EPIGLOTTIS

The epiglottis is a thin leaf-like plate of elastic fibrocartilage which projects obliquely upwards behind the tongue and hyoid body, and in front of the laryngeal inlet (Figs 34.2, 34.3; see Fig. 34.5). Its free end, which is broad and round, and occasionally notched in the midline, is directed upwards. Its attached part, or stalk (petiolus), is long and narrow and is connected by the elastic thyroepiglottic ligament to the back of the laryngeal prominence of the thyroid cartilage just below the thyroid notch. Its sides are attached to the arytenoid cartilages by aryepiglottic folds (which contain the aryepiglottic muscle). Its free upper anterior, or lingual, surface is covered by mucosa (the epithelium is non-keratinized stratified squamous), which is reflected onto the pharyngeal aspect of the tongue and the lateral pharyngeal walls as a median glossoepiglottic, and two lateral glossoepiglottic, folds. There is a depression, the vallecula, on each side of the median fold. The lower part of its anterior surface, behind the hyoid bone and thyrohyoid membrane, is connected to the upper border of the hyoid by an elastic hyoepiglottic ligament, and separated from the thyrohyoid membrane by adipose tissue, which constitutes the clinically important preepiglottic space. The smooth posterior, or laryngeal, surface is transversely concave and vertically concavo-convex, and is covered by ciliated respiratory mucosa: its lower projecting part is called the tubercle. This surface forms the oblique anterior wall of the laryngeal vestibule. The cartilage is posteriorly pitted by small mucous glands (Fig. 34.3D) and is perforated by branches of the internal laryngeal nerve and fibrous tissue, which means that the posterior surface of the epiglottis is in continuity through these perforations with the pre-epiglottic space.

Fig. 34.5 A and B, Sagittal sections of the left side of the larynx, showing the laryngeal membranes (A) and the interior aspect (B) of the left half of the larynx. C, The quadrangular membrane viewed from the left side.

(A, From Drake, Vogl and Mitchell 2005.)

THYROID CARTILAGE

The thyroid cartilage is the largest of the laryngeal cartilages (Figs 34.1-34.3). It consists of two quadrilateral laminae with anterior borders that fuse along their inferior two-thirds at a median angle to form the subcutaneous laryngeal prominence (‘Adam’s apple’). This projection is most distinct at its upper end, and is well marked in men but scarcely visible in women. Above, the laminae are separated by a V-shaped superior thyroid notch or incisure. Posteriorly, the laminae diverge, and their posterior borders are prolonged as slender horns, the superior and inferior cornua. A shallow ridge, the oblique line, curves downwards and forwards on the external surface of each lamina: it runs from the superior thyroid tubercle lying a little anterior to the root of the superior cornu, to the inferior thyroid tubercle on the inferior border of the lamina. Sternothyroid, thyrohyoid and thyropharyngeus (part of the inferior pharyngeal constrictor) are attached to the oblique line, usually as little more than a tendon (Fig. 34.3A).

The internal surface of the lamina is smooth. Above and behind, it is slightly concave and covered by mucosa. The thyroepiglottic ligament, the paired vestibular and vocal ligaments, the thyroarytenoid, thyroepiglottic and vocalis muscles, and the stalk of the epiglottis are all attached to the inner surface of the cartilage, in the angle between the laminae. The true vocal folds lie 6–9 mm below the median thyroid notch. The superior border of each lamina is concave behind and convex in front, and the thyrohyoid membrane is attached along this edge (Figs 34.1, 34.2). The inferior border of each lamina is concave behind and nearly straight in front, and the two parts are separated by the inferior thyroid tubercle. Anteriorly, the thyroid cartilage is connected to the cricoid cartilage by the anterior (median) cricothyroid ligament, which is a thickened portion of the cricothyroid membrane.

The anterior border of each thyroid lamina fuses with its partner at an angle of approximately 90° in men and approximately 120° in women. The shallower angle in men is associated with the larger laryngeal prominence, the greater length of the vocal cords, and the resultant deeper pitch of the voice. The posterior border is thick and rounded and receives fibres of stylopharyngeus and palatopharyngeus. The superior cornu, which is long and narrow, curves upwards, backwards and medially, and ends in a conical apex to which the lateral thyrohyoid ligament is attached. The inferior cornu is short and thick, and curves down and slightly anteromedially. On the medial surface of its lower end there is a small oval facet for articulation with the side of the cricoid cartilage: this facet is variable and is well defined only sometimes.

During infancy, a narrow, rhomboidal, flexible strip, the intra-thyroid cartilage, lies between the two laminae, and is joined to them by fibrous tissue.

CRICOID CARTILAGE

The cricoid cartilage is attached below to the trachea, and articulates with the thyroid cartilage and the two arytenoid cartilages by synovial joints. It forms a complete ring around the airway, the only laryngeal cartilage to do so (Fig. 34.3B). It is smaller, but thicker and stronger, than the thyroid cartilage, and has a narrow curved anterior arch, and a broad, flatter posterior lamina.

ARYTENOID CARTILAGE

The paired arytenoid cartilages articulate with the lateral parts of the superior border of the cricoid lamina (Figs 34.2, 34.3). Each is pyramidal, and has three surfaces, two processes, a base and an apex. The posterior surface, which is triangular, smooth and concave, is covered by transverse arytenoid. The anterolateral surface is convex and rough, and bears, near the apex of the cartilage, an elevation from which a crest curves back, down and then forwards to the vocal process. The lower part of this arcuate crest separates two depressions (foveae). The upper is triangular (fovea triangularis), and the vestibular ligament is attached to it. The lower is oblong (fovea oblonga), and vocalis and lateral cricoarytenoid are attached to it. The medial surface is narrow, smooth and flat, and is covered by mucosa: its lower edge forms the lateral boundary of the intercartilaginous part of the rima glottidis. The base is concave, with a smooth surface for articulation with the lateral part of the upper border of the cricoid lamina. Its round, prominent lateral angle, or muscular process, projects backwards and laterally: it gives attachment to posterior cricoarytenoid behind, and lateral cricoarytenoid in front. The vocal ligament is attached to its pointed anterior angle (vocal process), which projects horizontally forward. The apex curves backwards and medially and articulates with the corniculate cartilage.

