Pharyngeal-Oral Function and Speech Production
The pharyngeal-oral apparatus, together with the velopharyngeal-nasal apparatus (discussed in Chapter 4), forms what is called the upper airway. In this book, the term upper airway is used in the context of the anatomy and physiology of this region so as to be consistent with terms such as lower airways, laryngeal airway, velopharyngeal-nasal airway, and oral airway. When referring to the acoustic properties of these regions, different terms are used: the term vocal tract is used for the pharyngeal-oral air spaces and the term nasal tract is used for the nasal air spaces (see Chapters 8–11).
Although most textbooks consider only the oral component (or the combined oral and velopharyngeal components), this chapter includes the pharynx in its consideration of pharyngeal-oral function. This is because the middle and lower regions of the pharynx function like articulators and acoustic filters in much the same way as do oral structures.
This chapter begins with the anatomy, forces (including muscle forces), and movements of the pharyngeal-oral apparatus. The next sections cover the control variables of the apparatus, its neural control, and its general functions. The final sections are dedicated to articulatory descriptions and processes as well as variables that may influence speech production. Its role during swallowing is discussed in Chapter 16.
ANATOMY OF THE PHARYNGEAL-ORAL APPARATUS
The pharyngeal-oral apparatus is a flexible tube that extends from the larynx to the lips and undergoes an approximate 90-degree bend (like a plumber’s elbow joint) at the level of the oropharynx. There, the longer and vertical pharyngeal portion communicates through the oropharyngeal (faucial) isthmus with the shorter and horizontal oral portion. The pharyngeal-oral apparatus is supported by a skeletal structure that provides the framework around which its internal topography is organized.
The full skeletal framework that supports both the velopharyngeal-nasal apparatus and the pharyngeal-oral apparatus is presented and illustrated in Chapter 4. This framework consists of the cervical vertebrae, bones of the skull (see Figure 4–1), and bones of the face (see Figure 4–2). The cervical (neck) segments of the vertebral column lie behind the three subdivisions of the pharynx—laryngopharynx, oropharynx, and nasopharynx—and form part of the substance of their back walls. The skull, made up of several irregularly shaped bones, forms the framework of the head. The facial bones contribute to the formation of the roof, floor, and sides of the oral cavity. The maxilla, mandible, and the temporomandibular joints are particularly prominent pharyngeal-oral structures and are discussed in more detail here.
Figure 5–1 depicts the maxilla. The maxilla forms the upper jaw and most of the hard palate. It consists of two complex bones (one on the left and one on the right) that combine at the midline. The maxilla lends strength to the roof of the oral cavity (as well as to the floor of the nasal cavities) and provides a buttress for the facial skeleton. Each bone of the maxilla has a palatine process that extends horizontally to the midline and joins with the palatine process from the opposite side to form the front three-fourths of the hard palate. The back one-fourth of the hard palate is formed by the much smaller palatine bones, which have horizontal processes that extend to the midline from each side to complete that part of the hard palate.
The alveolar process of the maxilla (sometimes called the alveolar arch) is a thick spongy projection that extends downward and houses the upper teeth. This process accommodates 16 permanent teeth (8 on each side). As shown in Figure 5–2, they are the central incisors, lateral incisors, canines (or cuspids), first and second premolars (or bicuspids), first and second molars, and third molar (or wisdom teeth). Infants and young children have only 10 teeth, called deciduous teeth (or baby teeth or milk teeth), which are later replaced by the permanent teeth.
Figure 5–3 shows the salient features of the mandible. The mandible (lower jaw) is a large horseshoe-shaped structure when viewed from above or below. Its open end faces toward the back. The front and sides of the mandible together form what is called the body of the structure. The left and right halves of the mandible join at the front through a fibrous symphysis (line of union) that ossifies (turns to bone) during the first year of life. Like the maxilla, the mandible also has an alveolar process that accommodates 16 permanent teeth. These teeth bear the same names as the maxillary teeth.
On each side of the mandible toward the back, there is an upward projection called the ramus (meaning a branch from the body). The location along the bottom of the mandible where each ramus diverges upward is designated as the angle. The upper part of each ramus has two projections, one at the front called the coronoid process and one at the back called the condylar process (also called the condyle). The coronoid process is somewhat rounded, whereas the condylar process has a neck and a prominent head.
Figure 5–2. The upper teeth. The alveolar process of the maxilla contains 16 permanent teeth (8 on each side) that include incisors, canines, premolars, and molars. Infants and young children have fewer deciduous teeth.
The mandible articulates with the left and right temporal bones along the sides of the skull to form the temporomandibular joints. As illustrated in Figure 5–4, these joints are located just in front of and below the external auditory meatuses (ear canals). They can be palpated as the mandible is raised and lowered. The temporomandibular joints are enclosed by a fibrous capsule and lubricated by synovial fluid. Each joint is of the condyloid variety in that it consists of an ovoid (egg-shaped) process (the head of the condyle) that fits into an elliptical-shaped cavity within the temporal bone on the corresponding side (Dickson & Maue-Dickson, 1982). The condyle of the mandible (the more rearward of its two processes) is separated from its receiving cavity by a cartilaginous meniscus (crescent) called the articular disk. The surfaces of the condyle and the temporal bone are themselves covered with fibrocartilage (cartilage that contains fibrous bundles of collagen) that is devoid of vascular tissue (Sicher & DuBrul, 1975).
Figure 5–3. Mandible as seen from three different views. The mandibular alveolar process contains 16 teeth that have the same names as their maxillary counterparts.
Figure 5–4. Temporomandibular joints and ligaments. The temporomandibular, sphenomandibular, and stylomandibular ligaments limit the motions of the joints in downward, backward, and forward directions.
Three ligaments influence the function of each temporomandibular joint (see bottom of Figure 5–4): the temporomandibular ligament, the sphenomandibular ligament, and the stylomandibular ligament. The temporomandibular ligament, which extends between the outer surface of the zygomatic arch and the outer and back surfaces of the neck of the condyle, limits the degree to which the condyle can be displaced downward and backward. The sphenomandibular ligament, which extends between the angular spine of the sphenoid bone and the inner surface of the ramus below the condyle, limits downward and backward displacement of the mandible. The stylomandibular ligament, which extends between the styloid process of the temporal bone to near the angle of the mandible, limits downward and forward displacement of the mandible.
