Source : Webster's Revised Unabridged Dictionary (1913)

Mediastine \Me`di*as"tine\, Mediastinum \Me`di*as*ti"num\, n.
   [NL. mediastinum, fr. L. medius middle; cf. mediastinus
   helper, a menial servant, LL. mediastinus equiv. to medius:
   cf F. m['e]diastin.] (Anat.)
   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.

Source : WordNet®

mediastinum
     n : the part of the thoracic cavity between the lungs that
         contains the heart and aorta and esophagus and trachea
         and thymus
     [also: {mediastina} (pl)]