teeth

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teeth,

hard, calcified structures embedded in the bone of the jaws of vertebrates that perform the primary function of mastication. Humans and most other mammals have a temporary set of teeth, the deciduous, or milk, teeth; in humans, they usually erupt between the 6th and 24th months. These number 20 in all: 2 central incisors, 2 lateral incisors, 2 canines, and 4 premolars in each jaw. At about six years of age, the preliminary teeth begin to be shed as the permanent set replaces them. The last of the permanent teeth (wisdom teeth) may not appear until the 25th year, and in some persons do not erupt at all. The permanent teeth generally number 32 in all: 4 incisors, 2 canines, 4 bicuspids, and 4 (or 6, if wisdom teeth develop) molars in each jaw. Human canines are the smallest found in any mammal.

Among all mammals, the tooth consists of a crown, the portion visible in the mouth, and one or more roots embedded in a gum socket. The portion of the gum surrounding the root, known as the periodontal membrane, cushions the tooth in its bony socket. The jawbone serves as a firm anchor for the root. The center of the crown is filled with soft, pulpy tissue containing blood vessels and nerves; this tissue extends to the tip of the root by means of a canal. Surrounding the pulp and making up the greater bulk of the tooth is a hard, bony substance, dentin. The root portion has an overlayer of cementum, while the crown portion has an additional layer of enamel, the hardest substance in the body. Most nonmammalian vertebrates do not have the outer layer of enamel on their teeth, but instead have a substance known as vitrodentine, similar to dentine, though much harder.

Proper diet is necessary for the development and maintenance of sound teeth, especially sufficient calcium, phosphorus, and vitamins D and C. The most common disorder that affects the teeth is dental caries (tooth decay). A widely accepted explanation of the process of tooth decay is that salivary bacteria convert carbohydrate particles in the mouth into lactic acid, which attacks the enamel, dentin, and, if left untreated, the pulp of the teeth. Regular cleansing and semiannual dental examinations (see dentistrydentistry,
treatment and care of the teeth and associated oral structures. Dentistry is mainly concerned with tooth decay, disease of the supporting structures, such as the gums, and faulty positioning of the teeth.
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) are important in preventing dental caries and gum disorders. Fluoridationfluoridation
, process of adding a fluoride to the water supply of a community to preserve the teeth of the inhabitants. Tooth enamel ordinarily contains small amounts of fluorides and when the amount is augmented through the intake of fluoridated water, especially during the
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 of public water supplies and use of fluoride toothpastes also help prevent caries. In the study of fossil remains done in paleontology and physical anthropology, teeth are the most frequently found remains, a testament to their high mineral content and resistance to deterioration over time. See dentitiondentition,
kind, number, and arrangement of the teeth of humans and other animals. During the course of evolution, teeth were derived from bony body scales similar to the placoid scales on the skin of modern sharks.
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.

Bibliography

See P. S. Ungar, Mammal Teeth: Origin, Evolution, and Diversity (2010).

Teeth

 

the bonelike structures found in the mouth in man and most gnathostomatous vertebrates (in some fish, also in the pharynx) that are used for grasping and holding food and for its mechanical processing, or mastication. Three anatomical parts of teeth may be distinguished: the crown, neck, and root, or roots. The main bulk of the tooth is composed of dentin, covered by enamel on the crown area and, in mammals, by cementum on the neck and root. Inside the tooth is a cavity, the root canal, which is filled with pulp. The canal opens at the end of the root, forming the apical foramen, through which blood vessels and nerve fibers enter the tooth cavity.

In the process of animal evolution teeth originally derived from the dermal teeth, or placoid scales, offish. Teeth appear during embryonic development as epithelial folds— the “dental lamina” with the rudiments of certain teeth. Mesenchymal cells of the dermis form a thickening (dental papillae) under each rudiment, over which epithelial cells of the dental lamina are suspended in the form of a cap, the so-called enamel organ. The internal layer of cells of the dental lamina participates in the formation of enamel; the external cells of the dental papilla, or odontoblasts, form dentin, and the mesenchymal cells surrounding it form cementum. The developing teeth emerge by cutting through the mucous membrane of the gum.

