The larynx is a respiratory organ, set in the respiratory tract between the pharynx and trachea.
Although phonation is important in man, the main function of the larynx is to provide a protective sphincter for the air passages. The larynx lies below the hyoid bone in the midline of the neck at the level of C4–6 vertebrae.
Skeleton of the larynx
The framework of the larynx consists of cartilages, ligaments and membranes. There are three single cartilages (thyroid, cricoid and epiglottic) and three pairs of cartilages (arytenoid, corniculate and cuneiform). The ligaments and membranes are extrinsic (thyrohyoid membrane and cricotracheal, hyoepiglottic and thyroepiglottic ligaments) and intrinsic (quadrangular membrane and cricothyroid ligament). The vocal cords are the upper part of the cricothyroid ligament (cricovocal membrane).
Cartilages
The thyroid, cricoid and arytenoid cartilages are composed of hyaline cartilage and with age parts of them may calcify or ossify; the epiglottic, corniculate and cuneiform cartilages are elastic fibrocartilage.
The thyroid cartilage consists of two laminae whose anterior borders are fused at a median angle, o r laryngeal prominence. A thyroid notch marks the upper end of the prominence. The posterior borders are free and projected upwards and downwards as the superior and inferior horns (Fig.
6.43). Each inferior horn articulates with the cricoid cartilage to form the cricothyroid joint. The outer surface of each lamina possesses an oblique ridge running downwards and forwards, and bounded above and below by a tubercle.
Figure 6.43 Thyroid cartilage: A from the right; B from the right and above and slightly in front.
The cricoid cartilage is the foundation of the larynx; to this signet-ring structure (Fig. 6.44) the thyroid and arytenoid cartilages are articulated by synovial joints. It is the only complete cartilaginous ring in the whole of the air passages. The anterior part of the ring is the arch;
posteriorly it is projected upwards as a quadrangular flat lamina. Near the junction of the arch and lamina is an articular facet for the inferior horn of the thyroid cartilage. The upper part of the lamina has sloping shoulders, which carry articular facets for the arytenoids. A vertical ridge in the midline of the lamina produces a shallow concavity on each side for the attachment of the posterior
cricoarytenoid muscle; the ridge gives attachment to longitudinal muscle fibres of the oesophagus (see p. 209).
Figure 6.44 Cricoid cartilage: A from the right; B from the right, above and in front.
The epiglottic cartilage is a slightly curled, leaf-shaped structure, prolonged below into a slender process (the stalk of the leaf) attached in the midline to the back of the laryngeal prominence, below the thyroid notch. The epiglottic cartilage leans back from its attached stalk to overhang the vestibule of the larynx. Fibrous tissue attaching the front and sides of the epiglottis to the body and greater horns of the hyoid bone (hyoepiglottic ligaments) form the framework of the glossoepiglottic folds that bound the valleculae (see p. 387). The posterior surface below the apex is pitted by mucous glands. A bulge on the lower part of this surface is the tubercle of the epiglottis.
Each of the pair of arytenoid cartilages is a three-sided pyramid with anterolateral, medial and posterior surfaces. The inferior base has a forward projection, the vocal process, and a lateral projection, the muscular process. The base articulates with the sloping shoulder on the upper border of the cricoid lamina. A very small corniculate cartilage articulates with the apex of each arytenoid cartilage and a tiny cuneiform cartilage lies nearby in the aryepiglottic fold; they are unimportant.
Joints
The cricothyroid joint, between the inferior horn of the thyroid cartilage and the facet on the side of the arch of the cricoid, is synovial. Movement between the cricoid and thyroid occurs round an axis that passes transversely between the two joints, so that one cartilage can rock backwards and forwards on the other. The recurrent laryngeal nerve lies immediately behind this joint.
The cricoarytenoid joint is also synovial. The capsule here is lax, allowing both rotary and lateral gliding movements. When the arytenoids are pulled laterally and downwards they slide apart from one another along the sloping shoulders of the cricoid lamina. This gliding of the arytenoids opens the gap between the vocal folds (the rima of the glottis) in the shape of a V; rotation opens the glottis in the shape of a diamond. In man there is a greater range of gliding than of rotary movement, and the open human glottis resembles a V and not a diamond (Fig. 6.45).
Figure 6.45 Movements of the arytenoid cartilages. In A, the vocal folds are adducted. In B, rotation of the arytenoids, as in animals, produces a diamond-shaped opening. In C, lateral excursion of the arytenoids produces the human V-shaped opening.
Ligaments and membranes
Of the extrinsic membranes, the thyrohyoid membrane connects the whole length of the upper border of the thyroid laminae and the superior horns to the body and greater horns of the hyoid bone (Fig. 6.37). The thyrohyoid membrane passes up behind the body of the hyoid bone to be attached to its upper border; a bursa lies between the membrane and the back of the bone. It is here that remnants of the thyroglossal duct (see p. 26) may persist, necessitating resection of the central part of the bone to give adequate removal.
Figure 6.46 Skeleton of the larynx: A interior, viewed from the right with the right quadrangular membrane and the right halves of the thyroid cartilage and hyoid bone removed; B similar view showing muscles attached to the right arytenoid cartilage.
The thyrohyoid membrane forms the lateral wall of the piriform recess and is perforated by the internal laryngeal nerve and the superior laryngeal vessels. It is not part of the larynx, but anchors the skeleton of the larynx to the hyoid bone.
The epiglottis is attached to the hyoid bone and thyroid cartilage by the hyoepiglottic and thyroepiglottic ligaments. The former are described above; the latter is a strong band attaching the stalk of the cartilage to the angle between the thyroid laminae just below the thyroid notch.
T h e cricotracheal membrane connects the lower border of the cricoid cartilage to the first cartilaginous ring of the trachea.