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THE HUMAN VOICE.

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tongue, free and movable in its anterior part, forms the floor of the whole passage between the root of the epiglottis and the incisor teeth of the lower jaw. The muscular mass forming the cheeks contracts the cavity of the mouth on the sides, and the lips by their mobility variously modify the aperture by which the air issues.

Thus the air issuing from the larynx may pass out either by the nostrils or the mouth. It passes out by the nostrils when the mouth is closed, or even when the veil of the palate descends. When the veil of the palate is raised, and the mouth is opened, a free passage is afforded, through what has been called the oral canal, outwards. The oral canal is manifestly capable of much greater modification as to size, than the passage of the nostrils.

"The tongue, the lips, articulate; the throat,

With soft vibration, modulates the note."-Darwin.

On the Human Voice.-In the investigation of the human voice, two points in particular deserve attention-first, the inquiry into the precise seat of the sounds; and secondly, into the mode in which these sounds are produced.

As to the first question, it is now determined, beyond all doubt, that the sound of the voice is generated in the glottis, and neither above nor below that point. Before going further, it should be remarked that this word glottis has not always been used in exactly the same sense. "By turns," says the eminent French physiologist, Adelon, "the superior aperture of the larynx, its inferior aperture, and the intermediate space between these two apertures, has borne the name of glottis; but, according to the etymology of the word, derived from yawsoa, the tongue, the speech, no other part of the larynx should be called by that name but that where the vocal sound is formed—and we shall see that that part is the inferior aperture or chink."-Physiologie de L'Homme, ii. 256. In this sense alone, then, the word glottis is here employed, namely, to signify the aperture between the two vocal ligaments, that is, between the two inferior vocal cords, as they are sometimes called.

Among the proofs that this chink, or glottis, is the seat of voice, it may be mentioned, that if an aperture exist in the windpipe, the sound of the voice ceases. Such an aperture is frequently formed in man as a surgical operation, and an opening has often been made in the same situation in animals for the purpose of experiment. Also, when an opening exists above the glottis, that the voice is not lost. Again, that though the epiglottis, the superior vocal ligaments of the larynx, and the upper part of the arytenoid cartilages, be injured, the voice is not lost: moreover, that in living animals, when the glottis is laid bare, it is seen that the inferior ligaments of the larynx which form the boundaries of the fissure termed glottis, are

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SEVERAL THEORIES OF VOICE.

thrown into vibration: it is known, too, that the division of the laryngeal nerves supplying the muscles, which regulate the states of the aperture, and make the vocal cords tense, destroys the power of producing vocal sounds. It is also found that sounds can be produced in the dead human body by forcing a current of air from the windpipe through the larynx, provided the vocal cords be in some degree tense and the glottis be narrow. The larynx has been cut from the body, and freed from all the parts in front of the glottis; thus, the epiglottis, the upper vocal ligaments, and the ventricles of the larynx between the superior and inferior, or vocal ligaments, the greater part of the arytenoid cartilages, namely, their upper part, may be removed in short, if nothing remain but the inferior ligaments or vocal cords, and these be so approximated that the glottis shall be narrow, clear tones will be produced by forcing air through it from the windpipe.

Such facts as these entitle us to regard the glottis and the vocal cords, which form its immediate boundaries, as the essential source of voice, while the windpipe simply conveys air, and the cavities above the glottis, comprehending the upper part of the larynx and the air passages through the mouth and nostrils, correspond to the tube of a musical instrument, by which the sound is modified, but not generated.

It has been already remarked that the vocal ligaments are composed of elastic tissue, and that it is owing to this elasticity that they are adapted to the office which they perform. While, then, it is quite certain that no proper vocal sounds can be produced, except in the glottis, it seems manifest that the adjacent somewhat abundant tissue of the same kind is susceptible of a vibration and resonance in unison, so as at least to modify the sounds of the voice.

In reference to the second question · - what is the nature of the change produced in the glottis during the formation of voice-no inconsiderable difficulty is met with. The points of debate which have arisen on this subject are, whether the vocal ligaments be a set of membranous cords obeying the laws of musical strings; if the aperture of the glottis be a reeded instrument, in which the vocal ligaments play the part of vibrating tongues; or even whether the real source of the sounds of the voice be not a molecular vibration of the air, produced by its passage through the narrow aperture of the glottis; and, lastly, whether the organ of the voice does not in part combine all these three sources of sound, so as to be at once, in some respects, a stringed instrument, a tongued instrument, and a simple wind instrument.

The ancients regarded the sounds of the voice as analogous to those of a flute. According to this view, the vibrations of the larynx are of little account, the actual sounds being produced by a

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molecular undulation of the air. That the organ of voice is not, in some degree, analogous to this kind of musical instrument, is not to be absolutely denied, but it is certain that this is not the principal mode in which the sounds are produced.

One of the earliest ideas of modern times on the subject of the voice is, that the larynx is analogous to a horn; that is to say, to a wind instrument, in which the vocal cords act the same part as the lips of the performer on a horn. Not much more than a hundred years ago arose the idea that the larynx is a set of musical cordsnamely, that the vibrations of these cords, on the same principle as a stringed instrument, produce the sound, which is then conveyed outwards by the air.

