Mē'di-ās'tine or Mē'di-as-ti'num, n.

n. [Fr. médiastin, Sp. & It. mediastino, N. Lat. mediastinum, from Lat. medius, middle; Lat. mediastinus, helper, a common servant, employed in all kinds of menial occupations, L. Lat. mediastinus, equivalent to medius.] (Anat.) The membranes and their contents, which separate the two lungs from each other, in the median line. It extends from the back-bone to the breast-bone, and is a continuation of the pleura.