The mediastinum is a thick midline partition that goes from the sternum anteriorly to the thoracic vertebrae posteriorly, and from the superior thoracic aperture to the inferior thoracic aperture.

A horizontal plane going through the sternal angle and the intervertebral disc between vertebrae TIV and TV splits the mediastinum into superior and inferior parts. The inferior part is further subdivided by the pericardium, which encloses the pericardial cavity enclosing the heart. The pericardium and heart make up the middle mediastinum.

The anterior mediastinum is located between the sternum and the pericardium; the posterior mediastinum is located between the pericardium and thoracic vertebrae.