THE PRODUCTION OF VOICE.
Voice is then produced by arrangements precisely similar to those of some musical in struments. The organ of voice is, indeed, a reed-instrument, the sound produced by the vibrations of the reed or vocal cords being modified by the tubes above and below.
Musical sounds are due to movements or vibrations occurring with a certain regularity, and they differ in loudness, in pitch, and in quality.
Loudness of the sound depends on the extent of the vibrations.
Pitch of the sound depends on the rapidity of the vibrations.
Quality of the sound depends on the ad mixture of tones produced by vibrations of vary ing rates of rapidity related to one another. A vibrating 100 times per second gives out what is called a pure tone. If several other tuning-forks, vibrating at rates which are mul tiples of 100, say 200, 300, 400, &c., be thrown into vibration, the sounds emitted by them will blend with the sound of the first or fun damental tone, and the quality of the sound will be changed though its pitch remains the same. The tones that have been added are called overtones. The quality of a musi cal sound, therefore, depends on the overtones in it. Thus if the same note be produced on a piano, a violin, and a trumpet, the quality of the note of each will be markedly different, though the three notes are of the same pitch, the difference being due to the different over tones present in each. Human voices uttering the same note differ in quality, because the construction of the vocal apparatus of one in dividual favours the production of a different set of overtones from those of another.
In the production of the voice, then, there are to be noted the arrangements for (1) the vibration of the cords to produce sound, (2) the regulation of the loudness of the sound, (3) the variation of the pitch of the sound, and (4) the determination of the quality of the sound. • The Vibration of the Vocal order that the cords may be thrown into proper vibration they must be brought close together and parallel to one another, so that only a narrow slit intervenes between them, through which air is driven with some degree of force. The vocal cords are brought to gether, so that the space between them is narrowed to a slit, by the action of a muscle which passes from the cricoid cartilage to the outer angle of the arytenoid cartilages. When
the muscle of each side contracts, the arytenoid cartilages are caused to turn on their bases, so that the vocal cords attached to them are brought close up to one another. Other muscles assist in the manoeuvre. This having been accomplished, a strong expiration drives the air from the lungs through the slit between the cords, and throws them into movement. Other muscles are connected with the ary tenoid cartilages which cause them to rotate in an opposite direction, so separating the vocal cords and widely opening the glottis.
By means of an arrangement of mirrors, called the laryngoscope, devised in modern times, this operation can be easily seen. A person is seated upright in a chair, a lamp, throwing a strong light, being placed on a table at his side and slightly behind him. In front of him an observer sits, who has a slightly concave mirror fixed on his forehead. The person opens his month, and the observer arranges the mirror so as to catch the light from the lamp and reflect it in a bright beam into the person's throat. The observer then takes a small plane mirror, not so large as a shilling, set at an angle on a long stem, and, having slightly warmed it, passes it to the back of the person's throat, the person's tongue being pulled gently forwards meanwhile. The observer so places the plane mirror that the light from the mirror on his forehead is thrown on to it, and is then reflected downwards to illuminate the larynx. It requires, of course, some practice and dex terity to arrange the mirrors properly. When it is skilfully done, the observer is able to see in the plane mirror a view of the illuminated larynx, with its lining membrane and vocal cords. When the person is breathing quietly, the chink of the glottis is seen widely open, but as soon as he utters a sound, as that of a in far, the cords are seen to advance quickly towards the middle line, so that the passage is completely closed but for the narrow slit between them. A good view of the cords is thus obtained, and they should be pearly white and glistening, with clean-cut edges. The vibra tion is little in amount, but very rapid.