Movements at the temporomandibular joints are conceptualized as movements of the mandible relative to a stabilized skull (although the opposite is possible, such as when the chin is rested on the top of a table and separation of the jaws causes the skull to rotate upward and backward). Movements of the mandible, as mediated through its condyloid processes, has three displacement possibilities: (a) upward and downward, made possible by a hingelike action, (b) forward and backward in a gliding action, and (c) side to side in a gliding action. These displacement possibilities are portrayed individually in Figure 5–5; however, temporomandibular joint movements are often multidimensional and very complex.
Figure 5–5. Temporomandibular joint movements. These movements include a hinge-like action to lower the mandible, a forward-backward gliding action, and a side-to-side gliding action.
The internal topography of the pharyngeal-oral apparatus is fashioned around a hollow tube that bends at a right angle at the junction between the pharyngeal and oral portions of the structure. The pharyngeal, oral, and buccal cavities and their mucous lining deserve individual consideration.
Recall from Chapter 4 that the pharynx (throat) is a tube of tendon and muscle that extends from the base of the skull to the larynx. This tube is widest at the top and narrows down its length and is larger side to side than front to back (oval shaped). The lower and middle parts of the pharyngeal tube are designated as the laryngopharynx and oropharynx, respectively, and are the parts of greatest interest in this chapter (see Figure 4–4). The back and sides of the laryngopharynx and oropharynx are ringed by pharyngeal muscles (see Figure 4–10). The lower part of the oropharynx is bounded by the tongue and epiglottis and the upper part of the oropharynx opens into the oral cavity at the front through the anterior faucial pillars (palatoglossal arch). The back wall of the upper part of the oropharynx can be seen when looking back through the faucial isthmus (see Figure 4–5).
Figure 5–6 depicts the oral cavity (mouth cavity). The front entryway to the oral cavity is designated as the oral vestibule and is defined to include the lips, cheeks, front teeth, and forwardmost segments of the alveolar processes of the maxilla and mandible. The oral cavity is bounded at the back by the anterior faucial pillars (palatoglossal arch), above by the hard palate and velum, and below by its floor comprising mainly the tongue.
The tongue is a prominent feature of the oral cavity, and its importance in activities such as speaking and singing has long been recognized (Figure 5–7). Although unitary in nature, it is sometimes subdivided into different regions. The subdivisions chosen may have either anatomical or functional bases, depending on their purpose. Anatomical schemes usually recognize a root, pertaining to the vertically oriented back part of the tongue (front wall of the laryngopharynx and oropharynx), and a blade, pertaining to the horizontally oriented part of the tongue (floor of the oral cavity) (Zemlin, 1998). In contrast, functional schemes usually recognize regions of the tongue that are considered important to the behavior of the structure (Kent, 1997).
Figure 5–6. Oral cavity. The oral vestibule is the front entryway to the oral cavity and is bounded at the front by the lips and at the back by the front parts of the alveolar processes. The back of the oral cavity is bounded by the anterior faucial pillars.
Figure 5–8 shows the tongue as consisting of five functional components, called the tip, blade, dorsum, root, and body. The tip of the tongue is the part of its surface nearest the front teeth at rest. The blade is the part of its surface that lies behind the tip and below the alveolar ridge of the maxilla and the front part of the hard palate. The dorsum of the tongue constitutes the surface that lies behind the blade and below the back part of the hard palate and the velum. The root of the tongue designates the part of the surface of the structure that faces the back of the pharynx and the front of the epiglottis. The body of the tongue represents its central mass and underlies the other four surface parts.
Fairbanks was a giant in speech science. He also trained others who became distinguished scientists. Fairbanks was a key figure in the development of speech science as a discipline and had a major influence in bringing it to the fore as an integrated science. One of his best-known works was the development of the notion that speech production was controlled in the manner of a servomechanism that relied on sensory feedback. His book titled Voice and Articulation Drillbook (Fairbanks, 1960) is a classic and contains the famous Rainbow Passage that has been used in more speech research studies than any other reading. Fairbanks died while on a flight between Chicago and San Francisco. The flight was diverted to Denver, where the coroner ruled that he had choked to death while eating. Fairbanks was greatly admired. The three of us are honored to be able to directly trace our professional lineages to him.
Figure 5–7. “That skull had a tongue in it, and could sing once.” Hamlet in Hamlet, Act 5, Scene 1, William Shakespeare.
Figure 5–8. Five functional components of the tongue. From front to back, they are the tip, blade, dorsum, and root. The body is the central mass of the tongue that lies below the other four.
The buccal cavity lies to the sides of the oral cavity. This cavity constitutes the small space between the gums (gingivae) and teeth internally and the lips and cheeks (buccae) externally. The buccal cavity connects to the oral cavity through spaces between the teeth and behind the last molars. The status of the lips and cheeks are major determinants of the size of the buccal cavity.
The pharyngeal-oral apparatus contains a mucous lining on its internal surfaces. This lining consists of an outer layer of epithelium and an inner layer of connective tissue (lamina propria). The details of this layering differ at different locations within the pharyngeal-oral apparatus, especially the outer layer of epithelium (Dickson & Maue-Dickson, 1982). The most prominent mucosa lining has a shiny appearance and covers all of the soft tissues of the apparatus except the gums, hard palate, and tongue. A so-called masticatory mucosa covers the gums and the hard palate and has a collagen subflooring that causes its epithelium to hold firmly against adjacent bone. The upper surface of the tongue is covered with a specialized mucosa that contains an array of small pockets and crypts that house taste buds.
He was a pleasant young man who was honorably discharged after serving in the military. He had made his way to a large Veterans Administration Medical Center. His only complaint was hearing loss, but the audiologist thought his speech was inconsistent with his hearing test results. The moment he said his name to the speech-language pathologist, there was suspicion that he had impairment of one or both cranial nerves serving the tongue. When asked to open his mouth, the beam of a flashlight revealed a shrunken and wrinkled tongue that seemed to dance around under the surface like a bagful of jumping beans. The signs were classic of lower motor neuron disease and a neurologist reported presumptive bilateral congenital agenesis (failure to develop) of the hypoglossal nerves (motor nerves to the tongue). How had this escaped detection during his physical examination for the military? Perhaps he was only asked to say “ah.”
