4. These tubes are somewhat flexible, sufficiently so to bend when the parts move in which they are situated; but they are greatly strengthened by bands or rings of cartilage which keep the passages always open; otherwise there would be a constantly-recurring tendency to collapse after every breath. The lung-substance essentially consists of these bronchial tubes and terminal air-cells, with the blood-vessels ramifying about them (Fig. 37). At the top of the trachea is the larynx, a sort of {126} box of cartilage, across which are stretched the vocal cords. Here the voice is produced chiefly by the passage of the respired air over these cords, causing them to vibrate. [Illustration: FIG. 37.--SECTION OF THE LUNGS.] [Sidenote: 5. The epiglottis? When it does not close in time, what is the consequence?] 5. Over the opening of the larynx is found the _epiglottis_, which fits like the lid of a box at the entrance to the lungs, and closes during the act of swallowing, so that food and drink shall pass backward to the oesophagus, or gullet (Fig. 38). Occasionally it does not close in time, and some substance intrudes within the larynx, when we at once discover, by a choking sensation, that "something has gone the wrong way," and, by coughing, we attempt to expel the unwelcome intruder. The epiglottis is one of the many safeguards furnished by nature for our security and {127} comfort, and is planned and put in place long before these organs are brought into actual use in breathing and in taking food. [Illustration: FIG. 38.--SECTION OF MOUTH AND THROAT. A, The Tongue. B, The Uvula C, Vocal Cord. E, Epiglottis. L, Larynx. N, Trachea. O, Oesophagus.] [Sidenote: 6. Lining of the air-passages? Ciliated cells? Their uses? The three diseases of the lungs?] {128} [Illustration: FIG. 39.--CILIATED CELLS.]