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1. A septum between two parts of an organ or a cavity. 2. The median dividing wall of the thoracic cavity, covered by the mediastinal pleura and containing all the thoracic viscera and structures except the lungs. It is divided arbitrarily into four parts: the superior m. is that part lying above the pericardium; it contains the arch of the aorta and the vessels arising from it, the innominate veins, and upper portion of the superior vena cava, the trachea, the esophagus, the thoracic duct, the thymus, and the phrenic, pneumogastric, cardiac, and left recurrent laryngeal nerves. The middle m. contains the pericardium and its contents and the phrenic nerves and accompanying vessels. The anterior m. is the narrow space between the pericardium and the sternum containing some lymphatic glands and vessels and branches of the internal mammary artery. The posterior m. lies between the pericardium and the vertebral column, below the level of the fourth thoracic vertebra; it contains the descending aorta, thoracic duct, esophagus, azygos veins, and pneumogastric nerves.

Division of cavity of chest between pleural sacs.

A membraneous partition which divides the cavity of the chest into two parts, separating the two lungs from each other; a continuation of the pleura.

The space between the right and left pleura in and near the median sagittal chest plane; an incomplete vertical septum of the testis.

A membranous space formed by the approximation of the pleura, dividing the chest into two parts, the one right, the other left. The mediastinum, formed by a double reflection of the pleura, extends from the spine to the posterior surface of the sternum. Its anterior part, called Anterior mediastinum, Mediastinum pectorale, lodges, at its upper part, the thymus gland in the foetus, and is filled below with fatty, areolar tissue. Its posterior part, parallel to the spine, is occupied by the oesophagus, vena azygos, thoracic duct, the lower part of the windpipe, the origin of the brouchia and a number of lymphatic glands. This part is called the posterior mediastinum- Mediastinum dorsale. By some anatomists, Mediastinum is defined to be the pleural walls of the membranous space, whilst the space itself is termed Cavum mediasti'ni.

The median septum between the lateral cavities of the thorax.

Of the ancients, any structure serving as a partition, especially those portions of the pleurae that form the lateral boundaries of the m. of modern anatomists, which is the interpleural space, or median space of the thorax, between the two pleurae, containing all the thoracic viscera except the lungs.