262 A MANUAL OF PHYSIOLOGY 



spread out. on every side, impinge on the tympanic mem- 

 brane, 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 windpipe by an expiratory act, and 

 of a suitable tension in the vibrating cords. An approxima- 

 tion 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 mer- 

 cury 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, and tension ; the shorter, thinner, 

 more tense and less dense 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 lengthened, 

 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. 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 lateral crico- 



