Mouth and Pharynx

Development of Mouth and Pharynx

The lining of the primitive gut tube is derived from the embryonic endoderm and the supporting tissues and mesentery from the visceral layer of lateral plate mesoderm. The amniotic cavity expands around the developing embryo to create the body wall but leaves the endoderm/ectoderm connection at the oropharyngeal membrane and caudally at the cloacal membrane. The early mouth, stomodeum, is formed by the ectoderm and the early pharynx by the endoderm. The developing pharynx is properly part of the foregut and it extends from the oropharyngeal membrane to the respiratory diverticulum. During the fourth week of development, the oropharyngeal membrane ruptures.

A large fold of mesenchyme located over the eventual forehead, the frontonasal prominence, extends inferiorly to form the superior border of the stomodeum. Two maxillary prominences are found on each side of it, and a single, fused mandibular prominence borders the stomodeum inferiorly. The frontonasal prominence develops two nasal placodes that hollow out to become the nasal cavities. On either side of the placodes are the lateral and medial nasal prominences. The lateral prominences fuse with the maxillary prominences to create the nasolacrimal grooves. The medial nasal prominences fuse with each other on the midline to form the philtrum of the upper lip and also with the maxillary prominences to create the upper lip. The medial nasal prominence also carries the mesenchyme that will differentiate into the primary palate, which fuses with the palatine processes of each maxilla to form the hard palate.

The pharyngeal (branchial or gill) arches are found on both sides of the developing neck. These arches consist of mesenchyme from neural crest cells, which form the connective tissue structures of the face and direct the formation of the other structures thereafter. Each arch contains a cartilage core, an aortic arch, a skeletal muscle, and a cranial nerve.

The first pharyngeal arch, which is supported by the Meckel cartilage, gives rise to the maxillary and mandibular prominences, mandible, incus, malleus, and other facial bones. The muscular mesenchyme therein is innervated by the trigeminal nerve, and it differentiates into all the muscles innervated by that nerve.

The second pharyngeal arch is supported by the Reichert cartilage, and it gives rise to the lesser horn and part of the body of the hyoid bone, styloid process, and stapes. The muscular mesenchyme in the second arch is innervated by the facial nerve, and it differentiates into all the muscles which that nerve innervates. The second arch, in conjunction with the first, will also form the auricle of the ear.

The cartilage of the third pharyngeal arch creates the remainder of the hyoid bone. The only muscle derived from it is the stylopharyngeus, which is innervated by the glossopharyngeal nerve.

The fourth pharyngeal arch gives rise to (most of) the thyroid cartilage. The vagus nerve innervates the muscle mesenchyme of the fourth arch, which will give rise to all the pharyngeal and palatine muscles apart from the stylopharyngeus (IX) and the tensor veli palatini (V3). The external branch of the superior laryngeal nerve (vagus) innervates the cricothyroid and cricopharyngeus muscles.

The fifth pharyngeal arch exists in some animals but not in humans. However, the sixth pharyngeal arch and its cartilages give rise to the rest of the thyroid and all the other laryngeal cartilages. The muscle mesenchyme in this arch is innervated by the recurrent laryngeal nerve, a branch of the vagus, which innervates the intrinsic laryngeal muscles.

The spaces between arches on the exterior of the neck form pharyngeal grooves, and those spaces on the interior side form pharyngeal pouches. The first pharyngeal groove is located between the first and second pharyngeal arches; it deepens toward the first pharyngeal pouch until only a thin membrane separates them. The external auditory meatus is the derivative of the first groove, and the middle ear and auditory tube are remnants of the first pouch, with the tympanic membrane separating them.

The second, third, and fourth pharyngeal grooves fuse and migrate into the developing neck as they are overgrown by mesenchyme, briefly forming a cervical cyst that dwindles and disappears. The second pharyngeal pouch forms the bed of the palatine tonsil as lymphocytes migrate into its hollow, medial side. A dorsal extension of the third pouch forms the inferior parathyroid gland, and a ventral extension forms the thymus. Similarly, a dorsal extension of the fourth pouch forms the superior parathyroid gland and a ventral extension to the calcitonin-producing C cells of the thyroid gland.

The tongue forms in the anterior part of the developing mouth as two lateral lingual swellings and one medial lingual swelling, which enlarge and fuse. The muscles developing deep to these swellings are derived from somites innervated by the hypoglossal nerve. The lingual swellings are located in the floor of the first arch, and their surface lining is innervated by the mandibular branch of the trigeminal nerve. The rest of the tongue forms deep to the second, third, and fourth arches. A small diverticulum forms in the floor of the mouth between the second and third arches. This is the foramen cecum, and it will extend inferiorly and eventually separate from the tongue, travel along a pathway called the thyroglossal duct, and become the thyroid gland.

Oral Cavity

The mouth, or oral cavity, is the beginning of the alimentary canal. Its roof is formed by the palate, the tongue rises up out of its floor, and the cheeks and lips form its boundaries laterally and anteriorly. The mouth communicates anteriorly with the external environment by the rima oris, or oral orifice, and posteriorly with the pharynx through the isthmus of the fauces. The oral cavity is divided into the vestibule and oral cavity proper by the teeth and alveolar processes of the mandible and maxilla. When the mouth is closed, these two parts are connected only by the small spaces between the teeth and a variable gap between the last molar tooth and the ramus of the mandible, through which a catheter can be passed for feeding when the jaws are closed tightly by muscle spasm.

When the lips are everted, a midline fold of mucous membrane, known as the frenulum, can be seen extending from each lip to the adjacent gum. These frenula may cause problems when fitting artificial dentures. Also in the vestibule, opposite the crown of the second maxillary molar tooth, is a small eminence through which the duct of the parotid gland opens. These structures of the vestibule are readily visible and can usually be felt by the tongue. Many small glands are located in the mucous membrane of the lips (labial glands) and of the cheeks (buccal glands), which empty their secretions directly into the vestibule.

The lips (upper and lower) are extremely mobile folds, which form the margins of the rima oris and meet laterally at the right and left angles of the mouth, where they become continuous with the cheeks. The framework of the lip is formed by the orbicularis oris muscle, external to which is skin with its subcutaneous tissue and internal to which is the mucous membrane. The red area of the lip has an intermediate appearance between the cheek skin and the mucous membrane.

The general structure of the cheek is similar to that of the lip. The framework is formed by the buccinator muscle, strengthened by a firm fascial layer, with skin and subcutaneous tissue external to it and a mucous membrane on the internal side. On the external surface of the buccinator muscle, at the anterior border of the masseter muscle, lies the buccal fat pad, which is especially prominent in the infant.

When the tip of the tongue is turned superiorly and posteriorly, several structures come into view. In the midline is the frenulum of the tongue. Immediately lateral to each side of the frenulum is a sublingual caruncle, at the apex of which is the opening of the submandibular duct. Running posterolaterally from the sublingual caruncle is a raised fold of mucosa caused by the underlying sublingual gland, with openings of several small ducts of this gland scattered along it. At each side of the undersurface of the tongue is the fimbriated fold and, medial to that, the deep lingual vessels are visible through the mucous membrane.

By direct examination of the open mouth, in addition to the structures described above, one can see the palate, the palatoglossal fold, and the palatopharyngeal fold, with the palatine tonsil between them, the teeth, and the tongue.

In an at-rest state, the upper and lower teeth are apt to be slightly separated from each other, the tongue is at least partially in contact with the palate, and the vestibule is nearly obliterated by the lips and cheeks lying against the teeth and gums.

Mandible

The mandible, or jawbone, forms the bony framework for the lower part of the oral cavity and the skeleton of the lower part of the face. It has a U-shaped body, with a broad flat ramus running superiorly from each end of the body.

The area of fusion of the right and left halves of the body of the mandible at the anterior midline is the mandibular symphysis. At the inferior anterior surface of the symphysis is a triangular elevation called the mental protuberance, the lower outer angles of which are the mental tubercles. At the inferior part of the inner surface of the symphysis is a variable elevation, the mental spine or spines, which may be present as a single eminence or as two eminences, one superior and the other inferior. These give origin to the geniohyoid and genioglossus muscles.

