1478 



VOICE. 



throat. In this case the larynx was divided 

 immediately above the vocal cords, and in 

 consequence of the oblique direction of the 

 wound, the arytenoid cartilage and the vocal 

 cord on one side were injured. During 

 respiration the glottis was observed to assume 

 a triangular form, but when a sound was 

 uttered, the chordee vocales became nearly 

 parallel, and the rima glottidis of a linear 

 ibrm. The posterior part of the aperture 

 did not appear to be closed. In a second 

 case of this kind, he observed that the aryte- 

 noid cartilages, as long as the vocalization of 

 the breath continued, maintained the position 

 which they had assumed when the glottis 

 was closed entirely.* The vibrations of the 

 thyro-arytenoid ligaments are considered by 

 Ferrein-f- to be analogous to those of strings ; 

 hence he denominated these ligaments (though 

 improperly) chorda; vocales. He imagined 

 that the longitudinal tension of these cords 

 alone governed the pitch of the voice. Mr. 

 Willis j has embraced the hypothesis of 

 Ferrein ; he observes, that to obtain the 

 various notes of the glottis, it is only ne- 

 cessary to vary its longitudinal tension after 

 the ligaments have been placed in the proper 

 position j but M. Biot remarks, " Qu'y 

 a-t-il en effet dans la glotte qui ressemble a 

 une corde vibrante? Comment pourroit-on 

 en tirer jamais des sons d'un volume com- 

 parable a ceux que 1'homme produit ? Les 

 plus simples notions d'acoustiques suffisent 

 pour faire rejeter cette etrange opinion." 



On inspecting the larynx from above, we 

 see two very nearly rectangular-shaped la- 

 minae, one on each side of the chink of the 

 glottis, but nothing resembling an isolated 

 cord. The mucous membrane which lines 

 the thyro-arytenoid ligaments (to which it 

 closely adheres), as well as the rest of the 

 vocal tube, must be considered as forming a 

 part of the weight of the vibrating surface 

 upon which the air acts ; the thyro-arytenoid 

 ligaments confer on this membrane the re- 

 quisite tension and resistance during vocaliza- 

 tion, and it is this membrane which gives the 

 sides of the glottis their laminated figure. 

 The vocal ligaments, with their lining mem- 

 brane, are stretched by the thyro-cricoid 

 muscles, not all round like a drum, but in 

 one direction only, namely, in that of their 

 length, being attached on three sides, leaving 

 one only free to vibrate. The vocal cords 

 are, as h'-is been seen, rectangular-shaped 

 membranes, and from experiments made on 

 the larynx after death by Ferrein, Miiller, 

 and others (which the author lias repeatedly 

 verified), are found to vibrate like cylindrical 

 cords ; we will therefore apply to the former 

 the well-known formulae which regulate the 

 vibrations of the latter. 



In cords composed of the same material, 



* Mayo, Outlines of Physiology, p. 991. 



f Mcmoires de 1'Academie. 1741. p. 400. 



J Cambridge Philosophical Transactions, vol. iv. 



Precis Elem. de Phys. toin. i. p. 398. 



and of uniform thickness, the time of a com- 

 plete musical vibration, or double oscillation, is 



t=2 /JP_ 

 V 2 gP 



where I is the length of the cord, p its weight, 

 P the force with which it is stretched, and 

 gzzrlGJ, feet. 



In order to apply this formula to the vocal 

 ligaments, let a be their depth, b their breadth, 



T/te apparatus employed for making experiments on 

 the Human Larynx. (After Miiller.) 



N, shaft or column for the attachment of the larynx ; 

 /; the forceps fur compressing the larynx late- 

 'rally ; u, the bellows pipe ; v, the manometer con- 

 nected with the tube 11, for estimating the tension 

 of the air used in the experiments. M, o, columns 

 for the attachments of the pulleys x 1 and y' ; ,r, 

 a line by means of which the vocal cords are ex- 

 tended in the direction of their length ; it passes 

 over the pulley x< ; ?/, a line passing over the 

 pulley y', by means of which the vocal cords may 

 be relaxed and reduced to their minimum length, 

 thus performing the office of the crico-tliyroid 

 muscle ; z, a line by means of which the vocal 

 cords may be extended by drawing them down- 

 wards and forwards. 





