The Air Passages consist of the interior of the nose, pharynx, larynx, windpipe or trachea, and the bronchial tubes. When we breathe, we draw in the air through the nose, in which it is warmed by contact with the mucous membrane, which is richly supplied with blood, and after it has passed through the pharynx and larynx it passes into the trachea or windpipe, which subdivides into numerous tubes called the bronchial tubes (bronchia), which in turn subdivide into and terminate in minute subdivisions in all the small air spaces in the lungs, of which the lungs contain millions. A writer has stated that if the air cells of the lungs were spread out over an unbroken surface, they would cover an area of fourteen thousand square feet. The air is drawn into the lungs by the action of the diaphragm, a great, strong, flat, sheet-like muscle, stretched across the chest, separating the chest-box from the abdomen. The diaphragm's action is almost as automatic as that of the heart, although it may be transformed into a semi-voluntary muscle by an effort of the will. When it expands, it increases the size of the chest and lungs, and the air rushes into the vacuum thus created. When it relaxes the chest and lungs contract and the air is expelled from the lungs. Now, before considering what happens to the air in the lungs, let us look a little into the matter of the circulation of the blood. The blood, as you know, is driven by the heart, through the arteries, into the capillaries, thus reaching every part of the body, which it vitalizes, nourishes and strengthens. It then returns by means of the capillaries by another route, the veins, to the heart, from whence it is drawn to the lungs. The blood starts on its arterial journey, bright red and rich, laden with life-giving qualities and properties. It returns by the venous route, poor, blue and dull, being laden down with the waste matter of the system. It goes out like a fresh stream from the mountains; it returns as a stream of sewer water. This foul stream goes to the right auricle of the heart. When this auricle becomes filled, it contracts and forces the stream of blood through an opening in the right ventricle of the heart, which in turn sends it on to the lungs, where it is distributed by millions of hair-like blood vessels to the air cells of the lungs, of which we have spoken. Now, let us take up the story of the lungs at this point.