Your smile is often the first thing people notice when they look
at you. It's the facial expression that most engages others.
With the help of the teeth - which provide structural support for
the face muscles - your mouth also forms your frown and lots of
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 enables us to 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 kinds of taste buds are grouped together on certain
areas of 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,
sometimes a dentist will 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 consisting
mostly of mineral salts and water, 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, falling-off, 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 fully grown, 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 12 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 not come
through at all. 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, lips, tongue, soft palate, or 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 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 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).
Children 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 saliva 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 isn't treated, the decay
process progresses to involve the dentin. 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
"milk bottle mouth," 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 will usually 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.
Reviewed by:
Lisa A. Goss, RDH, BS, and Garrett B. Lyons Jr., DDS
Date reviewed: August 2006
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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