Each half of the body of the mandible has an upper and a lower part, the arch of the lower part being wider than that of the upper part. The upper part is the alveolar process, so called because it contains the sockets for the lower arcade of teeth. The lower part, or body of the mandible, has a much greater proportion of compact bone. Just lateral to the symphysis on the lower border is an oval depression or roughened area for the attachment of the anterior belly of the digastric muscle, the digastric fossa. On the external surface of the mandibular body, inferior to the second premolar tooth, is the mental foramen, by which the mental branches of the inferior alveolar nerve and vessels leave the mandibular canal. Also on the external surface is an ill-defined oblique line that runs from the mental tubercle to the anterior border of the ramus. Sometimes it may start from the lower border of the mandible inferior to the molar teeth, or there may be lines from each of these places that meet as they run posterosuperiorly. On the internal surface of the mandibular body, a ridge of bone runs obliquely from the digastric fossa to the level of the socket of the last molar tooth. This ridge gives attachment to the mylohyoid muscle and is therefore known as the mylohyoid line. Superior to the mylohyoid line is a shallow sublingual fossa, in which lies the sublingual gland, and inferior to the mylohyoid line is the fossa for the submandibular gland.

The ramus presents a medial and a lateral surface as well as anterior, superior, and posterior borders. The remaining border of the ramus depends on an arbitrary decision as to the dividing line between the ramus and the body; the angle of the mandible is the area of junction of the posterior and inferior borders of the mandible. It is usually slightly obtuse in the young adult and flares slightly laterally. In the center of the medial surface of the ramus is the mandibular foramen, the beginning of the mandibular canal that transmits the inferior alveolar nerve, artery, and vein. The lingula projects partly over the foramen from its anterior edge, and the mylohyoid groove runs anteroinferiorly from it for a short distance. Projecting superiorly from the superior border of the ramus are the triangular coronoid process anteriorly and the condylar process posteriorly, with the mandibular notch between the two. The condylar process is subdivided into a head and neck.

The left and right halves typically fuse by the second year. The position of the mental foramen indicates changes that occur over time. At birth it is near the mandible's inferior border because the alveolar process makes up most of the body. After full development it is about halfway between the upper and the lower borders. When an individual becomes edentulous, much of the alveolar process is resorbed, and the mental foramen comes to lie near or on the superior border. The angle of the mandible is more obtuse in the infant than after it has become fully developed. Again, in the edentulous state, it appears more obtuse, although this may be at least in part due to a backward tilt of the condylar process.

Temporomandibular Joint

The bony structures that enter into the formation of this joint are the head of the mandibular condyle and the mandibular fossa and articular tubercle of the temporal bone above. The head of the mandible is ellipsoidal, with the long axis directed medially and slightly posteriorly. This articular surface is markedly convex in the sagittal and coronal planes. The articular surface on the temporal bone is concave posteriorly but becomes more convex anteriorly. A fibrocartilage articular disk is interposed between the two articular surfaces just described. Each surface of the disk more or less conforms to the articular surface to which it is related, but the shape of the disk between individuals is quite variable. The cartilage that covers the bony articular surfaces differs from that of most joints in that it is constituted from fibrocartilage tissue rather than hyaline cartilage, although its gross appearance is similar to that of the articular cartilages of other joints.

The temporomandibular joint is a true, or synovial, joint, with two synovial spaces, one superior to the articular disk and one inferior to it. This joint can be further described as having a hinge motion in the lower space and a sliding motion in the upper space.

The capsular ligament is rather loosely arranged, being attached superiorly to the margin of the articular surface on the temporal bone and affixed inferiorly around the neck of the mandible. The capsular ligament is firmly attached to the entire circumference of the articular disk. Forming a pronounced thickening of the lateral aspect of the capsule is the lateral temporomandibular ligament, which runs inferiorly and posteriorly from the inferior border of the zygomatic process of the temporal bone to the lateral and posterior sides of the neck of the mandible. Two accessory ligaments are not blended with the capsule. The rather thin sphenomandibular ligament runs from the spine of the sphenoid bone to the lingula of the mandible, and the stylomandibular ligament, a thickened band of deep cervical fascia, runs from the styloid process to the lower part of the posterior border of the ramus of the mandible.

The temporomandibular joint receives its nerve supply from the auriculotemporal and masseteric branches of the mandibular division of the trigeminal nerve. Its arterial supply comes via branches of the maxillary and superficial temporal arteries from the external carotid artery.

The basic movements that are allowed in the temporomandibular joint are (1) gliding of the articular disk anteriorly and posteriorly on the articular surface of the temporal bone, accompanied by the head of the mandible (which moves with the disk because the disk is attached near the joint capsule's attachment to the neck of the mandible and the external pterygoid muscle is attached to both) and (2) the hinge movement that takes place between the head of the mandible and the articular disk. In opening of the mouth, both movements are involved, with the hinge movement predominating in slight opening and the gliding movement predominating in wide opening. When chewing, one condyle remains more or less in position, while the other moves backward and forward. This is combined with slight elevation and depression of the mandible. If the mouth is opened just enough so that the upper and lower incisor teeth can clear each other, the jaw can be protracted and retracted, with the movement occurring in the upper joint.

Floor of Mouth

The term floor of the mouth is used differently by different authors, but in all cases it is applied to the floor of the oral cavity proper and does not include the vestibule. It is sometimes used to mean the structures that actually serve as boundaries of the cavity inferiorly. In this sense, the structures that form it would be the superior and lateral surfaces of the anterior part of the tongue and the mucous membrane that is reflected from the side of the tongue to the inner aspect of the mandible. Other authors have used the term to mean the muscular and other structures that fill the interval bounded by the mandible and the hyoid bone. This would mean primarily the mylohyoid muscle, which is then thought of as the boundary between the mouth above and the submandibular triangle of the neck below the muscle.

The right and left mylohyoid muscles form a diaphragm that is stretched between the two mylohyoid lines of the mandible and the body of the hyoid bone. The posterior fibers of each muscle insert on the body of the hyoid bone, and from there forward to the symphysis of the mandible the right and left muscles meet each other in a midline raphe. The mylohyoid muscle is supplied by the mylohyoid nerve, which is a branch of the inferior alveolar nerve, which itself is a branch of the mandibular division of the trigeminal nerve.

Slightly off of the midline, the anterior belly of the digastric muscle lies along the inferior surface of the mylohyoid muscle. Anteriorly it attaches to the digastric fossa of the mandible, and posteriorly it ends in the intermediate tendon, by means of which it is continuous with the posterior belly of the digastric muscle, which attaches to the mastoid notch of the temporal bone. The intermediate tendon is anchored to the hyoid bone by a fascial loop. The anterior belly is also supplied by the mylohyoid nerve and the posterior belly by a branch from the facial nerve.

Closely related to the posterior belly of the digastric muscle, the stylohyoid muscle extends from near the root of the styloid process to the greater horn of the hyoid bone. It usually attaches to the hyoid by two slips, between which the posterior belly and intermediate tendon of the digastric muscle pass. The stylohyoid is supplied by a branch of the facial nerve.

The right and left geniohyoid muscles, one on each side of the midline, rest on the superior surface of the mylohyoid muscle. They are attached anteriorly to the mental spines and posteriorly to the body of the hyoid bone. The geniohyoid muscle is supplied by fibers from the first cervical nerve that accompanies the hypoglossal nerve.

With the foregoing description of the related muscles in mind, the hyoid bone can be thought of as held in a muscular sling hung between the mandible and the stylomastoid region of the temporal bone, thus making the floor of the mouth quite mobile. All of these muscles can help in the elevation of the hyoid bone and the floor of the mouth. The geniohyoid and stylohyoid muscles determine the anteroposterior position of the hyoid bone, lengthening and shortening the floor of the mouth. The infrahyoid (strap) muscles (omohyoid, sternohyoid, sternothyroid, and thyrohyoid) pull the hyoid bone and floor of the mouth inferiorly.

