The lungs are the organs of respiration properly; they are two in number, and situated in the chest, placed side by side, being separated from the abdomen by the diaphragm.

The size varies with the capacity and condition of the chest, age, inspiration, expiration, and disease. They are conical in shape, are longer posteriorly than anteriorly, and have concave bases. The color of the lungs is of a pinkish gray, mobbled with black; these black spots are more numerous in adult life than in infancy. The right lung is shorter but larger than the left, whose transverse diameter is somewhat diminished by the position of the heart. It has three lobes, the left having but two.

The structure of the lungs is spongy, and its compression between the fingers produces a crackling sound called orepitation. It consists of air-vesicles held together by cellular tissue, called parenchyma, through which blood-vessels and air-vessels are ramified. A certain number of air-cells communicate with each other, and with a single branch of the bronchial tube; these are separated from neighboring cells by partitions of parenchyma, and thus are formed the lobules in which the aeration of the blood is performed.