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V. § 68.] MECHANISM OF THE HUMAN VOICE. 135
duced with surprising rapidity. In this respect, the human 'reed' far exceeds any that we can artificially construct.
The size and shape of the cavity of the mouth may be altered by opening or closing the jaws, raising or dropping the tongue and tightening or loosening the lips. We should expect that these movements would not be without effect on the resonance of the contained air, and this proves on experiment to be the fact. If we hold a vibrating tuning-fork close to the lips, and then modify the resonating cavity in the ways above described, we shall find that it resounds most powerfully to the fork selected when the parts of the mouth are in one definite position. If we try a fork of different pitch, the attitude of the mouth, for the strongest resonance, is no longer the same.
Hence, when the vocal chords have originated a reed-sound containing numerous well-developed partial-tones, the mouth-cavity, by successively throwing itself into different postures, can favour by its resonance first one partial-tone, then another; at one moment this group of partial-tones, at another that. In this manner endless varieties of quality are rendered possible. Good vocalisation, therefore, requires the resonating cavity to be so placed as to modify in the way most attractive to the ear