LUNG, an organ of respiration.
Human, Anatomy.—The organs of res piration are on each side of the chest, conical, and separated from each other by the heart in front and a membranous partition, the mediastinum. Externally they are convex, to correspond with the chest walls, and internally concave to receive the heart; above they terminate in a tapering cone and below in a broad concavity resting on the diaphragm. In color they are mottled, pinkish-gray, speckled with black. Each is divided into two lobes, separated by a deep fis sure, and the right lung has a third lobe above of triangular shape; the right is also larger on account of the heart lying toward the left side. The lungs are kept in position by their roots, com posed of the bronchi, pulmonary artery, and pulmonary veins; the right side pre sents the bronchus above, then the ar tery, then the veins; but on the left side we find the bronchus between the artery and the veins. Each lung is inclosed in a serous membrane, the pleura, which extends to its root, and is then expanded on the chest wall. The lungs are com posed of minute ramifications of the bronchial tubes, terminating in intercel lular passages and quadrilateral or hexag gonal air-cells, along with ramifications of the pulmonary artery and veins, bron chial arteries and veins, lymphatics and nerves, the whole bound together by areolo-fibrous tissue constituting the parenchyma of the lungs. See PHYSI OLOGY: RESPIRATION.
Comparative Anatomy.—In the lowest and simplest forms of animal life (aquatic), we find no trace of respira tory organs, the interchange between the layer of water with the aerating surface being effected by the general movement of the body, or by cilia. In
most of the Mollusca we find gills in the place of lungs, except in the terrestrial species, as the snail or slug, where we have a lung which is a simple cavity in the back communicating directly with the air, and covered with minute blood vessels; in bivalve mollusks again, as in the oyster, it is the internal surface of the mantle or skin-lining which is the special organ, with the same essen tial structure as gills. In the Articulata, as tapeworm, marine worms, Crustacea, as the crab tribe, we find a somewhat similar arrangement to that of the Mol lusca, but in insects, and other proper air-breathing Articulata, we have a reg ular series of air-sacs along each side of the body, opening by pores, called spira cles or stigmata, so in the spider-tribe, but in a more concentrated form, and more resembling the lung of the Verte brata. The gills of fishes come next in the scale, accompanied in many cases with an air bladder, especially in those approaching the Reptilia in their organ ization, and in some of these it is a double sac, the analogue of the double lung. The lungs of the reptiles are, for the most part, capacious sacs occupying a good deal of the trunk cavity, but not filled, like those of the Mammalia, by an act of inspiration, but chiefly by the process of swallowing. In birds we have the connecting link between the types of structure in the two classes.
Pathology.—There are various dis eases of the lungs; two of the most important are tubercular phthisis and pneumonia.