FORCES OF THE PHARYNGEAL-ORAL APPARATUS
Two types of forces are applied to the pharyngeal-oral apparatus: passive and active. Passive force is inherent and always present, but subject to change. The passive force of the pharyngeal-oral apparatus arises from the natural recoil of structures that line its walls, the surface tension between structures in apposition (lips, tongue, gums, hard palate, velum), the pull of gravity, and aeromechanical forces within the pharyngeal and oral portions of the apparatus.
The active force of pharyngeal-oral function comes from the contraction of muscles, some intrinsic (both ends attached within a component) and some extrinsic (one end attached within a component and one end attached outside the component). The function described here for individual muscles assumes that the muscle of interest is engaged in a shortening (concentric) contraction, unless otherwise specified as being involved in a lengthening (eccentric) contraction or a fixed-length (isometric) contraction. The tongue presents a somewhat more complex situation because of its special status as a muscular hydrostat (explained below). Discussed here are muscles of the pharynx, mandible, tongue, and lips.
The muscles of the pharynx are located within the laryngopharynx, oropharynx, and nasopharynx and are discussed in detail in Chapter 4. For the purposes of this chapter, those within the laryngopharynx and oropharynx are of primary interest and are reviewed here briefly.
Muscles of the laryngopharynx and oropharynx can influence the lumen (cavity) of the pharynx (the cross section along its length) in the region that lies behind the tongue, epiglottis, and oral cavity (the back wall of which is easily visualized through the faucial isthmus). The lumen of the pharynx in this region can also be influenced by adjustments of the tongue and epiglottis (see Figure 4–15). Muscles that attach to the laryngopharynx and oropharynx fabric proper (within the posterior and lateral pharyngeal walls) are revisited here. These include the inferior constrictor muscle, middle constrictor muscle, and stylopharyngeus muscle (see Figure 4–10).
The inferior constrictor muscle of the pharynx is located toward the bottom of the structure and is sometimes conceptualized as two muscles, the thyropharyngeus and cricopharyngeus muscles. Its fibers arise from the sides of the thyroid and cricoid cartilages and diverge in a fanlike configuration as they course backward and toward the midline. There they interdigitate with fibers of the paired mate from the opposite side. The middle and upper fibers of the muscle ascend obliquely, whereas the lowermost fibers run horizontally and downward and are continuous with those of the esophagus. When the inferior constrictor muscle contracts, it pulls the lower part of the back wall of the pharynx forward and draws the sidewalls of the lower pharynx forward and inward. These actions cause the lumen of the lower pharynx to reduce in cross-section.
The middle constrictor muscle of the pharynx is located midway along the length of the pharyngeal tube and is sometimes conceptualized as two muscles, the chondropharyngeus and ceratopharyngeus muscles. Its fibers arise from the greater and lesser horns of the hyoid bone and the stylohyoid ligament and course backward and toward the midline where they insert into the median raphe (seam) of the pharynx. The uppermost fibers of the middle constrictor muscle course obliquely upward and overlap the lower fibers of the superior constrictor muscle, whereas the lowermost fibers of the muscle run obliquely downward beneath the fibers of the inferior constrictor muscle. Recall that this fiber arrangement is akin to the way in which roof shingles partially overlap. When the middle constrictor muscle contracts, it decreases the cross-sectional area of the oropharynx by pulling forward on the posterior pharyngeal wall and forward and inward on the lateral pharyngeal wall. Simultaneous contraction of the left and right middle constrictor muscles causes the pharyngeal lumen to constrict regionally in the manner of a sphincter.
The stylopharyngeus muscle extends between the styloid process of the temporal bone and the lateral wall of the pharynx near the juncture of the superior constrictor and middle constrictor muscles of the pharynx. Its fibers run downward, forward, and toward the midline. When the stylopharyngeus muscle contracts, it pulls the pharyngeal tube upward and draws the lateral wall of the pharynx toward the side. Together with similar action of its paired mate from the opposite side, it widens the lumen of the pharynx, especially in the region of the oropharynx, but also elsewhere along the length of the pharyngeal tube.
Seven muscles provide active forces that operate on the mandible. These muscles are depicted in Figure 5–9 and are responsible for positioning the mandible in accordance with the movements allowed by the temporomandibular joints. Included among these muscles are the masseter, temporalis, internal pterygoid, external pterygoid, digastric, mylohyoid, and geniohyoid. Their general force vectors are depicted in Figure 5–10.
The masseter muscle is a flat, quadrilateral structure that covers much of the outer surface of the ramus of the mandible. Fibers of this muscle are in two layers. An outer layer forms the bulk of the muscle and courses from an aponeurosis along the front two-thirds of the zygomatic arch downward and backward to insert on the angle and nearby outer surface of the ramus of the mandible. An inner layer of fibers courses from the entire length of the zygomatic arch downward and forward to insert into the outer surface of the upper half of the ramus and its coronoid process. Contraction of the outer layer of the masseter muscle results in elevation of the mandible and approximation of the mandible and maxilla. The elevation is along a path that is at a right angle to the plane of occlusion of the molars. If the elevation is sufficient, pressure is brought to bear on the molars. Contraction of the inner layer of the muscle also results in elevation of the mandible and additionally exerts a force on the mandible that pulls it backward and aids in approximating the jaws.
The temporalis muscle is a broad, fan-shaped structure that covers much of the side of the cranium. Fibers of this muscle originate from the inferior temporal line of the parietal bone and the greater wing of the sphenoid bone. These converge as they course downward under the zygomatic arch and insert on the inner surface and front border of the coronoid process and the front surface of the ramus of the mandible. Fibers toward the front and middle of the muscle course vertically, whereas those toward the back of the muscle have a more horizontal orientation. Contraction of the temporalis muscle results in an upward and backward pull on the mandible, with vertically oriented fibers contributing to the upward component and horizontally oriented fibers contributing to the backward component. Activation of the temporalis muscle on only one side may result in retraction of the mandible on the same side and movement of the front of the mandible toward the activated side.