In most vertebrates teeth are replaced by new ones as they wear out. Teeth may be replaced continuously throughout life (polyphyodont dentition; most vertebrates, excluding mammals), only once at a young age (diphyodont dentition; the majority of mammals), or not at all (monophyodont dentition; certain mammals such as the edentates and cetaceans). In most fish and reptiles and in amphibians all the teeth are identical (isodont, or homodont, dentition); some fish and reptiles and, as a rule, mammals have teeth of various forms (heterodont dentition). In cyclostomes special horny teeth develop, which are not homologous to the teeth of gnathostomatous vertebrates. In fish the teeth occur in the soft tissues and on the bones of the mouth and on the branchial arches (pharyngeal teeth). Some fish, for example, loach and carp, have only pharyngeal teeth, while others, such as adult sturgeon, have no teeth at all. In amphibians the teeth occur on the bones of the mouth. In reptiles the teeth are usually ankylosed to the jaws (to their rim or to the inside) and are seldom found on the other bones of the mouth. In crocodiles the teeth are set in sockets (alveoli) in the jaws. In venomous snakes, poison teeth develop in the upper jaw that have a canal connecting with the poison gland. Turtles have no teeth; their function is performed by the cutting edges of the horny jaw cases. The modern birds have no teeth, although fossil birds, such as the archaeopteryx and ichthyornis, had teeth in the alveoli of the jaws. In mammals the teeth are implanted in the alveoli of the jaws. There are no teeth in the adults of some edentates, in monotremes, and in whalebone whales.

The front teeth, or incisors, are scalpriform and are used for grasping and cutting food; there may be from one to five pairs in each half of the upper and lower jaws. Next to the incisors are the conelike canines (one pair), which have the function of grasping and tearing food (in predators) and are weapons of defense (in artiodactyl and perissodactyl omnivores and in some pinnipeds and cetaceans). The back teeth, or molars (up to eight pairs), are complex in shape and are used to grind food; molars are divided into premolars (three-four pairs), and true molars (three-four pairs), which have two or more roots. The shape and number of teeth in various species of mammals are more or less constant and are an important taxonomic characteristic. The composition and number of teeth in mammals are conventionally expressed by a dental formula, in which the number of teeth in one-half of the upper (numerator) and lower (denominator) jaw are indicated. Thus the dental formula of a dog is:

where i is incisors (incisivi), c canines (canini), pm premolars (praemolares), m molars (molares), and the total number of teeth 44. The initial letters of the Latin names are usually omitted:

In predators the number of teeth does not exceed 44; in artiodactyls and perissodactyls and in rodents the number of teeth varies greatly; some edentates have 18–20 teeth and some marsupials as many as 58; dentate cetaceans of the dolphin family may have up to 250 teeth. Different teeth develop unequally, for example, tusks, depending on their function. In predators the canines are well developed and in rodents, the incisors and molars; in rodents the canines are lost and in their place is a toothless space, the diastema. In some mammals the teeth are continuously growing, for example, the incisors of rodents. In lemurs and monkeys the number of teeth is reduced to 32 (Old World monkeys), which coincides with the number of teeth in man.

B. S. MATVEEV

In man, besides grinding, biting, and holding food, teeth play a major role in the pronunciation of certain sounds and affect facial appearance (especially the front teeth). The teeth are arranged in the jaws in the form of an arch. The teeth of the upper and lower jaws interlock according to their anatomical structures, creating the bite, or occlusion. The formula of the milk (deciduous) teeth is:

The formula of the permanent teeth is:

In these formulas the numbers above the horizontal line indicate the upper teeth, those below the line the lower teeth; the numbers to the left of the vertical line represent the right side of a dentition, those to the right of the vertical line the left side. Individual teeth are designated accordingly, for example: Teeth is the deciduous third upper left tooth and Teeth is the permanent third lower right.

