- 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.
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.
Delicate, hair-like filaments, not unlike the pile on velvet, called cilia, spring from the epithelial lining of the air tubes. Their constant wavy movement is always upwards and outwards, towards the mouth. Thus any excessive secretion, as of bronchitis or catarrh, is carried upwards, and finally expelled by coughing. In this way, the lungs are kept quite free from particles of foreign matter derived from the air. Otherwise we should suffer, and often be in danger from the accumulation of mucus and dust in the air passages. Thus these tiny cilia act as dusters which Nature uses to keep the air tubes free and clean ([Fig. 5]).