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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 parts by a horizontal plane passing through 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 as follows:
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- Anterior mediastinum. The region between the sternal angle, the deep sternal surface, the pericardial sac, and the diaphragm. The anterior mediastinum contains fat and areolar tissue and the inferior part of the thymus or its remnant.
- Middle mediastinum. This region contains the pericardial sac and heart (see Chapter 4 for further details).
- Posterior mediastinum. The region containing anatomic structures deep to the pericardial sac, including the thoracic portion of the descending aorta, the azygos system of veins, the thoracic duct, the esophagus, and the vagus and sympathetic nerves (Figure 5-1B). This chapter will focus on the structures located in the posterior mediastinum and their projection into the superior mediastinum.
- Superior mediastinum. The region superior to the sternal angle containing the aortic arch and its three branches, the superior vena cava (SVC) and the brachiocephalic veins, the trachea, the esophagus, and the phrenic and vagus nerves. The superior mediastinum also contains the thymus; however, in an adult, the thymus is usually atrophied and presents as a fatty mass.
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The autonomic nervous system of the thorax consists of both sympathetic and parasympathetic motor neurons through which cardiac muscle, smooth muscle, and the glands of the thorax and the abdomen are innervated. Autonomic innervation involves two types of neurons: preganglionic neurons and postganglionic neurons. The sympathetic nerves in the thorax and in other areas of the body include visceral sensory fibers that course along the general sensory neurons.
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Sympathetic Nerves of the Thorax
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The thoracic sympathetic chain or trunk courses vertically across the heads of the ribs along the posterior thoracic wall, deep to the parietal pleura (Figure 5-2A). This location parallels the vertebral column; the sympathetic chain, therefore, is also referred to as the paravertebral ganglia. The sympathetic chain descends posterior to the diaphragm to continue its descent in the abdominal cavity.
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