A smile is the facial expression that most engages others. With the
help of the teeth — which provide structural support for the face
muscles — the mouth also forms a frown and other expressions that show
on your face.


The mouth also plays a key role in the digestive system,
but it does much more than get digestion started. The mouth —
especially the teeth, lips, and tongue — is essential for speech. The
tongue, which allows us to taste, also helps form words when we speak.
The lips that line the outside of the mouth both help hold food in while
we chew and pronounce words when we talk.





With the lips and tongue, teeth help form words by controlling air
flow out of the mouth. The tongue strikes the teeth as certain sounds
are made.


The hardest substances in the body, the teeth are also necessary for
chewing (or mastication) — 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 food.


Here's how each aspect of the mouth and teeth plays an important role in our daily lives.
Basic Anatomy of the Mouth and Teeth




The entrance to the digestive tract, the mouth is lined with mucous
membranes. 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 and the nasal passages above. The soft palate forms a curtain
between the mouth and the throat, or pharynx, to the rear. The soft
palate contains the uvula, the dangling flesh 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 bumps
called papillae. These contain tiny pores that are our taste buds. Four
main kinds of taste buds are found on the tongue — those that sense
sweet, salty, sour, and bitter tastes. Three pairs of salivary glands
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, allows for the lips' mobility. The reddish
tint of the lips comes from underlying blood vessels. The inside portion
of both lips is connected to the gums.


There are several types of teeth. Incisors are the squarish,
sharp-edged teeth in the front of the mouth. There are four on the
bottom and four on the top. On either side of the incisors are the sharp
canines. The upper canines are sometimes called eyeteeth. Behind the
canines are the premolars, or bicuspids. There are two sets, or four
premolars, in each jaw.


The molars, situated behind the premolars, have points and grooves.
There are 12 molars — three sets in each jaw called the first, second,
and third molars. The third molars are the wisdom teeth, thought by some
to have evolved thousands of years ago when human diets consisted of
mostly raw foods that required extra chewing power. But because they can
crowd out the other teeth or cause problems like pain or infection, a
dentist might need to remove them.


Human teeth are made up of four different types of tissue: pulp,
dentin, enamel, and cementum. The pulp is the innermost portion of the
tooth and consists of connective tissue, nerves, and blood vessels,
which nourish the tooth. The pulp has two parts — the pulp chamber,
which lies in the crown, and the root canal, which is in the root of the
tooth. Blood vessels and nerves enter the root through a small hole in
its tip and extend through the canal into the pulp chamber.


Dentin surrounds the pulp. A hard yellow substance, it makes up most
of the tooth and is as hard as bone. It's the dentin that gives teeth
their yellowish tint. Enamel, the hardest tissue in the body, covers the
dentin and forms the outermost layer of the crown. It enables the tooth
to withstand the pressure of chewing and protects it from harmful
bacteria and changes in temperature from hot and cold foods. Both the
dentin and pulp extend into the root. A bony layer of cementum covers
the outside of the root, under the gum line, and holds the tooth in
place within the jawbone. Cementum is also as hard as bone.
Normal Development of the Mouth and Teeth




Humans are diphyodont, meaning that they develop two sets of teeth.
The first set of 20 deciduous teeth are also called the milk, primary,
temporary, or baby teeth. They begin to develop before birth and begin
to fall out when a child is around 6 years old. They're replaced by a
set of 32 permanent teeth, which are also called secondary or adult
teeth.


Around the 8th week after conception, oval-shaped tooth buds
consisting of cells form in the embryo. These buds begin to harden about
the 16th week. Although teeth aren't visible at birth, both the primary
and permanent teeth are forming below the gums. The crown, or the hard
enamel-covered part that's visible in the mouth, develops first. When
the crown is formed, the root begins to develop.


Between the ages of 6 months and 1 year, the deciduous teeth begin to push through the gums. This process is called eruption or teething.
At this point, the crown is complete and the root is almost fully
formed. 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 molars' purpose is to
grind food, and the incisors and canine teeth are used to bite into and
tear food.


