Pleura

(wrsvex.) A membrane which ~ lines the internal surface of the thorax, and — covers its viscera. It forms a great proce , — the mediastinum, which divides the thorax into two cavities. Its use is to render the surface of the thorax moist by the vapour it ex hales.. The cavity of the thorax is every where” lined by this smooth and glistening membrane, ~ which is in reality two distinct portions oF bags, which by being applied to each other — laterally, form the septum called mediastinum ¢~ this divides the cavity into two parts, and is _ attached posteriorly to the vertebre of the back; and anteriorly to the sternum. But the two laminz of which this septum is forms ed, do not every where adhere to each other; — for at the lower part of the thorax they are separated to afford a lodgement to the heart, and at the upper part of the cavity they receive — between them the thymus. The pleura is— plentifully supplied with arteries and veins from the internal mammary, and the inter~_ costals. Its nerves, which are very inconsiderable, are derived chiefly from the dorsal and intercostal nerves. The surface of the pleura, - like that of the peritoneum and other membranes lining cavities, is constantly bedewed with a serous mixture, which prevents adhesions of the viscera. The mediastinum, by dividing the breast into two cavities, obviates many inconveniences to which we should otherwise be liable. It prevents the two lobes of the lungs from compre ing each other when we lie on one side, and consequeptly contributes to the freedom of respiration, which is disturbed by the least pre ure on the lungs. If the point of a sword penetrate between the ribs into the cavity of the thorax, the lungs on “that side cease to perform their office, because the air being admitted through the wound, prevents the dilatation of that lobe, while the other lobe, which is separated from it by the mediastinum, remains unhurt, and continues to perform its functions as usual.