Biology of the Mouth
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.
 See the figure A View of the Mouth.
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 lips are distinctly divided by a wet-dry border (vermilion border); the inside surface is moist and the outside surface is skinlike.
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 (villi) that contain taste buds, which sense the taste of food. The sense of taste is relatively simple, distinguishing only sweet, sour, salty, and bitter. 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 Section 6, Chapter 97).
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. It aids in chewing and eating by gathering food into lumps so that food can slide out of the mouth and down the esophagus, by beginning digestion, and by dissolving foods so that they can more easily be tasted. After food is eaten, saliva cleanses the mouth. 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.
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, 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 beyond the last baby molars and, therefore, do not replace any teeth. This lack of replacement is also true for the permanent second molars 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.
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.
Cutting food with the front teeth and chewing with the back teeth break down food into more easily digestible particles. Saliva from the salivary glands coats food particles with digestive enzymes and begins digestion. Between meals, the flow of saliva washes away bacteria that can cause tooth decay (cavities) and other disorders.
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.
 See the sidebar Color Changes in the Mouth.
Effects of Aging
With age, taste sensation decreases. Older people are less able to identify food by taste, and many foods taste bitter. Older people may also have disorders or take drugs that affect their ability to taste.
Many older people retain their teeth, especially those people who do not develop cavities or periodontal disease, a destructive disease of the gums and supporting structures caused by the long-term accumulation of bacteria. Some older people lose some or all of their teeth and need partial or full dentures. Tooth loss is the major reason that older people cannot chew as well and thus may not consume enough calories.
Tooth enamel tends to wear away with age, making the teeth vulnerable to damage and decay. Periodontal disease, however, is the major cause of tooth loss. Periodontal disease is more likely to occur in people with poor oral hygiene, in people who smoke, and in people with certain disorders, such as diabetes mellitus, poor nutrition, leukemia, and AIDS.
A modest decrease in saliva production occurs with age. The significance of this change is uncertain; some experts believe that it may make the lining of the esophagus more susceptible to injury.
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