A usage of the term floor of the mouth which is less technical than the two previously given is to think of the structure as the mucous membrane that is reflected from the side of the tongue to the mandible. The attachment of the mucous membrane of this area to the mandible, where it is continuous with the gum, is along a line drawn from the posterior end of the mylohyoid line to a point just above the mental spine.

Roof of Mouth

The roof of the mouth, or palate, forms the superior and posterosuperior boundaries of the “oral cavity proper,” which it separates from the nasal cavity and nasopharynx. The region of approximately the anterior two thirds of the palate has a bony framework and is, therefore, the hard palate; the posterior third is the soft palate. The palate is variably arched both anteroposteriorly and transversely, the transverse curve being more pronounced in the hard palate.

The bony framework of the hard palate is formed by the palatine processes of the two maxillae and the horizontal processes of the two palatine bones that meet in the midline. These bony structures also form the floor of the nasal cavity, and this common bony wall is traversed near the midline anteriorly by the incisive canals, which transmit blood vessels and nerves between the mucous membrane of the nose and the mucous membrane of the palate. In a posterolateral position at each side of the bony palate are the greater and lesser palatine foramina for the transmission of the greater and lesser palatine vessels and nerves. The oral surface of the bony palate is covered by mucoperiosteum (mucous membrane and periosteum fused together), which exhibits a faint midline ridge, the palatine raphe, at the anterior end of which is a slight elevation called the incisive papilla. Running laterally from the anterior part of the raphe are about six transverse ridges, the transverse plicae.

Anteriorly, the soft palate is continuous with the hard palate and ends posteroinferiorly in a free margin, which forms an arch, with the palatoglossal and palatopharyngeal folds on each side as its pillars. The uvula, greatly variable as to length and shape, is a projection that hangs inferiorly from the free margin of the soft palate on the midline. The framework of the soft palate is formed by a strong, thin, fibrous sheet, known as the palatine aponeurosis, which is partially formed by the tendons of the tensor veli palatini muscles . In addition to the aponeurosis, the thickness of the soft palate is made up of the palatine muscles, many mucous glands on the oral side, and a mucous membrane on both the oral and pharyngeal surfaces. The mass of glands extends forward onto the hard palate as far anteriorly as a line between the canine teeth.

The muscles of the soft palate can be briefly described as follows: (1) the levator veli palatini arises from the posteromedial side of the cartilaginous portion of the auditory tube and the adjacent inferior surface of the petrous portion of the temporal bone. Its anterior fibers insert in the palatine aponeurosis, and the posterior ones are continuous with those of the opposite side; (2) the tensor veli palatini arises from the anterolateral side of the cartilaginous portion of the auditory tube and the adjacent angular spine and the scaphoid fossa of the sphenoid bone. Its tendon passes around the pterygoid hamulus, which acts as a pulley, and then spreads out into the palatine aponeurosis; (3) the uvular muscle arises from the posterior nasal spine and palatine aponeurosis, and unites with its counterpart on the other side to end in the mucous membrane of the uvula; (4) the palatoglossus muscle runs from the soft palate to the side of the tongue; and (5) the palatopharyngeus muscle runs from the soft palate inferiorly into the pharyngeal wall. These muscles are supplied by vagus nerve fibers, probably from the cranial part of the spinal accessory nerve, except for the tensor veli palatini, which is supplied by the mandibular branch of the trigeminal nerve.

By means of the actions of the described muscles, the soft palate can be positioned as necessary for swallowing, breathing, and phonation. It can be brought into contact with the dorsum of the tongue and it can be brought up against the wall of the pharynx, which is important in closing off the nasopharynx from the oropharynx during swallowing.

Muscles Involved in Mastication

Chewing, or mastication, is one of the important functions carried on in the mouth, and a number of muscles are involved either directly or indirectly in this activity. However, the four muscles that are primarily responsible for the forceful chewing movements of the mandible are classified by most authors as the “muscles of mastication.” These are the masseter, temporalis, lateral pterygoid, and medial pterygoid muscles.

The masseter muscle is a thick, quadrangular muscle that is readily palpable on the side of the jaw. It is described as having a superficial and a deep part, which can be rather easily separated on the posterior aspect of the muscle but are blended together anteriorly. The superficial part arises from the inferior border of the anterior two thirds of the zygomatic arch (zygomatic process of maxilla, zygomatic bone, and zygomatic process of temporal bone) and runs medially and a little posteriorly to insert on the lateral surface of the lower part of the ramus of the mandible. The area of insertion continues to the inferior border of the mandible. The deep portion of the masseter muscle arises from the inner surface of the whole length of the zygomatic arch and runs almost vertically inferiorly to insert on the lateral surface of the coronoid process and upper part of the ramus of the mandible. The deepest fibers frequently blend with the adjacent portion of the temporalis muscle. The masseter muscle is supplied by a masseteric branch from the mandibular division of the trigeminal nerve, which reaches the deep surface of the muscle by passing through the mandibular notch.

The temporalis muscle, spread out broadly on the lateral side of the skull, is a thin sheet, except where its fibers converge toward the tendon of insertion. It arises from the whole temporal fossa (the extensive area between the inferior temporal line and the infratemporal crest) and from the inner surface of the temporal fascia that covers the muscle. The temporalis muscle inserts by means of a thick tendon that passes medial to the zygomatic arch and attaches to the apex and deep surface of the coronoid process of the mandible and the anterior border of the ramus almost as far as the last molar tooth, with some of the fibers frequently becoming continuous with the buccinator muscle. Two or three deep temporal branches of the mandibular nerve enter the deep surface of the temporalis muscle.

The lateral pterygoid muscle is somewhat conical in shape and runs horizontally in the infratemporal fossa. It arises as the fusion of superior and inferior heads. The superior head attaches to the infratemporal surface of the greater wing of the sphenoid bone, and the inferior head attaches to the lateral surface of the lateral pterygoid plate. The two heads join and form a tendon of insertion that ends on the front of the condylar neck of the mandible and on the anterior aspect of the capsule and articular disk of the temporomandibular joint. A lateral pterygoid nerve from the mandibular branch of the trigeminal enters the deep surface of this muscle.

The medial pterygoid muscle, located medial to the ramus of the mandible, is thick and quadrangular. Its main origin is from the medial surface of the lateral pterygoid plate and from the pyramidal process of the palatine bone between the two pterygoid plates. A small slip of muscle originates from the tuberosity of the maxilla and the adjacent surface of the pyramidal process of the palatine bone. The medial pterygoid muscle inserts on the medial surface of the ramus of the mandible between the mylohyoid groove and the angle. The medial pterygoid nerve from the mandibular runs along the medial side of the muscle to enter it.

The muscles of mastication all pass across the temporomandibular joint, and they are the major muscles producing the movements allowed at this joint. Elevation of the mandible is brought about by the masseter, temporalis, and medial pterygoid. They are able to bring the lower teeth powerfully up against the upper teeth. They also are acting against gravity in most positions of the head in keeping the mouth closed. If they are relaxed, the weight of the jaw can gap the mouth to a small degree. The muscle of mastication that actively opens the mouth is the lateral pterygoid. It does this by pulling the articular disk and condyle of the mandible anteriorly. Other muscles that help in opening the mouth against resistance are the suprahyoid, infrahyoid, and platysma muscles. Protrusion of the jaw is brought about primarily by bilateral contraction of the lateral pterygoid, because in this movement, also, the articular disk and condyle of the mandible are brought anteriorly. The superficial portion of the masseter and the medial pterygoid can give some minor aid in protrusion. Retraction of the mandible is accomplished mostly by the posterior part of the temporalis muscle, some of the fibers of which run nearly horizontally. The digastric and geniohyoid muscles can contribute to retraction when the hyoid bone is anchored.

All the muscles of mastication are employed in the act of chewing, because it involves the four movements of the mandible described above (i.e., elevation, depression, protrusion, and retraction) and at least one of the muscles of mastication is involved in each of these movements. For the most part, chewing is done either on one side or the other, and the condyle of the side on which the chewing is being done remains more or less in position while the condyle of the other side moves back and forth, as in protrusion and retraction. This is combined in proper sequence with slight elevation and depression to bring about the grinding action on the food.

