Teeth.

« treatments to reduce the risk of tooth decay; clear plastic coatings painted on the teeth, called dental sealants, and applications of the mineral fluoride, which fortifiestooth enamel, are two such treatments.

Fluoride is also added to public water supplies in a process called fluoridation, which benefits more than 150 million Americans. Gum disease, or periodontal disease, is a progressive condition that worsens with age.

Gum disease occurs when bacteria eat away at gum tissue, causing it to pullaway from the teeth.

This space between the tooth and gum, called a periodontal pocket, traps even more bacteria.

Gum disease develops in two stages.

Gingivitis, theearly stage, causes red, swollen gums that bleed easily.

Gingivitis can be eliminated through good oral hygiene and dental care.

If not treated, gingivitis can progress toperiodontitis, when bacteria attack the bone supporting the teeth.

To treat periodontitis, dentists may have to surgically cut out the infected portion of the gum so thebacteria can be removed. Malocclusions—teeth that are crowded, crooked, or out of alignment—make it more difficult to clean teeth, which can lead to other oral health problems such as toothdecay and gum disease.

Many of these disorders start to appear between the ages of 6 and 12, when permanent teeth begin to erupt.

Generally, malocclusions resultwhen the jaw is too small to hold all of the teeth.

Malocclusions are often genetic, tending to run in families.

In other cases, dental injury or chronic thumb sucking maylead to poorly aligned teeth.

Malocclusions are treated by dentists specially trained to correct them, called orthodontists. III TEETH IN OTHER ANIMALS Animal teeth have the same four tissues that make up human teeth: enamel, dentin, cementum, and pulp.

The composition and structure of each substance may differin each animal.

In horses, for example, enamel is found inside the tooth as well as on the outer surface, rather than simply encasing the dentin and the pulp as it doesin human teeth. Some animals are monophyodont, developing only one set of teeth that grow continuously throughout an animal’s lifetime.

These animals have open-rooted teeth,which have wide openings at the root that permit dentin-forming cells to grow and multiply.

Most rodents, for example, have open-rooted teeth.

The gnawing habits ofthese animals wear down their teeth, otherwise the teeth would grow very long.

The front teeth of beavers, for example, can grow up to 1.2 m (4 ft) a year.

Sharksand some other fish are polyphyodont—that is, they continuously lose their teeth and develop new ones. Most mammals that depend on catching, chewing, and digesting food for survival have developed teeth that meet these needs.

Mammal teeth are classified by the typeof food the animal eats.

Insectivores are animals that eat only insects, such as bats, shrews, anteaters, and armadillos.

These animals have square teeth with special V-shaped edges that efficiently grind the hard coverings of insects.

Carnivores, such as dogs, cats, hyenas, and walruses, generally have large and well-developed teethwith long canines for clamping down on prey or fighting.

A walrus also uses its canines, which grow up to 1 m (about 3 ft) long, as hooks in climbing on ice.Herbivores—cows, sheep, deer, and horses, for example—eat only plants and have sharp incisors for cutting vegetation and flat teeth with complicated ridges forgrinding and mashing.

Piscivores, or fish-eaters, have sharp teeth that angle backward to catch and hold their prey.

Seals and dolphins swallow food whole withoutchewing it first; they are equipped with many identical, conical-shaped teeth that are used to catch and grasp their slippery prey before swallowing. Fish teeth have evolved to perform different functions in each species.

For example, fish that feed on crab, shrimp, and other crustaceans have developed strong, bluntteeth for crushing and grinding the hard outer shells.

Piranhas have serrated teeth that fit together like scissors, enabling the small fish to cut the flesh from their prey.Many fish have teeth on their tongues or gills.

A hagfish uses the rasping teeth on its tongue to bore holes in its victims and drain their blood.

Sawfish have long, flatbeaks with a row of weaponlike tooth projections in each jaw that can cut their prey in half. Only some reptiles and amphibians have teeth.

Salamanders have rows of small pointed teeth, but frogs and toads do not have teeth after infancy.

Some snakes andfrogs develop an egg tooth that enables a hatching young to chip its way out of its egg.

This tooth eventually disappears.

Many reptiles have teeth growing on thetongue or the palate, and some even have a second set in the throat.

Some snakes, such as rattlesnakes, have prominent fangs for delivering injections of venom totheir victims.

Crocodiles have between 30 and 40 teeth in each jaw.

These reptiles use their daunting teeth not for chewing, but to gradually tear food into bits as theythrash violently with their prey in the water. Contributed By:Chris MartinMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

All rights reserved.. »