The mediastinum is the space that separates the 2 lungs and contains the heart, thymus, esophagus, trachea, the large blood vessels, and lymph nodes. A mediastinoscopy is a procedure in which a lighted instrument (mediastinoscope) is inserted through a neck incision to visually examine the structures in the top of the chest cavity. Most commonly this procedure is used to examine lymph nodes in a patient with lung cancer, for disease staging purposes. A sample ( biopsy ) may be taken with the mediastinoscope.