from
The Collaborative International Dictionary of English v.0.48
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.
[1913 Webster]