Mediastinum

A middle portion or septum separating adjacent parts. The pleural folds or septa extending from the spine to the sternum, and dividing the thoracic cavity into right and left chambers. It is divided into the anterior, the middle, and the posterior mediastinum, and otherwise into an upper (or superior) and a lower mediastinum. M. testis, a cavity in the posterior portion of the testicle formed by a projection inward of the tunica albuginea. It occupies about one -third of the transverse, and one-fourth of the longitudinal diameter of the testicle.