The larynx acts as a sphincter guarding the lower respiratory tract and is responsible for phonation. It lies in the neck and its inlet is continuous with the laryngopharynx (Figs 7.61 & 7.62). Inferiorly, the larynx is continuous with the trachea. It consists of a framework of cartilages and bone, which supports the vocal and vestibular folds and the muscles that move them. Anteriorly lie the infrahyoid strap muscles.
The skeleton of the larynx comprises the thyroid, cricoid and arytenoid cartilages, the epiglottis and the hyoid bone.
The thyroid cartilage (Figs 7.67 & 7.68) has two flat laminae joined anteriorly to form the midline laryngeal prominence (Adam’s apple). The posterior margin of each lamina is free and bears a superior and an inferior horn. The tips of the inferior horns articulate by tiny synovial joints with the lateral surfaces of the cricoid cartilage.
The cricoid cartilage (Figs 7.67 & 7.71) lies below the thyroid cartilage and is signet ring-shaped, with a narrow anterior arch and a wide posterior lamina. Its inferior border lies horizontally, while the superior border slopes upwards posteriorly. Movement at the cricothyroid joints allows the arch of the cricoid to tip upwards, while the lamina tips backwards.
In addition to its articulations with the thyroid cartilage, the upper border of the cricoid lamina bears articular surfaces, one on each side of the midline, for synovial joints with the two arytenoid cartilages. Each arytenoid (Figs 7.64, 7.67 & 7.71) is pyramidal in shape, with its base on the cricoid lamina and its apex superiorly. It has four surfaces (medial, posterior, anterolateral and inferior) and two processes. Anteriorly lies the vocal process, to which the vocal fold is attached, while its muscular process projects laterally. Each arytenoid is able to rotate around its own vertical axis. In addition, the arytenoids can slide laterally and downwards away from the midline on the slope of the articular surface on the cricoid.
The epiglottis (Figs 7.64 & 7.67) is attached in the midline to the inner surface of the thyroid cartilage, just below the laryngeal prominence. It extends upwards and backwards behind the tongue to which it is attached in the midline by the median glossoepiglottic fold. From each side of the epiglottis a lateral glossoepiglottic fold (Fig. 7.61) extends to the side of the tongue. These folds bound a recess on each side of the midline, the vallecula (Fig. 7.61), which is a site where swallowed foreign bodies may lodge. The epiglottis overlies the laryngeal inlet when the larynx is raised against the tongue when swallowing.
The hyoid bone (Figs 7.62 & 7.67) is the uppermost skeletal element of the larynx. It consists of a midline body bearing greater horns projecting posteriorly. On each side a lesser horn is located on the upper surface where the body and greater horn unite.
Several membranes are attached to the laryngeal framework and fill the gaps between the skeletal structures. The conus elasticus (cricovocal membrane) (Fig. 7.66) is elastic and attaches inferiorly to the upper border of the cricoid cartilage. In the midline anteriorly, it reaches the inferior border of the thyroid cartilage, forming the inelastic cricothyroid ligament. Posteriorly it is attached to the vocal processes of the arytenoids. The membrane has a free upper border, which runs between the arytenoids and the thyroid cartilage and forms the basis of the vocal folds (true vocal cords).
The inelastic quadrangular (aryepiglottic) membrane (Fig. 7.66) attaches posteriorly to the body and apex of the arytenoid and anteriorly to the lower part of the lateral edge of the epiglottis. It has a lower free border which forms the vestibular fold (false vocal cord), while the upper free edge forms the aryepiglottic fold (part of the laryngeal inlet).
The thyrohyoid membrane (Fig. 7.68) fills the gap between the thyroid cartilage and the hyoid bone. Its posterior free edges are thickened to form the lateral thyrohyoid ligaments, while the midline thickening is the median thyrohyoid ligament.