Depending on their shape and function, teeth are divided into incisors, with flat crowns and cutting edges (for biting off food); canines, with a conelike crown (for tearing food); premolars (with a cuboid crown and two cusps on the chewing surface); and molars (with four-five cusps on the chewing surface), used for grinding food. The incisors, canines, and premolars (except the first and second upper) have one root, the first and second upper premolars and the lower molars have two roots, and the upper molars have three roots. An individual’s teeth are unique, corresponding to the shape of the face; this is of great significance for prosthodontics and for forensic medicine.

The tooth root is connected to the alveolus of the jaw by fibers of the root covering (pericementum, periodontal membrane), which are attached to the cementum at one end and to the wall of the alveolus at the other. The neck of the tooth is firmly implanted in the gum, the edge of which loosely adjoins the tooth, forming a crevice-like space (the pocket) that extends around the periphery of the tooth to a depth of 1–2 mm.

Each jaw has ten milk teeth, consisting of four incisors, two canines, and four molars and 16 permanent teeth, including four incisors, two canines, four premolars, and six molars.

The formation of teeth in the fetus begins in the fifth week of the embryonic period. When the infant is born, all 20 milk teeth are lying in the alveoli of the jaws. Some permanent teeth—the first molars, incisors, and canines—are also formed intrauterinely. The rest of the permanent teeth begin to form after birth. Eruption of teeth occurs twice in a human life: the milk teeth in the infant erupt between the ages of six months and 24–30 months and the permanent teeth between 5–6 and 14 years (the third molars or wisdom teeth between 17 and 25 years).

Diseases of the teeth may develop through the influence of unfavorable factors on the teeth or as a result of diseases of internal organs and systems of the body, or they may be congenital. Diseased teeth may become a focus of infection and may lead to serious disturbances in the activity of the gastrointestinal tract, the cardiovascular system, the kidneys, and joints. The most common affections of teeth are tartar, dental caries, pulpitis, and periodontitis.

A. I. RYBAKOV

The form and structure of human teeth are also studied by anthropologists. Teeth are preserved better than other bony fossil remains of ancient people, so that the study of teeth has played a large role in solving problems of the origin of man. In the process of man’s evolution his teeth have undergone reduction, resulting in smaller teeth and more simplified crowns. The canines and incisors were already smaller in prehominids and early hominids. The premolars and molars changed during the entire process of anthropogenesis under the influence of various factors—changes in diet, general changes in the dimensions and form of the facial cranium, and genetic factors.

The tooth structure of modern man shows considerable variation. Long periods of past isolation in different regions of the world has caused certain small morphological details to appear frequently in some groups and rarely in others. These differences are studied in ethnic anthropology and are used, along with other anthropological data, to solve problems of the origin of races and peoples.

REFERENCE

Zubov, A. A. Odontologiia. Moscow, 1968.

A. A. ZUBOV

What does it mean when you dream about teeth?

Something that one can “get one’s teeth into” relates to a power or control issue. Losing the teeth may reflect a loss of power as well as a loss of one’s grasp of life circumstances. Biting or being bitten suggests struggling for control in an aggressive manner. (See also Bite, Dentures).

Teeth

(dreams)
Dreaming about teeth is very common in all cultures and age groups. Most dreams about teeth leave people feeling uneasy and anxious. Consider the overall content and context of the dream and note if you are having dental problems before making interpretation. Teeth usually symbolize power and/or control. Animals use their teeth for defense and nourishment and show their teeth when angry. Humans often display similar behaviors. Look and see if you are losing or abusing power and control in any area of your life (especially if you are losing teeth in your dream). Old dream interpretations say that dreaming about teeth is a bad omen that suggests financial difficulties.