CORNICULATE CARTILAGES

The corniculate cartilages are two conical nodules of elastic fibrocartilage which articulate with the apices of the arytenoid cartilages, prolonging them posteromedially (Fig. 34.3E). They lie in the posterior parts of the aryepiglottic mucosal folds, and are sometimes fused with the arytenoid cartilages.

CUNEIFORM CARTILAGES

The cuneiform cartilages are two small, elongated, club-like nodules of elastic fibrocartilage, one in each aryepiglottic fold anterosuperior to the corniculate cartilages, and are visible as whitish elevations through the mucosa (see Fig. 34.5).

TRITIATE CARTILAGES (CARTILAGO TRITICEA)

The tritiate cartilages are two small nodules of elastic cartilage, situated one on either side above the larynx within the posterior free edge of the thyrohyoid membrane, about halfway between the superior cornu of the thyroid cartilage and the tip of the greater cornu of the hyoid bone (Figs 34.1, 34.2). Their functions are unknown, although they may serve to strengthen this connection.

CALCIFICATION OF LARYNGEAL CARTILAGES

The thyroid, cricoid, and most of the arytenoid cartilages consist of hyaline cartilage, and may therefore become calcified. This process normally starts at about 18 years of age. Initially it involves the lower and posterior part of the thyroid cartilage, and subsequently spreads to involve the remaining cartilages, calcification of the arytenoid cartilage starting at its base. The degree and frequency of calcification of the thyroid and cricoid cartilages appear to be less in females. There is some evidence to suggest that a predilection for tumour invasion may be enhanced by calcification of the laryngeal cartilages.

The tip and upper portion of the vocal process of the arytenoid cartilage consists of non-calcifying, elastic cartilage. This may have considerable functional significance: the vocal process may bend at the elastic cartilage during adduction and abduction, and the two arytenoid cartilages will contact mainly at their ‘elastic’ superior portions during adduction.

CRICOTHYROID JOINT

The joints between the inferior cornua of the thyroid cartilage and the sides of the cricoid cartilage are synovial. Each is enveloped by a capsular ligament strengthened posteriorly by fibrous bands (Figs 34.1, 34.2, 34.4). Both capsule and ligaments are rich in elastin fibres. The primary movement at the joint is rotation around a transverse axis which passes transversely through both cricothyroid joints. The effect of this rotation is to move the cricoid and thyroid cartilages relative to one another in such a way as to bring together the lamina of the thyroid cartilage and the arch of the cricoid cartilage (‘closing the visor’). There is some controversy as to which cartilage moves, but it seems most likely that the cricoid cartilage rotates to a greater extent. When the joint is in a neutral position, the ligaments are slack and the cricoid can glide, to a limited extent, in different directions on the thyroid cornua. The effect of these movements is to lengthen the vocal folds, provided the arytenoid cartilages are stabilized at the cricoarytenoid joint. This may also increase vocal fold tension.

Fig. 34.4 A, Lateral view of the cricothyroid joint. B, Anterosuperior view of the cricoarytenoid joint.

CRICOARYTENOID JOINT

The cricoarytenoid joints are a pair of synovial joints between the facets on the lateral parts of the upper border of the lamina of the cricoid cartilage and the bases of the arytenoids. Each joint is enclosed by a capsular ligament and strengthened by a ligament that, although traditionally called the posterior cricoarytenoid ligament, is largely medial in position (Figs 34.1-34.4).

The cricoid facets are elliptical, convex and obliquely directed laterally, anteriorly and downwards. The long axes of the two facets intersect posteriorly at an angle of about 50°. Two movements occur at this joint. The first is rotation of the arytenoid cartilages at right angle to the long axis of the cricoid facet (dorso-medio-cranial to ventro-latero-caudal), which, because of its obliquity, causes each vocal process to swing laterally or medially, thereby increasing or decreasing the width of the rima glottidis. This movement is sometimes referred to as a rocking movement of the arytenoid cartilages. There is also a gliding movement, by which the arytenoids approach or recede from one another, the direction and slope of their articular surfaces imposing a forward and downward movement on lateral gliding. The movements of gliding and rotation are associated, i.e. medial gliding occurs with medial rotation and lateral gliding with lateral rotation, resulting in adduction or abduction of the vocal folds respectively. When viewed from above, foreshortening can give the illusion that the arytenoid cartilages are rotating about their vertical axes, but the shape of the facets prevents such movement occurring. The posterior cricoarytenoid ligaments limit forward movements of the arytenoid cartilages on the cricoid cartilage. It has been suggested that the ‘rest’ position of the cricoarytenoid ligament is a major determinant of the position of a denervated vocal cord.

ARYTENOCORNICULATE JOINTS

Synovial or cartilaginous joints link the arytenoid and corniculate cartilages.

INNERVATION OF THE CRICOTHYROID, CRICOARYTENOID AND ARYTENOCORNICULATE JOINTS

The cricothyroid, cricoarytenoid and arytenocorniculate joints are innervated by branches of the recurrent laryngeal nerves, which arise either independently or from branches of the nerve to the laryngeal muscles. The capsules of the laryngeal joints contain numerous lamellated (Pacinian) corpuscles, Ruffini corpuscles and free nerve endings.

EXTRINSIC LIGAMENTS AND MEMBRANES

INTRINSIC LIGAMENTS AND MEMBRANES

The fibroelastic membrane of the larynx lies within the cartilaginous skeleton of the larynx, beneath the laryngeal mucosa (Fig. 34.5). It forms a discontinuous sheet separated on both sides of the larynx by a horizontal cleft between the vestibular and vocal ligaments. Its upper part, the quadrangular membrane, extends between the arytenoid cartilages and the sides of the epiglottis. Its lower part, the cricothyroid membrane and conus elasticus, connects the thyroid, cricoid and arytenoid cartilages.

