CHAPTER LXXV 

 SOUND PRODUCTION AND SPEECH 



THE power of speech is indicative of the high cerebral development 

 of man. Animals possess the power of sound production, but not of 

 speech. Among the insects sound is produced chiefly by a rapid 

 rhythmic movement of one hard part against another, such as the 

 chirping of crickets and grasshoppers. The groan of the death's- 

 head moth is possibly produced by the moth drawing air through its 

 tracheae, but more probably by a rapid movement of the palps 

 against the proboscis. Among the animals higher in the scale of 

 evolution sound is produced by a blast of air upon elastic mem- 

 branes. The voice of amphibians is due to the driving of air past 

 the tightly stretched membranes of the larynx; the same is true of 

 the song of the bird and of the voice of mammals and man. 



The speech organs are (1) The wind chest (the lungs) with the 

 wind tubes (the bronchi and trachea); (2) the sound-producer (the 

 larynx); (3) the sound modifier (the cavities of the mouth, nose, and 

 throat). 



The wind chest drives the air through the reed instrument (the 

 larynx); the vibration of the reeds (the vocal cords) produces the 

 pitch of the sound, and the mouth and nose cavity, resonating like 

 the tube of a trumpet, amplifies certain overtones and gives the quality 

 which deter mines speech. 



The proper management of the breath is most important. We 

 have seen that the breathing is regulated by the partial pressure of 

 C0 2 in the alveolar air, or, rather, by the concentration of acid 

 (H+) ions in the blood. We can alter the breathing by means of 

 the will only within narrow limits. A shortage of air and a rise 

 of the partial pressure of C0 2 will inevitably cut short the most 

 eloquent periods, and give the speaker a catch in his breath. We 

 must learn to speak with a full lung, on the complemental and not 

 on the tidal air-supply. The contraction of the belly muscles must 

 balance the positive pressure in the thorax during the singing of a 

 sustained note, so that the circulation may continue unimpaired. 

 The stammerer fails to use his diaphragm properly; the bellows 

 of his wind chest is at fault, or, more correctly, the nerve centre 

 which plays upon the bellows. The vocal cords vibrate in much 

 the- same way as do the lips in blowing a trumpet. In a reed 

 pipe the air is stopped intermittently for a moment, and then 

 let pass by the vibrating tongue or reed, so ' that a set of pulses 



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