in PHONATION AND ABTICULATION 147 



obstructs the passage of air, and increases the pressure in the 

 trachea necessary for throwing the vocal cords into vibration. 

 The loss of voice when the trachea is opened depends on the fall 

 of the pressure of the expiratory air below the minimum necessary 

 for the vibration of the vocal cords. 



But the pressure of the expiratory air would in itself produce 

 no musical effect if the vocal cords were not thrown into a proper 

 degree of tension by their tensor muscles. As we have seen, 

 paralysis of the anterior crico-thyroid muscles makes the voice 

 hoarse, and hinders the formation of high tones. 



The following general laws of the mechanism of the production 

 of laryngeal sounds may be deduced from experiments on animals 

 and observations on man : 



(a) The membranous glottis is the exclusive seat of voice 

 production. Lesions of the vocal cords render voice production 

 impossible. 



(b) The vocal cords acting as membranous tongues are thrown 

 into vibration by the pressure of the expiratory blast, and vibrate 

 synchronously with the air-current. The vibrations of the vocal 

 cords certainly produce a note, but its intensity is very low, hardly 

 to be compared with that of the tones arising from the larynx. 

 The true sounding body is the air, but the vibrations of the air 

 are determined by the vibrations of the vocal cords. 



(c) The vibrations of the air which are started in the glottis 

 are transmitted to the mass of air lying below as well as above 

 the vocal cords. The vibrations of air in the windpipe, bronchi, 

 and lungs are communicated to the thoracic wall, and can easily 

 be detected by applying the hand to the chest. This resonance 

 of the chest must certainly produce increased intensity of the 

 laryngeal notes, though it is difficult to appreciate its importance. 



(d) The resonator proper consists of the parts lying above the 

 vocal cords, the laryngeal vestibule, and upper portions of the 

 pharynx, mouth, and nose. It is on the vibrations of the air in 

 this tube that the special qualities which characterise the human 

 voice depend. The necessary coincidence between the vibration 

 of the vocal cords and that of the air in the resonator is obtained 

 by the varying tension of the walls, and the alterations in length, 

 breadth, and shape of the cavity, by upward and downward move- 

 ments of the larynx, and alterations of the tongue, soft palate, 

 pillars of fauces, cheeks, and lips. 



(e) Morgagni's ventricles are of little importance as resonators, 

 but they give space for the free vibration of the vocal cords, and 

 produce a secretion by which the laryngeal mucous membrane is 

 kept moist. 



(/) The false vocal cords can alter the form of the laryngeal 

 vestibule by their approximation towards the middle line, and 

 thus change the character of the tone produced by the vibrations 



