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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: a superior m. [TA] (m. superus [TA]), which lies directly superior to a horizontal plane intersecting the sternal angle and approximately the T4–5 intervertebral disk, and an inferior m. [TA] (m. inferius [TA]) inferior to that plane; the latter is, in turn, subdivided in 3 parts: a middle m. [TA] (m. medium [TA]), which is coterminus with the pericardial sac containing the heart, a nearly potential anterior m. [TA] (m. anterius [TA]) lying in front, and a posterior m. [TA] (m. 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]

- anterior m. [TA] the narrow nearly potential space region between the pericardium posteriorly and the sternum anteriorly containing the thymus or its remnants, some lymph node s and vessels and branches of the internal thoracic artery. SYN: m. anterius [TA].

- m. anterius [TA] SYN: anterior m..

- inferior m. [TA] the region below a horizontal plane transecting approximately the T4–5 intervertebral disk posteriorly and the sternal angle anteriorly, demarcating the inferior limit of the superior m.. It is subdivided into three regions: middle, anterior, and posterior. SYN: m. inferius [TA].

- m. inferius [TA] SYN: inferior m..

- m. medium [TA] SYN: middle m..

- middle m. [TA] the large central portion of the inferior m., which includes the pericardium and the contained heart, as well as the phrenic nerves and cardiacophrenic vessels. SYN: m. medium [TA].

- posterior m. [TA] lies between the pericardium anteriorly and the vertebral column posteriorly and below the level of the plane that interesects the sternal angle and the T4–5 intervertebral disk. It contains the descending aorta, thoracic duct, esophagus, azygos veins, and vagus nerves. SYN: m. posterius [TA], postmediastinum.

- m. posterius [TA] SYN: posterior m..

- superior m. [TA] part of the m. lying superior to the horizontal plane intersecting the sternal angle and approximately the T4–5 intervertebral disc ( i.e., above the pericardium); it contains the arch of the aorta and the vessels arising from it, the brachiocephalic veins, and upper portion of the superior vena cava, the trachea, the esophagus, the thoracic duct, the thymus, and the phrenic, vagus, cardiac, and left recurrent laryngeal nerves. SYN: m. superius [TA].

- m. superius [TA] SYN: superior m..

- m. testis [TA] SYN: m. of testis.

- m. of testis [TA] a mass of fibrous tissue continuous with the tunica albuginea, projecting into the testis from its posterior border; testicular septa radiate as continuations surrounding the testicular lobules. SYN: m. testis [TA], corpus highmori, corpus highmorianum, Highmore body, septum of testis.

me·di·as·ti·num .mēd-ē-ə-'stī-nəm n, pl -na -nə

1) the space in the chest between the pleural sacs of the lungs that contains all the viscera of the chest except the lungs and pleurae also this space with its contents

2) MEDIASTINUM TESTIS

the space in the thorax (chest cavity) between the two pleural sacs. The mediastinum contains the heart, aorta, trachea, oesophagus, and thymus gland and is divided into anterior, middle, posterior, and superior regions.