We have just learned that the pitch of sound depends on the rapidity of the vibrations.  This depends on the length of cords and their tightness for the shorter and tighter a string is, the higher is the note which its vibration produces.  The vocal cords of women are about one-third shorter than those of men, hence the higher pitch of the notes they produce.  In children the vocal cords are shorter than in adults.[50] The cords of tenor singers are also shorter than those of basses and baritones.  The muscles within the larynx, of course, play a very important part in altering the tension of the vocal cords.  Those qualities of the voice which we speak of as sweet, harsh, and sympathetic depend to a great extent upon the peculiar structure of the vocal cords of the individual.

Besides the physical condition of the vocal cords, as their degree of smoothness, elasticity, thickness, and so on, other factors determine the quality of an individual’s voice.  Thus, the general shape and structure of the trachea, the larynx, the throat, and mouth all influence the quality of voice.  In fact, the air passages, both below and above the vibrating cords, act as resonators, or resounding chambers, and intensify and modify the sounds produced by the cords.  It is this fact that prompts skillful teachers of music and elocution to urge upon their pupils the necessity of the mouth being properly opened during speech, and especially during singing.

  Experiment 187. To show the anatomy of the throat.  Study the
  general construction of the throat by the help of a hand mirror.  Repeat
  the same on the throat of some friend.

Experiment 188. To show the construction of the vocal organs.  Get a butcher to furnish two windpipes from a sheep or a calf.  They differ somewhat from the vocal organs of the human body, but will enable us to recognize the different parts which have been described, and thus to get a good idea of the gross anatomy.

  One specimen should be cut open lengthwise in the middle line in front,
  and the other cut in the same way from behind.

354.  Speech.  Speech is to be distinguished from voice.  It may exist without voice, as in a whisper.  Speech consists of articulated sounds, produced by the action of various parts of the mouth, throat, and nose.  Voice is common to most animals, but speech is the peculiar privilege of man.

[Illustration:  Fig. 152.—­Diagramatic Horizontal Section of Larynx to show the Direction of Pull of the Posterior Crico-Arytenoid Muscles, which abduct the Vocal Cords. (Dotted lines show position in abduction.)]

The organ of speech is perhaps the most delicate and perfect motor apparatus in the whole body.  It has been calculated that upwards of 900 movements per minute can be made by the movable organs of speech during reading, speaking, and singing.  It is said that no less than a hundred different muscles are called into action in talking.  Each part of this delicate apparatus is so admirably adjusted to every other that all parts of this most complex machinery act in perfect harmony.