In order that grinding can be carried on efficiently, the food must be kept between the teeth by the tongue on one side and the cheek and lips on the other side. Naturally, the muscular framework of the cheek and lips is important in accomplishing this. The framework of the cheek is formed by the buccinator muscle, which takes its origin from the outer surfaces of the maxilla and mandible in the region of the molar teeth and between the posterior ends of these lines of attachment from the pterygomandibular raphe, by means of which it is continuous with the superior constrictor of the pharynx. From this U-shaped origin, the horizontal fibers of the muscle run anteriorly, apparently to continue into the orbicularis oris muscle, with the superiormost and inferiormost fibers going into the upper and lower lips, respectively, and the intermediate fibers crossing near the corner of the mouth, so that the superior fibers of this intermediate group go into the lower lip and the inferior fibers of the intermediate group go into the upper lip. The buccinator muscle is supplied by the facial nerve. The framework of the lips is formed by the orbicularis oris muscle. In addition to the fibers that appear to be the forward prolongations of the buccinator muscle, fibers come into the orbicularis oris from all of the muscles that insert in the vicinity. The orbicularis oris muscle is also supplied by the facial nerve.

Tongue

The tongue is an extremely mobile mass of striated muscle, covered by mucous membrane. Arising from the floor of the mouth, the tongue practically fills the oral cavity when all its parts are at rest and the individual is in an upright position. The shape of the tongue may change extensively and rapidly during the various activities it has to perform.

The areas of the tongue covered by the mucous membrane are the apex, the dorsum, the right and left margins, and the inferior surface. These are obvious topographic designations, except for the dorsum, which needs further description. The dorsal aspect of the tongue extends from the apex to the reflection of the mucous membrane to the anterior surface of the epiglottis at the vallecula, forming an arch that, in its anterior or palatine two thirds, is directed superiorly, whereas its posterior or pharyngeal one third is directed posteriorly. Several divisions of the tongue have been proposed. Sometimes the terminal sulcus has been said to separate the body and the root of the tongue, but in other instances the portion called the root has been limited to the posterior and inferior attachment of the tongue or has even been restricted to mean only the region of attachment through which muscles and other structures enter and leave the tongue. From the practical point of view, it is rather irrelevant where one permits the root to start and the body to end, or vice versa, but it is important to realize that the posterior third of the tongue and its epiglottic region are not visible by simple inspection even if the tongue is protruded unless the examiner uses a mirror or presses the tongue down with the aid of a spatula.

At the posterior end of the body is a small blind pit, known as the foramen cecum, the remnant of the thyroglossal duct, from which the thyroid gland developed during the fetal stage. Angling anterolaterally toward each side from the foramen cecum is the terminal sulcus, which is usually referred to as the dividing line between the anterior and posterior parts of the tongue. However, the real dividing line may run just anterior to the vallate papillae. A median sulcus is not always very distinct but is related to the interior, lingual septum.

The mucous membrane covering the apex and body of the tongue is moist and pink and is thickly studded with various papillae. The majority of the papillae are of the filiform type, in which the epithelium ends in tapered, rough points to provide friction for the handling of food. Scattered about the field of filiform papillae are the larger, rounded fungiform papillae. In front of the sulcus terminalis runs a V-shaped row of 8 to 12 (circum) vallate papillae, which rise far more prominently over the surface of the mucous membrane than do the two other types of papillae. The whole mucosa of the anterior two thirds of the tongue is firmly adherent to the underlying tissue.

The mucous membrane of the posterior one third of the tongue (the pharyngeal part), though smooth and glistening, has an uneven or nodular surface owing to the presence of a varying number (35 to 100) of rounded elevations with a crypt in the center. These nodules consist of lymphoid tissue lying deep to the epithelium. The lymphoid nodules are grouped around the epithelium-lined crypt or pit and, taken collectively, are called the lingual tonsil.

On both margins of the tongue, the mucous membrane is thinner and, for the most part, devoid of papillae, though a variable number of vertical folds may be found on the posterior part of each margin. They are called foliate papillae, and represent rudimentary structures similar to the well-developed foliate papillae seen in rodents.

The mucous membrane of the inferior surface of the tongue is thin, smooth, devoid of papillae, and more loosely attached to the underlying tissue. It exhibits the midline frenulum and some rather rudimentary fimbriated folds that run posterolaterally from the tip of the tongue. The frenulum is a duplication of the mucous membrane and connects the inferior lingual surface with the floor of the mouth. The deep lingual veins usually shine through the mucosa between the frenulum and the fimbriated folds on each side.

Many small glands are scattered beneath the mucous membrane and partly embedded in the muscle. Mucous glands are located in the posterior third of the dorsum, with their ducts opening on the surface and into the pits of the lingual tonsil. In the region of the vallate papillae, the purely serous lingual glands of von Ebner send numerous ducts (from 4 to 38) into the furrows, or moat, surrounding each of these papillae. Glands of a mixed type, the lingual glands of Blandin and Nuhn, are found to each side of the midline inferior and posterior to the apex of the tongue.

The receptor organs for the sense of taste, the taste buds, are pale oval bodies (about 70 µ in their long axis), seen microscopically in the epithelium of the tongue and to a much lesser extent in the epithelium of the soft palate, pharynx, and epiglottis. The taste buds are most prevalent in the epithelial lining of the furrows surrounding the vallate papillae. A few taste buds are present on the fungiform papillae and also scattered on the foliate papillae. A taste bud reaches from the basement membrane to the epithelial surface, where a pore is situated, into which the microvilli (taste hairs) of the neuroepithelial taste (gustatory) cells extend. From 4 to 20 taste cells are intermingled with the more numerous supporting sustentacular cells of the taste buds.

The majority of the tongue is made up of skeletal (striated) muscles, which are composed of muscular bundles, interlaced in many directions. An incomplete lingual septum divides the tongue into symmetric halves. One group of muscles, the extrinsic ones, originates outside of the tongue, whereas the intrinsic group of lingual muscles originates and inserts entirely within other muscles of the tongue. The genioglossus muscle arises from the superior mental spine of the mandible and fans out along the entire length of the dorsum of the tongue, with the lowest fibers having some attachment to the hyoid bone. Lateral to this muscle is the hyoglossus muscle, which arises from the body of the hyoid bone as well as the entire length of the greater and lesser horns from which it runs vertically upward. The styloglossus muscle arises from near the tip of the styloid process and an adjacent part of the stylomandibular ligament. It runs as a band inferiorly and anteriorly onto the lateral aspect of the tongue. The palatoglossus muscle descends from the soft palate, forming the framework of the palatoglossal fold. The intrinsic lingual muscles are named according to the three spatial dimensions in which their fascicles run. Of the two longitudinal muscles, the superior longitudinal band extends from anterior to posterior just deep to the mucous membrane of the dorsum. The inferior longitudinal band spreads between the genioglossus and hyoglossus muscles on the undersurface of the tongue.

The contraction of both longitudinal muscles shortens the tongue. The transverse lingual muscle, which is covered by the superior longitudinal muscle, furnishes nearly all of the transversely running fibers and is intermingled with fascicles of the extrinsic muscle group. The vertical lingual muscle is made up of all the vertical fibers, except those supplied by extrinsic muscles, with which it forms a closely woven network. By the combined actions of all these muscles, the shape of the tongue can be extensively altered: lengthened, shortened, broadened, narrowed, curved in various directions, protruded, and drawn back into the mouth.

The innervation of the tongue involves the following nerves: (1) motor from the hypoglossal nerve except for the palatoglossus muscle, which is innervated by the vagus nerve; (2) general sensory to the anterior two thirds via the lingual nerve, which is accompanied by the chorda tympani, a branch of the facial nerve, which is special sensory (taste) to the same area; and (3) glossopharyngeal nerve, which is general and special sensory to the posterior one third of the tongue. The vagus nerve is general and special sensory to the epiglottic region.

Teeth

The teeth are specialized structures that bite or tear off the pieces of solid food that enter the oral cavity and chop and grind this food as it is being mixed with saliva in preparation for swallowing. The muscles of mastication are responsible for the movements of the lower teeth in relation to the upper teeth, and the tongue and cheeks are responsible for positioning food between the teeth as necessary.

