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A MANUAL OF PHYSIOLOGY 



of the vagus. All the other intrinsic muscles are supplied by the 

 recurrent laryngeal branch of the vagus. It receives these motor 

 fibres from the spinal accessory, and supplies sensory fibres to 

 the mucous membrane of the larynx below the vocal cords and 

 to the trachea. 



The voice is produced, like the sounds of a reed instrument, 

 by the rhythmical interruption of an expiratory blast of air by 

 the vibrating vocal cords. When a bell is struck, vibrations are 

 set up in the metal, which are communicated to the air. It 

 is not the same with the vibrations of the vocal cords ; if they 

 were plucked or struck, they would only produce a feeble note. 

 The air in the mouth, pharynx, larynx, trachea, and lungs is 



FIG. 120. DIRECTION OF PULL 

 OF THF. LATERAL CRICO- 

 ARYTENOIDS, WHICH ADDUCT 

 THE VOCAL CORDS. 



Dotted lines show position in 

 adduction. 



FlG. 1 1.9. D 1 A G R A M M A T I C 



HORIZONTAL SFCTIOV OF 

 LARYNX TO SHOW THE DI- 

 RECTION OF PULL OF THE 

 POSTERIOR CRICO-ARYTENOID 

 MUSCLES, WHICH ABDUCT 

 THE VOCAL CORDS. 



Dotted lines show position in 

 abduction. 



the real sounding body ; a pulse of alternate rarefaction and con- 

 densation is set up in it by the interference, at regular intervals, 

 of the vocal cords with the expiratory blast. Forced abruptly 

 from their position of equilibrium as the blast begins, they almost 

 immediately regain and pass below it, in virtue of their elasticity, 

 and continue to vibrate as long as the stream of air continues to 

 issue in sufficient strength. Not only do they vibrate up and 

 down, but also towards and away from the middle line, so that, 

 at least in the chest voice, they come into contact with each other 

 at each swing. The sound-waves thus set up spread out on 

 every side, impinge on the tympanic membrane, 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 pro- 

 duction of a sufficient pressure in the blast of air driven through 

 the windpipe by an expiratory act, and of a suitable tension in 



