include the (_a_)
crico-thyroids, which tense and elongate them, (_b_) thyro-arytenoids,
which relax and shorten them. The crico-thyroid may be considered the
most important muscle of phonation, because it is so much used and so
effective. By its action the cricoid is pulled up in front and down
behind, so that the arytenoids are drawn back, and thus the vocal
bands tensed and lengthened. The lateral crico-arytenoids and the
thyro-arytenoids have the opposite effect--_i.e._, they relax and
shorten the vocal bands; hence when they come into play a new register
begins. The thyro-arytenoids, attached along the whole length of the
vocal bands externally, have a very important but not well-understood
action in the production of the higher tones, and probably also of the
falsetto.

The whole larynx is lined with mucous membrane, that covering the true
vocal bands being very thin. The false vocal bands are made up chiefly
of mucous membrane; the true vocal bands abound in elastic tissue. The
larynx rises during the production of high tones, and during
phonation its vibrations may be felt, as also those of the chest.

_Practical._

1. Feel in your own person the parts of the larynx, etc., from above
down.

2. Note the vibration of the larynx when a vowel is spoken or sung. A
similar vibration of the chest walls may be felt by the hands laid
over them.

3. Note the change of position of the larynx in singing a scale.

4. Dissect a pig's or sheep's pluck and some specimens of the larynx.

[Illustration: FIG. 35. These three figures illustrate perhaps more
clearly the _action_ of the muscles indicated FIGS. 26-34.

The arrows show the direction of the pull of the muscles. The result
of this action is the new position of the cartilages and vocal bands,
which is shown by red outlines. The muscle is also depicted in red.
The heavier outer rim is to indicate the thyroid cartilage. By
comparing the upper and the lowest figure it will be seen that they
are opposites. Of course, in phonation the vocal bands are never so
much separated as shown in the illustrations. Rather does the lower
figure indicate a case of extreme separation due to a very deep
inspiration. However, these illustrations are merely diagrams meant to
indicate in a general way the manner of the working of parts. For
exact pictures of the vocal bands and related parts, see Chapter
VII.]

CHAPTER VII.

SOUND--THE LARYNGOSCOPE--THE LARYNX RECONSIDERED