The internal pterygoid muscle is a quadrilateral structure that follows an orientation that generally parallels that of the masseter muscle. Fibers of the internal pterygoid muscle originate from the lateral pterygoid plate and the perpendicular plate of the palatine bone. From there, they course downward, backward, and outward to insert on the inner surface of the angle and ramus of the mandible. Contraction of the internal pterygoid muscle results in elevation of the mandible. Sufficient elevation causes pressure to be placed on the opposing teeth of the mandible and maxilla. Activation of the muscle on only one side may result in slight movement of the corresponding condyle toward the opposite side.
Figure 5–9. Muscles of the mandible. These muscles can raise the mandible (masseter, temporalis, internal pterygoid), lower the mandible (external pterygoid, digastric [anterior belly], mylohyoid, geniohyoid), move the mandible laterally (masseter, temporalis, internal pterygoid, external pterygoid), move the mandible forward (external pterygoid), and move the mandible backward (masseter, temporalis).
The internal pterygoid muscle has a special relationship with the masseter muscle. Together these two muscles form a muscular sling that surrounds the angle of the mandible. This anatomical sling holds the angle from above and effectively straps the ramus to the skull. The result is a functional articulation between the mandible and the maxilla, with the temporomandibular joint acting as an enabling guide for movements of the mandible (Zemlin, 1998).
Figure 5–10. Actions of muscles of the mandible. The masseter and temporalis muscles raise the mandible and pull it backward, the external pterygoid muscle slides the mandible downward and forward, and the internal pterygoid, digastric (anterior belly), mylohyoid, and geniohyoid muscles lower the mandible.
The external pterygoid muscle is one of the smaller muscles of the mandible. Fibers of the external pterygoid muscle have two origins toward the front, one from the greater wing of the sphenoid bone and one from the lateral pterygoid plate. Fibers from these two points of origin tend to converge as they run generally horizontally backward to insert into the neck of the condyle of the mandible. Contraction of the external pterygoid muscle causes the condyle to slide downward and forward. Contraction of the external pterygoid muscle on only one side tends to move the front of the mandible toward the opposite side.
Three other muscles have a role in actions of the mandible. These are the digastric (anterior belly), mylohyoid, and geniohyoid, all supplementary muscles of the laryngeal apparatus. The structure and function of these muscles are presented in detail in Chapter 3 (and illustrated in Figure 3–24) and are reviewed here only briefly.
The digastric is a two-bellied muscle arranged such that it can pull upward on the hyoid bone and/or downward on the mandible. Its action is dependent on the degree to which either or both of these structures are fixed in position by other muscles. With greater relative fixation of the hyoid bone, contraction of the anterior belly of the digastric muscle results in a lowering of the mandible.
The mylohyoid muscle is positioned along the floor of the oral cavity. It is oriented such that its fibers can exert an upward and forward pull on the hyoid bone or a downward pull on the mandible. With greater relative fixation of the hyoid bone, contraction of the mylohyoid muscle brings about a lowering of the mandible.
The geniohyoid is a cylindrical muscle that lies above the mylohyoid muscle. The course of its muscle fibers is essentially parallel to the fiber course of the anterior digastric muscle. Orientation of the muscle is such that it can pull upward and forward on the hyoid bone or downward on the mandible. With greater relative fixation of the hyoid bone, contraction of the geniohyoid muscle results in a lowering of the mandible.
Skeletal support for the overall pharyngeal-oral apparatus comes from vertebrae and bones of the skull and face, but the tongue is endowed with its own “soft skeleton.” This personal skeleton is largely connective tissue and serves to both surround and separate different components of the tongue, including its left and right halves (Dickson & Maue-Dickson, 1982). This skeleton also includes a dense felt-like network of fibrous elastic tissue that lies below the epidermis and constitutes an encapsulating structural bag around the tongue (Zemlin, 1998). It is through this special soft skeleton that the eight muscles of the tongue (four intrinsic and four extrinsic) are able to bring about the wide variety of tongue movements that are possible.
The intrinsic muscles of the tongue are the superior longitudinal, inferior longitudinal, vertical, and transverse muscles. They are depicted in Figure 5–11.
The superior longitudinal is a broad, flat muscle that lies just beneath the expansive upper surface (dorsum) of the tongue. Fibers originate within the root of the tongue from the hyoid bone and course forward in an imbricated pattern (like overlapping fish scales) along the long axis of the tongue. Forward attachments of the muscle are in the region of the front edges of the tongue and the upper surface of the tongue tip. Fibers near the midline course downward to their attachments, whereas fibers toward the side course obliquely toward the lateral boundary of the tongue. Contraction of the entire superior longitudinal muscle can shorten the tongue and increase its convexity from front to back. Because the muscle is composed of a series of short imbricated fibers, it can also activate in patterns that differentially affect the regional configuration of the tongue. For example, contraction of fibers toward the front of the tongue can pull the tongue tip upward and toward the side of muscle activation. Simultaneous contractions of comparable fibers in the paired superior longitudinal muscles elevate the tongue tip without deviation to either side. For another example, contraction of fibers that insert obliquely into the edges of the tongue can pull the lateral margins of the structure upward to create a longitudinal trough down the center of the tongue toward the front.
Figure 5–11. Intrinsic muscles of the tongue. These are the superior longitudinal, inferior longitudinal, vertical, and transverse muscles.
Arm and leg amputations occurred in large numbers during the Civil War. Amputees from this era reported that pain or other sensations continued to arise from where their missing limbs had been. Some described them as sensory ghosts and many doctors of the time thought that those who reported them had mental problems. Not so. Phantom limb pain or other sensations are now recognized to be common following amputation of any body part and scientists have embraced a number of theories about their origins. Those who have had their tongues amputated (usually surgically and because of cancer) also report tongue pain or other sensations that are analogous to those associated with their better known phantom-limb counterpart. Ghost tongues are most prominent right after surgery and tend to fade away with time, although they are known to abruptly reappear on occasion. All of this is really quite haunting when you think about it.
The inferior longitudinal muscle is positioned near the undersurface of the tongue somewhat toward the side. It arises from the body of the hyoid bone at the root of the tongue and courses forward through the body of the tongue to insert near the lower surface of the tongue tip. Fibers of the inferior longitudinal muscle blend with the fibers of different extrinsic muscles of the tongue (discussed below) within the body of the tongue. Contraction of the inferior longitudinal muscle shortens the tongue and pulls the tip of the structure downward and toward the same side. Simultaneous contraction of comparable fibers in the paired inferior longitudinal muscles pulls the tongue tip downward symmetrically.
