Inside of this voice box, one of the rings of muscle that run around the windpipe has stretched into a pair of straight, elastic bands, or strings, one on each side of the air pipe, known as the _vocal cords_, or voice bands. These are so arranged that they can be stretched and relaxed by little muscles; and, when thrown into vibration by the air rushing through the voice box, they produce the sounds that we call talking or singing. The more tightly they are stretched, the higher and shriller are the tones they produce; and the more they are slackened, or relaxed, the deeper and more rumbling are the tones. This is why, when you try to sing a high note, you can feel something tightening and straining in your throat, until finally you can stretch it no tighter, and your voice "breaks," as you say, into a scream or cry. All musical instruments that have strings, are played, or produce their sounds, upon this same principle. The thinner and shorter the string, or the more tightly it is stretched, the higher the note; the heavier and longer the string, the lower the note. But no musical instrument ever yet invented can equal the human voice in the music of its tones, in its range, in the different variety and quality of tones it can produce, and in its wonderful power of expression. The human voice is a combination of reed organ, pipe organ, trumpet, and violin; and can produce in its tiny music box--only about two inches long by one inch wide--all the tones and qualities of tones that can be produced on all these instruments, except that it cannot go quite so high or so low. All the musical instruments in the world, from the penny whistle to the grand piano, are but poor imitations of the human music box. The bellows, of course, of the human pipe organ are the lungs; while the tongue furnishes the stops; and the throat, mouth, and nose, the resonance, or sounding, chambers. Just as a violin, or guitar, has two main parts,--a string, which vibrates and makes the sound; and a box, or hollow body, which catches that sound and enlarges it and gives it sweetness and vibration and quality,--so the human voice has two similar parts--the vocal bands, which make the sound; and a sound box, or rather series of three resonance boxes,--the throat, the mouth, and the nasal passages,--which enlarge and soften it and improve its quality.