In the brass wind instruments such as horn, trombone, and trumpet, the lips of the player vibrate and excite the air within. Varying pitches are obtained partly by the varying wind pressure of the musician; if he breathes fast, the pitch rises; if he breathes slowly, the pitch falls. All of these instruments, however, except the trombone possess some valves which, on being pressed, vary the length of the tube and alter the pitch accordingly. In the trombone, valves are replaced by a section which slides in and out and shortens or lengthens the tube. 274. The Percussion Instruments. The percussion instruments, including kettledrums, bass drums, and cymbals, are the least important of all the musical instruments; and are usually of service merely in adding to the excitement and general effect of an orchestra. In orchestral music the various instruments are grouped somewhat as shown in Figure 192. CHAPTER XXIX SPEAKING AND HEARING [Illustration: FIG. 193.--The vibration of the vocal cords produces the sound of the human voice.] 275. Speech. The human voice is the most perfect of musical instruments. Within the throat, two elastic bands are attached to the windpipe at the place commonly called Adam's apple; these flexible bands have received the name of vocal cords, since by their vibration all speech is produced. In ordinary breathing, the cords are loose and are separated by a wide opening through which air enters and leaves the lungs. When we wish to speak, muscular effort stretches the cords, draws them closer together, and reduces the opening between them to a narrow slit, as in the case of the organ pipe. If air from the lungs is sent through the narrow slit, the vocal cords or bands are thrown into rapid vibration and produce sound. The pitch of the sound depends upon the tension of the stretched membranes, and since this can be altered by muscular action, the voice can be modulated at will. In times of excitement, when the muscles of the body in general are in a state of great tension, the pitch is likely to be uncommonly high. Women's voices are higher than men's because the vocal cords are shorter and finer; even though muscular tension is relaxed and the cords are made looser, the pitch of a woman's voice does not fall so low as that of a man's voice since his cords are naturally much longer and coarser. The difference between a soprano and an alto voice is merely one of length and tension of the vocal cords. Successful singing is possible only when the vocal cords are readily flexible and when the singer can supply a steady, continuous blast of air through the slit between the cords. The hoarseness which frequently accompanies cold in the head is due to the thickening of the mucous membrane and to the filling up of the slit with mucus, because when this happens, the vocal cords cannot vibrate properly.