The Jaw and Oral Cavity

If the eyes are the “windows of the soul,” then the mouth is the “doorway to the body. ” The oral cavity is situated at the end of the digestive tract and is surrounded by the lips and cheeks externally and by the gums and teeth internally. The mouth cavity is connected to the pharynx and is covered at the top by the hard and soft palates. The tongue forms the large part of the cavity’s floor. The activities of these component parts require fluid, which comes from the salivary glands.

The Jaw


It is hard to believe that a backache or a pain in the legs can originate in the jaw. However, an understanding of the relationship between the muscles and bones in the jaw and the rest of the body leads to an understanding of how an imbalance in one part affects another.

The jaw bears the teeth and forms the framework of the mouth. It consists of two bones: the maxilla (upper jaw) and mandible (lower jaw). The upper part of the jaw is stationary, while the lower part is the only movable bone in the face. The position of the jaws is dependent on the relationship of the teeth to each other when the mouth is closed.

The mandible or the lower part of the jaw functions like a hinge, permitting the mouth to open and close, as shown in Figure 1.1. This hinge action is accomplished by bones and muscles located in the skull, neck, and face. There are two phases involved in opening and closing the jaw. The first phase is a simple hinge action. The second phase involves a gliding action of the joint, which helps open the jaw to its maximum. The muscles that coordinate the movements of the jaw joints originate in the head, neck, and face areas. Due to the relationship of these muscles to the muscles of the shoulder and back, any imbalance in the jaw area may eventually affect the shoulder and back.

The temporal bone of the skull (the bone on the side of the head, above the ears), and the mandible fit together to form the jaw joint called the temporomandibular joint as shown in Figure 1.2. There are two such joints-one on the left side of the head and one on the right side.

Figure 1-2. Temporal Bone and Mandible Form the Temporomandibular Joint

Figure 1.3 shows the components of the temporomandibular joint. Each joint is surrounded by a saclike sheath called a capsule.This capsule contains synovial fluid, which lubricates and nourishes the joints. At each end of the lower jaw, there is a knoblike structure called a condyle. At the ends of bones throughout the body, condyles are attached to muscles that join the bone to nearby bones. Mounted on top of each condyle is a cushion made of cartilage and known as the disc. As with any joint in the body, a disc or cushion sits between the joint and its socket, which, in the case of the jaw, is located on the skull. This disc moves with the condyle and prevents it from hitting the temporal bone. When the disc is abnormally placed, the jaw may not function properly causing pain or discomfort in the head, neck, shoulder, and back area. Since the jaw joint sits right in front of the ear, any problems with this joint area may also affect the ears, eyes, and sinuses.

Opening and closing the jaw is a relatively simple movement compared with the joint movement that occurs while grinding the teeth during the act of chewing. During grinding, one jaw joint slides forward, while one slides back. This movement can be felt by placing your fingers in front of your ears while making grinding movements with your teeth. If no problems exist, this motion is performed with ease, and quietly. However, audible clicking or popping noises or pain may indicate that the joints are not functioning normally. (See TMJ in Part Two.)

The Teeth


Teeth-they’re not just for chewing. Our teeth perform important functions such as helping us to eat and speak, and keeping our facial muscles from sinking in. Teeth are not for opening bottles, holding nails and pins, or biting fingernails or pencils. If mistreated, teeth are subject to breakage, cavities, yellowing, and other forms of degeneration. Caring for your teeth requires that you see the mouth and teeth as important organs of your body.

Teeth are an integral and growing part of the body. The same blood supply that goes to the heart takes oxygen and nutrients to every tooth. And, as anyone who has ever had a toothache knows, the teeth are very much alive with nerves.

The better we understand the structure and development of teeth, the greater our appreciation of them will be. Practicing proper oral hygiene and maintaining good nutritional habits will help keep teeth and gums healthy. To better maintain our teeth, let’s begin with a discussion of their structure and development.

Structure


The same structure can be seen in all human teeth, whether they are baby teeth (also known as deciduous or milk teeth) or permanent teeth. The care given to the baby teeth will be reflected by the adult teeth. Preventive care must begin even before the first tooth appears because teeth begin to develop before they erupt. Parents are now aware, for instance, of the damage done to teeth when infants are allowed to sleep with bottles in their mouths. (See Bottle Mouth Syndrome in Part Two.)

As seen in Figure 1.4, there are three main parts of a tooth-the crown, neck, and root. When you look at a tooth, the top part, called the crown, is visible. The root is the part that is imbedded in bone and covered by the gums, unless gum disease exists. The junction between the crown and the roots is called the neck. The parts of a tooth are composed of various materials.

