mediastinum, n. [NL. mediastinum, fr. L. medius middle; cf. mediastinus helper, a menial servant, LL. mediastinus equiv. to medius: cf F. médiastin.].

A partition; a septum; specifically, the folds of the pleura (and the space included between them) which divide the thorax into a right and left cavity. The space included between these folds of the pleura, called the mediastinal space, contains the heart and gives passage to the esophagus and great blood vessels. [1913 Webster]

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OXFORD DICTIONARY

mediastinum, n. (pl. mediastina) Anat. a membranous middle septum, esp. between the lungs.

Derivative

mediastinal adj.

Etymology

mod.L f. med.L mediastinus medial, after L mediastinus drudge f. medius middle

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