Mediastinum

(me-de-as-ti’ ’-num) [in medio stare, to stand in the middle]. I. A partition separating adjacent parts. 2. The space left in the middle of the chest between the two pleurae, divided into the anterior, middle, posterior, and superior mediastinum. The anterior M. contains the origins of the triangular is stern i muscles, the internal mammary ve els of the left side, loose areolar ti ue, lymphatic ve els, and a few lymphatic glands. The middle M. contains the heart and pericardium, the ascending aorta, the superior vena cava, the bifurcation of the trachea, the pulmonary arteries and veins, and the phrenic nerves. The posterior M. contains a part of the aorta, the greater and le er azygos veins, the pneumogastric and splanchnic nerves, the esophagus, the thoracic duct, and some lymphatic glands. The superior mediastinum, that part lying above the pericardium, contains the origins of the sternohyoid and sternothyroid muscles, and part of the long us coll i muscles, the transverse portion of the aortic arch, the innominate, left carotid, and subclavian arteries, the superior vena cava and the innominate veins, the left superior intercostal vein, the pneumogastric, cardiac, phrenic, and left recurrent laryngeal nerves, the trachea, esophagus, thoracic duct, the remains of the thymus gland, and lymphatics. M. testis, a septum in the posterior portion of the testicle formed by a projection inward of the tunica albuginea. 383