The outer surface of the tooth, the enamel, is a hard layer covering the crown of the tooth and the upper part of the neck. The function of enamel is to resist abrasive wear and protect the tooth from damage and pain. It is the hardest tissue of the body and is composed almost entirely (97 percent) of inorganic salts. Inorganic salts, in general, are mineral constituents of the body and play specific roles in the functions of cells.

Most of the inner bulk of the tooth is made of dentin. Dentin is 67 percent inorganic salts and is not as strong as enamel. Specialized cells in dentin called odontoblasts form new dentin from minerals transported by the blood. Small tubelike structures in dentin transmit pain sensations to the nerves in the pulp, the soft tissue containing the nerve and blood supply of the tooth. When dentin is exposed by thinning or other forms of damage to enamel, the tooth may become sensitive. When tooth decay or a cavity reaches the pulp, treatment of the nerve or loss of the tooth may be inevitable.

Cementum, a bony substance, is very thin and covers the surface of the roots. Its purpose is to attach the tooth to the jawbone and the gums. When the gums are receded, cementum is exposed, causing sensitivity to hot, cold, or pressure.

The teeth rest in a bony socket. They are attached to this bone by collagen fibers, fibrous tissue that forms a bridge from the cementum to the socket. The gums or gingiva surround the teeth and the bone.

Development


When the fetus is only five weeks old, tooth buds appear. Initially, four tooth buds, representing baby teeth, appear on each side of the upper and lower jaw. These buds undergo several changes as each layer and part of the tooth begins to form. Enamel cells form and multiply. Some of these cells begin to become specialized, forming the dentin and pulp. Next, the cells for the future crown and root begin to be arranged, and layers of enamel and dentin begin to be deposited in incremental layers. The next stage is calcification or hardening of the calcium salts in the teeth, which is followed by eruption.

The process continues until approximately twenty-three weeks in utero for some teeth and up to eleven months after birth for others. Different teeth develop within this range of time; for example, front baby teeth begin to develop at five months in utero and continue until they erupt when an infant is five to seven months old. Different parts of a tooth also develop at different times. The enamel of the front baby teeth begins to develop at fourteen weeks in utero, and it is completed one and a half months after birth.

Trauma during birth, diarrhea or vomiting, feeding difficulties, rickets, and other chronic childhood diseases may cause disturbances that result in malformation of teeth. Because teeth begin to develop in utero, fetal trauma affects color or structure of teeth. Depending on when the trauma is experienced, a particular tooth or part of a tooth may be affected. For example, congenital syphilis, an infectious disease sometimes contracted during birth, will affect the teeth that are developing at that time. The front baby teeth will have notches on the edge, and the back teeth will have a pitted appearance.

Long-term treatment with the antibiotic tetracycline and excessively high amounts of fluoride in the water will cause tooth discoloration. With tetracycline, brownish bands will appear on teeth. If fluoride in excess of one part per million is ingested during the development of enamel and dentin, mottling of teeth will result. The mottling gives the teeth an opaque, chalky appearance. The severity of the condition is determined by the amount of fluoride ingested.

Eruption


There are twenty baby teeth that usually begin to appear (erupt) when a child is about six months of age. Additional teeth will then appear at the rate of about one per month. There is usually a range of plus or minus two to six months when teeth erupt and when they shed (fall out). Usually, the teeth of slender children erupt and shed earlier than the teeth of stocky children. The following table in the right column shows the approximate age at which each tooth appears.

The position of the teeth as they erupt depends on many factors. The teeth on either side and the teeth directly opposing each tooth help give guidance for proper position. If the teeth on either side or any opposing teeth are missing, a tooth may erupt incorrectly.

For example, if a bottom front tooth, which normally contacts the top front tooth, is lost, the top tooth will continue to erupt, since its opposing tooth is missing. If an adjacent tooth is lost, teeth on either side of the missing tooth will begin to shift into the space. This is why when a tooth is pulled, a replacement should be made to prevent shifting or extrusion of teeth. (See Tooth, Loss of, in Part Two.)

If baby teeth do not fall out on time, they may prevent the permanent teeth from developing properly. In the attempt to erupt, the permanent teeth may find another path in which to erupt, thus causing misalignment. Sometimes, more than the normal number of teeth erupt; these extra teeth are called supernumerary teeth. They may cause overcrowding and may have to be pulled. Some permanent teeth may not develop at all. In this case, the baby teeth will not fall out and should be kept as long as possible. The occurrence of too few or too many teeth tends to follow a genetic pattern, with other family members-past or present- displaying the same condition.

