The first thing that comes to mind when you think of your mouth is probably eating — or kissing! But your mouth's a lot more than an input slot for food or a tool for smooching your sweetie.

Where Would We Be Without Them?

Your mouth and teeth form your smile, which is often the first thing people notice when they look at you. The mouth is also essential for speech: The tongue (which also allows us to taste) enables us to form words with the help of our lips and teeth. The tongue hits the teeth to make certain sounds — the th sound, for example, is produced when the tongue brushes against the upper row of teeth. If a person has a lisp, that means the tongue touches the teeth instead of directly behind them when saying words with the s sound.

Without our teeth, we'd have to live on a liquid diet or a diet of soft, mashed food. The hardest substances in the body, the teeth are necessary for mastication — a fancy way of saying chewing — the process by which we tear, cut, and grind food in preparation for swallowing.

Chewing allows enzymes and lubricants released in the mouth to further digest, or break down, food. This makes the mouth one of the first steps in the digestive process. Read on to find out how each aspect of the mouth and teeth plays a role in our daily lives.

Basic Anatomy of the Mouth and Teeth

The mouth is lined with mucous membranes (pronounced: myoo-kus mem-branes). Just as skin lines and protects the outside of the body, mucous membranes line and protect the inside. Mucous membranes make mucus, which keeps them moist.

The membrane-covered roof of the mouth is called the palate. The front part consists of a bony portion called the hard palate, with a fleshy rear part called the soft palate. The hard palate divides the mouth from the nose above. The soft palate forms a curtain between the mouth and the throat (or pharynx — pronounced: fa-rinks) to the rear.

The soft palate contains the uvula (pronounced: yoo-vyoo-luh), the dangling fleshy object at the back of the mouth. The tonsils are located on either side of the uvula and look like twin pillars holding up the opening to the pharynx.

A bundle of muscles extends from the floor of the mouth to form the tongue. The upper surface of the tongue is covered with tiny projections called papillae. Our taste buds are located here. The four main types of taste buds — sweet, salty, sour, and bitter — are found on the tongue.

Three pairs of salivary glands in the walls and floor of the mouth secrete saliva, which contains a digestive enzyme called amylase that starts the breakdown of carbohydrates even before food enters the stomach.

The lips are covered with skin on the outside and with slippery mucous membranes on the inside of the mouth. The major lip muscle, called the orbicularis oris (pronounced: or-bik-yoo-lar-iss or-iss), allows for the lips' mobility. The reddish tint of the lips comes from underlying blood vessels, which is why the lips can bleed so easily with injury. The inside part of the lips connects to the gums.

All About Teeth

The types of teeth are:

• Incisors are the squarish, sharp-edged teeth at the front and middle of the mouth. There are four on the bottom and four on the top.
• To the sides of the incisors are the long, sharp canines, two on the bottom and two on the top. The upper canines are sometimes called eyeteeth or cuspids.
• Behind the canines are the premolars, or bicuspids. There are two sets, or a total of four premolars, in each jaw — two behind each of the canines on the bottom and two behind each canine on the top.
• The molars, situated behind the premolars, have points and grooves. There are 12 molars in the adult mouth — three sets in each jaw called first, second, and third molars. The third molars are called wisdom teeth. Wisdom teeth get their name because, as the last teeth to erupt, they break through when a person is becoming an adult and is supposedly wiser. Wisdom teeth are not essential today, but some people believe they evolved thousands of years ago when human diets consisted of mostly raw foods that required extra chewing power.
  
  Because wisdom teeth can crowd out the other teeth or cause problems like pain and infection, a dentist may need to remove them. This often happens during a person's teenage years.

Each tooth is made of four types of tissue: pulp, dentin, enamel, and cementum. The pulp is the innermost portion of the tooth. Unlike the outer parts of the tooth, the pulp is soft. It is made of connective tissue, nerves, and blood vessels, which nourish the tooth. The pulp has two parts: the pulp chamber, which lies in the crown (or top part of the tooth) and the root canal, which is in the bottom part of the tooth that lies beneath the gums. Blood vessels and nerves enter the root through a small hole at the very bottom of the tooth and extend through the canal into the pulp chamber.

Dentin surrounds the pulp. A hard yellow substance, dentin makes up most of the tooth. It is the dentin that gives the tooth its slightly yellowish tint.

Both the dentin and pulp cover the whole tooth from the crown into the root. But the outermost layer covering the tooth is different, depending on whether it sits above the gum or below it. Enamel, the hardest tissue in the body, covers the crown.

Under the gum line, a bony layer of cementum covers the outside of the root and holds the tooth in place within the jawbone. Cementum is as hard as bone but not as hard as enamel, which enables the tooth to withstand the pressure of chewing and protects it from harmful bacteria and changes in temperature from hot and cold foods.

Normal Development of the Mouth and Teeth

Humans are diphyodont (pronounced: dy-fy-uh-dant), meaning that they develop two sets of teeth. The first set of teeth, the deciduous (pronounced: duh-sid-you-wus) teeth are also called the milk, primary, temporary, or baby teeth. These teeth begin to develop before birth, start to push through the gums between the ages of 6 months and 1 year (this process is called eruption), and usually start to fall out when a kid is around 6 years old. They are replaced by a set of 32 permanent teeth, which are also called secondary or adult teeth.

Although teeth aren't visible at birth, both the deciduous and permanent teeth are forming beneath the gums. By the time a child is 3 years old, he or she has a set of 20 deciduous teeth, 10 in the lower and 10 in the upper jaw. Each jaw has four incisors, two canines, and four molars.

