RESPIRATION 



261 



glottis are supplied by the superior laryngeal branch of the 

 vagus, which also contains the sensory fibres for the mucous 

 membrane of the larynx above the vocal cords. 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 instru- 

 ment, by the rhythmical interruption of an expiratory blast 

 of air by the vibrating vocal cords. When a bell is struck, 



FIG. 92. DIRECTION OF 



PULL OF THE LATERAL 



CRICO-ARYTENOIDS, 

 WHICH ADDUCT THE 

 VOCAL CORDS. 



Dotted lines show position in 

 adduction. 



FIG. 91. DIAGRAMMATIC 



HORIZONTAL SECTION OF 



LARYNX TO SHOW THE 



DIRECTION OF PULL OF 



THE POSTERIOR CRICO- 

 ARYTENOID MUSCLES, 

 WHICH ABDUCT THE 

 VOCAL CORDS. 



Dotted lines show position in 

 abduction. 



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 the real sounding body; a pulse 

 of alternate rarefaction and condensation 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. The sound-waves thus set up 



