1. THE OBJECT OF RESPIRATION.--In one set of capillaries, or hair-like vessels, the blood is impoverished for the support of the different members and organs of the body. In another capillary system the blood is refreshed and again made fit to sustain life. The former belongs to the greater or _systemic_ circulation; the latter to the lesser or _pulmonary_, so called from _pulmo_, the lungs, in which organs it is situated. The blood, as sent from the right side of the heart to the lungs, is venous, dark, impure, and of a nature unfit to circulate again through the tissues. But, when the blood returns from the lungs to the left side of the heart, it has become arterial, bright, pure, and no longer hurtful to the tissues. This marvellous purifying change is effected by means of the very familiar act of respiration, or breathing. [Sidenote: 2. What are the lungs? How many lungs are there? Lung-substance? Its properties? The pleura?] 2. THE LUNGS.--The lungs are the special organs of respiration. There are two of them, one on each side of the chest, which cavity they, with the heart, almost wholly occupy. The lung-substance is soft, elastic, and sponge-like. Under pressure of the finger, it _crepitates_, or crackles, and floats when thrown into water; these properties being {124} due to the presence of air in the minute air-cells of the lungs. To facilitate the movements necessary to these organs, each of them is provided with a double covering of an exceedingly smooth and delicate membrane, called the _pleura_. One layer of the pleura is attached to the walls of the chest, and the other to the lungs; and they glide, one upon the other, with utmost freedom. Like the membrane which envelops the heart, the pleura secretes its own lubricating fluid, in quantities sufficient to keep it always moist. [Illustration: FIG. 34.--ORGANS OF THE CHEST. A, Lungs. B, Heart. D, Pulmonary Artery. E, Trachea.] {125} [Illustration: FIG. 35.--LARYNX, TRACHEA, AND BRONCHIAL TUBES.] [Illustration: FIG. 36.--DIAGRAM AND SECTION OF THE AIR-CELLS.] [Sidenote: 3. Communication of the lungs with the external air? Bronchial tubes?] 3. THE AIR-PASSAGES. --The lungs communicate with the external air by means of certain air-tubes, the longest of which, the _trachea_, or windpipe, runs along the front of the neck (Fig. 34, E, and 35). Within the chest this tube divides into two branches, one entering each lung; these in turn give rise to numerous branches, or bronchial tubes, as they are called, which gradually diminish in size until they are about one-twenty-fifth of an inch in diameter. Each of these terminates in a cluster of little pouches, or "air-cells," having very thin walls, and covered with a capillary network, the most intricate in the body (Fig. 36). [Sidenote: 4. Office of the bronchial tubes? What further can you state of them?]