MICROSTRUCTURE OF THE LARYNX

The laryngeal mucosa is continuous with that of the pharynx above and the trachea below. It lines the entire inner surface of the larynx, including the ventricle and saccule, and is thickened over the vestibular folds, where it is the chief component. Over the vocal folds it is thinner, and is firmly attached to the underlying vocal ligaments. It is loosely adherent to the anterior surface of the epiglottis, but firmly attached to its anterior surface and the floor of the valleculae. On the aryepiglottic folds it is reinforced by a considerable amount of fibrous connective tissue, and it adheres closely to the laryngeal surfaces of the cuneiform and arytenoid cartilages.

The laryngeal epithelium is mainly a ciliated, pseudostratified respiratory epithelium where it covers the inner aspects of the larynx, including the posterior, laryngeal surface of the epiglottis, and it provides a mucociliary clearance mechanism shared with most of the respiratory tract (see Ch. 57). However, the vocal folds are covered by non-keratinized, stratified squamous epithelium where they contact each other: this important variation protects the tissue from the effects of the considerable mechanical stresses that act on the surfaces of the vocal folds. The exterior surfaces of the larynx, which merge with the laryngopharynx and oropharynx (including the anterior, lingual surface of the epiglottis and the aryepiglottic folds), are subject to the abrasive effects of swallowed food, and are therefore covered by non-keratinized, stratified squamous epithelium.

The laryngeal mucosa has numerous mucous glands, especially over the epiglottis, where they pit the cartilage, and along the margins of the aryepiglottic folds anterior to the arytenoid cartilages, where they are known as the arytenoid glands. Many large glands in the saccules of the larynx secrete periodically over the vocal folds during phonation. The free edges of these folds are devoid of glands, and their stratified epithelium is vulnerable to drying and requires the secretions of neighbouring glands: hoarseness as a result of excessive speaking is due to partial temporary failure of this secretion. The epithelial surfaces are ridged and this may help retain the lubricating secretions over the surfaces of the edges of the folds. Poorly lubricated folds offer increased resistance to airflow, which means that higher subglottal pressures are needed to initiate phonation. Taste buds, like those in the tongue, occur on the posterior epiglottic surface, aryepiglottic folds and less often in other laryngeal regions.

UPPER PART

The upper part of the laryngeal cavity contains the laryngeal inlet (aditus), the aryepiglottic fold and the laryngeal vestibule (introitus).

MIDDLE PART

The middle part of the laryngeal cavity is the smallest, and extends from the rima vestibuli above to the rima glottidis below. On each side it contains the vestibular folds, the ventricle and the saccule of the larynx.

Vocal folds (cords) and ligaments

The free thickened upper edge of the conus elasticus forms the vocal ligament. It stretches back on either side from the mid level of the thyroid angle to the vocal processes of the arytenoids. When covered by mucosa, it is termed the vocal fold or vocal cord (cord is the preferred clinical term) (Figs 34.5, 34.6). The vocal folds form the anterolateral edges of the rima glottidis and are concerned with sound production. Each fold consists of five layers, namely mucosa, lamina propria (three layers) and the vocalis muscle (Fig. 34.7).

Fig. 34.7 Coronal view of the laryngeal cavity, showing the distribution of the mucous membrane in the laryngeal cavity; the inset shows the structure of the true vocal folds.

The mucosa overlying the vocal ligament is thin and attached to the underlying lamina propria by a basement membrane. It lies directly on the ligament, and so the vocal fold appears pearly white in vivo. The lamina propria consists of three layers. The most superficial consists of loose collagen and elastic fibres, and is only loosely attached to the underlying vocal ligament, an arrangement that produces a potential space (Reinke’s space) that extends along the length of the free margin of the vocal ligament and a little way onto the superior surface of the cord: oedema fluid readily collects here in disease. The intermediate layer consists of elastic fibres, and the deep layer is formed of collagen fibres; these two layers collectively form the vocal ligament. Fibres of the vocalis muscle form the fifth layer of the vocal folds. The site where the vocal folds meet anteriorly is known as the anterior commissure, and is the region where fibres of the vocal ligament pass through the thyroid cartilage to blend with the overlying perichondrium, forming Broyle’s ligament. Since Broyle’s ligament contains blood vessels and lymphatics, it represents a potential route for the escape of malignant tumours from the larynx.

Reinke’s oedema

The mucous membrane is loosely attached throughout the larynx, and can accommodate considerable swelling which may compromise the airway in acute infections. At the edge of the true vocal folds the mucosal covering is tightly bound to the underlying ligament so that oedema fluid does not pass between the upper and lower compartments of the vocal cord mucosa. Any tissue swelling above the vocal cord exaggerates the potential space deep to the mucosa (Reinke’s space), causing accumulation of extracellular fluid and flabby swelling of the vocal cords (Reinke’s oedema). The oedema can persist because of the poor lymphatic drainage in this region of the larynx. Vocal abuse may initiate such changes, but the condition is nearly always confined to smokers.

Vocal cord nodules

Vocal fold nodules are chronic lesions of the vocal folds and develop most commonly as the result of persistent overuse of the voice which has caused an increase in vocal fold tension and a more forceful adduction. They normally develop at the point of maximum contact of the vocal folds, i.e. at the junction of the anterior third and the posterior two-thirds of the vocal ligament. Excessive trauma at this point, for example when singing with poor technique or forcing the voice, initially produces subepithelial haemorrhage or bruising; in time this results in pathological changes such as subepithelial scarring (‘singer’s nodes’ or ‘clergyman’s nodes’). Nodules increase vocal fold mass and affect vocal fold closure: the persistent posterior glottal opening causes hoarseness, a breathy voice, reduced vocal intensity and an inability to produce higher frequencies of vibration. These changes can cause a cycle in which increasing vocal effort is required by way of compensation, and this exacerbates the problem.

