oid.

The opening or widening of the glottis is effected on each side (one
muscle of the pair and its action being alone described in this
and other cases) by the antagonist of these muscles, the
_crico-arytenoid[=e]us posticus_, whose attachments are exactly as
indicated by the names--viz., to the posterior part of the two
cartilages named. When reading the description of these or other
muscles it is absolutely necessary to have a pictorial illustration or
the real object before one. The pull of this muscle is from the more
fixed point, as in all other cases; hence the force is applied in a
direction from below and outward, with the result that the
arytenoid cartilage is tilted outward, and with it the vocal band is
moved from the middle line.

[Illustration: FIG. 30 (Chapman). Diagram showing action of
crico-thyroid muscle, stretching of the vocal cords, and lengthening
of them. The dotted lines indicate the position assumed when the
muscle has contracted.]

[Illustration: FIG. 31 (Spalteholz). View of the larynx as looked at
from above. The illustration shows particularly well both the true and
the false vocal bands. The true vocal bands are placed much as they
are when a barytone is singing a very low tone. The part of the figure
lowest on the page represents the back part of the larynx.]

[Illustration: FIG. 32 (Spalteholz). A cross-section transverse to the
larynx, such as can be readily made with a strong knife.]

The _crico-thyroid_ also tends to open the glottis. Just as the
diaphragm is the most important muscle of breathing, so is the
crico-thyroid the most important in ordinary speaking and in singing
in the lower register. It is a relatively large and strong muscle with
an oblique direction in the main, though it is composed in reality of
several sets of fibres some of which are much more oblique in
direction than others (Fig. 28). As its name indicates, its points of
attachment are to the thyroid and the cricoid cartilages, but the most
fixed point (_origin_) is its point of attachment to the larger
cartilage; hence its direction of pull is from the thyroid, with the
result that the anterior part of the cricoid is drawn up, the
posterior part down, and the arytenoid cartilage, resting on the upper
part of the cricoid, backward, so that the vocal band is rendered
longer and more tense (see especially Fig. 29). It is important to
note that this is the muscle most used in singing the lower tones of
the