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A TEXTBOOK OF PHYSIOLOGY 



is given to the air, which is -alternately rarefied and condensed; 

 the number of these pulses is determined by the length of the " tongue " 

 (pendulum). The audible sound is produced, not by the " tonjme 

 itself, but by the pulses of the air. By placing the middle and fore 

 finger lightly on either side of the thyroid cartilage and singing a 

 sustained note, one can feel the cords vibrate. A blast of air blown 

 through a sheep's trachea and larynx will give forth a sound when the 

 cords of the larynx are tightened (Fig. 450). The blast forces the cords 

 towards the mouth, and they swing back owing to their elasticity. 



The larynx consists of a stiff-walled cartilaginous box divided by 

 the membranous vocal cords into two chambers of unequal size, one 

 above the other. The cords, placed one on either side and opposite 

 each other, can be tightened or slackened, thickened or thinned, 

 shortened or lengthened, moved towards or away from each other, 

 so as to leave either a wide or narrow slit the rima glottidis through 

 which the air blast passes. 



-FiG. 450. EXPERIMENTAL SOUNDS PRODUCED BY BLOWING THROUGH SHEEP'S 



TRACHEA. 



The vibration of the cords can be discerned in man by means of the 

 laryngoscope. A small flat mirror (Fig. 451 ) is attached at an angle to a 

 long handle, so that the mirror can be passed to the back of the throat 

 and the reflection of the rima glottidis seen, when a light is cast upon 

 it from another mirror. This second mirror, a concave one, is fastened 

 to the forehead, and has a central hole in it for the eye, and is placed 

 so as to reflect the light of a lamp on to the small mirror (Fig. 452). If 

 an intermittent source of illumination is used, and the number of 

 intermissions is the same as the number of vibrations of the vocal 

 cords, these appear as if stationary; but if the intermissions are not 

 quite at the same rate, then the cords appear to be slowly moving. 



