CHAPTER XVI. 



THE VOICE AND SPEECH. 



IK nearly all air-breathing vertebrate animals there are arrangements 

 lor the production of sound, or voice, in some parts of the respiratory 

 apparatus. In many animals, the sound admits of being variously modi- 

 fied and altered during and after its production; and, in man, one such 

 modification occurring in obedience to dictates of the cerebrum, is speech. 



MODE OF PKODUCTION or THE HUMAK VOICE. 



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It has been proved by observations on living subjects, by means of the 

 laryngoscope, as well as by experiments on the larynx taken from the 

 dead body, that the sound of the human voice is the result of the inferior 

 laryngeal ligaments, or true vocal cords (A, cv, Fig. 298) which bound 

 the glottis, being thrown into vibration by currents of expired air impelled 

 over their edges. Thus, if a free opening exists in the trachea, the sound 

 of the voice ceases, but returns if the opening is closed. An opening 

 into the air-passages above the glottis, on the contrary, does not prevent 

 the voice being formed. Injury of the laryngeal nerves supplying the 

 muscles which move the vocal cords puts an end to the formation of 

 vocal sounds; and when these nerves are divided on both sides, the loss 

 of voice is complete. Moreover, by forcing a current of air through the 

 larynx in the dead subject, clear vocal sounds are produced, though the 

 epiglottis, the upper ligaments of the larynx or false vocal cords, the 

 ventricles between them and the inferior ligaments or true vocal cords, 

 and the upper part of the arytenoid cartilages, be all removed; provided 

 the true vocal cords remain entire, with their points of attachment, and 

 be kept tense and so approximated that the fissure of the glottis may be 

 narrow. 



The vocal ligaments or cords, therefore, may be regarded as the proper 

 organs of the mere voice: the modifications of the voice being effected by 

 other parts tongue, teeth, lips, etc., as well as by them. The structure 

 -of the vocal cords is adapted to enable them to vibrate like tense mem- 

 branes, for they are essentially composed of elastic tissue; and they are 

 so attached to the cartilaginous parts of the larynx that their position 

 .and tension can be variously altered by the contraction of the muscles 

 which act on these parts. 



