THE APPARATUS OF BREATHING.
The Windpipe or Traehea.--The lungs may for the moment be regarded as sacs situated iu the chest, which communicate with the out side by means of a series of tubes. The chief of these is the trachea or windpipe, which passes down the front of the neck into the chest. It is about 4i inches long. It is surmounted by the larynx (the box of the windpipe), a box - like structure made of cartilage (gristle), which con tains the organ of voice, to be described later in this section. The larynx may be felt projecting at the upper part of the neck, and in some people it visibly projects. The pro jection is called the pomum Adami, Adam's apple. The upper end of the larynx opens into the pharynx or throat, and is pro vided with a lid—the epiglottis—which closes the opening under certain circumstances. The parts will be understood by referring to the accompanying figure, which represents the larynx and the windpipe. L is the larynx, formed mainly of two pieces of cartilage, called the thyroid (Th.) and the cricoid (Cr.). A little below Cr. the larynx passes into the windpipe. e of the figure points to the tongue-shaped epi glottis, shown raised, but when lowered capable of covering over the upper opening of the wind pipe. Now let the reader refer to Fig. 101, p. 195, and the relation of these to surrounding parts will be understood. On that figure ep points to the epiglottis; the line pointing to g is a little below the level where the larynx passes into the windpipe (w). It will be seen also that the larynx opens into the back of the throat, the opening being guarded by the epi glottis, and that the gullet (g) is just behind the windpipe Further, it will be seen that the finger, placed on the skin where the turn is made from under the chin ou to the front of the neck, will touch the upper edge of the larynx, and that this is below the root of the tongue. Before air can enter the trachea, then, it must pass through the mouth or nostrils (still refer to Fig. 101) into the pharynx (ph), past the raised epiglottis, through the larynx, and so into the trachea. Now it will at once occur to anyone that if air can enter into the larynx or windpipe, so can water, or anything one may be drinking, or, for that matter, any thing one may be eating. This accounts for
food or drink, or things being held in the mouth, a coin, &c., slipping accidentally into the windpipe. But why does this happen only accidentally? Why is it not a constant occur. rence? Let anyone put his finger on the front of the neck, on the notch at the top of the larynx, and then let him swallow. He will feel the larynx suddenly lifted up, and, when the swallowing is over, drop down again. By this action the larynx is raised up to be under cover of the root of the tongue, and at the same time the epiglottis folds down. These move ments are effected by nervous action, and it is only accidentally, when the parts are taken unawares, as it were, that anything can drop into the larynx or windpipe. The position of the gullet behind the windpipe explains how a mass of food that has been swallowed, and has stuck in the gullet, by pressing on the wind . pipe in front, produces the sensation of choking, though there is no difficulty in breathing.
The Bronchial lower end of the windpipe is situated in the chest, and there it divides into two branches (Fig. 149), one of which passes to each lung. Each branch is called a bronchus (Latin, bronchus, a wind pipe), and is called right or left as it passes to the right or left lung. The place where the tube enters the lung is called the root of the lung. Each bronchus, after passing into the lung, divides into smaller tubes, and these again into still smaller, and so the division and subdivision go on till the whole lung is penetrated by branches, the final subdivisions of which are of extreme fineness. To all these divisions and subdivisions the general term bronchial tubes is applied. The smallest of them are only about the h-th of an inch in diameter. Fig. 150 represents A, the windpipe, branching into B, the left, and c, the right bronchus, and these into smaller bronchial tubes, such as D. This figure is, however, only a representation, for the multitude of tubes of extreme fineness into which the larger tubes ultimately break up cannot, of course, be shown, though some attempt is made to repre sent them on the side on which a is placed.