1482 



VOICE. 



of the vocal cord, which, as we have seen, 

 are analogous to those of a musical string, 

 produce a corresponding shortening and elon- 

 gation of its axis, regarded as a tongue ; and 

 lastly, since one tone only is produced at a 

 time, the vibrations resulting from the double 

 action which appears to exist in the vocal 

 apparatus must be synchronous. 



We have seen how nearly, when we take 

 into account the delicacy and difficulty of the 

 experiments, their results agree with the 

 theory that the vocal cords are subject to the 

 same laws as other stretched lamina?, and it 

 would be highly interesting to compare these 

 results with the simultaneous variations which 

 they undergo transversely, and thus discover 

 how far the laws of vibrating elastic tongues 

 may be applied to them. It might possibly 

 be objected to the idea of this twofold action, 

 that the production of sound by the vocal 

 cords is sufficiently accounted for by suppos- 

 ing them to vibrate merely as elastic tongues ; 

 but then it is found by experiment, that by 

 artificially dividing their length into two ven- 

 tral segments, there results the octave of the 

 fundamental note, which proves that at all 

 events they vibrate as cords. In conclusion, 

 we must ever bear in mind the vast difference 

 between natural and artificial mechanism, and 

 however complicated a problem it may be to 

 determine that constitution of the vocal ap- 

 paratus, by which the thyro-arytenoid liga- 

 ments may simultaneously obey the laws of 

 cords and tongues, yet to a physiologist who 

 is accustomed to meet with, the most admir- 

 able contrivances and combinations in the 

 animal frame, the difficulty of finding a strictly 

 mathematical solution is, in such a case, no 

 objection to its truth, when the facts, as far 

 as they have been observed, are decidedly 

 favourable to its reality. Were the move- 

 ments of the glottis independent of any tube 

 or column of air, the study of the functions 

 of the vocal organs would be much more 

 simple ; but we find it situated nearly in the 

 centre of the vocal tube of which the trachea 

 and bronchi are the inferior, and the upper 

 part of the larynx, pharynx, nose and mouth, 

 the superior portion ; we have therefore to 

 consider the influence of this tube, and of its 

 inclosed column of air in the production of 

 voice. 



In order to investigate the mutual relations 

 between a reed and a pipe, two methods 

 may be adopted : one of these is to vary the 

 pitch of the reed while the length of the pipe 

 remains constant, and the other to vary the 

 length of the pipe with a reed sounding one 

 tone only when detached from the tube. In 

 the construction of reeded pipes for musical 

 purposes, it is incumbent on the mechanician 

 to adjust the length of the tube to the pitch 

 of the reed. When a free reed is used on 

 the principle of Kratzenstein or Grenie, it is 

 found that, if the pipe be not in perfect unison 

 with the reed, the purity of the tone de- 

 creases within certain limits, as the discord- 

 ance between the reed and pipe increases. 

 The researches of MM. Biot, Weber, Willis 



and Miillcr have greatly enlarged our know- 

 ledge on this subject. We learn from their ex- 

 periments how great an influence is mutually 

 exerted between a pipe and its reed, when 

 the pitch of the one is made to vary while 

 the other remains constant, and we may con- 

 clude that analogous effects are produced 

 between the vocal tube and the glottis. The 

 slightest knowledge of acoustics is sufficient 

 to inform us that the pitch of any pipe, such 

 as the organ, the flute, the trumpet, in short 

 of all musical tubes vibrating in a similar 

 manner, depends on the velocity of an im- 

 pulse propagated in the air within, and is 

 determined by the length of the pipe. As 

 long as the tubes of musical instruments re- 

 main rigid, the nature of the materials which 

 compose them does not affect the pitch of 

 the sound, but merely influences the quality 

 of the tone, and it is indifferent whether we 

 employ metal, wood, or paper in their con- 

 struction ; each of these substances will yield 

 a tone of a particular timbre, or quality, de- 

 pending on the nature of the motions pro- 

 duced among its particles by the friction of 

 the air on its surface; but the pitch will be 

 the same in each, if the lengths of the pipes 

 be equal, proving that the air itself is the 

 source of sound. When, however, the sides 

 of the tube are composed of flexible mem- 

 branes, the inclosed air has a vibratory mo- 

 tion, conjointly with, and subordinate to, that 

 of the parietes of the tube, whereby the pitch 

 of the sound is affected, as well as its quality. 

 M. Savart* found that by taking tubes com- 

 posed of layers of paper of constant length, 

 but varying in thickness, graver sounds were 

 produced as the parietes became thinner, and 

 that the gravity of the sound was increased 

 by moistening and relaxing the sides of the 

 tubes. We shall presently see the application 

 of these facts to the vocal apparatus. 



We find the flexibility of the trachea and 

 bronchi capable of being varied by the opera- 

 tion of two forces, the one longitudinal or 

 parallel to the axis of the tube, the other 

 transverse. The first of these comprises the 

 muscles which elevate and depress the larynx; 

 the latter, the cartilaginous segments of rings 

 perpendicular to the axis of the tube ha\ing 

 muscular fibres attached to their posterior 

 extremities, the contraction and elongation of 

 which regulate the diameter of the trachea. 

 The pharynx, mouth and nasal cavities, which 

 form the superior extremity of the vocal tube, 

 are also provided with muscles to modify the 

 tension of that part of the tube so that it 

 may vibrate synchronously with the rest. 

 The necessity for this change in the dimen- 

 sions of the tube, in order that it may vibrate 

 in unison with the glottis, is in accordance, 

 not only with the joint system of pipe and 

 reed above described, but also with what 

 actually takes place in the vocal organs of 

 living animals. When the voice is raised in 

 the scale from grave to acute, a corresponding 

 elevation takes place in the larynx towards 



* Annales de Chemie et de Physique, torn, xxxii. 