Humans develop two sets of teeth, a deciduous set (milk teeth), which begin to come in at about the age of 6 months, and a so-called permanent set, which gradually begin to replace the deciduous set at about the age of 6 years.

The deciduous teeth number 20 in all, 5 on each side of the upper and lower jaws. Starting at the midline of each jaw and progressing laterally and posteriorly to each side, the deciduous teeth are named in order: central (medial) incisor, lateral incisor, canine (cuspid), first molar, and second molar. The four teeth of the same name are differentiated by designating which jaw and which side of the jaw, as right or left upper (maxillary) or lower (mandibular) central incisor. The deciduous teeth are smaller than the permanent teeth that take their position.

The permanent teeth, once all have come in, number 32, 8 on each side of the upper and lower jaws. Starting at the midline of each jaw and progressing laterally and posteriorly to each side, the permanent teeth are named in order: central (medial) incisor, lateral incisor, canine (cuspid), first premolar (bicuspid), second premolar (bicuspid), first molar, second molar, and third molar (wisdom tooth). The incisors, and to some extent the canines, are adapted for biting the food, whereas the molars, and to some extent the premolars, are adapted for grinding and pounding food.

Normally, the upper dental arch is wider than the lower dental arch, and the upper incisors and canines overlap the lower incisors and canines. When the jaws are closed (in occlusion), the teeth of the two jaws come into contact in such a way that their chewing surfaces fit each other, which means that the teeth of one jaw are not exactly opposite the corresponding teeth of the other jaw. In spite of this, because the lower molars, especially the third molars, are longer anteroposteriorly, the dental arches end at approximately the same place posteriorly. Teeth are described as having a labial (buccal) surface, a lingual surface, and a contact surface.

Ordinarily, none of the teeth have erupted before birth, but all of the deciduous teeth usually come in between the sixth month and the end of the second year. The time of eruption varies considerably as does any timetable of development. The possible range of the time at which each deciduous tooth may erupt is indicated in the parentheses below the name of each tooth in the accompanying illustration.

From the end of the second year until the sixth year, no visible change in the teeth takes place. At about the sixth year, the first permanent molar comes in posterior to the second deciduous molar, and it is important that this be recognized as a permanent tooth and given the care that a permanent tooth merits. Starting in the seventh year, gradual replacement of the deciduous teeth by the permanent teeth takes place, which is usually completed by the twelfth year. The second molar, as a rule, emerges about this time, and the third molar, if it erupts at all, several years later. The approximate time at which each permanent tooth may erupt is specified in the accompanying illustration below the name of the tooth. The developing permanent teeth are present within the jaw long before they erupt. Obviously, during the eruption of the teeth, growth changes must occur in the jaws.

The crown of a tooth is the portion of the tooth projecting beyond the gum. It differs in shape in different types of teeth, the difference being related to the functional adaptation of the tooth. The crown of an incisor is chisel shaped, that of a canine is large and more conical, and the crowns of the premolars and molars are flattened and broad, with tubercles.

The neck of a tooth is the short, constricted portion that connects the crown and the root.

The root of a tooth is the portion embedded in the alveolar process of the jaw. It is long, tapering, and fitted to its socket. The root of an incisor is usually single, the canine has a single long root, and that of a premolar is usually single, flattened anteroposteriorly, and grooved, with some tendency to division. Each molar has two roots, an anterior root and a posterior root, which are apt to be wide, flattened anteroposteriorly, grooved, and perhaps partially divided. At the tip (or tips) of each root is a minute opening called the apical foramen, which allows passage for blood vessels and nerves to the root.

The interior of a tooth contains a space called the cavity of the tooth (pulp cavity), which is filled in the natural state by loose connective tissue, capillaries, nerves, and lymphatics, collectively called the pulp, on the outer surface of which is a layer of cells called odontoblasts. The cavity extends into each root as the tapering root canal, which ends at the apical foramen.

Surrounding the cavity is the dentin, which constitutes the mass of the tooth and is a hard, highly calcified (only 28% organic matter), homogeneous material. It is traversed by dentinal tubules (dental canaliculi) extending from the cavity to the outer margin of the dentin. The dentinal tubules are occupied by processes of the odontoblasts, which create the dentin.

Forming a cap over the dentin of the crown is the dense, white, and glistening enamel, the hardest (only about 3% organic material) and most resistant material in the body. It is made up of solid, hexagonal prisms (enamel prisms), which are oriented essentially perpendicular to the related surface of the crown. It is created within the gums by cells called ameloblasts.

Cementum, modified bone having lamellae, canaliculi, and lacunae, covers the dentin of the roots. It is very thin at its beginning at the neck and increases in thickness toward the root's apex.

The root of the tooth is united to the wall of the socket by an important layer of vascular fibrous connective tissue, the alveolar periosteum or periodontal ligament. This layer is continuous with the lamina propria of the gum at the alveolar process margin (near the neck of the tooth).

The covering of the internal and external surfaces of the alveolar processes of the maxilla and mandible, the gums or gingivae, is made up of stratified squamous epithelium, resting on a thick, strong lamina propria, which is firmly attached to the underlying bone. This, being a fusion of mucous membrane and periosteum, could be called mucoperiosteum. The gum forms a free fold, which surrounds the base of the crown of the tooth for a short distance like a collar.

The arteries and nerves that supply the teeth are branches of the superior and inferior alveolar arteries and nerves. These travel partly to the pulp cavity and partly to the surrounding periodontal ligament. Branches to the pulp cavity travel by way of the apical foramen and root canal. The vessels form a rich capillary plexus under the odontoblast layer.

The enamel of the tooth originates from the oral ectoderm, which differentiates into ameloblasts, and the rest of the tooth comes from the mesenchymal tissue of the maxillary and mandibular arches. The ameloblasts and odontoblasts create a bell-shaped structure as they lay down enamel and dentin to create the tooth within the mesenchyme. The mesenchyme remains in the dental pulp, allowing the alveolar vessels and nerves to reach the developing tissues.

Salivary Glands

Numerous glands secrete the watery, somewhat viscous fluid known as saliva into the oral cavity. Small salivary glands are widely scattered under the lining of the oral cavity and are named, according to their location, labial, buccal, palatine, and lingual glands. The three chief, large, paired salivary glands are the parotid, submandibular, and sublingual.

The parotid gland, the largest of the salivary glands, is roughly shaped as a three-sided wedge, which is fitted in anterior and inferior to the external ear. The triangular superficial surface of the wedge is practically subcutaneous, with one side of the triangle almost as high as the zygomatic arch and the opposing angle at the level of the angle of the mandible. The anteromedial side of the wedge abuts against and overlaps the ramus of the mandible and the related masseter and medial pterygoid muscles. The posteromedial side of the wedge turns toward the external auditory canal, mastoid process, sternocleidomastoid, and digastric (posterior belly) muscles. The parotid (Stensen) duct leaves the anterior border of the gland and passes superficial to the masseter muscle, at the anterior border of which it turns medially to pierce the buccinator muscle and then the mucous membrane of the cheek near the second maxillary molar.

The submandibular gland lies in the submandibular triangle but overlaps all three sides of the triangle, extending superficial to the anterior and posterior bellies of the digastric muscle and deep to the mandible, in the submandibular fossa. Most of the gland is inferior to the mylohyoid muscle, but a deep process extends superior to the muscle. The submandibular (Wharton) duct at first runs anteriorly with the deep process and then in close relation to the sublingual gland (first inferior and then medial to it) to reach the sublingual caruncle at the summit of which it opens, next to the lingual frenulum.

The sublingual gland, the smallest of the three paired salivary glands, is located deep to the mucous membrane of the floor of the mouth, where it produces the sublingual fold. It lies superior to the mylohyoid muscle in relation with the sublingual fossa on the mandible. In contrast to the parotid and submandibular glands, which have quite definite fibrous capsules, the lobules of the sublingual gland are loosely held together by connective tissue. About 12 sublingual ducts leave the superior aspect of the gland and open individually through the mucous membrane of the sublingual fold. Some of the ducts from the anterior part of the gland may combine and empty into the submandibular duct. This is apparently prone to considerable variation.

