VOICE AND SPEECH. 605 



work they perform ; and that the non-nitrogenous as well 

 as the nitrogenous foods may, in their combustion, afford the 

 requisite conditions for muscular action (see Chap. II). The 

 urgent necessity for nitrogenous food, especially after exer- 

 cise, 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. 



OHAPTEK XVIII. 



OF VOICE AND SPEECH. 



IN nearly aD. 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. 



Mode 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. 165) 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 



