518 RESPIRATORY SYSTEM OF THE HORSE 
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dorsally and medially. The most important movement is rotation of the arytenoid 
cartilage about a longitudinal axis so that the vocal process swings outward (ab- 
duction) or inward (adduction), carrying the vocal cord with it. Another move- 
ment here is rotation about a transverse axis, in which the apical part of the carti- 
lage is raised or lowered. The arytenoid is also capable of slight inward or outward 
gliding movement. 
The thyro-hyoid joints (Articulationes hyo-thyreoidexe) are formed by the 
anterior cornua of the thyroid cartilage and the cartilaginous extremities of the 
thyroid cornua of the hyoid bone. They are diarthroses and the joint capsule is 
reinforced by a relatively strong thyro-hyoid hgament. The thyroid cornu has 
here a cartilaginous prolongation about a quarter of an inch (ca. 6 mm.) in length. 
The chief movement is rotation around a transverse axis passing through the two 
joints. 
The crico-thyroid membrane (Membrana crico-thyreoidea) occupies the thy- 
roid notch and extends backward to the arch of the cricoid cartilage. It is tri- 
angular in form, and is attached by its base to the anterior border of the arch of 
the cricoid cartilage, while its borders are attached to the margins of the thyroid 
notch. It is strong, tightly stretched, and composed largely of elastic tissue. It 
is reinforced ventrally by longitudinal fibers, dorsally by fibers which stretch across 
the thyroid notch. 
The thyro-hyoid membrane (Membrana hyo-thyreoidea) connects the body 
and anterior borders of the thyroid cartilage with the body and thyroid cornua of 
the hyoid bone. 
The hyo-epiglottic ligament (Lig. hyo-epiglotticum) attaches the lower part 
of the oral surface of the epiglottis to the body of the hyoid bone. It forms an 
elastic sheath for the hyo-epiglottic muscle. Ventrally it blends with the thyro- 
hyoid membrane, and dorsally it is not well defined. 
The thyro-epiglottic ligament (Lig. thyreo-epiglotticum) is strong and thick; 
it is composed chiefly of elastic tissue. It connects the base of the epiglottis with 
the body and the adjacent medial surfaces of the lamin of the thyroid cartilage. 
Other fibers attach the cuneiform processes somewhat loosely to the thyroid lamin. 
The transverse arytenoid ligament (Lig. aryteenoideum transversum) is a 
slender band which connects the medial angles of the bases of the arytenoid carti- 
lages. 
The vocal (or posterior thyro-arytenoid) ligament (Lig. vocale) underlies and 
is intimately adherent to the mucous membrane of the true vocal cord. It is thin 
and elastic and is attached ventrally to the body of the thyroid cartilage and the 
crico-thyroid membrane close to its fellow of the opposite side, and ends dorsally 
on the processus vocalis of the arytenoid cartilage. 
The ventricular (or anterior thyro-arytenoid) ligament (Lig. ventriculare) is 
included in the false vocal cord. It consists of loosely arranged bundles which 
arise chiefly from the cuneiform process and end on the lateral surface of the pro- 
cessus vocalis and adjacent part of the arytenoid cartilage. 
The crico-tracheal membrane (Membrana crico-trachealis) connects the cri- 
coid cartilage with the first ring of the trachea. 
MUSCLES OF THE LARYNX 
A. Exrrrnstc MuscLES 
The sterno-thyro-hyoideus is described on p. 268. 
The thyro-hyoideus (M. hyo-thyreoideus) is a flat, quadrilateral muscle, which 
lies on the lateral surface of the thyroid lamina and the lateral part of the thyro- 
hyoid membrane. It arises from the body and thyroid cornu of the hyoid bone 
(almost meeting the opposite muscle), and ends on the oblique line on the lamina 
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