The primary teeth help the permanent teeth erupt in their normal
positions; most of the permanent teeth form close to the roots of the
primary teeth. When a primary 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.


Kids start to lose their primary teeth, or baby teeth, at about 6
years old. This begins a phase of permanent tooth development that lasts
over the next 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 kids get braces.
What the Mouth and Teeth Do




The first step of digestion involves the mouth and teeth. Food enters
the mouth and is immediately broken down into smaller pieces by our
teeth. Each type of tooth serves a different function in the chewing
process. Incisors cut foods when you bite into them. The sharper and
longer canines tear food. The premolars, which are flatter than the
canines, grind and mash food. Molars, with their points and grooves, are
responsible for the most vigorous chewing. All the while, the tongue
helps to push the food up against our teeth.


During chewing salivary glands in the walls and floor of the mouth
secrete saliva, which moistens the food and helps break it down even
more. Saliva makes it easier to chew and swallow foods (especially dry
foods), and it contains enzymes that aid in the digestion of
carbohydrates.


Once food has been converted into a soft, moist mass, it's pushed
into the throat (or 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.
Problems of the Mouth and Teeth




Proper dental care — including a good diet, frequent cleaning of the
teeth after eating, and regular dental checkups — is essential to maintaining healthy teeth and avoiding tooth decay and gum disease.


Common mouth and dental diseases and conditions — some of which can be prevented, some of which cannot — are :


Disorders of the Mouth





* Aphthous stomatitis (canker sores). A common form
of mouth ulcer, canker sores occur in women more often than in men.
Although their cause isn't completely understood, mouth injuries,
stress, dietary deficiencies, hormonal changes (such as 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. They 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 lip and/or mouth don't
form properly during fetal development. Children born with these
disorders may have trouble feeding immediately after birth.
Reconstructive surgery in infancy and sometimes later can repair the
anatomical defects, and can prevent or lessen the severity of speech
problems later on.
* Enteroviral stomatitis is a common childhood infection caused by a family of viruses called the enteroviruses. An important member of this family is coxsackievirus,
which causes hand, foot, and mouth disease. Enteroviral stomatitis is
marked by small, painful ulcers in the mouth that may decrease a child's
desire to eat and drink and put him or her at risk for dehydration.
* Herpetic stomatitis (oral herpes). Kids can get a
mouth infection with the herpes simplex virus from an adult or another
child who has it. The resulting painful, clustered vesicles, or
blisters, can make it difficult to drink or eat, which can lead to
dehydration, especially in a young child.
* Periodontal disease. The gums and bones supporting
the teeth are subject to disease. A common periodontal disease is
gingivitis — inflammation of the gums characterized by redness,
swelling, and sometimes bleeding. The accumulation of tartar (a hardened
film of food particles and bacteria that builds up on teeth) usually
causes this condition, and it's almost always the result of inadequate
brushing and flossing. 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.




Disorders of the Teeth





* Cavities and tooth decay. When bacteria and food
particles stick to 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 isn't treated, the decay
process progresses to involve the dentin. Without treatment, serious
infections can occur. The most common ways to treat cavities and more
serious tooth decay problems are: filling the cavity; performing root
canal therapy, involving 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. A common cause of tooth
decay in toddlers is "baby bottle tooth decay," which occurs when a
child goes to sleep with a milk or juice bottle in the mouth and the
teeth are bathed in sugary liquid for an extended period of time. To
avoid tooth decay and cavities, teach your kids good dental habits —
including proper tooth-brushing techniques — at an early age.
* 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 isn't 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 the X-rays show there's a chance that
impacted teeth may cause problems, the dentist may recommend that the
tooth or teeth be extracted.
* Malocclusion is the failure of the teeth in the
upper and lower jaws to meet properly. Types of malocclusion include
overbite, underbite, and crowding. Most conditions can be corrected with
braces, which are metal or clear ceramic brackets bonded to the front
of each tooth. The wires connecting braces are tightened periodically to
force the teeth to move into the correct position.