The mediastinum is the middle compartment of the chest which is situated between the 2 pleural cavities. Its boundaries are (Fig. 241): anterior, the sternum; posterior, the bodies of the 12 thoracic vertebrae; superior, the thoracic inlet; inferior, the diaphragm. The sides are formed by the mediastinal pleurae. It is divided into superior and inferior mediastina by an imaginary line which extends from the sternal angle (manubriosternal joint) to the disk between the 4th and the 5th thoracic vertebrae. The inferior mediastinum is subdivided into 3 mediastina by the heart, which acts as the key structure in this subdivision. That part of the inferior mediastinum which contains the heart is called the middle mediastinum; that part in front of it makes up the anterior, and the part situated behind the heart constitutes the posterior mediastinum.
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