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Full text of "Diseases Of The Nose Throat And Ear"

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To comprehend how a sound is produced in the larynx it is necessary to
understand the Bernoulli effect. This principle states that during the steady
flow of a fluid or a gas, the pressure is less where the velocity is greater. In
other words, when air passes from one large space to another (i.e. from lung
to pharynx), through a constriction (the glottis), the velocity will be greatest
and the pressure least at the site of the constriction.

When we wish to phonate, the recurrent laryngeal nerves set the vocal
cords into the adducted position, but because the vocal processes are slightly
bulkier than the membranous cord a slight gap exists between the membranous
cords. The lungs then expel air and the air stream passes through this chink
between the vocal cords. According to the Bernoulli principle, therefore, there
is a drop of pressure at this site and this causes the mucosa of the vocal cords
to be drawn into the gap thus blocking it. At this time the subglottic pressure
rises causing another stream of air to 'flow through the cords with another
resultant pressure drop and closure of the gap. As this process is repeated a
vibratory pattern develops at the vocal cords and the resulting sound is what
we appreciate as voice. The change of this sound into speech is accomplished
by the tongue, teeth, lips and palate.

At birth the vocal cords are about 7 mm long, at puberty about 14 mm long,
in the adult female 15-16 mm long and in the adult male from 17 to 21 mm
long. Because the anterior commissure is situated at the thyroid prominence
this increased length in the male explains why men have larger 'Adam's
apples* than women. It can also be understood from the above that, as a
general principle, as the length of the cord is decreased so the pitch of the
voice rises. The alteration in the length of the cord is produced by the crico-
thyroid and the thyro-arytenoid muscles. Length is not the only factor in voice
pitch, however—both the tension or elasticity of the cord and the tracheal air
pressure are important. As the cord is shortened the 'slack' must be taken up
and the tension readjusted—this is done by the vocalis muscle. Increase in
tension, maintaining the same length of cord, will also cause some rise in
pitch. It is a well-known fact that as one speaks louder so the pitch of the
voice rises. Further reference will be made to this fact in the sections on voice
strain and vocal nodules.

In actual phonation, pitch and pressure are associated in such a way that a
slight increase of air pressure causes a considerable rise in pitch. If this were
the whole story, however, the pitch of the voice would rise linearly as the
loudness increased. In actual fact, increase of loudness is attained by rise of
air pressure associated with decrease of elasticity of the glottis. It is a truism to