Pleura (Gr. , the side), the thin serous membrane which lines the cavity of the chest on either side, and is reflected at the root of the lung over the external surface of these organs. That portion of the pleura which lines the wall of the chest is called the costal pleura, and that which covers the lung the pulmonary pleura. The space between the two is the pleural cavity. In the healthy condition, however, this cavity can hardly be said to exist, since the two opposite surfaces of the pleura, though not adherent, are in contact with each other, or separated only by an extremely thin layer of serous fluid, which enables them to glide readily over each other in the movements of respiration. (See Lungs, and Thorax.) In disease the pleural cavity on either side is liable to be distended by an exudation of serum or of pus, or sometimes of air which gains access from a wound of the lung or of the walls of the thorax. The pleural surfaces may also become adherent to each other, as a consequence of inflammation, thus obliterating more or less completely the pleural cavity, and interfering with the free action of the lung.