The vertical muscle originates from just beneath the dorsum of the tongue and courses downward vertically and toward the side through the body of the tongue. Fibers of the vertical muscle terminate near the sides of the tongue along its lower surface. There is some suggestion that not all fibers follow a course through the entire body of the tongue, but rather are found only in the upper half of the tongue (Miyawaki, 1974), or that there is a mixture of short and long fibers, some of which course only through the upper half of the tongue and some of which course all the way to the lower part of the tongue (Abd-El-Malek, 1939). Contraction of the vertical muscle results in a flattening of the tongue on the side of action, especially toward its lateral margins. More midline parts of the upper tongue surface may also be lowered on the side of action as a result of pull exerted during the contraction of this muscle.
The transverse muscle, as its name implies, courses side to side within the tongue. Fibers of the muscle arise mainly from the median fibrous skeleton of the tongue and course laterally, where they terminate in fibrous tissue along the side of the tongue. Upper fibers fan out in an upward direction, whereas lower fibers fan out in a downward direction. The intermingling of transverse muscle fibers with those of other intrinsic and extrinsic tongue muscles is extensive and makes it hard to determine their precise course and location within different parts of the tongue. Some fibers may not extend all the way to the sides of the tongue (Miyawaki, 1974) and the extent to which fibers are located at the back of the tongue is in question (Abd-El-Malek, 1939). The transverse muscle is, however, a major constituent in the mass of interwoven muscle fibers that make up the bulk of the tongue. Contraction of the transverse muscle results in a narrowing of the tongue from side to side and an elongation of the tongue.
The extrinsic muscles of the tongue include the styloglossus, palatoglossus, hyoglossus, and genioglossus muscles. These are depicted in Figure 5–12.
The styloglossus muscle originates from the front and side of the styloid process of the temporal bone and the stylomandibular ligament. Fibers of the muscle course forward, downward, and toward the midline to insert into the sides of the root of the tongue. From there, they run in various directions, but primarily toward the midline and forward within the body of the tongue. Some fibers of the styloglossus muscle interdigitate with fibers of the inferior longitudinal muscle, whereas others interdigitate with fibers of the hyoglossus muscle. Ultimate blending of different fibers makes it difficult to distinguish those of one muscle from another. Contraction of the styloglossus muscle can have multiple consequences, including that it can: (a) draw the body of the tongue upward and backward, (b) pull the side of the tongue upward to influence the tongue’s concavity, (c) shorten the tongue, and (d) pull the tongue tip toward the side.
The palatoglossus muscle is discussed in Chapter 4 (and illustrated in Figure 4–12) as a part of the velopharyngeal-nasal apparatus. There it is referred to as the glossopalatine muscle (its origin and insertion being reversed in that context). For present purposes, the palatoglossus muscle can be thought of as originating from the lower surface of the palatal aponeurosis with its fibers coursing downward, forward, and toward the side (forming the anterior faucial pillar) and inserting into the side of the root of the tongue. There the fibers of the palatoglossus muscle blend with those of the transverse, styloglossus, and hyoglossus muscles of the tongue. When the palatoglossus muscle contracts, it pulls upward, backward, and inward on the root of the tongue. Through its action, the muscle can displace the tongue mass backward in the oral cavity and increase the concavity of its upper surface. When the left and right palatoglossus muscles contract simultaneously, the result is a lengthwise grooving of the upper surface of the tongue.
Figure 5–12. Extrinsic muscles of the tongue. These are the styloglossus, palatoglossus, hyoglossus, and genioglossus muscles.
The hyoglossus muscle (see also Chapter 3 and Figure 3–24) is a quadrilateral structure that originates from the upper border of the body and greater cornua of the hyoid bone and extends upward and forward to insert into the side of the tongue toward the rear. Fibers of the hyoglossus muscle intermingle with those of the styloglossus and palatoglossus muscles. Some authors consider one particular bundle of fibers within the hyoglossus muscle to be a separate muscle (Zemlin, 1998). This bundle is identified as the chondroglossus muscle and has fibers that extend from the hyoid bone farther forward into the tip of the tongue where they intermingle with fibers from intrinsic tongue muscles such as the inferior longitudinal muscle. Contraction of the hyoglossus (and chondroglossus) muscle results in a lowering of the body of the tongue and a backward displacement of its mass. The lowering effect is most pronounced along the sides of the tongue (Dickson & Maue-Dickson, 1982).
The genioglossus muscle is complex and makes up a large portion of the tongue. This muscle is fan-shaped and originates as three groups of fibers from the inner surface of the body of the mandible near the midline. The lower fibers course backward to insert into the root of the tongue. The middle fibers course backward and extend upward into the tongue in the region of the juncture between the dorsum and blade of the structure. The upper fibers run vertically and forward to insert into the tip of the tongue (Langdon, Klueber, & Barnwell, 1978), although some authors report that fibers stop short of the tongue tip itself (Doran & Baggett, 1972; Miyawaki, 1974). Collectively, fibers of the genioglossus muscle travel through the body of the tongue between layers of muscle fibers formed by the vertical, transverse, and superior longitudinal muscles of the structure (Dickson & Maue-Dickson, 1982). Contraction of the genioglossus muscle can have a diverse set of consequences, depending on which particular fibers of the muscle are activated and in what patterns. Possible outcomes are that: (a) the root of the tongue can be moved forward so as to force the tip of the tongue against the teeth or out of the mouth, (b) the front of the tongue can be pulled backward, and (c) the center line of the tongue can be pulled downward so as to form a trough-like depression along the length of the upper surface of the structure.
Figure 5–13 depicts the general force vectors associated with actions of the eight muscles of the tongue. Although discussion of the individual capabilities of the tongue muscles is instructive, it does not do justice to the intricate and interacting forces that can operate on and within the tongue to move it in different ways. Much of this has to do with special properties of the tongue that qualify it as a muscular hydrostat (discussed below under Movements of the Tongue).
