A tooth is a hard structure found in the jaw of some animals. It is used to chew food and, in some animals, like the carnivores, as a weapon. Teeth are generally partly covered by the gums.
Types of tooth:
- Molar, used for grinding up foods
- Carnassial, slicing food. In carnivores only.
- Premolar, similar to molars but smaller and sometimes called "bicuspids"
- Canine, used for tearing apart foods and sometimes called "cuspids"
- Incisor, used for cutting foods
Human teeth consist of four tissues:
- Enamel is a hard outer layer consisting of calcium and phosphate.
- Dentine is the inner layer, the bulk of the tooth.
- Pulp is the core, containing nerves and blood vessels.
- Cementum is the thin layer around the root; a bone-like material which connects the teeth to the jaw.
Humans have 32 teeth evenly distributed across the quadrants. Each quadrant of 8 teeth consists of 2 incisors, 1 cuspid, 2 bicuspids and 3 molars. The last molar of each quadrant, commonly referred to as wisdom teeth, may or may not erupt.
Humans grow two sets of teeth, though some animals grow more. Sharks grow a new set of teeth every two weeks. Some other animals grow just one set. Rodent teeth grow continually and wear off to a relatively standard length. In humans, the first (or primary, or deciduous) set of teeth appears at about six months of age. This is known as teething and can be quite painful for an infant. The second, permanent set is formed between the ages of six and twelve years. A new tooth forms underneath the old one, pushing it out of the jaw. This set can last for life if cared for properly.
Teeth are among the most distinctive features of different mammal species, and one that fossilizes well. Paleontologists use them to identify fossil species and, often, their relationships. The shape of the teeth is related to the animal's diet, as well as its evolutionary descent. For example, herbivore diets are harder to digest thus herbivores have more molars for chewing. Carnivores need canines to kill and tear and since meat is easier to digest, can swallow without the need for molars to chew the food well.
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2 Other Animals
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Plaque is a soft white layer which forms on teeth, containing large amounts of bacteria of various types, particularly Streptococcus mutans. Left unchecked for a few days plaque will harden, especially near the gums, forming tartar.
Certain bacteria in the mouth live off the remains of foods, especially sugars. In the absence of oxygen they produce lactic acid, which dissolves the calcium and phosphorus in the enamel in a process known as demineralisation. Enamel demineralisation takes place below the critical pH of about 5.5
Saliva gradually neutralises the acids causing the pH of the tooth surface to rise above the critical pH. This causes 'remineralisation', the return of the dissolved minerals to the enamel. If there is sufficient time between the intake of foods (two to three hours) and the damage is limited the teeth can repair themselves.
Dental caries (cavitation) occurs when over a period of time the process of demineralisation is greater than remineralisation. Attempts to prevent dental caries involve reducing the factors that cause demineralisation, and increasing the factors leading to remineralisation. Unchecked demineralisatin leads to cavities, which may penetrate the underlying dentine to the tooth's nerve-rich pulp and lead to toothache.
In moderation, fluoride is known to protect the teeth against caries. It toughens the teeth by replacing the hydroxyapatite and carbonated hydroxyapatite minerals of which the enamel is made with fluorapatite, which is harder. It also reduces the production of acids by bacteria in the mouth by reducing their ability to metabolize sugars. The addition of fluoride (sodium monofluorophosphate) to toothpaste is now very common, and may explain the decline in dental caries in the Western world in the past 30 years.
Some believe that a diet rich in fluorine salts, particularly in childhood, can lead to a stronger enamel which is less susceptible to decay. Fluoridation of drinking water remains a controversial issue. However, in many parts of the world, the natural water supply may be sufficiently rich in fluorides to supply the needs of children without additional sources being required.
Caries may be treated by filling cavities with a long-lasting material. This was, traditionally, achieved using gold or a compound of metals called amalgam, which contains mercury. For cosmetic reasons, and because it is thought mercury may seep from fillings into the circulation over time, a ceramic or other white filler may be preferred to amalgam. As a last resort, teeth affected by caries may be extracted, preferably under local or general anaesthetic.
Some foods may protect against caries. Milk and especially cheese appear to be able to raise pH values in the mouth and so reduce tooth exposure to acid. Milk and cheese are both rich in calcium and phosphate and may also encourage remineralisation. Plus, they may increase saliva production which increases the pH level in the mouth. Foods high in fibre may also help to increase the flow of saliva. Unsweetened (sugar free) chewing gum stimulates saliva production, and helps to clean the surface of the tooth (even sugary gum may be helpful, since the sugar dissolves out very quickly).
Sugars are commonly associated with dental caries. Other carbohydrates, especially cooked starches, eg crisps, may also damage teeth, although to a much lesser degree. This is because starch is not an ideal food for the bacteria. It has to be converted (by enzymes in saliva) first.
Sucrose (table sugar) is most commonly associated with caries, although glucose and maltose seem equally cariogenic (likely to cause caries). The amount of sugar consumed at any one time is less important than how often sugar containing foods and drinks are consumed. The more frequently sugars are consumed, the greater the time during which the tooth is exposed to low pH levels, at which demineralisation occurs. It is important therefore to try to encourage infrequent consumption of food and drinks containing sugar so that teeth have a chance to repair themselves. Obviously, limiting sugar-containing foods and drinks to meal times is one way to reduce the incidence of caries.
Fresh fruit (and fruit juices) contains not only sugars, but some (oranges, lemons, limes, apples) also contain acids which lower the pH.
Another factor which affects the risk of developing caries is the stickiness of foods. Some foods or sweets may stick to the teeth and so reduce the pH in the mouth for an extended time, particularly if they are sugary. It is important that teeth are cleaned at least once a day, preferably with a toothbrush and fluoride toothpaste, to remove any food sticking to the teeth. Regular brushing and the use of dental floss also removes the dental plaque coating the tooth surface.