=The Air Passages.=--The air usually passes in at the nose and returns by the same way, except during talking or singing. Observe your mouth with a mirror (Fig. 68); at the back part, an arch is seen which is the rear boundary line of the mouth (Exp. 1). Just above the arch is likewise the rear boundary line of the nasal passages. The funnel-shaped cavity beyond, into which both the mouth and nasal passages open, is called the _pharynx_ (far′inks), or throat (see Fig. 68, also Fig. 83). Below, two tubes open from the pharynx. One is the _trachea_ (trā′kea) or windpipe, the other is the _esophagus_ or _gullet_. At the top of the trachea is the cartilaginous _larynx_, or voice box. If the finger is placed upon the larynx or Adam’s apple, it is plainly felt to move up and down when swallowing. The opening into the larynx is provided with a lid of cartilage, the epiglottis. Inside the larynx, the vocal cords are stretched from front to back. Just below the larynx comes the _trachea_ proper, which is a tube about three fourths of an inch in diameter and about four inches long (Fig. 69). It consists of hoops of cartilage (Fig. 69) which are not complete circles, but are shaped somewhat like the letter C, being completed at the rear by involuntary muscular tissue, whose function is to draw the ends together at times (for instance, during coughing) and reduce the size of the tube. The function of the hoops of cartilage is to keep the windpipe open at all times. If it should be closed by pressure, life might be lost. These rings of cartilage may be felt in the neck. [Illustration: FIG. 70.--LOBULE OF LUNG.] The lower end of the trachea is just behind the upper end of the breastbone; there it divides into two large tubes. These subdivide into a great number of smaller branches called _bronchial tubes_. Cartilage is found in the walls of all but the smallest of the tubes. The subdivision continues, somewhat like the branching of a tree, until the whole lung is penetrated by bronchial tubes. Each tiny tube finally ends in a wider funnel-shaped chamber called a _lobule_ (Fig. 70), into which so many dilated sacs, called _air cells_, open, that the walls of the terminal chamber or lobule may be said to consist of tiny cups, or air cells, placed side by side. The lobules, or clusters of air cells, are chiefly near the surface of the lung. (The word “cell” is here used in its original sense to denote a cavity or chamber, and not in the sense of a protoplasmic cell.) [Illustration: FIG. 71.--CAPILLARIES AROUND AIR SACS OF LUNGS (enlarged 30 diameters). Air sacs in white spaces. Dark lines are capillaries. (Peabody.)]