604 VOICE AXD SPEECH. 



that the non-nitrogenous as well as the nitrogenous foods 

 may, in their combustion, afford the requisite conditions 

 for muscular action. The urgent necessity for nitrogenous 

 food, especially after exercise, is probably due more to 

 the need of nutrition by the exhausted muscles and other 

 tissues for which, of course, nitrogen is essential, than to 

 such food being superior to" non-nitrogenous substances as a 

 source of muscular power. 



CHAPTER XVIII. 



OF VOICE AND SPEECH. 



IN nearly all air-breathing vertebrate animals there are 

 arrangements for the production of sound, or voice, in some 

 part of the respiratory apparatus. In many animals the 

 sound admits of being variously modified and altered 

 -during and after its production ; and, in man, one of the 

 results of such modification is speech. 



3Iode of Production of the Human Voice. 



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. 172) which bound 

 the glottis, being thrown into vibration by currents of 

 expired air impelled over their edges. Thus, if a free open- 

 ing exists in the trachea, the sound of the voice ceases, but 

 returns on the opening being 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 