The muscles of the lips are a subset of the muscles of the face. These muscles are more than a dozen in number and are portrayed in Figure 5–14 from different perspectives. The muscles of the lips include one intrinsic (contained within) and many extrinsic (one attachment within) muscles. They are the orbicularis oris, buccinator, risorius, levator labii superioris, levator labii superioris alaeque nasi, zygomatic major, zygomatic minor, depressor labii inferioris, mentalis, levator anguli oris, depressor anguli oris, incisivus labii superioris, incisivus labii inferioris, and platysma.
The folk language is filled with indications that the person on the street knows something about pharyngeal-oral function in speech production. Below are some expressions that we generated off the tops of our heads. Look at these and then try to add to the list from your own knowledge of the folk language. Our favorite from the list below is the last one, used during World War II to mean be careful to whom you are talking. Here goes. “That’s a real tongue twister.” “We were just jawing it.” “He’s bumping his gums.” “They were flapping their cheeks.” “She’s lying through her teeth.” “Don’t give me any of your lip.” “I don’t chew my cabbage twice.” “He’s running off at the mouth again.” “Hold your tongue, young man.” “She’s a big loudmouth.” And our favorite, “Loose lips sink ships.”
Figure 5–13. Actions of the eight muscles of the tongue as shown from side and front views (with parts cut away). These actions are extremely complex and not easily summarized. Shown here are major actions of each muscle, although other actions are possible.
The orbicularis oris is a ring of muscle within the lips that forms a sphincter at the oral end (mouth opening) of the pharyngeal-oral apparatus. This ring of muscle is complex and is constituted of fibers from both intrinsic and extrinsic sources that intertwine to form an airway valve and the most mobile part of the face. Fibers of the orbicularis oris muscle that are exclusive to the lips (intrinsic) are arranged in concentric rings around the border of the sphincter. These rings follow the outer contour of the upper and lower lips. The course of the intrinsic fibers of the orbicularis oris muscle changes with changes in the angular circumference of the mouth opening. Contraction of the orbicularis oris muscle can result in several positional changes of the lips. These include movements of the lips toward one another and forward, which, if extensive enough, can result in closure of the mouth and a forcing together of the lips. The corners of the mouth may also move as a result of activation of the orbicularis oris muscle. Such movement can be upward, downward, toward the side, or toward the midline. Action of the orbicularis oris muscle may also force the lips and/or corners of the mouth against the teeth.
Those lip muscles that are extrinsic are sometimes subgrouped into sets that follow fiber courses that are transverse (horizontal), angular (oblique to the corners of the mouth), vertical (from above or below), and parallel (adjacent to and alongside the lips). These subsets are considered, in turn, below, and are summarized in Table 5–1. The platysma, which is classified as a cervical (neck) muscle, is also discussed because it has extrinsic influences on the lower lip.
The transverse facial muscles that influence the lips are the buccinator muscle and the risorius muscle. The buccinator is sometimes called the bugler’s muscle and the risorius is often referred to as the laughter muscle.
The buccinator muscle is a broad muscle that forms part of the cheek. It originates from the pterygomandibular ligament, the outer surface of the alveolar process of the maxilla, and the mandible from the region of the last molars. Fibers course horizontally forward and toward the midline to insert into the upper and lower lips near the corner of the mouth. Uppermost fibers of the muscle enter the upper lip, whereas lowermost fibers enter the lower lip. Fibers of the central part of the muscle converge near the corner of the mouth and cross such that the lower fibers of that part of the muscle insert into the upper lip and the upper fibers insert into the lower lip. Contraction of the buccinator muscle can pull the corner of the mouth backward and toward the side. It can also force the lips and cheek against adjacent teeth.
Figure 5–14. Muscles of the lips. All but one of these 14 muscles are extrinsic lip muscles—the exception being a subset of fibers of the orbicularis oris muscle. These extrinsic muscles are subgrouped into the transverse (buccinators and risorius), angular (levator labii superioris, levator labii superioris alaeque nasi, zygomatic major, zygomatic minor, depressor labii inferioris), vertical (mentalis, levator anguli oris, depressor anguli oris), and parallel (incisivus labii superioris, incisivus labii inferioris) muscles. The platysma muscle is a neck muscle and is included here because it has extrinsic influences on the lower lip.
Table 5–1. Extrinsic Tongue Muscles Organized According to Their Subgroupings
The risorius is a small muscle located within the cheek, but closer to the surface than the buccinator muscle. It arises from fascia of the masseter muscle and courses horizontally forward and toward the midline to insert into the corner of the mouth and the lower lip. Contraction of the risorius muscle draws the corner of the mouth backward and toward the side; it may also force the lips against adjacent teeth.
The angular muscle group includes five muscles. These are the levator labii superioris, the levator labii superioris alaeque nasi, the zygomatic major, the zygomatic minor, and the depressor labii inferioris.
The levator labii superioris muscle has a broad origin from below the orbit of the eye, the front of the maxillary bone, and the zygomatic bone. Its fibers course downward and slightly inward and insert into the upper lip. Contraction of the levator labii superioris muscle results in elevation of the upper lip; it may also cause an outward turning (eversion) of the upper lip.
The levator labii superioris alaeque nasi muscle originates as a slender slip from the front of the maxilla and courses vertically downward and slightly toward the side. The muscle divides into a nasal segment and a lip segment. Fibers from the lip segment of the muscle insert into the upper lip where they intermingle with fibers of the orbicularis oris muscle. Contraction of the lip segment of the levator labii superioris alaeque nasi muscle causes elevation of the upper lip. Contraction of the nasal segment of this muscle dilates the anterior naris on the corresponding side, as described in Chapter 4 (and illustrated in Figure 4–14).
The zygomatic major muscle has its origin on the side of the zygomatic bone and runs down and toward the midline where it inserts into the corner of the mouth. Fibers associated with its insertion intermingle with those of the orbicularis oris muscle. Contraction of the zygomatic major muscle pulls backward on the corner of the mouth. At the same time, action of this muscle lifts the corner of the mouth upward and toward the side.
The zygomatic minor muscle originates from the inner surface of the zygomatic bone. Its fibers course downward and toward the midline where they insert into the upper lip and interweave with fibers of the orbicularis oris muscle. Contraction of the zygomatic minor muscle results in elevation of the upper lip. It also pulls the corner of the mouth upward.