The position of teeth may also be altered when baby teeth are lost too early due to decay, accidents, or other causes. The teeth on either side of and opposing a lost tooth will then begin to shift and cause problems with the proper sequence of erupting new teeth. In this situation, a splint is used to prevent shifting and to keep the space open. (See Trauma to Children’s Teeth in Part Two.)

Other factors affecting tooth position may involve permanent teeth that have become locked in the bone (a condition called ankylosis) and are unable to erupt. These teeth may have to be moved with surgery and braces.

Spaces between the baby teeth are normal. As the jaw grows, the baby teeth are spaced in order to accommodate their larger base. When the permanent teeth come in, they fit the larger adult jaw and fill in the spaces. Excess space may be present until a child is twelve to fourteen years old.

Types of Teeth


Eventually, there will be thirty-two permanent teeth: eight incisors, four canines, eight premolars, and twelve molars as shown in Figure 1.5. These permanent teeth will begin to appear as the baby teeth are lost-when a child is about six years

old-and should all be present by the time a child is seventeen. Only the last three teeth in each half of each jaw are not replacements for baby teeth. The teeth in these positions erupt only once.

Incisors are the sharp, chisel-shaped front teeth used for cutting food. Cone-shaped canines or cuspids are used for tearing food. The premolars or bicuspids have two cusps that are used for tearing and crushing food. The molars are located in the back of the mouth and have several cusps that are used for grinding food. The furthest (third) set of molars in the back of the mouth are also known as wisdom teeth.

The Palate


As the mouth is explored, it becomes apparent how important every structure is, no matter how much we take it for granted. For example, eating would become impossible without a roof dividing the mouth from the nasal cavity. The roof of the mouth or palate is the horizontal structure separating the mouth and the nasal cavity. The palate is divided into two sections, as seen in Figure 1.5.

The front portion, directly behind the teeth, is the hard palate, which consists of bone that is covered by soft tissue. The hard palate contains some major nerves that affect the teeth. It is concave or vaulted and is the area in which the tongue rests when the mouth is closed. Ridges contained here aid in manipulation of food during mastication (chewing) and swallowing.

The soft palate is located behind the hard palate and blends into the pharynx, the cavity behind the soft palate where the digestive and respiratory passages meet. The soft movable palate is where the gag reflex begins. On either side of the soft palate are the tonsils. When we eat, the soft palate automatically seals the area that connects to the nasal areas so that food does not get into the respiratory system.

The palate plays an important role in dental care and dental health. During the developmental stage of the embryo, if the palate does not fuse in the middle, a condition called cleft palate results. (See Cleft Palate and Lip in Part Two.) In this condition, an opening exists between the two parts of the palate, making eating and speaking very difficult if not impossible. Surgery or appropriate appliances can correct cleft palate. In other cases, the palate may be too narrow, and the teeth will not come together properly. In this situation, the palate is expanded with orthodontic appliances.

When dentures are worn, the palate is used for suction to support the false teeth. However, this becomes difficult when excess bone (tori) develops. Tori usually occur in the middle of the palate and may have to be surgically removed if dentures are to be worn or if the tori become irritated or painful.

With certain kinds of gum disease, tissue is taken from the palate and transplanted on the gums. Some nerves that supply the teeth originate in the palate. During difficult procedures, such as those performed during oral surgery, these nerves on the palate are anesthetized.

Eating very hot or crusty foods such as pizza may damage the delicate soft tissues covering the palate. The palate also shows specific signs during illnesses such as AIDS, chicken pox, or herpes.

The Tongue


Forked tongue, tongue-lashing, tongue-twister…. Even the English language recognizes the importance of the tongue, without which we would not be able to speak, swallow, or taste. This important part of our body can even tell us, in a general way, the state of our health. And what more graphic way to express disgust than by sticking out your tongue?

The tongue is a mass of “voluntary” muscles, called intrinsic and extrinsic muscles. The intrinsic muscles allow the tongue to change size and shape quickly. The flexible extrinsic muscles allow the tongue to rapidly change position.

The bottom of the tongue is attached to the floor of the mouth. This is where the salivary glands are located. Many nerves are situated in the tongue and floor of the mouth. If a person becomes unconscious, the tongue tends to fall back into the airway, causing suffocation. It is, therefore, necessary to look in the person’s mouth and clear the airway before commencing CPR (cardiopulmonary resuscitation).

The tongue is not the lively red color of the gums; rather, it has a greyish coat on the surface, caused by the specialized cells of which it is made. The muscle is covered with a mucous membrane formed into nipplelike elevations called papillae. Papillae roughen the tongue’s surface to help it guide foods during chewing and swallowing. Papillae also contain nerves for touch sensations, and most contain taste buds. Examination of the tongue with a mirror reveals a row of v-shaped, rounded, raised areas toward the back of the tongue where the taste buds responding to bitterness are located. In front of this row are tall, thin, cone-shaped raised areas that respond to sweet, sour, or salty substances. At the sides of the tongue are taste buds that react to acidic ingredients.

