struments. Within the throat, two elastic bands are attached to the
windpipe at the place commonly called Adam's apple; these flexible
bands have received the name of vocal cords, since by their vibration
all speech is produced. In ordinary breathing, the cords are loose and
are separated by a wide opening through which air enters and leaves
the lungs. When we wish to speak, muscular effort stretches the cords,
draws them closer together, and reduces the opening between them to a
narrow slit, as in the case of the organ pipe. If air from the lungs
is sent through the narrow slit, the vocal cords or bands are thrown
into rapid vibration and produce sound. The pitch of the sound depends
upon the tension of the stretched membranes, and since this can be
altered by muscular action, the voice can be modulated at will. In
times of excitement, when the muscles of the body in general are in a
state of great tension, the pitch is likely to be uncommonly high.

Women's voices are higher than men's because the vocal cords are
shorter and finer; even though muscular tension is relaxed and the
cords are made looser, the pitch of a woman's voice does not fall so
low as that of a man's voice since his cords are naturally much
longer and coarser. The difference between a soprano and an alto voice
is merely one of length and tension of the vocal cords.

Successful singing is possible only when the vocal cords are readily
flexible and when the singer can supply a steady, continuous blast of
air through the slit between the cords. The hoarseness which
frequently accompanies cold in the head is due to the thickening of
the mucous membrane and to the filling up of the slit with mucus,
because when this happens, the vocal cords cannot vibrate properly.

The sounds produced by the vocal cords are transformed into speech by
the help of the tongue and lips, which modify the shape of the mouth
cavity. Some of the lower animals have a speaking apparatus similar to
our own, but they cannot perfectly transform sound into speech. The
birds use their vocal cords to beautiful advantage in singing, far
surpassing us in many ways, but the power of speech is lacking.

276. The Ear. The pulses created in the air by a sounding body are
received by the ear and the impulses which they impart to the auditory
nerve pass to the brain and we become conscious of a sound. The ear is
capable of marvelous discrimination and accuracy. "In order to form an
idea of