(1) A septum between two parts of an organ or a cavity. (2) [TA], The median partition of the thoracic cavity, covered by the mediastinal part of the parietal pleura and containing all the thoracic viscera and structures except the lungs. It is divided arbitrarily into two major divisions: the superior mediastinum [TA] (mediastinum superus [TA]), which lies directly superior to a horizontal plane intersecting the sternal angle and approximately the T4-5 intervertebral disc, and the inferior mediastinum [TA] (mediastinum inferius [TA]) inferior to that plane; the latter is, in turn, subdivided in three parts: a middle mediastinum [TA] (mediastinum medium [TA]), which is coterminus with the pericardial sac containing the heart: a nearly potential anterior mediastinum [TA] (mediastinum anterius [TA]) lying in front: and a posterior mediastinum [TA] (mediastinum posterius [TA]) behind, containing the esophagus, descending aorta, and thoracic duct. SYN: interpleural space, interpulmonary septum, mediastinal space, septum mediastinale. [Mod. L. a middle septum, fr. Mediev. L. mediastinus, medial, fr. L. mediastinus, a lower servant, fr. medius, middle]