270 Mr. Bishop's Experimental Researches ifito 



of the vibrating position, whilst, at the same time, the air is 

 acting in an opposite direction. 



The vibration of the edges of the glottis thus produced, by 

 communicating to the adjacent air an alternate state of con- 

 densation and rarefaction, the sounds of the voice are ac- 

 complished. 



The relative length of the vibrating surface of the glottis is 

 regulated conjointly by the pressure of the column of air in 

 the trachea, and the tension and resistance of the vocal liga- 

 ments. 



A certain degree of condensation of the air is therefore re- 

 quisite, the maximum varying with the proportion of the area of 

 the chest to that of the trachea. It is upon these hydrostatic 

 principles that the small muscles which close the glottis are ca- 

 pable of resisting the whole force of the respiratory muscles*. 



If a person possessing a deep bass voice be directed to expel 

 his breath in a manner not quite sufficient to yield the lowest 

 possible note, on applying the ear to his mouth a clicking mo- 

 tion is perceived. If the tension and velocity of the air are 

 now increased, the clicking ceases, and a continued sound is 

 produced, but of an exceedingly grave pitch. During the 

 previous state, Dr. Young f observes, " a delicate ear may 

 detect the vocal chords vibrating twenty-six times in a second, 

 or about two octaves below the A of a common bass voice." 



The intensity of the voice in the same medium:!^ > ^"'^ under 

 similar collateral circumstances, depends on the pressure of the 

 column of air in the trachea, and the range of motion performed 

 by the vibrating edges of the glottis. 



The true vibrating surface of the glottis is the mucous mem- 

 brane. The vocal chords confer on it the tension, resistance, 

 position, and probably other conditions necessary for vibra- 

 tion. Aphonia often results from undue relaxation of this 

 membrane. 



When we consider the fundamental pitch of the human 

 voice, and compare the length of the vocal ligaments with that 

 of stringed instruments, or the length of the vocal tube with 

 that of wind instruments, we must at once perceive that the 



* The force of the expiratory muscles is about a pound on every square 

 inch of the surface of the chest. 



t Nat. Phil., vol. ii. 



+ Acording to Derham, the human voice has been heard at the distance 

 often miles at Gibraltar. Boussingault, in his ascent of Chimborazo with 

 Colonel Hall,'at the height of 6004 metres, found the voice scarcely audible, 

 inhaling hydrogen gas also greatly enfeebles the voice. The intensity of 

 the tone varies reciprocally with the density of the air under similar colla- 

 teral circumstances. 



