1476 



VOICE. 



futile in the present state of the science of 

 acoustics to attempt any mathematical solu- 

 tion of the laws of the equilibrium and move- 

 ments of the heterogeneous masses of the 

 vocal tube. When, however, a membrane is 

 stretched in one direction only, it obeys the 

 same laws as a string. 



Fig. 890. 



An outline of the transverse section of the Human 



Larynx. (Outline of fig. 18, Vol. HI. p. 100, in 



which, the different parts are indicated.) 



x, x, the plane of the vibrating position of the vocal 

 cord ; y, y y, y, the plane of the respiratory posi- 

 tion. 



Having adapted two laminas of India rubber 

 to a pipe connected with the bellows of an 

 organ, M. Biot caused a current of air to 

 pass over their free edges, by which means 

 he obtained with facility sounds of different 

 pitch. Professor Willis made similar expe- 

 riments with leather and caoutchouc, but 

 could not produce with these substances so 

 great a range of tones as the glottis will 

 yield, and therefore concluded that the vocal 

 ligaments possess greater elasticity. Mr. 

 Willis has also investigated the position in 

 which it is necessary that membranous laminae 

 should be placed, in order that they may be 

 excited and sustained in a state of vibration. 

 He has likewise given a satisfactory explana- 

 tion of the mode of action of the air on reeds, 

 such as those of the organ pipe, which applies 

 also to free reeds, and every other case where 

 a vibratory motion is maintained by a current 

 of air. 



The experiments and theory of Mr. Willis 



are exceedingly important, for he has shown 

 that in ordinary breathing the vocal cords 

 remain inclined to each other, at an angle 

 which prevents any vibratory motion ; whereas 

 when their surfaces lie in the same plane, the 

 breath immediately excites them into a state 

 of vibration ; the natural position of the vocal 

 cords in these two states is seen in (fig. 890). 

 Mtiller also made some experiments on 

 stretched membranous bands, both isolated, 

 and in connection with a tube ; from which he 

 concludes that the force of the current of air 

 influences the pitch of the note produced; so 

 that a strong current will produce a more 

 acute tone than a weak one, and vice versa ; 

 but the author has not found this to be the 

 case in any of the experiments which he has 

 made. To obtain a pure quality of tone 

 when two membranous bands are stretched 

 across a tube, it is necessary that they should 

 be of equal weight and length, and subjected 

 to equal tension, otherwise they cannot 

 vibrate freely in equal periods of time. Ac- 

 cording to Cagniard De la Tour, if two mem- 

 branous laminae of equal length and weight 

 be stretched by unequal forces, so that there 

 is an interval of a fifth between the notes 

 they yield separately, the note resulting from 

 their combined action is the intervening third. 

 Miiller is disposed to doubt the accuracy of De 

 La Tour, but his own views do not differ ma- 

 terially from those of the latter, as he says that 

 when one tongue is most readily thrown into 

 vibrations bv the current of air, the sound is 

 emitted by it alone, but if the blast is such 

 that it throws them both into motion, they 

 may both vibrate together, and by reciproca- 

 tion produce a simple sound intermediate 

 between the fundamental note of the two 

 vibrating separately ; they may also emit two 

 distinct sounds, or the blast being modified, 

 the two sounds may be produced in suc- 

 cession. From these researches it appears, 

 that membranous laminae, stretched in imi- 

 tation of the thyro-arytenoid ligaments, will 

 not only vibrate readily, but produce a range 

 of musical tones. It has been remarked that 

 sounds are most readily produced when the 

 two laminae are stretched in the same plane, 

 and that a smaller volume of air is required 

 the nearer the edges of the laminae approach 

 each other, and a still smaller one when their 

 edges actually touch. De Kempelen states, 

 that to produce sound, the edges of the 

 glottis must be approximated to within -^ 

 or at least y 1 ^, of an inch. These experi- 

 ments upon artificial vibrating tongues per- 

 fectly agree with those the author has made 

 on the larynxes of animals. Owing to the 

 nature of the articulation of the thyroid with 

 the cricoid cartilage, and the manner in which 

 the crico-thyroid muscles act, an equal ten- 

 sion of both the thyro-arytenoid ligaments is 

 simultaneously secured, supposing the aryte- 

 noid cartilages to be at the same time in 

 corresponding positions, which is a necessary 

 condition for the production of a synchronous 

 vibratory motion in the two lips of the 

 glottis. 



