STRUCTURE AND ACTIONS OF THE LARYNX. 459 



concerned in the acts of Coughing and Sneezing ( 381), as well as in the 

 more prolonged impediments to the ingress and egress of air, which have been 

 already noticed as resulting from disordered states of the Nervous system 

 ( 504). A slight examination of the recent Larynx is sufficient to make it 

 evident that, when once the borders of the Rima Glottidis are brought toge- 

 ther by muscular action, the effect of strong aerial pressure on either side, 

 whether produced by an expulsory blast from below, or by a strong inspiratory 

 effort, occasioning a-'partial vacuum below, and consequently an increased 

 pressure above, will be to force them into closer apposition. With this 

 action, then, 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 Arytenoid 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 comparing the time occupied by an ordinary expiration, 

 with that required for the passage of the same quantity of air during the sus- 

 tenance 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 pas- 

 sage 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 approximation, 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 experimentally shown to be necessary, for their being thrown 

 into sonorous vibration. 



606. 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. 



a. These are chiefly of three kinds, strings, flute-pipes, and reeds or tongues. The Vocal 

 Ligaments 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 proportion 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 varia- 

 tion 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 a certain length, 

 which is adapted to give out a clear and distinct note, equal in depth to the lowest of the 



