212 HUMAN PHYSIOLOGY 



Children do not show this difference in voice. The change of 

 the voice in man takes place at puberty. Castration prevents this 

 change. 



One and the same individual produces tones of various 

 pitch by: 



(1) Changes in the tension of the vocal cords, produced 

 by: 



(a) Changes in the contraction of the muscles which 

 cause the tension of the cords to vary. 



(<) By changes in the force with which expired air is 

 emitted. 



If the air is exhaled forcibly, the vocal cords are brought 

 into a new and slightly raised position which produces a 

 greater tension than when they are stretched straight 

 between their points of insertion. 



The force of the exhaled air is generally 13-17 mm Hg. 



(2) Variations in the length and breadth of the vibrating 

 parts of the vocal cords. 



(a) Variation in the length of the vibrating part is pro- 

 duced by the greater or less pressing together of the aryte- 

 noids. If these are loosely pressed together, the edges of 

 the arytenoids also vibrate, but if they are firmly held, the 

 vocal cords only vibrate. In the first case the vibrating 

 reed is longer than in the second case. 



(/>) By peculiarities in the contraction of the thyro-aryte- 

 noid muscle or by the so-called false vocal cords being 

 placed upon the true vocal cords so that only the small 

 inner margins of the true cords are allowed to vibrate. Be- 

 cause of the small extent of the vibrating part, the pitch is 

 high. ' It is used in the so-called falsetto. 



(3) Variations in the thickness of the vocal cords. 



Fibres of the thyro-arytenoid muscle, having a perpendic- 

 ular direction, by their contraction cause the upper and 

 lower surfaces of the vocal cords to approach each other and 

 thus change their thickness. 



The range of the voice includes all the tones which an 

 individual can produce; generally it embraces two octaves. 



