THE LARYNX, AND ITS ACTIONS. 301 



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be to force them into closer apposition. With this action, th^u, the muscles 

 which regulate the tension of the vocal ligaments have nothing to do. In the 

 ordinary condition of rest, it seems probable that the Ary enoid cartilages are 

 considerably separated from each other ; so as to cause a wide opening to 

 intervene between their inner faces, and between the vocal ligaments, through 

 which the air freely passes ; and the vocal ligaments are at the same time in 

 a state of complete relaxation. In order to produce a vocal sound, it is not 

 sufficient to put the ligaments into a state of tension ; they must also be brought 

 nearer to each other. That the aperture of the Glottis is greatly narrowed 

 during the production of sounds, is easily made evident to one's self, by com- 

 paring the time occupied by an ordinary expiration with that required for the 

 passage of the same quantity of air during the sustenance of a vocal tone. 

 Further, the size of the aperture is made to vary in accordance with the note 

 which is being produced ; of this, too, any one may convince himself, by noting 

 the time during which he can hold out a low and a high note ; from which it 

 will appear, that the aperture of the Glottis is so much narrowed in producing 

 a high note, as to permit a much less rapid passage of air than is allowed 

 when a low one is sounded. This adjustment of the aperture to the tension 

 of the Vocal Ligaments, is a necessary condition for the production of a clear 

 and definite tone. It further appears that, in the narrowing of the Glottis 

 which is requisite to bring the vocal ligaments into the necessary approxima- 

 tion, the upper points of the Arytenoid cartilages are caused to approximate, 

 not only by being made to rotate horizontally towards each other, but also by 

 a degree of elevation ; so that the inner faces of the Vocal Ligaments are 

 brought into parallelism with each other, a condition which may be ex- 

 perimentally shown to be necessary for their being thrown into sonorous 

 vibration. 



405. We have now to inquire what is the operation of the Vocal Ligaments 

 in the production of sounds; and in order to comprehend this, it is necessary 

 to advert to the conditions under which tones are produced, by instruments of 

 various descriptions, having some analogy with the Larynx. These are chiefly 

 of three kinds, strings, flute-pipes, and reeds or tongues. The Vocal Liga- 

 ments were long ago compared by Ferrein to vibrating Strings ; and at first 

 sight there might seem a considerable analogy, the sounds produced by both 

 being elevated by increased tension. This resemblance disappears, however, 

 on more accurate comparison ; for it may be easily ascertained by experiment, 

 that no string so short as the vocal ligaments could give a clear tone at all to 

 be compared in depth with that of the lowest notes of the human voice ; and 

 also, that the scale of changes produced by increased tension is fundamentally 

 different. When strings of the same length, but of different tension, are made 

 the subject of comparison, it is found that the number of vibrations is in pro- 

 portion to the square roots of the extending forces. Thus, if a string extended 

 by a given weight produce a certain note, a string extended by four times that 

 weight will give a note, in which the vibrations are twice as rapid, and this 

 will be the octave of the other. If nine times the original weight be employed, 

 the vibrations will be three times as rapid as those of the fundamental note, 

 producing the twelfth above it. Now by fixing the larynx in such a manner 

 that the vocal ligaments can be extended by a known weight, Miiller has 

 ascertained that the sounds produced by a variation of the extending force 

 will not follow the same ratio ; and therefore the condition of these ligaments 

 cannot be simply that of vibrating cords. Further, a cord of certain length, 

 which is adapted to give out a clear and distinct note, equal in depth to tfce 

 lowest of the human voice, may be made by increased tension to produce all 

 the superior notes, which, in stringed instruments, are ordinarily obtained by 

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