The depressor labii inferioris is a small, flat muscle located off the midline of the lower lip. Fibers of the muscle originate from the front surface of the mandible and course upward and inward to insert into the lower lip from near the midline to the corner of the mouth. Contraction of the depressor labii inferioris muscle pulls the lower lip downward and toward the side. It may also cause the lower lip to turn outward.
The vertical facial muscles are three in number. They include the mentalis muscle, levator anguli oris muscle, and the depressor anguli oris muscle.
The mentalis muscle lies on the front of the chin. It is a small muscle that arises from the front and side of the mandible near the midline and inserts into the orbicularis oris muscle and the skin overlying the chin. Contraction of the mentalis muscle results in upward displacement of the soft tissue of the chin, a forcing of the lower part of the lower lip against the alveolar process of the mandible, and an outward curling of the lower lip. The lower lip may also elevate somewhat during contraction of the mentalis muscle. These actions are consistent with the familiar signs of pouting, and, indeed, the mentalis muscle is sometimes called the “pouting muscle.”
The levator anguli oris muscle (also referred to as the caninus muscle) originates from the front of the maxilla and courses downward and forward to insert into both the upper lip and the lower lip near the corner of the mouth. There, its fibers intermingle with those of the orbicularis oris muscle. Contraction of the levator anguli oris muscle draws the corner of the mouth upward and toward the side. Activation of this muscle can also elevate the lower lip against the upper lip and force the lips together.
The depressor anguli oris muscle is also sometimes referred to as the triangularis muscle. As its alternate name implies, the muscle is roughly triangular in form. This muscle has a broad origin from the outer surface of the mandible. Its fibers course upward and converge before inserting into the orbicularis oris muscle at the corner of the mouth and into the upper lip. Contraction of the depressor anguli oris muscle pulls the corner of the mouth downward. It also forces the lips together by drawing the upper lip downward against the lower lip.
There are two parallel facial muscles. These are the incisivus labii superioris muscle and the incisivus labii inferioris muscle.
The incisivus labii superioris is a small, narrow muscle that lies beneath the levator labii superioris muscle. Fibers of the incisivus labii superioris muscle originate from the maxilla in the region of the canine tooth and course parallel to the transverse fibers of the orbicularis oris muscle of the upper lip. This muscle inserts near the corner of the mouth where its fibers intermingle with the fibers of other muscles. Contraction of the incisivus labii superioris muscle pulls the corner of the mouth upward and toward the midline.
The incisivus labii inferioris muscle constitutes the lower lip counterpart of the incisivus labii superioris muscle. The incisivus labii inferioris muscle lies below the corner of the mouth and underneath the depressor labii superioris muscle. It originates on the mandible in the region of the lateral incisor tooth and courses parallel to the transverse fibers of the orbicularis oris muscle of the lower lip. The insertion of the incisivus labii inferioris muscle is into the region of the corner of the mouth. Contraction of the muscle results in a downward and inward pull on the corner of the mouth. The downward component of this action is antagonistic to the upward pull provided by the incisivus labii superioris muscle.
The platysma is a very board muscle that covers most of the front and side of the neck and much of the side of the face. The muscle has an extensive origin from a sheet of connective tissue within the neck above the clavicle and may even extend from as far below as the front of the chest wall and regions of the back of the torso. Fibers of the platysma muscle run upward and forward to attach to the lower edge of the mandible along the side and interweave with fibers of the opposite side at the front of the mandible. Its fibers have a broad distribution about the face, which includes a blending of fibers associated with different muscles of the lower lip and the corner of the mouth. Contraction of the platysma muscle draws the skin of the neck toward the mandible. It may also pull the lower lip and corner of the mouth to the side and downward and/or force the lower lip against the lower teeth and the alveolar process of the mandible.
Figure 5–15 portrays the general force vectors for the 14 muscles of the lips. These vectors summarize the active forces operating on the lips and their consequences on the positioning of the lips and corners of the mouth up and down, side to side, and with regard to compression against the teeth and/or alveolar processes of the maxilla and mandible. When the actions of these muscles are combined, they create a seeming infinite variety of lip adjustments that are intricately involved in human expression.
MOVEMENTS OF THE PHARYNGEAL-ORAL APPARATUS
Movements of the pharyngeal-oral apparatus allow it to perform a variety of functions involved in speech production and swallowing as well as many other activities. Movements of its component parts—the pharynx, mandible, tongue, and lips—are considered individually below.
The potential movements of the overall pharynx are discussed in detail in Chapter 4 and illustrated there (see Figure 4–15). Focus here is on potential movements of the laryngopharynx and oropharynx and their roles in changing the regional lumen of the pharynx and the degree of coupling between the oropharynx and the oral cavity through the palatoglossal arch (anterior faucial pillars). These portions of the pharyngeal tube are relatively mobile and present three movement capabilities: (a) inward and outward movement of their sidewalls, (b) forward and backward movement of their back wall, and (c) forward and backward movement of their front wall (tongue and/or epiglottis). These movements enable the lumen of the laryngopharynx and oropharynx to be changed in size and shape.
The pharynx can change in size from a maximally enlarged pharyngeal airway to one that is fully obstructed. In the case of complete obstruction, the walls of the pharynx may not only come in contact, but also undergo forceful compression against one another. Inward movements of the sides of the pharynx are effected mainly through contractions of the inferior and middle constrictor muscles, and outward movements are effected through contractions of the stylopharyngeus muscle. The sides of the pharynx can also be moved inward by lowering of the mandible (thereby creating a smaller and more circular lumen) and moved outward by raising the mandible again (Minifie, Hixon, Kelsey, & Woodhouse, 1970). Inward movements of the back wall can be effected by those same constrictor muscles, and inward movement of the front wall is usually accomplished by the tongue and epiglottis. The position of the upper front wall of the pharyngeal lumen can be changed by the velum and the lower boundary of the lumen is changed when the height of the larynx changes.
The degree of coupling between the pharyngeal cavity and oral cavity can be changed by: (a) upward and downward movements of the tongue, (b) upward and downward movements of the velum, and (c) side-to-side movements of the pillars of the palatoglossal arch (anterior faucial pillars). Maximum coupling is brought about by a combined maximum elevation of the velum and maximum depression of the tongue. Decoupling results when the undersurface of the velum and the upper surface of the tongue are placed in full apposition and the oropharyngeal airway is occluded. When contact between the tongue and the velum occurs, it is also possible to have openings on the left and right sides through which the pharynx and oral cavity are coupled.