Rima glottidis

The rima glottidis or glottis is the fissure between the vocal cords anteriorly and the arytenoid cartilages posteriorly (see Figs 34.8, 34.9). It is bounded behind by the mucosa that passes between the arytenoid cartilages at the level of the vocal cords. The glottis is customarily divided into two regions, an anterior intermembranous part, which makes up about three-fifths of its anteroposterior length and is formed by the underlying vocal ligament, and a posterior intercartilaginous part formed by the vocal processes of the arytenoid cartilages. It is the narrowest part of the larynx, having an average sagittal diameter in adult males of 23 mm, and in adult females of 17 mm: its width and shape vary with the movements of the vocal cords and arytenoid cartilages during respiration and phonation (see p. 590).

Fig. 34.8 The true vocal folds viewed through a fibre endoscope.

(By permission from Berkovitz BKB, Moxham BJ 2002. Head and Neck Anatomy. London: Martin Dunitz.)

Fig. 34.9 Intrinsic muscles of the larynx in posterior or lateral views with their actions shown alongside in a superior view. The arrows indicate the direction of movement of the cartilage in each case.

LOWER PART

The lower part of the laryngeal cavity, the subglottis or infraglottic cavity, extends from the vocal cords to the lower border of the cricoid. In transverse section it is elliptical above and wider and circular below, and is continuous with the trachea. Its walls are lined by respiratory mucosa, and supported by the cricothyroid ligament above and the cricoid cartilage below. The walls of this part of the laryngeal cavity are said to be exponentially curved, a feature that may serve to accelerate the airflow towards the glottis with the minimum loss of energy.

LARYNGOSCOPIC EXAMINATION

The laryngeal inlet, the structures around it, and the cavity of the larynx can all be inspected using fibreoptic endoscopy, through either the mouth or nasopharynx. The epiglottis is seen foreshortened, but its tubercle is visible. From the epiglottic margins the aryepiglottic folds can be traced posteromedially and the cuneiform and corniculate elevations recognized. The pink vestibular folds and pearly white vocal cords are visible and, when the rima glottidis is wide open, the anterior arch of the cricoid cartilage, the tracheal mucosa and cartilages may be seen (Fig. 34.8). The piriform fossae can also be inspected.

LARYNGEAL OBSTRUCTION AND TRAUMA

The mucosa of the upper larynx is highly sensitive, and contact with foreign bodies excites immediate coughing. Large foreign bodies may obstruct the laryngeal inlet or rima glottidis and suffocation may ensue. Smaller articles may enter the trachea or bronchi, or lodge in the laryng-eal ventricle and cause reflex closure of the glottis with subsequent suffocation. Inflammation of the upper larynx, e.g. secondary to infection or the effects of smoke inhalation, may swell the mucosa by effusion of fluid into the loose submucous tissue (oedema of the supraglottis). The effusion neither involves nor extends below the vocal cords, because the mucosa here is bound directly to the vocal ligaments and there is no submucous tissue. Laryngotomy below the vocal cords through the cricothyroid ligament, or tracheotomy below the cricoid cartilage, may be necessary to restore a free airway.

Suicidal wounds are usually made through the thyrohyoid membrane, damaging the epiglottis, superior thyroid vessels, external and internal carotid arteries and internal jugular veins. Less frequently, these wounds are inflicted above the hyoid, so that the lingual muscles and lingual and facial vessels are damaged.

PRE-EPIGLOTTIC SPACE

Its name implies that the pre-epiglottic space lies anterior to the epiglottis; in reality, it also extends beyond the lateral margins of the epiglottis, an arrangement that gives it the form of a horseshoe. The space is primarily filled with adipose tissue and does not appear to contain any lymph nodes.

The upper boundary is formed by the weak hyoepiglottic membrane, strengthened medially as the median hyoepiglottic ligament. The anterior boundary is the thyrohyoid membrane, strengthened medially as the median thyrohyoid ligament, and the lower boundary is the thyroepiglottic ligament, continuous laterally with the quadrangular membrane behind. The greater cornu of the hyoid bone forms its upper lateral border. Inferolaterally, the pre-epiglottic space is in continuity with the paraglottic space, from where it is often invaded by the laryng-eal saccule. It is also in continuity with the mucosa of the laryngeal surface of the epiglottis via multiple perforations in the cartilage of the epiglottis. Malignancies of the laryngeal surface of the epiglottis may invade the fat and areolar tissue of the pre-epiglottic space through these perforations.

PARAGLOTTIC SPACE

The paraglottic space is a region of adipose tissue that contains the internal laryngeal nerve, the laryngeal ventricle, and all or part of the laryngeal saccule. It is bounded laterally by the thyroid cartilage and thyrohyoid membrane, superomedially by the quadrangular membrane, inferomedially by the conus elasticus, and posteriorly by the piriform fossa. The lower border of the thyroid cartilage is inferior, and the paraglottic space is continuous inferiorly with the space between the cricoid and thyroid cartilages. Anteroinferiorly, there are deficiencies in the paramedian gap at the side of the anterior cricothyroid ligament, and posteroinferiorly, adipose tissue extends towards the cricothyroid joint. Some authorities exclude thyroarytenoid from the paraglottic space. Superiorly, the space is usually continuous with the pre-epiglottic space, although the two spaces may be separated by a fibrous septum.

Supraglottic tumours may spread into the paraglottic space and reach the subglottis, or extend beyond the limits of the larynx. Ventricular tumours may obstruct mucus outflow from the saccule and cause its expansion within the paraglottic space to form a saccular cyst: the tumour itself may also spread transglottically, and thereby fix the vocal cord either by invasion of the cricoarytenoid joint or by damaging the recurrent laryngeal nerve. Fixation of the vocal cord is a good indicator of a tumour within the paraglottic space. The proximity of the mucosa at the piriform fossa makes its removal in surgery mandatory for such disease.

SUBGLOTTIC SPACE

The subglottic space is bounded laterally by the cricovocal membrane, medially by the mucosa of the subglottic region, and above by the undersurface of Broyle’s ligament in the midline. It is continuous below with the inner surface of the cricoid cartilage and its mucosa.

