870 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



brought together, and the whole rim of the glottis or the vocal cords alone, 

 according to the pitch of the note, may be seen to vibrate. 



2. THE VOICE. 



The vocal machinery consists of (1) the motive power or breath ; (2) 

 the larynx, which forms the tone ; (3) the chest, the pharynx, the mouth, and 

 the nose, which color the tone ; and (4) the organs of articulation. 1 



The production of voice is undoubtedly accomplished by the vibration of 

 the vocal cords which have previously been approximated in the middle line 

 and made tense through action of the nerve-muscular apparatus already de- 

 scribed. A blast of air from below pressing against the cords so adjusted, 

 causes them to separate and fall into vibration. We have to distinguish in 

 voice the three features of loudness, pitch, and quality. 



The loudness of the tone depends on two factors : (1) the strength of the 

 tone-producing blast as determining not only the amplitude of vibration of 

 the vocal cords, but also the energy with which the air is expelled; (2) 

 the resonance of the two chambers between which the vocal cords are sus- 

 pended, the chest below and the cavities of the head above, whose walls and 

 contained air, by their sympathetic vibration, powerfully reinforce the oscilla- 

 tions imparted to them. 



The pitch of the voice is determined by the thickness, tension, and length 

 of the vocal cords, conditions which regulate the pitch of the note obtained 

 from any vibrating string. The thickness and the elastic quality of the cords 

 are probably largely under the control of the thyro-aryteuoid muscle. The 

 principal tensor of the cords is the crico-thyroid muscle. Other muscles, as 

 described above, may so fix the arytenoid cartilages that their vocal processes 

 may be prevented from taking part in the vibration of the cords throughout 

 the whole and also, possibly, throughout part only of their length. This 

 dampening of the vocal processes of the arytenoids may be accomplished either 

 by pressure applied to them throughout their whole length, in which case the 

 posterior part of the glottis is closed, or they may be pressed together at the 

 tips alone, leaving the respiratory glottis open as a triangular aperture. 



Quality. Variation in the quality of the voice depends on the fact that 

 vibrations of the vocal cords are composite in character, giving rise to notes 

 made up of a fundamental tone combined with upper partial tones (see p. 827). 

 By reason of the varied adjustments that may be imparted to it, the larynx is 

 capable of producing many more qualities of tone than is any artificial instru- 

 ment. 2 Change in the size and shape of the resonance-chamber above and 

 below the vocal cords produces a corresponding change in their fundamental 

 notes and, therefore, in the partial tones of the voice which they reinforce by 

 sympathetic vibration (see p. 829). According to Helmholtz, 3 the difference 

 in quality between the various vowel sounds of the human voice depends on 



1 C. H. Davis: The Voice, 1879. 



2 Helmholtz: Sensations of Tone, trans, by Ellis, 1885, p. 98. 

 8 Op. tit., p. 104. 



