Before the nutrients from your food can be absorbed into the bloodstream and taken to your body cells, the food must be broken down both physically and chemically. This process is called digestion and begins at the moment you put food into your mouth. Teeth are used to cut and grind food as you chew. Small pieces of food form a pulp, which is rolled into a ball by the tongue and mixed with a watery digestive fluid called saliva. Saliva is produced by salivary glands in the cheeks and under the tongue. It contains the first of a series of digestive enzymes that begin to break down the starch in food.
Babies are usually born without teeth. During their first two years of life, as they change from an all-milk diet to one that contains solid foods, they grow a set of 20 "milk teeth". At the age of about 6 years, a child's milk teeth begin to be replaced by permanent adult teeth. A full set of adult teeth numbers 32, although some people never develop their four back molars, the wisdom teeth.
Only the upper part of a tooth, the crown is a visible outside the gums. Most of the tooth is root and is concealed beneath the surface. The root may be three times as long as the crown. The root is anchored firmly in the jawbone and holds the tooth in place. A layer of tough, white enamel covers the crown. Enamel is the hardest substance found in the body and it gives teeth the strength they need to chew the hardest foods. Beneath the enamel is a softer bone-like dentin, each tooth has a cavity that contains blood vessels and nerves.
Tooth enamel is tough to stand up to most foods, but is easily attacked by the acids produced by some bacteria. These bacteria live on dirty teeth and feed on the sugar in sweet foods. They form into a thin, furry layer covering tooth, called plaque, and they produce the acid that may soften enamel. Enamel has no nerves, and a person may be unaware of the damage. Toothache occurs when food or drink penetrates the enamel to the dentin and the sensitive nerves inside.
One way to strengthen the tooth enamel against decay is the use of the chemical fluoride. In recent decades, fluoride has been used in toothpastes, and in many places it has been added to the water supply. This, and improved dental care, has reduced tooth decay in the developed world by have over the last twenty-five years.
The root of a molar tooth is surrounded by hard cement that binds it firmly in place in the jawbone. Above the gum, the soft dentin around the pulp cavity is covered by hard enamel. Enamel is made mostly of calcium carbonate and calcium phosphate, whereas dentin is similar to bone.
A full set of adult teeth contains 32 teeth. There are 8 flat incisors at the front for cutting food, 4 pointed canine teeth at the side for tearing and 8 premolars and 12 molars at the rear of the mouth with uneven surfaces that grind and crush food.
Types of Teeth
- Chewing raw vegetables and fruit keeps gums healthy because it stimulates them and encourages blood to flow through the veins in the gums.
- The parts of the jawbones in which the teeth sit are malleable and can change their shape. This is why dentists and orthodontists can move teeth around to treat some tooth diseases.
- The beneficial effects of fluoride on teeth were discovered when it was observed that in some areas where the water contained high levels of fluoride, people developed mottled brown spots on their teeth yet rarely suffered from tooth decay. The mottling was caused by fluoride deposits, which are resistant to decay.