14.  The Windpipe and Air-tubes.—­A large tube called the windpipe extends from the root of the tongue down the middle of the chest.  The windpipe divides into two main branches, which subdivide again and again, until the finest branches are not larger than a sewing-needle.  The branches are called bronchial tubes.  At the end of each tube is a cluster of small cavities called air-cells.  The air-tubes and air-cells are well shown on the following page.

15.  The Voice-box.—­If you will place the ends of your fingers upon your throat just above the breast-bone, you will feel the windpipe, and may notice the ridges upon it.  These are rings of cartilage, a hard substance commonly called gristle.  The purpose of these rings is to keep the windpipe open.  Close under the chin you can find something which feels like a lump, and which moves up and down when you swallow.

[Illustration:  AIR-TUBES AND AIR-CELLS.]

This is a little box made of cartilage, called the voice-box, because by means of this curious little apparatus we are able to talk and sing.  Two little white bands are stretched across the inside of the voice-box.  When we speak, these bands vibrate just as do the strings of the piano.  These bands are called the vocal cords.

16.  The Epiglottis.—­At the top of the voice-box is placed a curious trap-door which can be shut down so as to close the entrance to the air-passages of the lungs.  This little door has a name rather hard to remember.  It is called the epiglottis (ep-i-glot’-tis).  The cover of the voice-box closes whenever we swallow anything.  This keeps food or liquids from entering the air passages.  If we eat or drink too fast the voice-box will not have time to close its little door and prevent our being choked.  Persons have been choked to death by trying to swallow their food too fast.  Do you not think this is a very wonderful door that can open and shut just when it should do so without our thinking anything about it?

17.  The Nostrils and the Soft Palate.—­The air finds its way to the lungs through the mouth or through the two openings in the nose called the nostrils.  From each nostril, three small passages lead backward through the nose.  At the back part of the nasal cavity the passages of the two sides of the nose come together in an open space, just behind the soft curtain which hangs down at the back part of the mouth.  This curtain is called the soft palate.  Through the opening behind this curtain the air passes down into the voice-box and then into the lungs.

18.  The Pleura.—­In the chest the air tubes and lung of each side are enclosed in a very thin covering, called the pleura.  The cavity of the chest in which the lungs are suspended is also lined by the pleura.  A limpid fluid exudes from the pleura which keeps it moist, so that when the two surfaces rub together, as the lungs move, they do not become chafed and irritated.