The prevailing opinion of the present day is, that the larynx is a wind instrument, but a reeded wind instrument.

This common view may be expressed as follows:-the expired air is thrown into the larynx through the windpipe by the muscular action of the chest; the proper muscles of the larynx being contracted, create a sufficient tension of the vocal cords to permit them to be thrown into vibration by the impulse of the air. The sound so produced is conveyed through the mouth and nasal passages, undergoing various modifications in its passage outwards.

FIG. 90.

Let us consider then, in the first place, what evidence there is that the organ of the voice is a reeded instrument, with a double membranous tongue.

In short, the action of the organ of voice may be best explained in general terms, by comparing it with the pipe of an organ. Let us suppose Fig. 90 to be the wind-tube, into which the air is driven from below; b, the stopper, in which is placed the tongue; a and t the body-tube; and let there be a pipe, o (Fig. 91), to the wind-box, c c, and the air be t driven from the bellows, ffp, through t. The air throws the tongue, a (Fig. 90), into a state of vibration, and passes out in undulating movements from the body-tube. Such is

a general view of the nature of voice.

FIG. 91.

[graphic]

An experiment has been before referred to, which illustrates the

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RECEIVED THEORY OF VOICE.

effect of an elastic organic tissue, like that of the vocal ligaments, in producing sound on the principle of a double tongue. The extremity of a tube is closed by two bands of moist elastic tissue, for example, arterial tissue, so applied as to cover the whole end of the tube, with the exception of a slight fissure between the bands. In the experiments before referred to, India-rubber, or leather, was mentioned as being employed for this purpose. Both these substances produce a similar effect, but it appears that the middle arterial coat, being composed of the same tissue as the vocal ligaments, and having the same physical properties, forms the best kind of artificial larynx. When this tube is blown through at the free extremity, the tongues not only vibrate readily, but produce a range of musical tones. To obtain a pure quality of tone, it is necessary that the two membranous bands should be of equal weight and breadth, and subject to equal tension, otherwise they cannot vibrate equally in equal parts of time.

If the human larynx be dissected out, and the vocal cords be stretched, they will vibrate like a piece of artificial tissue, such as India-rubber or leather, in a current of air. In conducting these experiments, the same conditions must be secured as are required in the experiment with the tube, and the two membranous laminæ, before referred to. For example, the inner edges of the glottis, that is to say, of the vocal ligaments, must be turned outwards towards each other, so that they shall be in the same plane and parallel to each other, otherwise they will not produce any sound. Hence it may be inferred, that when the tension of the vocal ligaments takes place in the living animal, they turn upon their axis, till their planes which, in the state of relaxation, are inclined to the axis of the vocal tube, become perpendicular to it, and as the edges of the glottis approximate, its chink is nearly or entirely closed, and they acquire the true vibrating position. The production of the most simple tones of the voice requires the associated action of a most extensive range of organs; for it is calculated that in the ordinary modulation of the voice, more than one hundred muscles are brought into action at the same time.

As the air rushes up from the windpipe, a portion of each edge of the glottis yields to its pressure, and is curved upwards, so as to form a angle with the axis of the vocal tube, and leave between the two edges a narrow aperture, through which the air escapes. The tension and elasticity of the vocal ligaments tend to restore them to the plane of their former position. The air having been rarefied below the glottis during their elevation, becomes dense from their depression, and the necessary force being again accumulated, they are re-elevated, and thus an oscillating movement, consisting of an opening and closing of the glottis, takes place, which being com

EXPERIMENTS ON DEAD LARYNX.

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municated to the contiguous air, the sounds of the voice are produced.

FIG. 92.

The vibrating edge of the glottis varies in length according to the pressure of the column of air in the windpipe, and the resistance of the vocal ligaments. When other circumstances are alike, the intensity of the voice is determined by the pressure of the column of air in the windpipe, and the range of movement described by the vibrating edges of the glottis. The pitch of the voice does not depend solely on the tension of the vocal ligaments, but jointly on the variations, which they undergo in length and tension. Magendie observed, in the larynx of a dog, that a longer portion of the vocal ligaments vibrated while grave tones were produced, and that a diminution of length accompanied the succession of acute tones. Mayo has described the movements of the glottis in a man who had attempted to destroy himself by cutting his throat. The larynx in this case was cut through just above the vocal cords, and, owing to the oblique direction of the wound, an injury of the arytenoid cartilage and of the vocal cord on one side had occurred. When respiration was going on, the glottis was seen to be of a triangular form, but when the voice was exerted, the vocal cords passed into a parallel direction, and the glottis itself had a linear form. The posterior part of the aperture appeared to remain unclosed.

The cut represents the prepared head of a corpse, after Müller. A thread e, which passes over a roller to a scale, is so applied to the larynx that the tension of the vocal cords

can be increased by placing a greater weight on the scale. The action of the muscles is thereby imitated. The compressing apparatus seen on the wood-cut brings the vocal cords nearer to each other, and thus produces the requisite diminution in the width of the vocal fissure. The tube serves to convey the wind, which throws the tongue-apparatus into action. And thus, if we use the human head, or the head of the dog, or of the pig, or of any other animal, we can imitate the voice of man, the bark of the dog, the grunt of the pig, &c.

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