228 PRINCIPLES OF PHYSIOLOGY 



of the cords come closer together and the 

 glottis becomes much narrower. If voice is 

 now to be produced, as when a note is sung, 

 the margins touch and the glottis is entirely 

 closed for an instant. The pressure of the 

 air below the cords is increased by the expira- 

 tory effort, and there is a puff of air sent out 

 between the margins of the cords. This 

 relieves the pressure and instantly again the 

 glottis is closed by the elasticity of the cords. 

 Again the pressure rises and there is another 

 puff and so on, and the margins of the cords 

 thus move with each puff ; in other words, they 

 vibrate. So that the organ of voice is on the 

 principle of the siren, an acoustical instru- 

 ment by which musical tones are produced 

 by the fusion of individual puffs of air. 



136. The vocal cords can be tightened or 

 relaxed, and their free margins can be separ- 

 ated or approximated by the action of special 

 muscles. Thus, in singing a scale, beginning 

 with the low note, the cords are gradually 

 tightened by two muscles, the crico-thyroids, 

 passing from the sides of the signet of the 

 cricord upwards and forwards to the thyroid. 



