The =crico-thyroideus= (M. crico-thyreoideus) is a short muscle which
fills the groove on the lateral surface of the cricoid cartilage; its
fibers are directed dorsally and somewhat forward. It arises on the
lower half of the lateral surface and posterior edge of the cricoid
cartilage, and is inserted into the posterior border and adjacent part
of the surfaces of the lamina of the thyroid cartilage. Its action is to
draw the thyroid cartilage and the ventral part of the cricoid cartilage
together. In this action the cricoid cartilage is probably rotated,
carrying the bases of the arytenoid cartilages with it and thus tensing
the vocal cords.
The =crico-arytenoideus dorsalis s. posterior= (M. crico-arytænoideus
dorsalis) is a strong, somewhat fan-shaped muscle, which, with its
fellow, covers the dorsal surface of the lamina of the cricoid
cartilage. It is partially divisible into two layers. It has a broad
origin on half of the lamina of the cricoid cartilage, and its fibers
converge to be inserted into the processus muscularis of the arytenoid
cartilage. Its action is to dilate the rima glottidis by rotating the
arytenoid cartilage so as to carry the vocal process and cord outward.
The =crico-arytenoideus lateralis= (M. crico-arytænoideus lateralis)
lies on the inner face of the thyroid lamina. It arises from the
anterior border of the lateral part of the arch of the cricoid
cartilage. The fibers pass in a dorsal direction and converge on the
processus muscularis of the arytenoid cartilage. It closes the rima
glottidis by rotating the arytenoid cartilage inward.
The =arytenoideus transversus= (M. arytænoideus transversus) is an
unpaired muscle which stretches across the concave dorsal surface of the
arytenoid cartilages. Its fibers are attached on either side to the
processus muscularis and the ridge which extends forward from it. The
right and left parts of the muscle meet at a fibrous raphé which is
connected with the transverse arytenoid ligament. Filters of the
thyro-arytenoideus muscle overlap its anterior part. It narrows the rima
by drawing the arytenoid cartilages together.
[Illustration:
FIG. 355.—ADITUS LARYNGIS OF HORSE, EXPOSED BY OPENING PHARYNX AND
BEGINNING OF ŒSOPHAGUS ALONG MEDIAN DORSAL LINE.
_1_, False vocal cord; _2_, lateral ventricle; _3_, true vocal cords;
_4_, rima glottidis.
]
The =thyro-arytenoideus= (Mm. ventricularis et vocalis) is situated in
the lateral wall of the larynx, covered by the lamina of the thyroid
cartilage. It consists of two parts, between which the mucous membrane
of the lateral ventricle pouches to form the saccule. The =anterior
part= (M. ventricularis) arises from the anterior part of the
crico-thyroid membrane and the ventral border of the thyroid lamina. Its
fibers pass upward and backward to end partly on the processus
muscularis, partly on the arytenoideus transversus, meeting its fellow.
The =posterior part= (M. vocalis) has a more extensive origin on the
crico-thyroid membrane. Its direction corresponds with that of the true
vocal cord. It is inserted into the outer surface of the arytenoid
cartilage below the processus muscularis. The muscle closes the rima and
slackens the vocal cords. With the arytenoideus transversus and
crico-arytenoideus lateralis it forms a sphincter which closes the
entrance to the larynx.