January 21, 2007

The Human Teeth


Teeth (singular, tooth) are structures found in the jaws of many vertebrates. The primary function of teeth is to tear and chew food, and in some animals, particularly carnivores, for fighting and/or defence. The roots of the teeth are covered by gums. Adult teeth naturally darken with age as the pulp within the tooth shrinks and dentin is deposited in its place.

Teeth are among the most distinctive features of mammal species and fossils. Paleontologists use them to identify fossil species and their relationships. The shape of the teeth is related to the animal's diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing. Carnivores need canines to kill and tear meat.

While humans develop two sets of teeth throughout life (diphyodont), some animals develop only one set (monophyodont) or develop many (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks which is very useful since they munch a lot of food to keep alive in the sea like turtules and porpoises, and the wear and tear would mean they would die of starvation. Rodent teeth grow and wear away continually through the animal's gnawing, maintaining approximately constant length.

Humans are diphyodont, meaning that they develop two sets of teeth throughout life. The first set (the "baby," "milk," "primary" or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal eruption of teeth starting at about six months is known as teething and can be quite painful for an infant. Human children have 20 deciduous teeth evenly distributed across the mouth's quadrants. Each quadrant of five teeth has a:

central incisor
lateral incisor
cuspid (canine)
first molar
second molar

The second, permanent set of teeth consists of 32 teeth. Twenty-eight of them appear between the ages of about 6 and 12 years. Secondary teeth do not push deciduous teeth out of their sockets; instead, a group of cells (odontoclasts) forms in front of tip of second tooth and dissolves the base of first tooth. Finally, the first tooth is held in place only by tissues of gum. Deciduous molars are replaced by premolars. The third molars (the wisdom teeth) are the final teeth to erupt, usually around age 20. However, it is common for the wisdom teeth not to erupt at all (they are congenitally missing); this is often the case in small jaws without room to support the extra teeth. It is possible, though rare, for a person to have fourth molars, and there have been instances where fifth molars have been present in the dentition.
Adapted from WIKIPEDIA
People use their teeth to bite and chew food - they are the first step in the digestion of food. The long, sharp canine teeth tear up food (like meat). The wide, flat molars grind and mash up food. While we chew food, the tongue pushes the food to the teeth and saliva helps digestion and wets the food. Teeth also help us say certain sounds.
People have two sets of teeth in their lives, the primary teeth (also called the baby, milk or deciduous teeth) and the permanent teeth (also called the adult or secondary teeth). Children have 20 primary teeth; they are replaced by the permanent teeth by about age 13. Adults have 32 permanent teeth.
Primary Teeth:Most babies are born with no visible teeth -- the teeth are forming inside the gums. The 20 primary teeth (also called baby teeth or first teeth) erupt (poke through the gums) over the time from when a baby is from about 6 months to a year old.
Permanent Teeth:Primary teeth fall out and are replaced by 32 permanent teeth (also called the adult teeth). This happens over the time from when a child is from about 6 to 14 years old.
As a permanent tooth forms under the gums and in the jawbone, the roots of the primary tooth it is replacing dissolve. Then the primary tooth becomes loose and falls out. The permanent tooth will fill the space.
Wisdom Teeth:
Wisdom teeth (also called the third molars) are molars that usually erupt from the ages of 17 to
Some meaning
Cementum - a layer of tough, yellowish, bone-like tissue that covers the root of a tooth. It helps hold the tooth in the socket. The cementum contains the periodontal membrane.
Crown - the visible part of a tooth.
Dentin - the hard but porous tissue located under both the enamel and cementum of the tooth. Dentin is harder than bone.
Enamel - the tough, shiny, white outer surface of the tooth.
Gums - the soft tissue that surrounds the base of the teeth.
Nerves - nerves transmit signals (conveying messages like hot, cold, or pain) to and from the brain.
Periodontal membrane/ligament - the fleshy tissue between tooth and the tooth socket; it holds the tooth in place. The fibers of the periodontal membrane are embedded within the cementum.
Pulp - the soft center of the tooth. The pulp contains blood vessels and nerves; it nourishes the dentin.
Root - the anchor of a tooth that extends into the jawbone. The number of roots ranges from one to three.
Adapted from: EnchantedLearning.