The nerve supply of the large salivary glands is discussed in a later segment on the innervation of the mouth and pharynx and the autonomic nervous system.

Microscopically, the large salivary glands appear as compound tubular-alveolar glands. The secretions of these glands are serous and mucous and mucous with serous demilunes, with different proportions of these in different glands. The parotid gland is almost entirely serous, the submandibular gland is predominantly serous but with some mucous alveoli containing serous demilunes, and the sublingual gland varies to quite an extent in composition in different parts of the gland but, for the most part, is predominantly mucous with serous demilunes. In the parotid and submandibular glands, the alveoli are joined by intercalated ducts with low epithelium to portions of the duct system, which are thought to contribute water and salts to the secretion and, hence, are called secretory ducts. The epithelium of the ducts is at first cuboidal, then columnar, and may finally be stratified cuboidal near the opening of the duct. It should be noted that the appearance of serous demilunes is an artifact of specimen preparation and that during life, the serous-secreting cells of each acinus sit side by side with the mucous-secreting cells.

Sections through Mouth and Jaw

Fauces

The connotation given to the term fauces varies. Though complete agreement exists as to the general region to which the term refers, the precise contents and boundaries of this region vary between sources. In general, the area covers the space from the oral cavity into the pharynx. By most authors, the designation isthmus of the fauces, or oropharyngeal isthmus, is taken to mean the aperture by which the mouth communicates with the pharynx (i.e., the dividing line between the oral cavity and the oropharynx). The boundaries of this isthmus are the soft palate superiorly, the dorsum of the tongue in the region of the terminal sulcus inferiorly, and the left and right palatoglossal folds, also known as the anterior pillars of the fauces, which rise archlike on each side in the posterior limit of the oral cavity.

Closer to the oropharynx, a second arch is formed by the palatopharyngeal folds, also called the posterior pillars of the fauces. As a result of the projecting prominence of the anterior and posterior folds on each side, a fossa (tonsillar fossa or tonsillar sinus) comes into existence, which houses the palatine tonsil. On the free surface of this oval mass, which may bulge medially into the cavity of the pharynx for varying distances, 12 to 15 orifices (fossulae tonsillares) can be recognized. These are the openings of the tonsillar crypts. The latter branch and extend deeply into the substances of the tonsils. Several quite variable folds may overlap the medial surface of the tonsils in different degrees. Most frequently found is a triangular fold located anteriorly and inferiorly to the tonsils. Also, between the superior portions of the palatoglossal and palatopharyngeal folds, one may encounter frequently a supratonsillar fold that contains tonsillar tissue, a fact that has prompted some authors to call the recess below this fold the infratonsillar recess (or fossa) and others to designate it as “supratonsillar.” The lateral surface of the tonsil has a fibrous capsule, which is separated by some loose connective tissue from the superior constrictor muscle of the pharynx and, to a lesser and variable degree, from the palatopharyngeus muscle that sits deep to the fold of the same name.

The chief blood supply of the tonsil is the tonsillar branch of the facial artery, but the tonsillar branches of the lesser palatine, ascending palatine, ascending pharyngeal, and dorsal lingual arteries also participate in the arterial blood supply. Lymphatic fluid from the tonsil drains primarily to the jugulodigastric lymph node of the superior deep cervical group. The tonsil is innervated primarily by the glossopharyngeal nerve, though a few branches of the lesser palatine nerves also enter the tonsils.

A stratified squamous epithelium covers the tonsil and also lines the crypts, where it may be obscured by lymphocyte infiltration. The mass of the tonsils consists of lymphatic (lymphoid) tissue, which presents itself mostly in the form of lymph nodules or follicles, which, particularly in younger individuals, contain many germinal centers. Expansions from the above-mentioned fibrous capsule on the lateral tonsillar surface enter the lymphoid tissue, forming septa between the follicles surrounding the adjacent crypts.

Present at birth and increasing in size rapidly during the first few years of life, the tonsils usually decrease in size about puberty and may become atrophic in old age.

Histology of Mouth and Pharynx

The mouth and pharynx are lined by a mucous membrane that is attached in much of the area to the supporting wall (bone, cartilage, or skeletal muscle) by a fibroelastic, gland-containing submucosa that varies greatly in size, looseness, and the distinctness with which it can be delimited from the mucous membrane. The submucosa is interpreted as absent on most of the hard palate, the gums, and the dorsum of the tongue. The mucous membrane is composed of epithelium, which is predominantly nonkeratinized, stratified, and squamous in type, a basement membrane, and the fibroelastic lamina propria, which has vascular papillae indenting the epithelium to varying degrees in different areas. The muscularis mucosae, which is present in the digestive tube in general, is missing in the mouth and pharynx. Its place is occupied by an elastic network in the pharynx.

The lip has a framework of skeletal muscle, chiefly the orbicularis oris muscle. External to this are typical subcutaneous tissue and skin with hair follicles, sebaceous glands, and eccrine sweat glands. On the inner side of the muscular framework is the submucosa containing rounded groups of mixed, predominantly mucous glands (labial glands). The submucosa is not definitely delimited from the covering mucous membrane, which is composed, as described above, of lamina propria and nonkeratinized, stratified, squamous epithelium. The free margin of the lip has its characteristic red (or vermillion) color because the epithelial cells contain much translucent eleidin, and the vascular papillae of the tunica propria indent the epithelium more deeply here. The blood in the capillaries thus shows through to a greater extent.

The general structure of the cheek is very similar to that of the lip, the muscular framework being formed by the buccinator muscle. Here some glands are external to the muscular framework. In most of the lip and the cheek, the mucous membrane is quite closely bound to the muscular framework, preventing large folds of mucous membrane from being formed that might be easily bitten. Near the continuity of the mucous membrane with the gums, the attachment is much looser to allow for freedom of movement.

The soft palate has a fibromuscular framework, with the fibrous constituents (the expansion of the tendons of the tensor veli palatini muscles) being more prominent near the hard palate. On each side of the framework is a mucous membrane. That on the oral side has an elastic layer separating the lamina propria from a much thicker submucosa containing many glands. The epithelium is the typical nonkeratinized, stratified, squamous variety, which rounds the free margin of the soft palate and extends for a variable distance onto the pharyngeal surface.

The wall of the pharynx is for the most part composed of a mucous membrane, muscular layer, and thin fibrous sheath outside of the muscle that attaches the pharynx to adjacent structures. The epithelium in the nasopharynx (except for its lowest portion) is pseudostratified, ciliated, and columnar, whereas that of the rest of the pharynx is nonkeratinized, stratified, and squamous . The lamina propria is fibroelastic, with scattered small papillae indenting the epithelium. The deepest part of this lamina is a definite elastic tissue layer with many longitudinally oriented fibers. A well-developed submucosa is present only in the lateral extent of the nasopharynx and near the continuity of the pharynx with the esophagus. Scattered seromucous glands are present, mostly where there is pseudostratified columnar epithelium. The muscular layer, made up of skeletal muscle, is present as somewhat irregularly arranged layers.

Pharynx

The pharynx is a musculomembranous tube; much of its anterior wall is absent due to the fact that the right and left nasal cavities, oral cavity, and larynx open into its anterior side. It extends from the base of the skull to the inferior border of the cricoid cartilage at the level of the lower margin of the sixth cervical vertebra, at which time it becomes continuous with the esophagus. In addition to the cavities already listed, the pharynx also communicates with the middle ear on each side by means of the auditory (eustachian) tube; this fact explains how infections spread from the pharynx to the middle ear, making a total of seven cavities with which it has communication. The transverse diameter of the pharynx exceeds the anteroposterior diameter, which is greatest superiorly and is diminished to nothing inferiorly where the anterior and posterior walls are in contact unless separated during the act of swallowing. The transverse diameter does not differ greatly throughout the length of the pharynx, except where it narrows at the lower end.