INTRINSIC MUSCLES

LYMPHATIC DRAINAGE

The vocal cords, with their firmly bound mucosa and paucity of lymphatics, provide a clear demarcation between the upper and lower areas of the larynx. Above the vocal cords, the lymph vessels draining the supraglottic part of the larynx accompany the superior laryngeal artery, pierce the thyrohyoid membrane, and end in the upper deep cervical lymph nodes, often bilaterally. The supraglottic lymphatics also communicate with those at the base of the tongue. Below the vocal cords, some of the lymph vessels pass through the conus elasticus to reach the prelaryngeal (Delphian) and/or pretracheal and paratracheal lymph nodes, while others accompany the inferior laryngeal artery and join the lower deep cervical nodes.

OVERVIEW OF SPEECH PRODUCTION

All speech requires an input of energy. For all sounds in Western European languages, and most sounds in other languages, this energy takes the form of a pulmonary expiration. This continuous airflow is converted into a vibration within the larynx by a mechanism called phonation, in which the vocal folds vibrate periodically, interrupting the column of air as it leaves the lungs and converting it into a series of discrete puffs of air. Speech sounds that are produced by vocal fold vibration in this way are said to be voiced. Speech sounds that are produced without vocal fold vibration are termed unvoiced sounds.

The larynx is an inadequate sound source: the laryngeal ‘buzz’ that is produced by phonation is very quiet and cannot be varied sufficiently to produce the complex range of sounds that is human speech. Amplification and modification of the sound occur in the supralaryngeal vocal tract, which may be considered as a 17 cm long tube, narrow at the larynx and broadening out proximally as it passes through the pharynx, and oral and nasal cavities. This tube acts as a passive amplifier of the sound. (The analogy here is of a megaphone: cupping the hands round the lips lengthens the vocal tract and increases the volume of speech.) The supralaryngeal vocal tract modifies the basic vibration of the larynx by altering its geometry, length and calibre: it provides a series of resonators that can dampen or amplify certain sound frequencies and can transiently interrupt the exhaled air flow and modify it to produce speech. This process is known as articulation. The range of sounds that the human vocal tract is capable of producing is very wide, although any one human language will employ a subset of these sounds to convey meaning.

MUSCULAR CONTROL OF THE AIRSTREAM

Normal vegetative ventilation involves rhythmic movements of the thoracic cage that are produced by intercostal muscles and the diaphragm and a number of accessory muscles in the neck, arm and abdomen that have one attachment to some part of the thoracic cage. The thorax is capable of responding mechanically to widely varying demands for oxygen. From a tidal volume at rest of 500 mL and a respiratory rate of 12 per min, ventilation can increase in fit individuals during vigorous exercise to tidal volumes of 4.5 L and respiratory rates of 20–25.

Normal ventilatory patterns are considerably modified during speech, reflecting the special demands that speech places upon ventilation. The main source of energy for the production of speech sounds is a pulmonary expiration, although other mechanisms are possible. In order that speech is produced, sufficient pressure has to be generated beneath the vocal folds. This subglottal pressure (the difference between the air pressures above and below the vocal folds) has to be sustained above a minimum level throughout an utterance. It sets the vocal folds into vibration if the sound is to be voiced or generates airflow for an unvoiced sound. The minimum subglottal pressure needed for speech production is 7 cmH₂O, and this increases when loud sounds are produced or when sounds are stressed.

The need to generate sufficient sustained pressure means that speech ventilation is markedly non-rhythmical. At the onset of an utterance, inspiration is typically 1.5 L, which is deeper than for normal quiet ventilation, ensuring that sufficient air is taken in to maintain adequate subglottal pressure for the duration of the utterance. Inspiration is also quicker, 0.5 sec rather than 2–3 sec. Expiration is much longer than normal, perhaps lasting up to 30 sec, reflecting the fact that the vocal tract is more constricted at the larynx to ensure that pauses for further inspirations are made at suitable points in an utterance. At the end of the first inspiration during running speech, lung volumes do not fully return to resting levels. Conversational speech normally takes place at a higher range of lung capacities than operate in normal quiet ventilation.

The non-rhythmical pattern during speech requires greater inspiratory effort, and for most people it involves a greater use of the diaphragm, usually in combination with the abdominal muscles that are attached to the lower ribs (they stabilize the costal attachments of the diaphragm and increase the effectiveness of its action) (see p. 1011). However, the main differences are seen in expiration. Speech takes place at higher lung volumes, which means that greater recoil forces are stored in the elastic tissues of the lungs and the ribcage. The generation of subglottal pressure is the product of these elastic recoil forces and the muscular forces generated by the expiratory muscles. At the onset of an utterance, unrestrained recoil forces would generate excessive subglottal pressures that would be wasteful of air, and hence energy, and would affect the loudness of speech. Conversely, towards the end of an utterance, as recoil forces decline, subglottal pressure would fall without additional muscular exertion. Therefore, early in an utterance, inspiratory muscles, particularly the external intercostals and parasternal parts of the internal intercostals, continue to contract, relaxing slowly to counteract the effects of excessive passive elastic recoil. As the recoil forces decline below the point where they can maintain the minimum subglottal pressure needed for phonation, expiratory muscles contract to maintain subglottal pressure as lung air volume nears its resting expiratory level. The main muscles involved are the costal parts of the internal intercostals and the subcostal and transversus thoracis muscles. Their actions are aided by contraction of the anterior abdominal muscles to compress the abdomen. Accessory muscles such as latissimus dorsi may also come into play, but normally these accessory muscles are only active at the end of a very long or loud utterance, or in patients whose ventilatory function is compromised.

Though subglottal pressure tends to remain fairly constant during an utterance, it rises when sounds are stressed, and falls during the production of unvoiced sounds when the larynx is less constricted. At these times, compensatory mechanisms to ensure that pressure is maintained are required: the precise mechanisms have yet to be elucidated, but the internal intercostal muscles have been implicated. The anterior abdominal muscles are also active in singing, shouting and in attempts to speak without the pause necessary for inspiration. Contrary to popular belief, the diaphragm plays little part in the regulation of expiratory force. Unlike the intercostal muscles, the diaphragmatic musculature is sparsely supplied with muscle spindles, and therefore control of the diaphragm is poorly regulated: minute changes can be effected more successfully using the intercostal and anterior abdominal muscles.

