[Illustration:  Fig. 150.—­View of the Cartilages and Ligaments of Larynx.  (Posterior view.)

  A, epiglottis;
  B, thyroid cartilage;
  C, arytenoid cartilage;
  D, ligament connecting lower cornu of the thyroid with the back of the
     cricoid cartilage;
  E, cricoid cartilage;
  F, upper ring of the trachea.
]

352.  The Mechanism of the Voice.  The mechanism of the voice may be more easily understood by a study of Fig. 150.  We have here the larynx, viewed from behind, with all the soft parts in connection with it.  On looking down, the folds forming the true vocal cords are seen enclosing a V-shaped aperture (the glottis), the narrow part being in front.

The form of this aperture may be changed by the delicately coordinate activities of the muscles of the larynx.  For instance, the vocal cords may be brought so closely together that the space becomes a mere slit.  Air forced through the slit will throw the edges of the folds into vibration and a sound will be produced.

The Variations in the form of the opening will determine the variations in the sound.  Now, if the various muscles of the larynx be relaxed, the opening of the glottis is wider.  Thus the air enters and leaves the larynx during breathing, without throwing the cords into vibration enough to produce any sound.

We may say that the production of the voice is effected by an arrangement like that of some musical instruments, the sounds produced by the vibrations of the vocal cords being modified by the tubes above and below.  All musical sounds are due to movements or vibrations occurring with a certain regularity, and they differ in loudness, pitch, and quality.  Loudness of the sound depends upon the extent of the vibrations, pitch on the rapidity of the vibrations, and quality on the admixture of tones produced by vibrations of varying rates of rapidity, related to one another.

[Illustration:  Fig. 151.—­Longitudinal Section of the Larynx. (Showing the vocal cords.)

  A, epiglottis;
  B, section of hyoid bone;
  C, superior vocal cord;
  D, ventricle of the larynx;
  E, inferior vocal cord;
  F, section of the thyroid cartilage;
  H, section of anterior portion of the cricoid cartilage;
  K, trachea;
  L, section of the posterior portion of the cricoid cartilage;
  M, arytenoid cartilage;
  N, section of the arytenoid muscle.
]

353.  Factors in the Production of the Voice.  Muscles which pass from the cricoid cartilage to the outer angle of the arytenoids act to bring the vocal cords close together, and parallel to one another, so that the space between them is narrowed to a slit.  A strong expiration now drives the air from the lungs through the slit, between the cords, and throws them into vibration.  The vibration is small in amount, but very rapid.  Other muscles are connected with the arytenoid cartilages which serve to seperate the vocal cords and to widely open the glottis.  The force of the outgoing current of air determines the extent of the movement of the cords, and thus the loudness of the sound will increase with greater force of expiration.