CHAPTER XII. 



VOICE AND SPEECH. 



IT has long been established that the sounds of the voice in man 

 and mammalia are produced by the vibratory action of the vocal cords. 

 It is usually the blast of expired air under certain circumstances the 

 inspiratory blast also in its passage through the glottis that causes 

 the tense vocal cords to vibrate. These cords vibrate according to 

 the laws which regulate the vibration of stretched membranous cords. 

 As a result of these vibrations sound is produced which, in man, is 

 capable of being so modified as to constitute articulate speech. 



Experiments upon living animals show that the vocal cords alone 

 are the essential factors in the production of sound. For, so long 

 as these remain untouched, although all other parts in the interior of 

 the larynx are destroyed, the animal is still able to emit vocal sounds. 



The existence of an opening in the larynx of a living animal, or 

 of man, above the glottis in no way prevents the formation of vocal 

 sounds; however, should such an opening occur in the trachea, it 

 causes total loss of voice. By simply closing the opening sounds can 

 be again produced. Such openings in man are usually met with as 

 the result of accident, of suicidal attempts, or of operations performed 

 upon the larynx or trachea for the relief of disease. 



Production and Modification of Sounds. Whenever a solid body 

 surrounded by air is thrown into vibration the sensation of sound is 

 carried to the ear. The vibrations must, however, be of certain 

 strength and follow one another with certain rapidity. It is usually 

 stated that if the vibrations be fewer than 32 or exceed 33,768 per 

 second no effect is produced upon the nerve of hearing. 



For the production of a musical sound the vibrations must suc- 

 ceed each other at regular intervals; if the vibrations occur at 

 irregular intervals, only a noise results. 



The pitch of a sound depends upon the number of vibrations 

 dthin a given period of time. The pitch becomes higher in direct 

 )roportion to the rate of increase in the rapidity of the vibrations. 



The strength, or intensity, of the sound depends upon the extent 

 )f the vibratory action of the sonorous body. 



Tone, or timbre, is that peculiar character of a musical note 

 /hereby it can at once be distinguished from another note of exactly 

 ie same pitch and strength. 



(563) 



