transverse thoracic plane, which is a horizontal plane passing from the sternal angle to the T4–T5 intervertebral disc (Figure 5-1A). The inferior mediastinum is classically subdivided into anterior, middle, and posterior parts. Therefore, the four subregions of the mediastinum are the anterior mediastinum, middle mediastinum, posterior mediastinum, and superior mediastinum.

  • Anterior mediastinum. The anterior mediastinum is deep to the sternum and bounded by the sternal angle, pericardial sac, and diaphragm; contains the following:

    • Adipose tissue.

    • Thymus. The thymus in adults is involuted and is primarily a connective tissue remnant.

  • Middle mediastinum. The middle mediastinum contains the pericardial sac and heart (see Chapter 4 for further details).

  • Posterior mediastinum. The posterior mediastinum contains the following anatomic structures, which are posterior to the pericardial sac (Figure 5-1B):

    • Descending aorta. The thoracic portion of the aorta that gives rise to posterior intercostal arteries.

    • Azygos system of veins. Receives venous blood from the thoracic wall.

    • Thoracic duct. The primary lymphatic duct that receives lymph from all tissues below the diaphragm and from the left side of the head, neck, upper limb, and thorax.

    • Esophagus. The esophagus courses vertically directly posterior to the left atrium.

    • Sympathetic nerves.

Figure 5-1:

A. The lateral view of the thorax illustrating the mediastinal subdivisions. B. The posterior mediastinum in axial section (superior view).

This chapter focuses on the structures located in the posterior mediastinum and their projection into the superior mediastinum.

  • Superior mediastinum. The superior mediastinum is the region above the sternal angle and contains the following structures:

    • Aortic arch. The aortic arch arises at the level of the transverse thoracic plane, ascends up into the superior mediastinum, and descends in the posterior mediastinum. The aortic arch gives rise to the following three primary branches:

      • Brachiocephalic trunk. Supplies the right side of the head and neck and right upper limb.

      • Left common carotid artery. Supplies the left side of the head and neck.

      • Left subclavian artery. Supplies the left upper limb.

    • Superior vena cava (SVC). The SVC collects all venous blood from tissues above the diaphragm. The primary tributaries of the SVC are the azygos vein and the left and right brachiocephalic veins.

    • Trachea. The trachea bifurcates at the level of the transverse thoracic plane into the left and right primary bronchi.

    • Esophagus. The esophagus is a vertical, muscular tube that is located posterior to the trachea and transports food from the pharynx to the stomach.

    • Nerves. The phrenic nerves (en route to the diaphragm) and vagus nerves (en route to thoracic and abdominal organs) course through the superior mediastinum.

    • Thymus. In ...

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