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Definition of: mediastinum
(mē′dē·as·tī′nəm) noun plural ·na (-nə) Anat.
1. A median partition or septum separating two cavities of the body.
2. The partition between the two pleural sacs of the chest, extending from the sternum to the thoracic vertebrae and downward to the diaphragm. [<NL <Med. L mediastinus medial <medius]
—me′di·as·ti′nal adjective
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