The tongue has several functions. It is involved in speech, manipulation and positioning of food, tasting, and swallowing. The tongue aids chewing by crushing food against the roof of the mouth (the palate) and by rolling the food between the teeth. Swallowing is accomplished as the tongue presses the food against the palate and pushes it backward into the oropharynx (entrance into the digestive and respiratory systems). The act of chewing involves coordinated movements of certain muscles that close the mouth and raise and lower the mandible, causing the teeth to grind and crush the food. The smell and taste of food cause saliva to be secreted immediately, which helps dissolve, dilute, and lubricate chewed food. The cheeks become tense and the tongue moves the food between the teeth and backwards towards the stomach. If disease necessitates surgical removal of the tongue or if it is missing at birth due to genetic disturbances, chewing is assisted by the cheeks and the floor of the mouth.

The tongue is, of course, very active in speech. If the tongue is incorrectly attached to the floor of the mouth from birth, speech is impeded and a “lisp” develops. (See Speech Problems; and Tongue-Related Problems in Part Two.) The tongue must be placed near or against the upper front teeth to form the consonants D and T. Producing the sound made by the letter L also requires the interplay of tongue and teeth.

The Tongue as a Diagnostic Tool


The tongue’s appearance is often used as an aid in the diagnosis of various diseases and conditions. We know, for instance, that during illness, some of the papillae become engorged and change color, becoming strawberry in appearance. When people have pernicious anemia, their tongues may be sore, appear beefy red, and have patchy white spots on the surface. An enlarged tongue is a sign of hypothyroidism.

According to Chinese medical science, the tongue is divided into certain sections, with each part pertaining to an organ. In this approach to the diagnosis and treatment of disease, the tip of the tongue represents the heart and lung; the central part represents the spleen and stomach; the root of the tongue represents the kidneys; and the sides of the tongue represent the liver and gall bladder. Color, texture, size, and shape of the tongue are taken into consideration as a diagnostic tool to help determine the organ to be treated.

Although self-diagnosis by using the tongue is certainly not practical, understanding what a healthy tongue looks like may give you clues about your general health. Any variation in color (too red or pale), texture (thickly coated), size (enlarged or swollen), or shape (scalloped border) may indicate an unhealthy condition.

The Salivary Glands


Cleansing of the teeth and mouth as well as good digestion depend a great deal upon the salivary glands. A number of these glands secrete saliva into the mouth. Saliva-which contains water, salts, enzymes, and mucus-moistens and softens foods for ease in swallowing, and cleanses the teeth and mouth. The function of the salivary glands is affected by hormones produced by the pancreas, testes, ovaries, and the thyroid and pituitary glands. The nature and quantity of saliva is affected by reflexes for which the taste buds act as receptors. The presence of soft moist foods in the mouth will, for instance, stimulate less secretion of the lubricating component of saliva. Salivation or watering of the mouth can also be a conditioned reflex, as when we think about a certain food or hear the mention of a particular food.

There are a few minor salivary glands situated around the lips, inside the cheeks, in the palate, and on the tongue. There are also three major pairs of salivary glands. On each side of the face, just in front of the ears, are the parotid glands, the largest of the salivary glands. The duct for each parotid gland, called Stenson’s duct, opens into the mouth from each cheek opposite the upper second molar. The parotid glands produce a clear watery secretion that functions as a cleansing, dissolving, and digestive agent. The saliva produced by the parotid gland contains a substance called ptylin, a salivary enzyme that breaks down starch. When a sour food such as a lemon is introduced into the mouth, it stimulates the parotid glands.

The sublingual glands are located in the floor of the mouth, under the tongue. The duct for these glands, called Rivinus’ duct, opens into the mouth from the floor of the mouth directly behind the lower front teeth. The saliva secreted by this gland is thicker and ropey compared with the secretion from the parotid. Sublingual-gland secretions serve as lubricating agents; bland substances such as milk and bread stimulate its production.

The submandibular glands are located deeper in the floor of the mouth, under the base of the tongue and more to the side of the lower jaw. These glands secrete a mixed type of saliva that is thin at first and becomes thicker. This secretion is also used for lubricative and digestive purposes.

From The Complete Book of Dental Remedies by Flora Parsa Stay, DDS, ©1996. Published by Avery Publishing, New York. For personal use only; neither the digital nor printed copy may be copied or sold. Reproduced by permission.