Though the expiratory airflow from the lungs is the source of energy for most speech sounds, other sources of airflow are also used. The larynx can be used to generate airflow. The sound /p/ is produced by closing the larynx and then raising it with the lips closed, using the larynx like a piston. Opening the lips then releases a puff of air. This is called an ejective, and is an example of egressive glottalic airflow. Ingressive glottalic airflow is also possible, though less effective; examples are not found in English, but do occur in some African and native American languages. A third kind of airflow is velaric, in which the back of the tongue is raised against a lowered soft palate and the vocal tract is closed anterior to that point, either at the lips or with the tongue against the hard palate. This produces a click and is rare. A non-linguistic example in English is the sound made in encouraging a horse. Ingressive pulmonary airflow, such as is found in a groan or a gasp, is theoretically possible, but none of the world’s languages employ this as a source of airflow.

After removal of the larynx, e.g. following laryngeal cancer, patients can be taught to swallow air, store it in a segment of the oesophagus and then use it as the energy source to produce egressive oesophageal airflow (oesophageal speech). Speech in these circumstances tends to have a belching quality and may be badly phrased. Laryngectomy patients always produce phrases that are shorter than normal, and so prostheses incorporating valves and surgical shunts are often inserted to provide a larger egressive airstream by diverting air from the respiratory tract into the oesophagus.

PHONATION

The default position of the rima glottidis is open, to maintain patency of the airway during respiration. In quiet respiration, the anterior intermembranous part of the rima glottidis is triangular when viewed from above. Its apex is anterior and its base posterior, and it is represented by an imaginary line approximately 8 mm long connecting the anterior ends of the arytenoid vocal processes. The intercartilaginous part between the medial surfaces of the arytenoids is rectangular as the two vocal processes lie parallel to each other. During forced respiration, the rima glottidis is widened and the vocal cords are fully abducted to increase the airway. The arytenoid cartilages rotate laterally, and this moves their vocal processes apart, and converts the rima glottidis into a diamond shape in which both intermembranous and intercartilaginous parts are triangular. The greatest width of the rima glottidis is at the point of the attachments of the vocal cords to the vocal processes.

During speech the true vocal folds vibrate to act as a source of sound for subsequent speech. There have been a number of theories to explain the mechanism that produces this vibration, but these are now only of historic interest. The aerodynamic–myoelastic theory is generally accepted as the mechanism underlying vocal fold vibration, although it does not account for all aspects of phonation.

At the onset of an utterance, during an expiration, the true vocal folds are adducted: the lateral cricoarytenoids and interarytenoids bring together both the intermembranous and intercartilaginous parts of the glottis, actions which either close the glottis completely or reduce the space between the vocal folds to a linear chink. The mucous membrane covering the interarytenoid muscles, the interarytenoid fold, intrudes into the larynx when these muscles adduct the arytenoids, and so aids closure of the intercartilaginous part of the rima glottidis. The vocal cords are also tensed, an essential prerequisite for vibration. These actions cause a build-up of subglottal pressure that continues until a point is reached when the muscular force of adduction is no longer sufficient to resist the rising pressure, and the vocal folds are forced open a little, releasing air into the supralaryngeal vocal tract. The subglottal pressure falls when the subglottic and supraglottic cavities become continuous and the vocal folds begin to close. Two mechanisms bring about closure. If adductive tension is sustained, then the vocal folds will close. In addition, rapid closure is aided by a physical process, the Bernoulli effect. The forcing of air from a region of high to low pressure through a narrow space causes an increase in the kinetic energy of the molecules at the edge of the space. The effect of this is to lower pressure in the space between the folds at the level of the folds themselves, and this negative pressure simply sucks the folds together because they are mobile. This causes a rise once more in the subglottal pressure and the cycle is repeated. The effect is to cause the release of a series of puffs of air into the supralaryngeal vocal tract at a frequency of many times per second, which is perceived as a sound of a particular frequency (Figs 34.12, 34.13).

Fig. 34.12 Video montage of the normal phonatory cycle, obtained using a rigid fibre endoscope with stroboscopic illumination.

(By courtesy of Professor Paul Carding.)

Fig. 34.13 Position of the vocal folds during a cycle of vocal fold vibration (phonatory cycle) in superior (A) and coronal (B) views.

The source of energy for the vibration does not come from the larynx itself. The vocal folds do not behave like the prongs of a tuning fork: their predominant motion is in the horizontal plane at right angles to the movement of the air column, and there is little vertical movement. The energy is derived from the motion of the air generated by the muscular and recoil forces in the thorax, and the larynx is simply chopping that column into a series of segments.

The aerodynamic–myoelastic theory does not explain how vocal fold vibration is sustained, nor does it account for the very obvious muco-undulatory component of vocal fold vibration that is visible when the larynx is viewed stroboscopically. Without further input of energy the vibration of the vocal folds as described above would not be sustained but would be damped and gradually diminish. For vibration to continue there has to be additional energy input. The analogy here is with a child on a swing. For the motion to continue either the parent has to push the child at an appropriate point in the cycle, or the child has to sustain the motion for themselves by swinging their legs at the crucial point in the swing cycle. In the case of phonation, the source of this additional energy is unclear. It may come from the inertia of the air column itself, i.e. once the vocal folds close, the air column will continue moving upwards because inertia creates a negative pressure above the folds. Alternatively, the energy could come from the manner in which the folds open and close. As the subglottal pressure rises, the lower portion of the fold opens first and the upper edge of the fold is last to open, and when the subglottal pressure falls, the folds close from the bottom edge. It has been suggested that this non-uniform closure creates different shapes within the glottis that may result in differing negative pressures at different phases of the cycle. It also produces a vertical wave-like motion in the folds, termed the muco-undulatory component. The analogy here is with a flag blowing in the wind, and it reflects the differing stiffnesses of the various layers of the vocal folds described above. This vertical component will impart a negligible amount of energy to the air column, but it is likely to impart harmonics to the basic laryngeal vibration.

The sound that results from the process of phonation has three characteristics: a frequency that is perceived as pitch, an intensity that is perceived as loudness, and a timbre perceived as voice quality.

