Like most physiological questions, that of the emission of the voice is differently explained by writers on that subject. In order to explain its functions, the larynx has been compared to different musical instruments. Gerdy thought "that we should do better to endeavour to show that this instrument in man has no resemblance to any one formed by art." This, doubt less, is true; the human larynx is as inimitable in its perfection as it is admirable in the results it produces; but, in comparing the most ingenious machines of this character that man has ever constructed, with the larynx, we do precisely what Gerdy recommends, for this is the surest means of establishing its evident superiority. The analogy is besides incontestable, in spite of the distance which separates an inert mechanical production from a living organic apparatus and it is only by studying the formation of sounds in instruments that we can, if not explain, at least seek to comprehend their formation in the larynx.
The vocal apparatus of man is composed of the lungs, acting as a bellows; the trachea, which conducts the air from the lungs to the larynx, where the sound is formed; and of the pharynx, the buccal, and nasal cavities, which increase the sounds and modify their character.
The air driven through the glottis by the lungs causes a vibration of the vocal cords, and sound is produced; it is increased by passing through the upper part of the larynx, the mouth, and nasal fossæ; it acquires more or less volume, and its character varies according as these cavities are more or less open and free; but it does not change its nature as regards the tone. If, for example, the glottis emits a C, it may be heard as a muffled, a natural, or a nasal sound, according to the condition in which the cavities are through which it passes; but the tone does not change, it is always a C.
Savants have held different opinions on the formation of sounds in the larynx, and upon the functions of the constituent parts of the vocal organs. It being impossible to consider all these opinions, or the many experiments which have been made, the physical laws upon which they were founded, or which were opposed to them, we shall confine ourselves to a summary explanation of a few of them.
We may be permitted, however, first to quote from the Magasin Pittoresque an anecdote which very well illustrates this point of our subject:—
In 1798 Cuvier, in reading an essay on the voices of birds before the Academy of Sciences, remarked that some physiologists considered the larynx as a stringed instrument, others as a wind-instrument An academician spoke and denied this distinction, affirming that everybody knew that the larynx was a wind-instrument "You are in error," immediately exclaimed another member, "it is a stringed instrument"
These two theories have long divided philosophers.
Galen looked upon the glottis as a reed; Fabricius Acqua-pendente gave a remarkable description of the larynx in the sixteenth century; he recognized the glottis as the essential organ of the voice, and compared its action to that of an organ-pipe; the air in breaking against it produced the sound, the glottis being less open for acute than for grave sounds.
Dodart at the end of the seventeenth century, after hesitating between the vibration of the air or the vibration of the vocal cords as the origin of sound, compares the glottis to the mouth-piece of a hautboy. This great physiologist, in giving successive explanations, differing as widely as possible from each other, of the phenomena of the voice, has advanced or hinted at most of the theories which have been projected since his time.
In 1741 Ferrein compared the vocal cords to the strings of a violin, the air acting as the bow.
Biot could see nothing in the glottis which resembled a vibrating cord. "The simplest principles of acoustics," said this illustrious physicist, "are sufficient to make us reject this strange opinion." muller advocated the theory of Ferrein against that of Biot, and yet he admits with him and with Magendie, Cagniard de la Tour, G. Weber, and other learned men, that the glottis is a reed with two membranous lips vibrating under the action of the air, and producing the sound by their vibrations.
Savart compared the human glottis to a bird-catcher's whistle surmounted by a supply-pipe, the cavities of the whistle being represented by the ventricles of the larynx, the openings by the interval between the vocal cords. The air vibrated, he thought, in traversing the inferior glottis, and divided into two columns against the superior vocal cords, which act as the stop in an organ-pipe, one of these columns of air in vibrating causes the resonance of the air in the ventricles, the other causes the vibration of the air in the vocal tube. In this last hypothesis it is not the vibration of the vocal cords, but that of the air which produces the sound.
The theory of Savart on this latter point has been admitted by Longet and Masson. They believe the sound is produced at the orifice of the inferior glottis the same as at the mouth-piece of wind-instruments, by the periodically variable passage of the air, which becomes the seat of a vibratory movement. The inferior vocal cords and the ventricles are necessary for voice; the superior vocal cords should be considered simply as a means of perfection in regard to the variation and modulation of sounds.
Haller thought the epiglottis had no influence on voice, but that it permitted the swelling of the sounds into grave or acute without changing the tone, as did also Magendie and Biot Longet thinks that it assists in the expulsion of the air by the nasal fossæ in the production of very acute sounds, and that it may contribute to the timbre of the voice.
The part of the pharyngeal, buccal, and nasal cavities in the production and modification of sound is differently stated by different authors. Savart thought that they regulated the height of vocal sounds; they have been considered as supplementary apparatus, and that it is to their peculiar resonance that the quality of the voice is due.
The invention of the laryngoscope, by enabling us to see the interior of the larynx, has given us exact notions of its function. It enables us to verify the changes of form, the appearance of the glottis at different ages, and during the emission of the voice. This method of studying the larynx has resulted in late years in works of the greatest interest, M. Fournie considers the larynx as a membranous reed-instrument, and the mechanism of the voice, according to this learned observer, is as follows:—The vocal cords produce the sound by their vibration, but do not vibrate in their totality. Fixed in front and at the back, at the level of their free border, they may be separated by the air, but not caused to vibrate in their entire thickness, while the mucous membrane which covers their free border, and which adheres only very slightly, detaches itself under the influence of the passing air, and thus forms the free vibrating portion of the reed. On this part only of the larynx the epithelium of the mucous membrane is of the same nature as that of the membranes in the other parts of the organism which are subjected to constant friction, as those of the articulations for example, and this gives it the power of resisting the friction of the air and of the vibrations which it causes. In the emission of the voice the vocal cords are stretched lengthwise and breadthwise. By producing mechanically this double tension of the vocal cords in the larynx after death, M. Fournie obtained all the notes comprised between two octaves.
The ventricles of the larynx are contracted and almost effaced during the emission of the voice; their function seems to be to moisten the vocal cords with a mucous fluid, and to aid in their movements, as well as those of the walls of the vestibule and glottis.
The superior thyro-arytenoid ligaments adapt the tube formed by the vestibule of the glottis to the sounds emitted by the vocal cords. During the emission of sound their borders are never on the same line as the opening of the glottis; sometimes they approach the vocal cords, sometimes they almost disappear, or, on the contrary, enlarge into the vestibule of the glottis so as almost to fill it. By experiments on the dead subject, M. Fournie has proved that if these ligaments are drawn apart during the emission of a note by the larynx, the sound is lower by one tone; if only one is drawn aside it falls a semitone. The same result is produced in all the notes comprised in an octave.
The epiglottis descends and- nearly closes the superior opening of the larynx in grave sounds, and rises more and more as the sound becomes more acute. In the grave notes the soft palate permits the sounds to pass through the mouth and the nasal fossæ, and in proportion as the voice is elevated it rises toward the posterior orifice of the nasal fossæ, so as to prevent the echoing of the sound in these cavities.
The nasal fossæ give exit to the air when the disposition of the vocal tube is such, in the formation of certain letters, as more or less to hinder its passage through the mouth. The isthmus of the throat and the mouth have no influence on the note, but they perfect and modify its character. The trachea and bronchia, as well as the vocal tube, resound like a harmonic table, of which each part corresponds to one of the notes of the voice; and lastly, the intensity of the sound is in direct proportion to the force of the impulsion of the , air, to the extent of the vocal cords put in vibration, and to their tension.