THE VOICE. 



225 



eessory nerves supply the muscles of the larynx with motor 

 nerves. The sensory nerves, arising in the epithelium of the 

 larynx, are also branches of the vagus. Mechanical stimulation 

 of the mucous membrane of the larynx or electrical stimulation 

 of the superior laryngeal nerve will cause a cough or a forced 

 expiratory movement. 



The Changes Which Occur in the Position of the Vocal Cords 

 during the production of certain sounds may be studied by the 

 use of the laryngoscope, the principle of which is shown in Fig. 

 33. The view obtained from such an instrument is shown in 

 Figs. 34 and 35. The base of the tongue appears at the top ; be- 



Mirr 



Uxryn 



Tongue 

 depressor 



Fig. 33. Diagram of laryngoscope. 



I 



low this is the edge of the epiglottis, the flap of tissue guarding 

 the entrance to the larynx, and below in the middle line are seen 

 the true vocal cords as white shining membranes. Just above 

 these, on both sides, are two pink flaps of tissue, the false vocal 

 cords. These secrete a fluid which moistens the true cords. 



The Production of the Voice. If the vocal cords are put in 

 a state of tension and the aperture between them be narrowed, 

 causing them to offer a resistance to the passage of air issuing 

 from the lungs, they may be made to vibrate and to produce 

 sounds. It has been experimentally determined that a pressure 

 of expired air of from 140 to 240 mm. of water is required to 



