The cavity of the chest is lined with a serous membrane, called the pleura, which forms in each half of this cavity a sac without an opening. There are therefore two pleurae, a right and a left Proceeding from the edges of the sternum and the costal cartilages, the pleurae cover the lateral walls of the chest and a portion of the body of the vertebræ. They then approach each other, leaving a space between them called the posterior mediastinum. On reaching the root of the lungs, they turn from within outward, covering a portion of the pericardium and of the internal surface of the lungs, their posterior borders and external surface; they then penetrate the interlobular fissures, and fold back upon the anterior border of the lungs and upon their internal surface quite to their roots; then turning again forward, they cover the sides of the pericardium, in front of which they turn back to back, and then separating anew, they reach the borders of the sternum from whence they sprang. The space left between the pleurae behind the sternum is the anterior mediastinum, separated, as is seen, from the posterior mediastinum by the heart and the root of the lungs. At the top of the chest the pleurae form a conical cavity, which receives the apex of the lung; at the bottom they cover the superior surface of the diaphragm. In the posterior mediastinum are the oesophagus, the aorta, the azygos vein, the thoracic duct, and the lower portion of the trachea. In the anterior mediastinum are the pericardium—the envelope of the heart—and the thymus gland, an organ whose uses are unknown. That part of the pleurae which envelops the organs of the chest, and that which lines the walls of this cavity, are thus in contact with each other without adhering, in a normal condition, and they allow the expansion and contraction of the lungs and the walls of the chest The serous nature of the pleurae insures freedom of movement, and prevents all roughness in the constant friction of the surfaces.