PLEURÆ
Each lung is invested externally by a very delicate serous membrane termed the pleura, which, after enclosing the whole organ, except at its root, where the great vessels enter it, is reflected upon the inner surface of the thorax or chest. That portion of the pleura which is in contact with the surface of the lung is called the pleura pulmonalis, or visceral layer; whilst
that which lines the interior of the chest is called the pleura costalis, or parietal layer; while the space intervening between these two layers is called the cavity of the pleura. Each pleura, as will be at once seen by a reference to the figure, is a closed sac, and quite independent of the other. The interspace between the pleurae on the right and left side, is termed the mediastinum, and contains all the viscera of the thorax excepting the lungs.
The inner surface of each pleura is smooth, glistening, and moistened by a serous fluid; the outer surface is closely adherent to the surface of the lung, to the roots of the pulmonary vessels as they enter the lung, to the upper surface of the diaphragm, and to the walls of the chest. The lobes of the lungs are separated from one another by involutions or in-foldings of the visceral layer; two such involutions�one on either side�are shown in the figure. The use of these serous sacs is much the same as that of the Peritoneum (q. v.); each pleura retains the lung and, to a certain extent, the greater vessels in position, while it at the same time facilitates, within certain limits, the movements of those parts which are essential to the due performance of the act of respiration.