Across the larynx are stretched the true vocal cords.
[Illustration: =SHAPE OF THE GLOTTIS WHEN AT REST.=]
[Illustration: =THE GLOTTIS DILATED.=]
[Illustration: =GLOTTIS, CLOSED, AND MUSCLES CLOSING IT.=]
Each cord consists of a band of yellow tissue, covered by mucous
membrane.
By means of the action of the muscles of the larynx that connect with
the cartilages which enter into its structure, the vocal cords are so
adjusted that when the muscles of expiration force the air, which is
compressed in the lungs, out between these cords, their edges are set in
vibration. This is the beginning of the sound which we call voice, but
before it is heard in speech or song it is reinforced by the chambers of
resonance.[1]
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[1] For the function of the false, or superior vocal cords, see pp.
68-71, _Physical Culture_.