st 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
singing from the lowest note to the highest note of the register.

Thus the compass of the--

Bass voice is D to f 75- 354 vibs. per sec.
Tenor " c " c'' 133- 562 " "
Contralto " e " g'' 167- 795 " "
Soprano " b " f''' 239-1417 " "

The complete compass of the human voice therefore ranges from about D 75 to
f''' 1417 vibrations per second, but the quality of the same notes varies
in different individuals.

[Illustration: Fig. 9]

[Description: Fig. 9.--_Description of the laryngoscope and its mode of
use_.--The laryngoscope consists of a concave mirror which is fixed on the
forehead with a band in such a way that the right eye looks through the
hole in the middle. This mirror reflects the light from a lamp placed
behind the right side of the patient, who is told to open the mouth and put
out the tongue. The observer holds the tongue out gently with a napkin and
reflects the light from the mirror on his forehead on to the back of the
throat. The small mirror, set at an angle of 45 deg. with the shaft, is of
varying size, from half an inch to one inch in diameter, and may be fixed
in a handle according to the size required. The mirror is warmed to prevent
the moisture of the breath obscuring the image, and it is introduced into
the back of the throat in such a manner that the glottis appears reflected
in it. The light from the lamp is reflected by the concave mirror on to