The trachea is surmounted by a box-like structure of cartilage, about four and one-half inches long, called the larynx. The upper end of the larynx opens into the pharynx or throat, and is provided with a lid,— the epiglottis,—which closes under certain circumstances (secs. 137 and 349). The larynx contains the organ of voice, and is more fully described in Chapter XII. The continuation of the larynx is the trachea, a tube about three-fourths of an inch in diameter, and about four inches long. It extends downwards along the middle line of the neck, where it may readily be felt in front, below the Adam’s apple. Illustration: Fig. 85.—Larynx, Trachea, and the Bronchi. (Front view.) A, epiglottis; B, thyroid cartilage; C, cricoid-thyroid membrane, connecting with the cricoid cartilage below, all forming the larynx; D, one of the rings of the trachea. The walls of the windpipe are strengthened by a series of cartilaginous rings, each somewhat the shape of a horseshoe or like the letter C, being incomplete behind, where they come in contact with the œsophagus. Thus the trachea, while always open for the passage of air, admits of the distention of the food-passage. 204. The Bronchial Tubes. The lower end of the windpipe is just behind the upper part of the sternum, and there it divides into two branches, called bronchi. Each branch enters the lung of its own side, and breaks up into a great number of smaller branches, called bronchial tubes. These divide into smaller tubes, which continue subdividing till the whole lung is penetrated by the branches, the extremities of which are extremely minute. To all these branches the general name of bronchial tubes is given. The smallest are only about one-fiftieth of an inch in diameter. Illustration: Fig. 86.—Relative Position of the Lungs, Heart, and its Great Vessels. A, left ventricle; B, right ventricle; C, left auricle; D, right auricle; E, superior vena cava; F, pulmonary artery; G, aorta; H, arch of the aorta; K, innominate artery; L, right common carotid artery; M, right subclavian artery; N, thyroid cartilage forming upper portion of the larynx; O, trachea. Now the walls of the windpipe, and of the larger bronchial tubes would readily collapse, and close the passage for air, but for a wise precaution. The horseshoe-shaped rings of cartilage in the trachea and the plates of cartilage in the bronchial tubes keep these passages open. Again, these air passages have elastic fibers running the length of the tubes, which allow them to stretch and bend readily with the movements of the neck. 205. The Cilia of the Air Passages. The inner surfaces of the windpipe and bronchial tubes are lined with mucous membrane, continuous with that of the throat, the mouth, and the nostrils, the secretion from which serves to keep the parts moist.