mediastinum /mē′dē·əstī″nəm/ pl. mediastina [L, mediastinus, midway] , a part of the thoracic cavity in the middle of the thorax, between the pleural sacs containing the two lungs. It extends from the sternum to the vertebral column and contains all the thoracic viscera except the lungs. It is enclosed in a thick extension of the thoracic subserous fascia and is divided into the anterior mediastinum, middle mediastinum, posterior mediastinum, and superior mediastinum. −mediastinal, adj.
