s as indicated
above. The mucous membrane, that naturally covers all parts within the
vocal mechanism, has been dissected away to show the muscles.]

[Illustration: FIG. 27 (Spalteholz). Showing the parts indicated
above; and of these the crico-thyroid muscle is to be especially
observed. The oblique (especially so in the posterior part) direction
of its fibres is evident, so that when it contracts, it must pull up
the ring cartilage in front, and so tilt back its hinder portion and
with it the arytenoid cartilages, and so lengthen and tense the vocal
bands, as in the utterance of low tones.]

[Illustration: FIG. 28 (Spalteholz). A back (posterior) view of the
larynx, etc. Note how the arytenoid cartilages rest on the cricoid;
how the epiglottis overhangs, as its name implies, the glottis; and
that the posterior part of the windpipe is closed in by soft
structures, including (unstriped) muscle.]

It is important to remember the relative position of parts and to bear
in mind that most of the laryngeal structures are in pairs. To this
last statement the thyroid and cricoid cartilages and the epiglottis
are exceptions, being single.

Of the _epiglottis_, a flexible cartilage, it is necessary to say
little, as its function in voice-production, if it have any, has never
been determined. It hangs as a flexible protective lid over the
glottis, and food in being swallowed passes over and about it. It no
doubt acts to keep food and drink out of the larynx, yet in its
absence, in some cases, owing to disease, no very great difficulty was
experienced, probably because certain muscles acted more vigorously
than usual and tended to close up the glottic chink.

The following simple diagram will, it is hoped, make the relative
position of parts plain so far as the anterior (front) attachments of
parts to the thyroid cartilage are concerned. It will be understood
that the inner anterior surface is meant, and that by "middle line"
is intended the middle line of the body, the imaginary vertical
diameter passing like a plumb-line from the middle plane of the head,
let us suppose, downward just in front of the larynx.

[Illustration: FIG. 29.]

The angle made above and in front where the two wings of the thyroid
cartilage meet is termed _Adam's apple_ (_Pomum Adami_), and in some
cases, mostly males, is very prominent. Adam's apple has in itself,
however, no special significance in voice-production.

The little concavity between