The fundamental frequency of the human voice is determined by the resting length of the vocal cords and varies with age and sex. The frequency range of human speech is from 60 to 500 Hz, with an average of approximately 120 Hz in males, 200 Hz in females and 270 Hz in children. The mechanism of frequency alteration is not entirely clear. An increase in subglottal pressure will cause the frequency of phonation to rise. However, during an utterance, subglottal pressure appears to remain fairly constant, which suggests that the mechanism of frequency alteration resides in intrinsic changes within the vocal folds. Variations in frequency (pitch changes) during an utterance are determined by the complex interrelationships between length, tension and thickness of the vocal cords: one of these variables cannot be altered without affecting the other two parameters to some extent. Gross changes to the vocal cords demonstrate the effects of these variables. Inflamed and swollen vocal cords are much thicker than normal and result in a hoarse voice. At puberty, growth of the thyroid cartilage in males lengthens the vocal cords and lowers the fundamental frequency, and the voice ‘breaks’ as a result. During panic, the vocal cords may be tensed, which means that the cry for help is a high-pitched squeak.

Pitch is increased by increasing the length of the vocal folds, as may be confirmed during direct endoscopic examination of the larynx. At first sight this may seem counterintuitive, but, as the vocal cords are lengthened, there will be a consequent thinning and change in tension. Although an analogy is often drawn between the vocal cords and vibrating strings, a better analogy is a rubber band: if a rubber band is lengthened, the tension will increase, but the thickness will decrease. The vocal cords may be lengthened by up to 50% of their resting length. It is likely that the initial pitch setting is achieved by action of the cricothyroids, and that fine adjustments can then be made using the vocales. Paralysis of both cricothyroids, which is usually associated with loss of the neurones that are distributed via the superior laryngeal nerve (as a result of damage to the vagal nuclei in brainstem stroke), results in permanent hoarseness and inability to vary the pitch of the voice. It is important to remember that once the vocal cords are set in motion they will deviate from their original setting as they vibrate. Auditory feedback of the sounds produced is used to make minute compensatory adjustments to length, tension and thickness in order to maintain a constant pitch.

Any lengthening of the vocal cords tends to thin them. The thickness can be increased by the vocalis part of thyroarytenoid: as vocalis shortens, it will relax the vocal cords and at the same time increase their thickness. Changes in the tension of the vocal cords are produced by the same muscles that change their length, namely cricothyroid, posterior cricoarytenoid and vocalis, probably acting isometrically.

The mechanism by which loudness is increased is the subject of less debate. Loud sounds are produced by increasing subglottal pressure. This is achieved, in turn, by changing the opening quotient of the glottis (the ratio of the time spent in the open phase of the cycle to the total cycle time). In normal speech, this ratio is usually around 0.5, but in very loud speech it can fall to 0.3.

Timbre or voice quality refers to the harsh or mellow quality of the voice. At high volume the voice tends to be harsher, especially in untrained voices: higher-frequency components predominate because higher subglottal pressures are needed to sustain the increased volume. This can be overcome to an extent by increasing the airflow rather than the pressure.

A fundamental distinction in speech needs to be made between voiced and unvoiced sounds: nearly all languages make this distinction. Voicing has been described above. In unvoiced sounds, the vocal folds are not vibrating and will usually be opened under the action of posterior cricoarytenoid. The energy from the airstream is then used by other parts of the vocal tract to generate sound, normally by constricting or stopping the airflow. However, phonation is not an all-or-nothing process, but is subject to considerable modification and adjustment. In modal voice, i.e. speaking using habitual pitch, forces acting on the vocal folds are moderate, pressures are sustained, and air is conserved. However, phonation can occur when the vocal folds are more open than usual, resulting in breathy phonation with more air escaping per phonatory cycle than usual. Some languages in South Asia exploit the difference between breathy and non-breathy sounds, whereas in spoken English, a breathy voice is simply recognized as a feature of some speakers. At the other end of the spectrum is vocal creak, in which the vocal folds are more closed than normal. Different speakers will habitually employ different laryngeal settings which contribute to their particular voiced quality. In whispering, the intramembranous part of the glottis is closed, but the intercartilaginous part remains open, which produces a characteristic Y-shaped glottis and a greater loss of air at each phonatory cycle.

The main function of the larynx is to act as a sound source, but it can also function in speech as an airstream generator and as an articulator.

ARTICULATION

The sound produced by the phonation is not a pure tone because several harmonics at multiples of the fundamental frequency are also generated. Harmonics give a note of a particular frequency its defining characteristics. An ‘A’ played on an oboe or violin is immediately recognizable because of the different harmonics generated by the design of the instrument. The harmonic spectra of individual voices differ and will also vary depending upon the mode of phonation adopted. In the human vocal tract, the fundamental frequency and its harmonics are transmitted to the column of air which extends from the vocal cords to the exterior, mainly through the mouth. Part of the airstream can also be diverted through the nasal cavities when the soft palate is depressed to allow air into the nasopharynx. The supralaryngeal vocal tract acts as a selective resonator whose length, shape and volume can be varied by the actions of the muscles of the pharynx, soft palate, fauces, tongue, cheeks and lips; the relative positions of the upper and lower teeth, which are determined by the degree of opening and protrusion or retraction of the mandible; and alterations in the tension of the walls of the column, especially in the pharynx. Thus, the fundamental frequency (pitch) and harmonics produced by the passage of air through the glottis are modified by changes in the supralaryngeal vocal tract.

Harmonics may be amplified, or dampened. The fundamental frequency and its associated harmonics may also be raised or lowered by appropriate elevation or depression respectively of the hyoid bone and the larynx as a unit by the selective actions of the extrinsic laryngeal muscles. Effectively, these movements shorten or lengthen the resonating column, and to some extent also alter the geometry of the walls of the air passages. Analysis of the human voice shows that it has a very similar pattern of harmonics for all fundamental frequencies, determined by the vocal tract acting as a selective filter and resonator. This maintains a constant quality of voice without which intelligibility would be lost (recorded speech played back without its harmonics is completely unintelligible). Each human voice is unique: it has been suggested that the unique frequency spectrum of each individual voice could be used for personal identification.

