action of the abductor muscle upon the pyramid cartilages in separating the
vocal cords; (ii.) the action of the adductor muscles in approximating the
vocal cords.]

[Illustration: FIG. 8]

[Description: FIG. 8.--Diagram after Testut (modified) with hinder portion
of larynx and windpipe cut away, showing the conical cavity of the
sound-pipe below the vocal cords. The ventricle above the vocal cords is
seen with the surface sloping upwards towards the mid line.]

A diagram showing a vertical section through the middle of the larynx at
right angles to the vocal cords shows some important facts in connection
with the mechanism of this portion of the vocal instrument (_vide_ fig. 8).
It will be observed that the sound-pipe just beneath the membranous reed
assumes the form of a cone, thus the expired air is driven like a wedge
against the closed glottis. Another fact of importance may be observed,
that above the vocal cords on either side is a pouch called a ventricle,
and the upper surfaces of the vocal cords slope somewhat upwards from
without inwards, so that the pressure of the air from above tends to press
the edges together. The force of the expiratory blast of air from below
overcomes the forces which approximate the edges of the cords and throws
them into vibration. With each vibration of the membranous reeds the valve
is opened, and as in the case of the siren a little puff of air escapes;
thus successive rhythmical undulations of the air are produced,
constituting the sound waves. The pitch of the note depends upon the number
of waves per second, and the _register_ of the voice therefore depends upon
two factors: (1) the size of the voice-box, or larynx, and the length of
the cords, and (2) the action of the neuro-muscular mechanism whereby the
length, approximation, and tension of the vocal cords can be modified when