There is a very intricate system of nerve centres and fibres by which the muscles of the larynx and resonating cavities are connected with the brain cells in which tones are conceived. To each minute muscular fibre the right amount of nerve impulse is transmitted. Each fibre instantaneously contracts with the necessary degree of strength, and the correct adjustment for the tone is made. This takes place automatically, without our being in any way made conscious of it. We cannot feel what takes place in either the brain or the larynx. All we are conscious of is the imagined sound of the tone mentally conceived, the impulse to sing the tone, and the sound of the tone as the ear hears it. The actual operation is highly complex, but we are aware only of its simple features.
This can best be understood by actually singing some tones of various kinds. Sing the scale, do, re, mi, fa, etc., with a pure tone but no particular expression. Sing it soft, medium, and loud. Think how it should sound if the syllables meant “Alas! Alas! All hope is o’er!” and sing the scale with the tone color expressive of despair. Then imagine the tone quality which would be appropriate to Rejoice! Our triumph is at hand! and give your voice to this quality in singing the passage. Try some single sustained tones, first in a harsh, ugly quality, then in a soft, smooth, rich tone. In every case you will find that your voice responds instantly to your mental conception and faithfully reproduces the tone quality you have in mind. Finally, sing the scale several times, striving for the most beautiful and musical quality of which your voice is capable. With each repetition try to improve on the preceding effort.
Taylor, David C. “Natural Singing and the Old Italian Method.” Etude Magazine, August 1916: 594.