VOICE 309 



body ; a pulse of alternate rarefaction and condensation is set up in 

 it by the interference, at regular intervals, of the vocal cords with 

 the expiratory blast. Forced abruptly from their position of equi- 

 librium as the blast begins, they almost immediately regain and 

 pass below it, in virtue of their elasticity, and continue to vibrate as 

 long as the stream of air continues to issue in sufficient strength. 

 Not only do they vibrate up and down, but also towards and away 

 from the middle line, so that, at least in the chest voice, they come 

 into contact with each other at each swing. The sound-waves thus 

 set up spread out on every side, impinge on the tympanic membrane, 

 set it quivering in response, and give rise to the sensation of sound. 

 We may say, in a word, that the whole exquisite mechanism of 

 cartilages, ligaments, and muscles, has for its object the production 

 of a sufficient pressure in the blast of air driven through the wind- 

 pipe by an expiratory act, and of a suitable tension in the vibrating 

 cords. An approximation of the cords, a narrowing of the glottis, 

 is essential to the production of voice; with a widely-opened glottis 

 the air escapes too easily, and the necessary pressure cannot be 

 attained. The pressure in the windpipe was found in a woman 

 with a tracheal fistula to be about 12 mm. of mercury for a note of 

 medium height, about 15 mm. for a high note, and about 72 mm. 

 for the highest possible note. The period of vibration of structures 

 like the vocal cords depends on their length, thickness, density and 

 tension ; the shorter, thinner, more dense and less tense a stretched 

 string is, the greater is the vibration frequency, the higher the note. 

 In the child the cords are short (6 to 8 mm.), in woman longer 

 (10 to 12 mm. when slack, 13 to 15 mm. when stretched), in man 

 longest of all (14 to 18 mm. in the relaxed, and 18 to 22 mm. in the 

 stretched position) ; and the lower limit of the voice is fixed by the 

 maximum length of the relaxed cords. A boy or a woman cannot 

 utter a deep bass note, because their vocal cords are relatively 

 short, and do not vibrate with sufficient slowness. It is true that 

 by the action of the crico-thyroid muscle the cords can be length- 

 ened, and that the maximum length in a woman approaches or 

 exceeds the minimum length in a man. But the lengthening of the 

 vocal cords in one and the same individual is always accompanied 

 by other changes increase of tension, decrease of breadth and 

 thickness which tell upon the vibration frequency in the opposite 

 way, and more than compensate the effect of the increase of length, 

 so that for high notes the cords are longer than for low. The con- 

 traction of the thyro-arytenoid muscle is a more influential factor 

 in altering the tension of the cords than the contraction of the crico- 

 thyroid. It is probable that, when the highest notes are uttered, 

 only the anterior portions of the cords are free to vibrate, their 

 posterior portions being damped by the approximation of the vocal 

 processes of the arytenoid cartilages by the contraction of the 



