The lungs are two spongy organs quite distinct from each other, placed on either side of the middle line. A large portion of them is occupied by the bronchial tubes and the air-cells in which they terminate ; otherwise they are made up of numerous bloodvessels, lymphatics and nerves bound together by connective tissue.

Each lung is enclosed in a serous sac—the pleura, one layer of which is attached to the walls of the chest and diaphragm ; the other closely covers the lungs. The sacs are moistened by a serous secretion, so that the surfaces in contact can move easily on each other during the movements of respiration.

The apices of the lungs rise about 1.5 inches above the clavicle, passing upwards through the thoracic inlet into the root of the neck. The base of each lung rests on the diaphragm ; the right lung is shorter than the left, as the diaphragm rises higher on the right side, owing to the position of the liver. The left lung is narrower than the right, owing to the position of the heart. The base of the right lung rests on the right lobe of the liver ; the base of the left lung is in relation to the left lobe, the stomach and the spleen, the diaphragm being between. The right lung has three lobes, the left has two. On the inner border of each lung, nearer to the apex than to the base, is the root ; it is formed by the bronchial tubes, pulmonary arteries, pulmonary veins, nerves and lymphatics, held together by an investment of the pleura, which there enter by the hilum, a slit on the inner surface, and connect the lungs with the heart and trachea.

The root of the right lung lies behind the superior vena cava and upper part of the right auricle and below the vena azygos. The root of the left lung is beneath the arch of the aorta. The phrenic nerve lies in front of each, and the pneumogastric nerve behind each.