The mandible is capable of a wide range of movements that derive from actions of the temporomandibular joints (see Figure 5–5). Upward and downward movements of the mandible are rotational and take place about a lateral axis that passes through the condyloid processes on the left and right sides of the skull (resembling the swinging of a two-hinged trap door about an axis that extends through the center pins of its hinges). Forward and backward and side to side movements are accomplished through gliding movements of the mandible along the articular facets of the temporomandibular joints. These three movement possibilities (pitching the mandible upward or downward about a lateral axis, rolling it to one side or the other about a longitudinal axis, and yawing it to the left or right about a vertical axis) often combine in activities such as the crushing and grinding associated with chewing.
The mandible can be adjusted in position, but not in shape. Such adjustment is usually considered in relation to the maxilla because it constitutes the opposing jaw for the mandible and is critical to functions that the two structures carry out collaboratively, such as chewing and speech production. Adjustments that lower the mandible result from the action of one or more muscles that include the external pterygoid, digastric (anterior belly), mylohyoid, and geniohyoid muscles. In contrast, adjustments that elevate the mandible result from the action of one or more muscles that include the masseter, temporalis, and internal pterygoid muscles. Side-to-side movements are the domain of the masseter, temporalis, internal pterygoid, and external pterygoid muscles. Forward movements are caused by actions of the external pterygoid muscle, and backward movements are caused by actions of the masseter and temporalis muscles.
This isn’t about sports, but about x-rays. Early uses of x-rays to study structures of the upper airway during speech production were quite interesting. A few tidbits should give you an appreciation. A narrow gold chain was often placed down the midline of the tongue to make its longitudinal configuration easy to visualize on single shot lateral head x-rays. Some of the first findings from different laboratories were not in agreement concerning tongue positions during vowel productions. Despite public arguments about linguistic bases for the differences, it turned out that the head had not been fixed in position and variation was related to its rotation from one exposure to another. Then, there were dangers. A pioneer in the use of x-rays for speech research entered old age unable to grow a beard on one side of his face, the side he had frequently bombarded with x-rays to get the view of the speech production apparatus he wanted to study.
The tongue is a fleshy muscular structure that is exceedingly mobile. Its mobility derives from the fact that: (a) it rides with the mandible and goes as a whole where the mandible goes, (b) its position within the oral cavity can be shifted en masse as a body (akin to moving a closed fist around in space), and (c) its shape can be changed markedly and relatively independently of the first two sources of mobility. Movements of the tongue are often segmental and differ along its major and minor axes. Movements of different points on the surface of the structure can be upward and downward, forward and backward, side to side, or different combinations of these. Vertical movements can extend from the trough to the roof of the oral cavity. Front-to-back movements can range from a maximally forward displacement of the tongue out of the mouth to a maximally rearward displacement of the structure against the back wall of the pharynx. Side-to-side movements can range from the stretchable limits of one cheek to the other.
The enormous variety of possible tongue adjustments is truly amazing. The tongue can protrude, retract, lateralize, centralize, curl, point, lick, bulge, groove, flatten, rotate, and do many other things such as “picking between one’s teeth.” What seems to be a near infinite array of adjustments relates to its special mechanical endowment that allows it to function as a muscular hydrostat (Kier & Smith, 1985; Smith & Kier, 1989). A muscular hydrostat is a pliable structure without bones that has connective tissue that allows it to change shape while maintaining its overall volume. It is a pliable structure that is incompressible and behaves somewhat like a water-filled balloon. Examples of other muscular hydrostats are octopus tentacles and elephant trunks, as depicted in Figure 5–16.
This special property of the tongue, along with its personal soft skeleton that encapsulates it, provides leverages for the eight muscles that give rise to its motive force. Because of its hydrostatic properties, inward displacement of one part of the tongue brings about outward displacement of another part (like squeezing one part of a water-filled balloon and seeing another part bulge outward). Through the selective contraction of different muscle fibers, a relatively rigid but changing support system is created in which the contraction of different muscle fibers can change the location and shape of the tongue.
Take It Away
The tongue rides with the mandible and goes where it goes. Thus, when trying to interpret changes in the configuration of the tongue surface, it’s necessary to determine how much is attributable to adjustment of the tongue and how much is attributable to adjustment of the mandible. Suppose you had a client with a hyperkinetic disease in which both the tongue and mandible went through adventitious involuntary movements. How could you go about parsing them in your evaluation? Not to worry! Have the client speak through clenched teeth or while biting down on a small stack of tongue depressors. Then, the abnormal movements of the tongue are on their own and not confounded by the abnormal movements of the mandible. It’s called removing a degree of freedom of performance, and the principle can be applied in many ways when analyzing different speech structures.
Although conceptualization of the tongue as a muscular hydrostat has been largely accepted for decades, it has been difficult to study tongue movements from this perspective until recently. New technological advances have allowed the entire tongue volume to be tracked during various activities such as resting tidal breathing (Cheng, Butler, Gandevia, & Bilston, 2008) and speaking (Woo, Xing, Lee, Stone, & Prince, 2016). The challenges to quantification of tongue movement in a 4D landscape (3-dimensional space by time) are still being worked out (e.g., Woo et al., 2017).
The mobility of the face in the region of the lips rivals that of the fine movements of the fingers. Movements of the lips can occur along vertical, side-to-side, and front-to-back dimensions. Each lip can be moved independently of the other or the two lips can be coordinated in their movements. The upper lip is fixed in spatial coordinates to the fixed position of the maxilla, whereas the lower lip rides with the mandible so that its movements are dictated, in part, by the prevailing position of the mandible. The lips can be puckered, protruded, retracted, spread, pointed, curled inward and outward, rounded, and plumped and can be associated with a host of facial expressions, such as smiling, smirking, sulking, and sneering. The multitude of possible lip adjustments are effected by different combinations of the more than a dozen muscles that impart forces to the lips. Experimenting with lip movements in front of a mirror gives one an appreciation for the degrees of freedom of lip movement.