The study of the formation of sounds in the glottis includes that faculty which man possesses of producing the sounds of whistling. This is certainly a much less important and less elevated function; but it is nevertheless very interesting to the physiologist, as it evidently nearly resembles that of the voice in its mechanism.
In order to produce the sound of whistling, the lips form a real glottis, which Dodart has named the labial glottis. The opening between the lips varies in form; in the grave tones it is nearly round in shape, and at its maximum in diameter; in the acute sounds it becomes elliptic, and is reduced to a narrow slit; the tongue regulates the intonation, by approaching more or less to the lower front teeth, touching them in the acute sounds, and withdrawing itself in the grave sounds. The space which separates the lips from the teeth varies also, in the same relative degree, for the same reason. The tongue sharpens the notes as in flute-playing; the grave sounds may be produced in drawing in the air, as in breathing; in short, the sound is acute or intense in proportion to the impulsion of the air by the lungs.
If a disk of cork be placed between the lips, about one-fifth of an inch in thickness, with a hole about one line in diameter in the centre, the sound of whistling can be produced through this aperture, and modulated the same as with the lips. Cagniard de la Tour, to whom we are indebted for this experiment, concludes from it that the sound does not proceed from the vibrations of the lips; but has its origin in those of the air, excited by an intermittent friction against their walls. Longet and Masson compare the apparatus for whistling in man to the whistle of a bird-catcher, and they find a close analogy between the labial and the laryngeal glottis.
Fournie rejects this theory, and supposes the sound of whistling to be produced by mechanism analogous to that of an organ-pipe, the air breaking against the stop, which is represented by the upper incisors. Whichever doctrine we may accept, it is certain that the lips, or the perforated disk which replaces them, play an important part in the production and modification of sound in ordinary whistling, for when these sounds are made without the aid of the lips, by a peculiar disposition of the tongue, it is only a single sound. It is the same in whistling through the teeth with the lips drawn apart, or when the tongue being doubled, and the fingers placed in the mouth, we produce an intensely acute sound, but which cannot be modulated.
In the apparatus for whistling, as in that of the voice, the functional disposition, and its modifications in relation to the sounds emitted, takes place by movements under the control of the will, although they are, so to speak, instinctive. The changes in the dimensions of the orifices and of the buccal tube, in the tension of the walls of the mouth, the impulsion of the air, etc, are all effected instantaneously, and in such a manner as to produce all the notes. No instrument of music equals the perfection of this apparatus.