The posterior wall of the pharynx is attached superiorly to the pharyngeal tubercle on the inferior surface of the basilar part of the occipital bone, its adjacent area, and the undersurface of the petrous portion of the temporal bone medial to the external aperture of the carotid canal. The lateral wall has several attachments. It is attached superiorly to the cartilaginous portion of the auditory tube, which pierces the wall in this area. Anteriorly it connects to the lower, posterior border of the medial pterygoid plate and its hamulus, as well as the pterygomandibular raphe , the inner surface of the mandible (near the posterior end of the mylohyoid line), the side of the root of the tongue, the hyoid bone, and finally the thyroid and cricoid cartilages. Inferiorly, the walls of the pharynx continue as the walls of the esophagus.

The pharyngeal lining is a mucous membrane that is continuous with the lining of the cavities communicating with the pharynx. External to the mucous membrane of the posterior and lateral walls is a sheet of fibrous tissue, more defined superiorly than inferiorly, known as the pharyngeal aponeurosis (pharyngobasilar fascia), and external to this is the muscular layer of the pharynx. On the outer surface of the muscular layer is another fascial covering, the buccopharyngeal fascia. The posterior pharyngeal wall is separated from the prevertebral fascia overlying the anterior arch of atlas and the bodies of the second to the sixth cervical vertebrae (partially covered by the longus colli and longus capitis muscles) by a minimal amount of loose fibrous connective tissue. This allows the pharynx some degree of freedom of movement and forms a retropharyngeal space. Under anesthesia it is possible to palpate these bony structures as far caudally as the fourth or fifth cervical vertebra.

On the basis of openings in its anterior wall, the pharynx is divided into the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx typically has a purely respiratory function (acting as a passageway for air and not for food) and remains patent because of the bony framework to which its walls are related. The anterior wall is entirely occupied by the choanae (posterior nares), with the posterior border of the nasal septum between them. The posterior wall and roof form a continuous arched wall, with the roof extending from the superior margin of the choanae (where it is continuous with the roof of the nasal cavities) to about the midpoint of the basilar portion of the occipital bone; the posterior wall extends from this point caudally to about the lower border of the anterior arch of the atlas. In the region where the roof and posterior wall meet, the mucous membrane is thrown into many variable folds, with an accumulation of nodular and diffuse lymphoid tissue (extensively developed in children, atrophied in adults) forming the pharyngeal tonsil (adenoids). In the midline, near the anterior margin of the pharyngeal tonsil, is a minute flask-shaped depression of mucous membrane, known as the pharyngeal bursa. Also in the midline, near the anterior limit of the roof, and submerged in the mucosa or lying in the periosteum, a microscopic remnant of the Rathke pouch (pharyngeal hypophysis) can be found, which is grossly visible only when it has become cystic or has formed a tumor.

The incomplete floor of the nasopharynx is formed by the posterosuperior surface of the soft palate with an opening from the nasal to the oral pharynx between the soft palate and the posterior wall of the pharynx. This opening is closed by bringing these two structures in contact.

On the lateral wall of the nasopharynx at the level of the inferior concha is the pharyngeal opening of the auditory tube, with the pharyngeal recess (fossa of Rosen­müller) posterior to it. The levator cushion (produced by the levator veli palatini muscle) bulges into the inferior margin of the triangular opening, and coursing inferiorly from the posterior lip is the salpingopharyngeal fold produced by the muscle of the same name. In childhood, a considerable mass of lymphoid tissue (tubal tonsil) may be present around the opening of the auditory tube and may enlarge, blocking drainage from the middle ear.

The oropharynx extends from the inferior border of the nasopharynx to the level of the pharyngoepiglottic folds, with the epiglottis protruding into it. This part of the pharynx carries both air and food. The posterior wall remains in relation to the bodies of the second to fourth cervical vertebrae, whereas the anterior wall is deficient superiorly where the oropharynx and oral cavity communicate by means of the isthmus of the fauces. Inferior to the isthmus, the anterior wall is formed by the posterior third of the tongue. Between the tongue and epiglottis are the valleculae, spaces where foreign bodies may lodge.

The laryngopharynx lies posterior to the larynx and anterior to the fifth and sixth cervical vertebrae. The superior part of the anterior wall contains the roughly triangular laryngeal inlet (aditus), the borders of which are formed by the margins of the epiglottis, the aryepiglottic folds, and the interarytenoid incisure. Inferior to the laryngeal opening, the laryngopharynx only transmits food and is purely alimentary in function. The mucous membrane of the anterior wall overlies the posterior surfaces of the arytenoid cartilages and the lamina of the cricoid cartilage (mostly covered by laryngeal muscles). Inferior to the laryngoepiglottic fold on each side is the piriform recess, located between the cricoid and arytenoid cartilages medially and the lamina of the thyroid cartilage laterally. This is another location where foreign bodies may lodge.

Bony Framework of Mouth and Pharynx

The bony framework of the mouth is composed largely of the two maxillae, immovably attached to other bones of the skull, and the mobile mandible. The portions of the maxillae contributing to the formation of the bony palate have been previously described, and the alveolar processes of the maxilla have been referred to as providing the sockets for the upper teeth. Other bony structures contributing to the framework of the mouth and pharynx, serving as attachments for muscles of the mouth and pharynx, are parts of the palatine, sphenoid, temporal, occipital, and hyoid bones as well as the zygomatic arch.

The palatine bone is interposed between the maxilla and the pterygoid process of the sphenoid bone, and its horizontal portion forms the framework of the posterior part of the hard palate. Its pyramidal process articulates with the lower portions of the medial and lateral pterygoid laminae and helps to complete the pterygoid fossa.

The sphenoid bone is located in the base of the skull posterior to the ethmoid, frontal, palatine, and maxillary bones. It is anterior to the occipital and temporal bones, and it has the right and left pterygoid processes extending inferiorly from its body. Each pterygoid process has a medial and lateral pterygoid plate, with a pterygoid hamulus projecting posterolaterally from the medial pterygoid plate, to which the pterygomandibular raphe attaches and around which the tendon of the tensor veli palatini muscle passes. The greater wing of the sphenoid forms the anterior parts of the temporal and infratemporal fossae. The spine of the sphenoid, to which the sphenomandibular ligament attaches, is just medial to the mandibular fossa of the temporal bone.

The external acoustic meatus is an obvious landmark in the temporal bone that extends toward the middle ear. Posterior to the meatus is the mastoid process, on the medial side of which is the mastoid notch, where the posterior belly of the digastric muscle attaches. Anteroinferior to the meatus is the mandibular fossa for the articulation with the condyle of the mandible. Inferior to the meatus and posterior to the mandibular fossa is the styloid process, which projects for a variable distance inferiorly and slightly anteriorly. The squamous portion of the temporal bone is the extensive flat portion of the bone superior to the meatus that, together with parts of the greater wing of the sphenoid, frontal, and parietal bones, forms the temporal fossa for the attachment of the temporalis muscle. The petrous portion of the temporal bone extends medially and somewhat anteriorly from the meatus to insinuate itself between the basilar portion of the occipital bone and the infratemporal portion of the greater wing of the sphenoid.

The zygomatic arch forms a buttress over the infratemporal fossa and gives origin to the masseter muscle. It is made up (from front to back) by the zygomatic process of the maxilla, the zygomatic bone, and the zygomatic process of the temporal bone.

The basilar portion of the occipital bone is posterior to the body of the sphenoid bone and forms the bony framework of the roof and the superior part of the posterior wall of the pharynx. The pharyngeal tubercle on the inferior surface of the basilar portion of the occipital bone, anterior to the foramen magnum, is the superior attachment of the median raphe of the pharynx.

The hyoid bone has a body, as well as a greater and lesser horn on each side. It is a key structure in the floor of the mouth (and related tongue) and is important in the movements of these structures through the muscles that attach to it. The hyoid bone is also important as the origin of the middle pharyngeal constrictor muscle.

Supplementing the bony framework in supplying attachments to the muscles of the pharynx are the thyroid and cricoid cartilages that give origin to the inferior pharyngeal constrictor and some insertion to the stylopharyngeus muscle.

