|Tooth, plural teeth, are hard structures found in the jaws
of many of the higher animals. They have various structures to allow them
to fulfil different purposes. They are mostly used to chew food and in some
animals, particularly carnivores, as a weapon. Teeth are generally partly
covered by gums.
Types of teeth
* Molars are used for grinding up foods
The form teeth take and their mode of development in a species is called the species' dentition. Dentists sometimes refer to the inner surface of teeth as the lingual surface (meaning towards the tongue), and the outer surface as the labial surface (meaning towards the lips) or "buccal" (meaning towards the cheek). Other terms are "Mesial", or toward the midline, "Distal", which is away from the midline, "Occlusal", the top surface, or incisal, the cutting surfaces.
Humans grow two sets of teeth, though some animals grow many more: sharks grow a new set of teeth every two weeks. Some other animals grow just one set. Rodent teeth grow and wear away continually through the animal's gnawing, maintaining constant length.
In humans, the first (a.k.a. milk, primary or deciduous) set of teeth appears at about six months of age. This stage is known as teething and can be quite painful for an infant. Human children have 20 milk teeth evenly distributed across the quadrants. Each quadrant of 5 teeth consists of:
* central incisor
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 through regular brushing with water or toothpaste.
Types of tissue in teeth
* Enamel is a hard outer layer consisting of calcium and phosphate.
* Teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata.
Adult humans have 32 teeth evenly distributed across the quadrants. Each quadrant of 8 teeth consists:
* central incisor
The last molar of each quadrant (i.e. the third molar and commonly referred
to as wisdom teeth) may or may not erupt.
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.
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.