Mucous membrane (Fig. 7.65) lines the interior of the larynx. The epithelium is of the respiratory type, except over the vocal and aryepiglottic folds where it is stratified and squamous. A pouch of mucous membrane protrudes laterally between the vocal and vestibular folds, forming the saccule. It contains numerous mucous glands, the secretions of which moisten the vocal folds.
The intrinsic laryngeal muscles control the position and tension of the vocal and aryepiglottic folds, therefore modifying the shape of the airway through the larynx, acting both at the glottis (rima glottidis), the gap between the vocal folds, and at the inlet (Fig. 7.62). During swallowing, both openings narrow, but in coughing and phonation only the glottis narrows. The glottis widens during inspiration, especially when deep. The muscles are symmetrical in their arrangement.
The cricothyroid muscle (Fig. 7.68) attaches to the anterolateral surface of the cricoid cartilage and to the inferior edge of the thyroid cartilage and adjacent part of the inferior horn. It raises the arch of the cricoid cartilage, tipping the lamina backwards and thereby increasing the tension and length of the vocal folds. This has the effect of raising the pitch of the voice and narrowing the glottis.
The thyroarytenoid muscle (Fig. 7.70) attaches to the posterior surface of the thyroid cartilage adjacent to the conus elasticus. Its fibres pass posteriorly to gain the vocal process and adjacent body of the arytenoid cartilage. The muscle opposes the action of cricothyroid, drawing the arytenoids forwards and relaxing the vocal folds. Part of the muscle (vocalis) lies in the free edge of the vocal fold. The vocalis gives rigidity to the edge of the fold and also modifies tension differentially along its length.
Two muscles attach to the muscular process of each arytenoid. The lateral cricoarytenoid (Fig. 7.69) attaches to the lateral surface of the cricoid, its fibres passing posterosuperiorly to reach the muscular process of the arytenoid. The muscle rotates the arytenoid around its vertical axis, adducting the vocal folds and thereby closing the glottis. The posterior cricoarytenoid (Figs 7.63 & 7.69) inclines anterosuperiorly from the lamina of the cricoid to the muscular process of the arytenoid. Its action opposes that of the lateral cricoarytenoid, rotating the arytenoid so that the folds are abducted and the glottis opened. Simultaneous contraction of the lateral and posterior cricoarytenoid muscles separates the arytenoids by sliding them down the cricoid lamina.
The arytenoid muscle (Figs 7.63 & 7.65) spans the gap between the bodies of the arytenoid cartilages and has transverse and oblique parts. The muscle approximates the arytenoids, closing the glottis. The oblique arytenoids continue anterosuperiorly in the aryepiglottic fold as the aryepiglottic muscles (Fig. 7.69). These assist in closing the laryngeal inlet and are important during swallowing.
The larynx is supplied by the right and left superior and recurrent laryngeal branches of the vagus (X) nerves. Each recurrent laryngeal nerve (Figs 7.68 & 7.69) ascends from the root of the neck (p. 331) in the groove between the trachea and oesophagus. Passing beneath the lower border of the inferior constrictor, the nerve enters the larynx behind the cricothyroid joint.
It supplies motor fibres to all of the intrinsic muscles (except cricothyroid) and carries sensory fibres from all of the structures within the larynx and laryngopharynx below the level of the vocal folds.
The superior laryngeal nerve (Figs 7.58 & 7.68) arises from the vagus nerve just below the skull and descends to the thyrohyoid membrane where it divides, forming a motor external laryngeal branch and a sensory internal laryngeal branch. The external laryngeal nerve (Fig. 7.68) descends on the outer surface of the larynx to supply the cricothyroid muscle. The internal laryngeal branch (Figs 7.62 & 7.68) pierces the thyrohyoid membrane and provides sensory fibres to the larynx and laryngopharynx above the vocal folds. These sensory nerves contribute to the cough reflex.
The laryngeal branches of the superior and inferior thyroid arteries (Figs 7.13 & 7.15) supply the larynx, while venous blood drains via superior and middle thyroid vessels into the internal jugular veins.