Musculature of Pharynx

Much of the framework of the lateral and posterior walls of the pharynx is formed by an outer and inner layer of musculature. These layers are not completely separable throughout, because in some places they are definitely intermingled and overlap. The outer layer is more nearly arranged in a circular fashion and is made up of the three constrictor muscles of the pharynx, designated as superior, middle, and inferior pharyngeal constrictors, which overlap each other. The inner layer, which falls far short of being a complete layer, is longitudinally arranged and is composed of the stylopharyngeus, palatopharyngeus, and salpingopharyngeus muscles plus some other variable and irregular bundles of muscle fibers.

The superior pharyngeal constrictor muscle is quadrilateral in shape and somewhat thin. It originates from the posteroinferior edge of the medial pterygoid plate, the hamulus of the medial pterygoid plate, the pterygomandibular raphe (which runs from the hamulus to the lingula of the mandible), the posterior one fifth or so of the mylohyoid line and the adjacent part of the alveolar process of the mandible, and the side of the root of the tongue (the glossopharyngeus muscle). From this extensive line of origin, the fibers course posteriorly, with the lower fibers passing somewhat inferiorly and medially to meet the ones from the opposite side in the median pharyngeal raphe. This raphe extends most of the length of the posterior wall of the pharynx, being attached superiorly to the basilar part of the occipital bone at the pharyngeal tubercle, to which the uppermost fibers of the superior constrictor are also attached. The curved upper edge of the muscle passes deep to the auditory tube and is thus separated by a short distance from the base of the skull except at the midline posteriorly. At this gap the only framework of the pharynx is the pharyngobasilar fascia. The buccinator muscle runs anteriorly from the pterygomandibular raphe, which serves as part of its origin, and this muscle and the superior constrictor thus form a continuous sheet, which is therefore the continuous framework of the lateral wall of the oral and oropharyngeal cavities. A slip of the superior part of the superior constrictor muscle blends into the palatine aponeurosis, forming the so-called palatopharyngeal sphincter, contraction of which produces a ridge (Passavant ridge) against which the soft palate is raised. A triangular gap filled with fibrous connective tissue can be noted between the lower border of the superior pharyngeal constrictor muscle, the posterior border of the hyoglossus muscle, and the upper border of the middle pharyngeal constrictor muscle. The stylopharyngeus muscle inserts into this gap between the superior and middle constrictors. The stylohyoid ligament and glossopharyngeal nerve also cross this gap.

The middle pharyngeal constrictor muscle has a V-shaped line of origin, with the V resting on its side and the angle pointing anteriorly. The superior arm of this V is formed by the terminal portion of the stylohyoid ligament and the lesser horn of the hyoid bone, whereas the inferior arm of the V is formed by the entire length of the greater horn of the hyoid bone. From this rather narrow origin the fibers fan out widely, with the upper fibers coursing superiorly while curving posteriorly and medially. The middle fibers course horizontally and curve posteriorly and medially. The inferior fibers course inferiorly and curve posteriorly and medially. The medial pharyngeal constrictor's superior fibers overlap the superior constrictor, and the inferior fibers are overlapped by the inferior constrictor and reach quite far inferiorly in the posterior wall of the pharynx, to about the level of the superior border of the cricoid cartilage. The middle constrictor's fibers fuse and blend with those of the other side in the median raphe. Between the lower border of the middle constrictor and the upper border of the inferior constrictor, a triangular gap is noted, which is bounded anteriorly by the thyrohyoid muscle.

The inferior pharyngeal constrictor muscle is relatively thick and strong. It arises from the oblique line of the thyroid cartilage and the area just dorsal to that line, from a tendinous arch extending from the inferior end of the oblique line of the thyroid cartilage to the lateral surface of the cricoid cartilage. That portion arising from the cricoid cartilage is frequently referred to as the cricopharyngeus muscle. As do the other constrictor muscles, the inferior constrictor passes posteriorly and then medially to blend with its counterpart at the pharyngeal raphe. The cranial fibers pass more and more obliquely as they approach the raphe and overlap the middle constrictor, reaching nearly as far superiorly as the middle constrictor does. The fibers of the cricopharyngeus portion of the muscle course horizontally and form an annular bundle with no median raphe. It does blend to some extent with the related circular fibers of the esophagus, being referred to by some as the superior esophageal sphincter. A zone of sparse musculature is present between the cricopharyngeus muscle and the rest of the inferior constrictor muscle, which creates a weaker area in the posterior wall of the pharynx, where an instrument may accidentally pierce the wall. Just below the inferior border of the cricopharyngeus muscle, a triangular area (sometimes called the Laimer triangle) occurs in which the posterior wall of the esophagus is variably deficient, because the longitudinal muscle fibers of the esophagus tend to diverge laterally and pass around the esophagus to attach on the cricoid cartilage. It is thus seen that there is more than one weakened area in the posterior wall of the general region of the pharyngoesophageal junction, where diverticula may occur. The recurrent laryngeal nerve and accompanying inferior laryngeal vessels pass deep to the inferior constrictor muscle to travel superiorly behind the cricothyroid joint in entering the larynx.

As their names indicate, the major action of the superior, middle, and inferior pharyngeal constrictor muscles of the pharynx is to constrict the pharynx. They contract in sequence, grasping the bolus of food as it passes from the mouth to the esophagus. The nerve supply of the constrictor muscles of the pharynx is derived from the pharyngeal plexus.

The stylopharyngeus muscle is long, slender, and cylindrical superiorly but flattens near its insertion. It originates from the medial aspect of the base of the styloid process and then passes inferiorly and anteriorly, going between the external and internal carotid arteries and then entering the wall of the pharynx in the interval between the superior and middle constrictor muscles. As it spreads out internal to the middle constrictor muscle, the greater horn of the hyoid bone, and the thyrohyoid membrane, some of its fibers join the palatopharyngeus muscle and insert on the superior and posterior borders of the thyroid cartilage. Some fibers pass into the pharyngoepiglottic fold, and they are primarily responsible for the production of this fold. The remaining fibers of the stylopharyngeus muscle spread between the constrictor muscles and the mucous membrane (blending to some extent with the constrictors) and pass caudally in the posterolateral wall of the pharynx, until they fade out and attach to the fibrous aponeurosis of the pharynx a short distance above the cricopharyngeus muscle. The stylopharyngeus muscle receives its nerve supply from the glossopharyngeal nerve, which curves around the posterior border of the muscle onto the lateral aspect in its course toward its final distribution on the posterior third of the tongue.

The salpingopharyngeus muscle consists of a slender bundle that produces the mucous membrane fold of the same name, which is rather variable in its degree of distinctness. This muscle arises from the inferior part of the cartilaginous part of the auditory tube, near its orifice, and passes into the wall of the pharynx, blending in part with the posteromedial border of the palatopharyngeus muscle. Some authors have described this muscle as a part of the levator veli palatini muscle, which gives a definite clue as to its action. The salpingopharyngeus muscle receives its nerve supply from the pharyngeal plexus.

The palatopharyngeus muscle, together with the mucous membrane covering it, forms the palatopharyngeal fold, also known as the posterior pillar of the fauces. This muscle takes its inferior origin from a narrow fasciculus on the dorsal border of the thyroid cartilage near the base of the superior horn and by a broad expansion from the pharyngeal aponeurosis in the area posterior to the larynx, just cranial to the cricopharyngeus muscle. As the fibers pass cranially, they form a rather compact muscular band that inserts into the aponeurosis of the soft palate by two lamellae, separated by the insertion of the levator veli palatini and the uvula. As indicated above, some of the fibers of the palatopharyngeus muscle intermingle with the stylopharyngeus muscle. The actions of the palatopharyngeus muscle include constriction of the pharyngeal isthmus by approximation of the palatopharyngeal folds, depression of the soft palate, and elevation of the pharynx and larynx. This muscle also receives its nerve supply from the pharyngeal plexus.

Additional muscle bundles are quite common, such as the one labeled accessory muscle bundle from petrous part of temporal bone (petropharyngeus muscle). Other additional muscles are brought about by the splitting of one of the usual muscles, quite commonly the stylopharyngeus. The majority of the additional muscles tend to run longitudinally.