Mediastinum

[Jessica]

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.

[Luis Fok]

a structure that divides a body part in the centre:

[Monty]

A membranous partition between two body cavities or two parts of an organ, especially that between the lungs.