During articulation the egressive airstream is given a rapidly changing specific quality by the articulatory organs, the lips, oral cavity, tongue, teeth, palate, pharynx, and nasal cavity. The discipline of phonetics primarily deals with the way in which speech sounds are produced, and consequently with the analysis of the mode of production of speech sounds by the vocal apparatus. In order to analyse the way in which the articulators are used in different speech sounds, words are broken down into units called phonemes, which are defined as the minimal sequential contrastive units used in any language.

The human vocal tract can produce many more phonemes than are employed in any one language. Not all languages have the same phonemes, and within the same language, the phonemes can vary in different parts of the same country and in other countries where that language is also spoken. Reproducing phonemes that are not used in native speech is difficult because such phonemes require unfamiliar positioning of the speech organs. A native speaker of any language can quickly recognize the origins of anyone attempting to use their language as a second language. The second-language speaker will usually speak it with an accent characteristic of their own first language because they are using the familiar configurations of their vocal tract for each phoneme instead of the correct positioning.

PRODUCTION OF VOWELS

All vowel sounds require phonation by vibration of the vocal cords. Each vowel sound has its own characteristic higher harmonics (frequency spectrum) that exhibit peaks of energy at certain frequencies. These energy peaks are always higher multiples of the fundamental frequencies and are called formants. Formants are the result of the combined effects of phonation, selective resonance of the vocal tract, and the properties of the head as a radiator of sound. The sounds of the different vowels are determined by the shape and size of the mouth, and the positions of the tongue and lips are the most important variables. The tongue may be placed high or low (close and open vowels), or further forwards or back (front and back vowels) and the lips may be rounded or spread.

PRODUCTION OF CONSONANTS

The production of consonants always involves some degree of constriction of the vocal tract. There are many more consonants than vowels, and, in general, consonants cannot be combined to produce syllables. The classification of consonants is complex and beyond the remit of this book: what follows is a summary (for fuller details, a textbook of phonetics should be consulted).

Consonants may be classified of the basis of where the constriction occurs, termed the place of articulation; the degree or extent of constriction, termed the manner of articulation; the shape of the constriction, termed the stricture; and whether or not there is vibration of the vocal folds, when consonants are described as voiced or unvoiced respectively.

Consonants may also be classified as labial, dental, alveolar, velar, uvular, pharyngeal or glottic, depending upon whether the point of maximum constriction occurs at the level of the lips, teeth, bony ridge behind the teeth, palate, uvula or pharynx (Fig. 34.14). Different parts of the tongue can be used in combination with the above places of articulation. Phoneticians divide the tongue into the tip, anterior edge, the front part of the dorsum, the centre and back parts of the remaining dorsum, and a most posterior part (the root). These divisions bear no obvious relationship to the anatomical landmarks on the tongue, but they are useful in describing the part of the dorsum of the tongue that contacts other areas of the mouth. Similarly, the manner of articulation can vary from a complete closure to a slight narrowing. An actual closure of the vocal tract is called a stop. A narrowing that is sufficient to produce turbulence of the air in the vocal tract, and which is perceived as a rustling sound, is termed a fricative. Approximants involve a degree of closure insufficient to produce turbulence but with closure greater than that for a vowel. Nasals involve a stoppage in the oral cavity with the soft palate lowered to allow airflow through the nose, and, unlike stops, they can be sustained. Stricture describes the shape of the constriction, e.g. a lateral consonant involves depression of the sides of the tongue, while a grooved consonant is produced by grooving the dorsum of the tongue. Consonants can be produced with the vocal folds vibrating, when they are termed voiced, or without vocal fold vibration, in which case they are termed unvoiced.

Fig. 34.14 Sagittal view of the left side of the head, showing the supralaryngeal vocal tract, the articulators and places of articulation. The broken line indicates the tongue position during retroflexion (10).

The best way to illustrate these classificatory systems in operation is by contrasting the production of different consonant pairs in which only one or two parameters have been changed. The /p/ of peat, the /b/ of beat and the /m/ of meat are all bilabial stops, meaning that they are produced by bringing together the lips. The /b/ and the /m/ are voiced but the /p/ is not. The contrast between the /b/ and the /p/ is in the differing way in which the airstreams are produced. The /b/ is produced with an egressive pulmonary airflow, while the /p/ is produced with the glottis closed, hence unvoiced, and the glottis is then raised using the larynx as a piston to compress the air in the supra-laryngeal vocal tract prior to the stop being released. The /m/ differs from the other two stops in being a nasal in which the soft palate is lowered to allow air to escape through the nasal cavity, and unlike the other two stops it can be sustained as in a sound of approval, e.g. encouraging a horse to move. Bilabial stops can be contrasted with the labiodental fricatives /f/ of feet and the /v/ of veal, both of which are produced by retracting the lower lip beneath the upper teeth. Neither involves a complete closure but both produce a significant constriction of the vocal tract with audible turbulence: the /f/ is unvoiced, while the /v/ is voiced. The sh sound (/∫/) in ship is also a fricative involving a grooving of the tongue and is associated with significant audible turbulence; it may be contrasted with the lateral approximant /l/ in law, in which the sides of the tongue are lowered. In this case it is the nature of the stricture which is different and the degree of closure, e.g. in the case of the approximant /l/, closure is insufficient to produce turbulence. The sh sound in ship can be compared to the /k/ in keen, where the position of the tongue is different: in /k/ the tongue blade contacts the soft palate, while in sh the tongue tip or the blade contact the postalveolar region. The most dramatic example of the difference between voiced and unvoiced sounds may be appreciated if the /s/ sound in sip is compared with the /z/ in zip. If the larynx is loosely palpated while making a sustained unvoiced ‘ssssss’ sound, no vibration is felt, but if the ‘ssssss’ is commuted into a prolonged voiced ‘zzzzzz’, then vibration in the larynx should be readily detectable. The position of the tongue and other articulators is exactly the same for both /s/ and /z/, the difference between them is the presence or absence of phonation.