MEDIASTINUM

[me.di.as.ti.num] n, pl -na [NL, fr. ML, neut. of mediastinus medial, fr. L medius] (1541): the space in the chest between the pleural sacs of the lungs that contains all the viscera of the chest except the lungs and pleurae; also: this space with its contents -- me.di.as.ti.nal adj