The mediastinum is the general term for all the tissues and organs located between the pleural cavities on both sides, and is generally used clinically in three divisions and four divisions. The tetralogy of the mediastinum is to make a horizontal line between the sternal angle and the lower edge of the fourth thoracic vertebra, above which is the upper mediastinum, below which is the lower mediastinum, and the lower mediastinum is bounded by the pericardium, with the anterior mediastinum in front of the pericardium, the posterior mediastinum behind the pericardium, and the middle mediastinum in the pericardial area. The trichotomy is simpler than the tetralogy, removing the horizontal line between the lower corner of the sternum and the lower edge of the chest wall, and dividing directly by the pericardium into the anterior mediastinum, the posterior mediastinum behind the pericardium, and the middle mediastinum in the pericardial region. There are many tissue structures in the mediastinum, and often tumorigenic lesions occur, and mediastinal subdivisions make an important classification for clarifying the location of tumors and the surgical approach.