LUNGS, the principal organs of respi ration in air-breathing vertebrates. They are enclosed the chest, an air-tight chamber which, mainly by means of the diaphragm acts as a bellows and moves the air in and out•of them. The purpose of the inhalation of air is to supply oxygen to the blood and to re move carbon dioxide. There are two lungs. one on the left, the other on the right side of the chest, and between them and almost com pletely covered by them are situated the heart and the great blood-vessels springing from it, the oesophagus, the thoracic duct, etc. The left lung is the smaller of the two, and both consist of a mass of ramified tubes, branches of the trachea, which unite them. The color of the lungs is pinkish white at birth, which changes to the mottled slate color of the adult lungs, and the still darker hue of the lungs of the aged. Through the walls of the lungs all the blood of the body passes, receiving oxygen and throwing off carbonic acid. The simplest lung imaginable would be an elastic membranous hag, well supplied with blood-vessels, and with a pipe connecting it with the air; the most com plicated lungs that exist are essentially of that construction, the purpose of the complications being merely the enlarging of the surface ex posed to the air.
In all air-breathing vertebrates, the atmos pheric air reaches the larynx through the nasal and the buccal cavities, then passes into the trachea, and into its ramifications which are called bronchi or bronchial tubes, and from these tubes into membranous pouches named alveoli. The terminal twig of a bronchial tube is a small canal, in which are found many open ings or orifices of very short tubes, which are the ultimate ramifications of the tube. The lung-substance is composed of the alveoli, the air-sacs, the small bronchi, blood and lymphatic vessels and nerves; the whole enclosed in a membrane that surrounds each of the lungs and is known as the pleura. The trachea or windpipe extends from the lower part of the larynx, of which it is the continuation, to the middle of the thorax, where it divides into the two large bronchi. It is situated in the middle line of the body, in front of the last cervical and the first five or six dorsal vertebra. In the back part of the trachea there are transversal muscular fibres of the unstriped variety. The fibrous tissue found in the trachea belongs to the two varieties of yellow elastic and white fibres. The mucous membrane is thin and in perfect continuity with that of the larynx and that of the bronchi. Its most remarkable fea ture is that its epithelial covering is composed of ciliated cells having in high degree the vibra tile movement. All along the trachea there are many mucous glands, and these are espe cially numerous upon its back part. The cilia
lash upward, and thus keep the passages free from mucus and remove foreign particles. As the passages become smaller they lose their cartilages, and the muscles form a continuous circular layer. The length of the trachea is from four to five inches. The average trans versal diameter of this tube is between 9 and 12 lines in adults. The anterior two-thirds of the trachea is cylindrical, the posterior third is a flattened wall. In the neck the trachea is covered by the skin and a few flat muscles; in the chest it is placed between the two lungs and covered by muscles, lymphatic glands and the bony and cutaneous walls of the thorax. The structure of the trachea is complicated. This tube is essentially composed of an internal layer which is a mucous membrane and an external one which is fibrous. Imbedded in these mem branes are from 16 to 20 cartilaginous pieces, with the shape of a horseshoe, or of a ring one third of which is missing. These incomplete rings are placed transversely at nearly equal distances one from the other; they give to the anterior two-thirds of the trachea the cylindri cal form.
The essential of the lungs are the air cells or air-sacs, in which the function of respi ration is performed. They are estimated to be about 600,000.000 in number and to be about one-two hundred and fiftieths of an inch in diameter. They consist of somewhat elongated cavities, which communicate with a bronchial ramification by a circular opening, usually smaller than the cavity of the cell. The air cells are arranged in groups and separated from each other by thin walls. Many small, shallow, cup-like depressions, separated from each other by portions of membrane, are found at the bot tom and on the lateral walls of the air-sacs. These, the alveoli, have no communication with each other except by their opening in the cavity of the air-sac. An epithelial layer exists in the air-cells and the alveoli. The walls of the air cells are formed of a thin membrane in which the blood and lymph-capillaries ramify. Minute openings lead from the air-cells into the lymph spaces of the membrane. The membranous walls are partly formed of elastic tissue. It is this that gives to the lungs their elasticity. At the root of the lungs the membrane known as the vis ceral pleura is continuous with a membrane which lines the chest-cavity (the parietal pleura). The space between the two is the pleural cavity; it is in reality a large lymph space, and communicates with the lymphatics of the pleura. Owing to the air-pressure within the lungs, the two pleura are closely pressed together, the lungs entirely filling the chest cavity.