=The Pleurae.=—Each lung is invested upon its external surface by an exceedingly delicate serous membrane, the pleura. This encloses the organ as far as its root, and is then reflected upon the inner surface of the thorax. The _pulmonary pleura_ is the portion investing the surface of the lung, and dipping into the fissures between its lobes. The _parietal pleura_ is that which lines the inner surface of the chest. The space between these two layers is called the cavity of the pleurae, (the pleural cavity); and contains nothing but a very little clear fluid. In the healthy condition the two layers are in contact and there is no real cavity, but after death the lungs become collapsed and separate from the walls of the chest. Each pleura is therefore a shut sac, one occupying the right, and the other the left half of the thorax, and they are perfectly separated from one another. The two pleurae do not meet in the middle line of the chest, excepting for a short distance between the second and third pieces of the sternum—a space being left between them, which contains all the viscera of the thorax excepting the lungs; this is called the mediastinum. The mediastinum then, is the space between the right and left pleural sacs. The arteries of the pleura are derived from the intercostal, internal mammary, musculo-phrenic, thymic, pericardiac, bronchial. The veins correspond to the arteries. =The Lungs.=—The lungs are the essential organs of respiration. They are two in number, placed one on each side of the chest, separated from each other by the heart and the contents of the mediastinum. A healthy lung hangs free within the pulmonary space. The lung is suspended by the root. The root of the lung is formed by the bronchial tubes, pulmonary artery, pulmonary veins, bronchial arteries, bronchial veins, etc., all of which are enclosed by the reflections of the pleurae. The _root of the lung_ may be described as being that part where all the great blood vessels and the bronchial tubes, enter the lungs. In many cases the lung does not hang free, but as a result of former pleurisy, the area of the pulmonary pleura is adherent to the parietal pleura. [Illustration: FIG. 18—The root of the left lung. (Toldt)] Each lung is conical in shape, and presents for examination, an apex, a base, and two surfaces. _The Apex_ forms a tapering cone which extends into the root of the neck about an inch and a half to two inches above the level of the top of the first rib. _The Base_ is broad and concave and rests upon the convex surface of the diaphragm, which separates the right lung from the upper surface of the right lobe of the liver and the left lung from the upper surface of the left lobe of the liver, the stomach, and spleen.