The mouth is the entrance to both the digestive and the respiratory systems. The inside of the mouth is lined with mucous membranes. When healthy, the lining of the mouth (oral mucosa) is reddish pink; the gums are paler pink and fit snugly around the teeth.

The roof of the mouth (palate) is divided into two parts. The front part has ridges and is hard (hard palate). The back part is relatively smooth and soft (soft palate). The moist mucous membranes lining the mouth continue outside, forming the pink and shiny portion of the lips, which meets the skin of the face at the vermilion border. The lip mucosa, although moistened by saliva, is prone to drying.

At the back of the mouth hangs a narrow muscular structure called the uvula, which can be seen when a person says "Ahh." The uvula hangs from the back of the soft palate, which separates the back of the nose from the back of the mouth. Normally, the uvula hangs vertically. Its nerve supply comes from the vagus (10th cranial) nerve.

On the floor of the mouth lies the tongue, which is used to taste and mix food. The tongue is not normally smooth. It is covered with tiny projections (papillae) that contain taste buds, which sense the taste of food. The sense of taste is relatively simple, distinguishing only sweet, sour, salty, and bitter. Sweet and salty taste receptors are located at and near the tip; sour, on the sides; and bitter, on the most posterior (back) part of the tongue. Smell is sensed by olfactory receptors high in the nose. The sense of smell is much more complex than that of taste, distinguishing many subtle variations. The senses of taste and smell work together to enable people to recognize and appreciate flavors (see Symptoms of Nose and Throat Disorders: Overview of Smell and Taste Disorders).

A View of the Mouth

The salivary glands produce saliva. There are three major pairs of salivary glands: parotid, submandibular, and sublingual. Besides the major salivary glands, many tiny salivary glands are distributed throughout the mouth. Saliva passes from the glands into the mouth through small tubes (ducts).

Saliva serves several purposes. Saliva aids in chewing and eating by gathering food into lumps so that food can slide out of the mouth and down the esophagus and by dissolving foods so that they can more easily be tasted. Saliva also coats food particles with digestive enzymes and begins digestion. After food is eaten, the flow of saliva washes away bacteria that can cause tooth decay (cavities) and other disorders. Saliva helps keep the lining of the mouth healthy and prevents loss of minerals from teeth. It not only neutralizes acids produced by bacteria but also contains many substances (such as antibodies and enzymes) that kill bacteria, yeasts, and viruses.

A tooth is divided into the crown, which is the part above the gum line, and the root, which is the part below the gum line. The crown is covered with shiny white enamel, which protects the tooth. Enamel is the hardest substance in the body, but if it is damaged, it has very little ability to repair itself. Under the enamel is dentin, which is similar to bone but is harder. Dentin surrounds the central (pulp) chamber, which contains blood vessels, nerves, and connective tissue.

The blood vessels and nerves enter the pulp chamber through the root canals, which are also surrounded by dentin. In the root, dentin is covered by cementum, a thin bonelike substance. Cementum is surrounded by a membrane (periodontal ligament) that cushions the tooth and attaches the cementum layer, and thereby the whole tooth, firmly to the jaw.

Color Changes in the Mouth
White areas can appear anywhere in the mouth and often are simply food debris that can be wiped away. However, because more persistent white areas can be an early sign of mouth cancer, they should always be evaluated by a dentist or doctor. White areas can indicate many other conditions besides cancer, such as a white spongy patch (a hereditary condition called white sponge nevus), a white line running along the inside of the cheek opposite the teeth (linea alba), and a grayish white area of the mucosa (leukoedema). The mouth may have dark blue or black areas due to silver amalgam from a dental filling, graphite from falling with a pencil in the mouth, or a mole. Heavy cigarette smoking can lead to dark brown or black discoloration called smoker's melanosis. Ingesting lead or drugs that contain silver can lead to a gray discoloration of the gums. Minocycline, an antibiotic, discolors bone, which may show through near the teeth as gray or brown. Children's teeth darken noticeably and permanently after even short-term use of tetracyclines (a class of antibiotic) by the mother during the second half of pregnancy or by the child during tooth development (specifically calcification of the crowns, which lasts until age 9). Brown areas in the mouth can also be hereditary. For example, darkly pigmented areas are particularly common among dark-skinned and Mediterranean people. Sometimes color changes in the mouth are a sign of a bodywide disease: Anemia may cause the lining of the mouth to be pale instead of the normal healthy reddish pink. Measles, a viral disease, can cause spots to form inside the cheeks. These spots, called Koplik's spots, resemble tiny grains of white sand surrounded by a red ring. Addison's disease and cancer (such as malignant melanoma) can cause color changes as well. In a person with AIDS, purplish patches caused by Kaposi sarcoma may appear on the palate. Small red spots on the palate can be a sign of a blood disorder or infectious mononucleosis.

People have two sets of natural teeth: baby (deciduous) teeth and adult (permanent) teeth. There are 20 baby teeth: one pair each of upper and lower central (front) incisors, lateral incisors, canines (cuspids), first molars, and second molars. There are 32 permanent teeth: one pair each of upper and lower central incisors, lateral incisors, canines, bicuspids (premolars), second bicuspids, first molars, second molars, and third molars (wisdom teeth). Wisdom teeth, however, vary—not everyone gets all four wisdom teeth, and some people do not get any wisdom teeth. The wisdom teeth are the last permanent teeth to come in, typically between the ages of 17 and 21.

There is a broad range of normal times for teeth to push through the gum tissue (erupt) into the mouth. For baby teeth, the central incisors are the first teeth to erupt, occurring at about 6 months of age. These are followed by the lateral incisors, first baby molars, canines, and, finally, second baby molars. By about 2½ years of age, all the baby teeth can usually be seen in the child's mouth. Each of these baby teeth will be pushed out by a permanent tooth, starting at about age 6. The permanent 6-year molars come into the mouth just behind the last baby molars and, therefore, do not replace any teeth. This lack of replacement is also true for the permanent second and third molars.

In rare cases, a child is born with a tooth (a natal tooth), or a baby tooth erupts in the mouth within a month of birth (a neonatal tooth). These teeth are usually baby lower incisors, but they may be extra (supernumerary) teeth. These teeth are removed only if they interfere with nursing or if they become exceedingly loose, which may pose a risk of choking.

In many children, the permanent lower incisors come in behind each other, resembling a cluster of grapes. Lack of space due to crowding or rotated permanent teeth may be the problem, and early orthodontic therapy (braces) may be necessary. Thumb or finger sucking may also affect the position of teeth, sometimes requiring early orthodontic therapy.

Last full review/revision March 2006 by Linda P. Nelson, DMD, MScD