The deciduous teeth help the permanent teeth erupt in their normal positions; most of the permanent teeth form just beneath the roots of the deciduous teeth above them. When a deciduous tooth is preparing to fall out, its root begins to dissolve. This root has completely dissolved by the time the permanent tooth below it is ready to erupt.

The phase during which permanent teeth develop usually lasts for about 15 years as the jaw steadily grows into its adult form. From ages 6 to 9, the incisors and first molars start to come in. Between ages 10 and 12, the first and second premolars, as well as the canines, erupt. From 11 to 13, the second molars come in.

The wisdom teeth (third molars) erupt between the ages of 17 and 21. Sometimes there isn't room in a person's mouth for all the permanent teeth. If this happens, the wisdom teeth may get stuck (or impacted) beneath the gum and may need to be removed. Overcrowding of the teeth is one of the reasons people get braces during their teenage years.

What Do the Mouth and Teeth Do?

The mouth and teeth play an important role in digesting food. Food is torn, ground, and moistened in the mouth. Each type of tooth serves a different function in the chewing process. Incisors cut foods when you bite into them. The sharper, longer canines tear food. The premolars grind and mash food. Molars, with their points and grooves, are responsible for the most vigorous grinding. All the while, the tongue helps to push the food up against our teeth.

As we chew, the salivary glands secrete saliva, which moistens the food and helps break it down further. As well as containing digestive enzymes, saliva makes it easier to chew and swallow foods (especially dry foods).

Once food has been converted into a soft, moist mass, it's pushed into the pharynx at the back of the mouth and is swallowed. When we swallow, the soft palate closes off the nasal passages from the throat to prevent food from entering the nose.

Things That Can Go Wrong With the Mouth

Proper dental care is essential to good oral health. This includes a good diet, brushing and flossing after eating, and regular dental checkups.

Common mouth diseases and conditions include:

• Aphthous stomatitis (canker sores). Canker sores are a common form of mouth ulcer that girls get more often than guys. Although their cause is not completely understood, mouth injuries, stress, dietary deficiencies, hormonal changes (as with the menstrual cycle), or food allergies can trigger them. They usually appear on the inner surface of the cheeks or lips, under the tongue, on the soft palate, or at the base of the gums, and begin with a tingling or burning sensation followed by a painful sore called an ulcer. Pain subsides in 7 to 10 days, with complete healing usually occurring in 1 to 3 weeks.
• Cleft lip and cleft palate are birth defects in which the tissues of the mouth and/or lip don't form properly as a fetus is developing in the womb. Children born with cleft lip or cleft palate can have reconstructive surgery in infancy — and sometimes later — to repair the cleft. This surgery can prevent or lessen the severity of speech problems later in life.
• Enteroviral stomatitis is a common type of infection. People with this condition have small, painful ulcers inside their mouths that may decrease their desire to eat and drink, putting them at risk of dehydration.
• Herpetic stomatitis (oral herpes). Oral herpes causes painful, clustered blisters inside the mouth or on a person's lip. People can get this infection when they have direct contact (such as kissing!) with someone with the herpes simplex virus, or from sharing utensils or cups with someone who has the infection.
• Periodontal disease. Periodontal (pronounced: pare-ee-oh-don-tul) disease affects the gums and tissues supporting the teeth. Gingivitis (pronounced: jin-jih-vy-tus), an inflammation of the gums characterized by redness, swelling, and sometimes bleeding, is one common form of periodontal disease. It's usually caused by the accumulation of tartar (a hardened film of food particles and bacteria that builds up on teeth). Gingivitis is almost always the result of not brushing and flossing the teeth properly.
  
  When gingivitis isn't treated, it can lead to periodontitis, in which the gums loosen around the teeth and pockets of bacteria and pus form, sometimes damaging the supporting bone and causing tooth loss.

Things That Can Go Wrong With Teeth

Proper dental care is essential to good oral health. This includes a good diet, brushing and flossing after eating, and regular dental checkups.

Common dental diseases and conditions include:

• Cavities and tooth decay. When bacteria and food particles are allowed to settle on the teeth, plaque forms. The bacteria digest the carbohydrates in the food and produce acid, which dissolves the tooth's enamel and causes a cavity. If the cavity is not treated, the decay process progresses to involve the dentin. Without treatment, serious infection can develop.
  
  The most common ways to treat cavities and more serious tooth decay problems are filling the cavity; performing a root canal procedure, which involves the removal of the pulp of a tooth; crowning a tooth with a cap that looks like a tooth made of metal, porcelain, or plastic; or removing or replacing the tooth.
  
  To avoid tooth decay and cavities, get in the habit of good dental care — including proper tooth brushing techniques.
• Malocclusion is the failure of the upper and lower teeth to meet properly when you bite down. The types of malocclusion include overbite, underbite, and crowding. Most of these conditions can be corrected with treatment, like braces. Braces are metal or clear ceramic brackets bonded to the front of each tooth. Wires connecting the brackets are tightened periodically to force the teeth to move into the correct position. There are also ways to correct the bite using removable clear appliances.
• Impacted wisdom teeth. In many people, the wisdom teeth are unable to erupt normally so they either remain below the jawline or don't grow in properly. Dentists call these teeth impacted. Wisdom teeth usually become impacted because the jaw is not large enough to accommodate all the teeth that are growing in and the mouth becomes overcrowded. Impacted teeth can damage other teeth or become painful and infected.
  
  Dentists can check if a person has impacted wisdom teeth by taking X-rays of the teeth. If, after looking at the X-rays, a dentist thinks there's a chance that impacted teeth may cause problems, he or she may recommend that the tooth or teeth be removed (extracted).

Reviewed by: Kenneth H. Hirsch, DDS
Date reviewed: October 2012