me·di·as·ti·num 
(mē′dē-ə-stīnəm)
(mē′dē-ə-stīnəm)n. pl. me·di·as·ti·na (-nə)
The region in mammals between the pleural sacs, containing the heart and all of the thoracic viscera except the lungs.
[New Latin mediastīnum, from neuter of Medieval Latin mediastīnus, medial, middle, from Latin, servant employed on general tasks (Medieval Latin anatomical sense probably influenced by Latin intestīnum, intestine), from medius, middle; see medhyo- in the Appendix of Indo-European roots.]
me′di·as·tinal (-nəl) adj.