Part 142

The =epiglottis= (Cartilago epiglottica) is situated above the body of the thyroid cartilage and curves toward the root of the tongue. It is shaped somewhat like a pointed ovate leaf, and presents two surfaces, two borders, a base, and an apex. The =oral= (or anterior) =surface= is concave in its length, convex transversely; the =pharyngeal= (or posterior) =surface= has the reverse configuration. The =borders= are thin, denticulated, and somewhat everted. The =base= is thick, and is attached to the dorsal surface of the body of the thyroid cartilage by elastic tissue. From each side of it a cartilaginous bar projects backward; these processes correspond to the cuneiform cartilages of man. The =apex= is pointed and curved ventrally. Nearly all of the epiglottis is covered with mucous membrane. It lies sometimes in front of, sometimes behind, the soft palate.

FIG. 352.—EPIGLOTTIS OF HORSE, RIGHT LATERAL VIEW. ]

The =arytenoid cartilages= (Cartilagines arytenoideæ) are situated on either side, in front of the cricoid, and partly between the alæ of the thyroid cartilage. They are somewhat pyramidal in form, and may be described as having two surfaces, two borders, a base, and an apex. The =internal surface= is concavo-convex, smooth, and covered by mucous membrane. The =external surface= is divided by a ridge into two areas: the lower flat area faces outward, and is covered by the thyro-arytenoid and lateral crico-arytenoid muscles; the upper concave area faces chiefly forward, and is covered by the transverse arytenoid muscle. The =anterior border= is convex, and at the base of the cartilage forms a projection termed the =vocal process= (Processus vocalis); it is so named because it gives attachment to the vocal or thyro-arytenoid ligament, which forms the basis of the true vocal cord. The =posterior border= is straight, and extends from the vocal process to the facet on the base. The =dorsal border= forms a deep notch with the apex. The =base= is concave and faces chiefly backward; it presents an oval, concave facet for articulation with the anterior border of the lamina of the cricoid cartilage. The external angle of the base forms a strong eminence termed the =muscular process= (Processus muscularis), which gives attachment to the crico-arytenoid muscles. The internal angle is attached to its fellow by the transverse arytenoid ligament. The =apex= (Cartilago corniculata) curves upward and backward, forming with its fellow the pitcher-shaped lip, from which the cartilages derive their name.

FIG. 353.—LEFT ARYTENOID CARTILAGE OF HORSE, INNER VIEW. ]

The cricoid and thyroid cartilages, and the greater part of the arytenoid cartilages, consist of hyaline cartilage. The apices and vocal processes of the arytenoid cartilages and the epiglottis (including the cuneiform processes) consist of elastic cartilage; they show no tendency toward ossification at any age. The thyroid and cricoid cartilages regularly undergo considerable ossification; the process begins in the body of the thyroid, and often involves the greater part of the cartilage.

JOINTS, LIGAMENTS, AND MEMBRANES OF THE LARYNX

The =crico-thyroid joints= (Articulationes crico-thyreoideæ) are diarthroses formed by the apposition of the convex facets on the ends of the posterior cornua of the thyroid cartilage with corresponding facets on the sides of the cricoid cartilage. The =capsule= is thin, but is strengthened by accessory bands dorsally, externally, and internally. The movements are chiefly rotation around a transverse axis passing through the centers of the two joints.

The =crico-arytenoid joints= (Articulationes crico-arytænoidea) are also diarthrodial. They are formed by the apposition of the convex facets on the anterior border of the cricoid cartilages with the concave facets on the bases of the arytenoid cartilages. Each has a very thin, loose =capsule=, strengthened by accessory bundles dorsally and internally. The movements are gliding and rotation. In the former the arytenoid cartilage glides inward or outward; in the latter it rotates around a longitudinal axis, so that the vocal process swings outward or inward.

The =thyro-hyoid joints= (Articulationes hyo-thyreoideæ) are formed by the anterior cornua of the thyroid cartilage and the extremities of the thyroid cornua of the hyoid bone. They permit rotation around a transverse axis passing through the two joints.

The =crico-thyroid membrane= (Ligamentum crico-thyreoideum) occupies the thyroid notch and extends backward to the arch of the cricoid cartilage. It is triangular 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 anterior border of the thyroid cartilage with the body and thyroid cornua of the hyoid bone.

The =hyo-epiglottic ligament= (Ligamentum 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= (Ligamentum 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 inner surface of the laminæ of the thyroid cartilage. Other fibers attach the cuneiform processes somewhat loosely to the thyroid laminæ.

The =transverse arytenoid ligament= (Ligamentum arytænoideum transversum) is a slender band which connects the inner angles of the bases of the arytenoid cartilages.

The =vocal= (or inferior thyro-arytenoid) =ligament= (Ligamentum vocale) forms the basis of the true vocal cord (Fig. 254). It is 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 superior thyro-arytenoid) =ligament= (Ligamentum 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 outer surface of the processus vocalis and adjacent part of the arytenoid cartilage.

The =crico-tracheal membrane= (Ligamentum crico-tracheale) connects the cricoid cartilage with the first ring of the trachea.

MUSCLES OF THE LARYNX

A. EXTRINSIC MUSCLES

The =sterno-thyro-hyoideus= is described on p. 226.

The =thyro-hyoideus= (M. hyo-thyreoideus) is a flat, quadrilateral muscle, which lies on the outer surface of the thyroid lamina and the lateral part of the thyro-hyoid membrane. It arises from the thyroid cornu of the hyoid bone, and ends on the oblique line on the lamina of the thyroid cartilage. Its action is to draw the larynx toward the root of the tongue.

The =hyo-epiglotticus= (M. hyo-epiglotticus) is a feeble muscle which occupies a median position above the central part of the thyro-hyoid membrane, inclosed by an elastic sheath, the hyo-epiglottic ligament. It arises from the body of the hyoid bone by two branches which unite and are inserted into the front of the base of the epiglottis. Its action is to draw the epiglottis toward the root of the tongue.

FIG. 354.—LARYNGEAL MUSCLES OF HORSE, SEEN FROM LEFT SIDE AFTER REMOVAL OF MAJOR PART OF LAMINA OF THYROID CARTILAGE.

a, Kerato-hyoideus; b, thyro-hyoideus; c, crico-thyroideus; d, sterno-thyroideus; e, ventricularis; f, vocalis; g, crico-arytenoideus lateralis; h, crico-arytenoideus post. s, dorsalis; i, arytenoideus transversus; 1, lingual process; 2, small cornu; 3, thyroid cornu, of hyoid bone; 4, thyroid cartilage; 5, cricoid cartilage; 6, epiglottis; 7, apex of arytenoid cartilage; 8, trachea; 9, muscular process of arytenoid; 10, lateral lobe of thyroid gland; 10′, isthmus of thyroid gland; 11, laryngeal saccule. (Ellenberger-Baum, Anat. d. Haustiere.) ]

B. INTRINSIC MUSCLES

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.