THE AIR AND THE LUNGS.
The air which enters through the nose and mouth passes into a tube of muscles and ring-like pieces of cartilage. The upper part of this tube is the voice-box or larynx, covered by a spoon-shaped lid which closes when we swallow; the lower part is the trachea, and the two parts are the windpipe. The trachea divides into two branches, the bronchial tubes, one for each lung. These tubes divide again and again like the branches of a tree, and end in exceedingly small sacs or bags. The air in these sacs, or air-cells, gives oxygen to the blood in the tiny blood-vessels of the lungs and takes from them the poison, carbonic-acid gas, water, and impurities, which it carries back through the windpipe into the outside air.
QUESTIONS ON THE LUNGS AND RESPIRATION.
Of what are the lungs composed?—"Of a soft, fleshy substance, full of small air-cells and tubes."
Of what use are the lungs?—"They are the breathing machines of the body."
How do the lungs appear when healthy?—"Porous and spongy."
How does the air get into the lungs?—"The air flows through the nose and mouth, into the windpipe and along the air-tubes, into the air-cells of the lungs."
What does the air do in the lungs?—"It swells the lungs and causes the chest to expand."