MEDIASTINUM

(Lat.), n. n. Med(mead)-ia-a2st(a'st)-in(en)'- U3m(u4m). Fr., medias tin. Ger., Mit tel fell (1st def.), Mittelklappe (1st def.). Supposed by Hyrtl to be a corruption of per medium tensum, something stretched between, like a partition. Gr., v^v, 6ia0paTT<or (cf. DIAPHRAGM). 1. Of the ancients, any structure serving as a partition, especially those portions of the pleura? that form the lateral boundaries of the m. of modern anatomists (more properly the cavum medias tin i), which is the interpleural space, or median space of the thorax, between the two pleurae, containing all the thoracic viscera except the lungs. [A, 500.] 2. The thin di epiment in cruciferous plants separating the silique or silicic into two parts, to which the seeds remain attached after the valves open. [B, 1 (a, 35).1 Anterior m. Fr., medias tin anterieur. Ger., vorderer Mittelfellraum. That portion of the inferior m. which lies in front of the pericardium. [L, 31, 142.] Inferior in., Lower m. That portion of the m. (1st def.) which is situated below the lower margin of the fifth dorsal vertebra; divided into the anterior, middle, and posterior mediastina. [A, 478; L. 31.] M. auris. Of Bartholin, the membrana tympani. [A, 500.] M. cerebelli. See FALX cerebelli. M. cerebri. Fr., medias tin du cerveau. See FALX cerebri. M. dorsale. See Posterior TO. M. pectorale. Ger., Mit tel fell. The interspace in the chest between the two pleural sacs. [LJ M. post i cum. See Posterior m. M. testis. See CORPUS Highmori. M. ventriculoriim lateralium. See SEPTUM hicidum. Middle m. Ger., mittleres Mit tel fell. That portion of the inferior m. which is situated between the anterior and posterior mediastina, and contains the heart, the ascending aorta, the superior vena cava, the bifurcation of the trachea, the pulmonary ve els, and the phrenic nerves. [A, 478.] Posterior m. Fr., medias tin poster i eur. Ger., hinterer Mittelfellraum. That portion of the inferior m. which is situated behind the pericardium and the roots of the lungs, and contains the descending thoracic aorta, the descending portion of the aortic arch, the oesophagus, the thoracic duct, the pneumogastric and splanchnic nerves, the azygous veins, and some lymphatic glands. [A, 478.] Superior in., Upper in. That portion of the m. (1st def.) which is situated above the level of the lower margin of the fifth dorsal vertebra. [L, 31.] [s. 724]