means of the hyoid-bone (lingual bone). Right through the centre of the larynx two highly elastic cords extend from the shield cartilage, anteriorly, to the two pyramid cartilages posteriorly. They are called the vocal cords. On the one side they are firmly attached to the laryngeal wall, and the opposite sharp edge projects into the interior of the larynx. They meet at an acute angle at the depression between the two wings of the shield cartilage, but diverge from there backward, and leave an interval for the passage of air. This interval or free space, the glottis, or rima glottidis (chink of the glottis), is either an equilateral triangle, or, as in very deep inspirations, an almost perfect oval (Fig. XX). By the air forced from the lungs, the vocal cords can be put into sounding vibrations, whereby voice is produced. Under the shield cartilage is the ring cartilage, which connects the larynx with the trachea (windpipe). Over the rima glottidis we find the epiglottis, a protecting cover, projecting from before and below in an oblique direction backward and upward over the larynx. It is a pear-shaped, flat cartilage, which shuts down upon and closes the glottis when food and drink are to pass into the stomach, so that they must glide over the cover (Figs. XIX, XX, XXI). Most changes of voice arise from narrowing and widening, and from lengthening and shortening, of the glottis, as well as from contraction and relaxation of the vocal cords. SECTION 6. TRACHEA (WINDPIPE, FIG. XIX). The trachea is a cartilaginous tube, which can, like the rest of the air-passages, expand, contract and shorten by means of elastic fibres. It measures from three and one-half to four and one-half inches in length. Its superior end is connected with the larynx; thence it descends vertically into the chest, and divides in the region of the third dorsal vertebra into two canals, the bronchi (the left and the right bronchus), one of which leads into the right, the other into the left lung. The interior of the trachea also is lined by mucous membrane. SECTION 7. THE AIR-RECEIVERS (LUNGS, FIG. XIX). The lungs occupy the greatest part of the thoracic cavity. They are divided into the right and left lung, connected above by the bronchi, branches of the windpipe, and between which the heart and the largest artery (aorta) are situated. Each lung has a broad, concave base, by which it rests upon the diaphragm, and a rounded apex, which stands behind the first rib. Each lung is divided by deep fissures, the right into three, the left into two lobes, and these again into numerous small lobules. VIEW OF THE INTERIOR OF THE LARYNX, THE POSTERIOR HALF BEING CUT AWAY. I, I, the greater horns of the hyoid-bone, cut across; 2, thyroid cartilages; 3, cricoid cartilages; 4, first ring of the trachea; 5, the thyro-hyoid membrane; 6, upper portion of the epiglottis; 7, cushion of the epiglottis; 8, ventricular fold, showing above it the wedge-shaped space of the upper laryngeal cavity; 9, left vocal cord; a, b, c showing the different portions of the interior muscle. The activity of the lungs consists in inspiration and expiration. In the former the chest expands; in the latter it contracts. The lungs do not represent a simple hollow space, like common bellows, but a very complicated tubular system. The substance of the lungs consists, especially at their periphery, almost entirely of very minute lobules, each of which possesses a narrow outlet (duct). These ducts combine to form gradually larger and larger canals, until, finally, the two beforementioned canals, the bronchi, are formed, which in their turn form, by their union, the trachea (windpipe). In inspiration the air passes through the glottis, trachea and bronchi to the air-cells of the lungs, whereby these are forced to expand. The expansion of the lungs depends upon the size or expansibility of the thorax. It is, therefore, necessary to strive, by appropriate exercises, to make the chest capable of expansion. The main function of the lungs is the removal of carbonic acid from the blood, and the introduction of oxygen. |