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Teeth | ||||||
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I INTRODUCTION Teeth, hard, bony structures in the mouths of humans and animals
used primarily to chew food, but also for gnawing, digging, fighting, and catching
and killing prey. Teeth are the body’s hardest, most durable organ—long
after bones and flesh have dissolved, archaeologists find well-preserved teeth
from humans and other animals that lived thousands of years ago.
Humans use teeth to tear, grind, and chew food in the first step of digestion,
enabling enzymes and lubricants released in the mouth to further break down
food. Teeth also play a role in human speech—the teeth, lips, and tongue
are used to form words by controlling airflow through the mouth. Additionally,
teeth provide structural support to muscles in the face and form the human smile. v4c14cz The visible portion of the tooth is called the crown. Projections on the top
of each crown, used primarily for chewing and grinding, are called cusps. The
portion of the tooth that lies beneath the gum line is the root. The periodontal
ligament anchors the tooth in place with small elastic fibers that connect the
cementum in the root to a special socket in the jawbone called the alveolus. Human tooth development occurs in stages. The hard tissue of the deciduous
teeth, or the dentin, forms while the fetus is in the womb. After the child
is born, tooth enamel develops in stages. Front tooth enamel, for example, is
usually complete around one month after birth, while the enamel on the second
molars is not completely developed until a child is about a year and a half
old. When the enamel is fully developed the tooth erupts. Front teeth usually
erupt when a child is from 6 to 12 months of age, second molars between 13 and
19 months old, and canines usually erupt at 19 months or older. The final stage
of tooth development is root completion, a slow process that continues until
the child is more than three years old. C Disorders of Human Teeth The three main diseases of human teeth are tooth decay, also called dental caries; gum disease, or periodontal disease; and problems with tooth alignment, called malocclusions. Human teeth problems are treated or prevented by dentists, professionals who are specially trained to practice dentistry. Tooth decay affects approximately 90 percent of all children by the time they are 14 years old. Tooth decay begins when bacteria are passed from mothers or caregivers to children between their first and second birthdays. When these bacteria are exposed to sugars commonly found in foods, the bacteria produce harmful acids that attack tooth enamel. Left unchecked, the acid eats holes in the enamel and forms cavities of tooth decay. Most tooth decay forms in the deep grooves on the chewing surfaces of the molars, called pits and fissures. Daily tooth brushing and proper dental care help prevent and reduce tooth decay. Dentists use preventive 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 fortifies tooth 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 pull away 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, the early 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 to periodontitis, 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 the bacteria can be removed. Some animals are monophydont, 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 of these 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. Sharks and some other fish are polyphydont—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 type of 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 teeth with 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 for grinding and mashing. Piscivores, or fish-eaters, have sharp teeth that angle backward to catch and hold their prey. Seals and dolphins swallow food whole without chewing 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, blunt teeth 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, flat beaks 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 and frogs 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 the tongue 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 to their 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 they thrash violently with their prey in the water. |
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