The trachea descends anterior to the esophagus, enters the superior mediastinum, and divides into right and left main bronchi. The trachea is a median structure but, near its lower end, deviates slightly to the right, resulting in the left main bronchus crossing anterior to the esophagus.he esophagus is a median structure that lies first behind the trachea and then behind the left atrium. It begins to deviate to the left below the left main bronchus. In the posterior mediastinum it is related to the vertebral column as a string is related to a bow.