The primary function of teeth is to break down food ready for digestion. Teeth also help us to pronounce sounds clearly and give the face shape and definition. Each of us grows two sets of teeth in a lifetime, the second set gradually replacing the first during childhood. The gums help to keep the teeth firmly in the jaw and protect their roots from decay. Teeth and gums are vulnerable to the build-up of plaque (a sticky mixture of bacteria, saliva, and food particles), which causes decay and gum disease.
At the beginning of the 20th century, it was common for people to have lost all of their secondary (adult) teeth by the age of 60 as a result of tooth decay and gum disease. Today, because of the advances that have been made in dental health care, improvements in nutrition, and widespread fluoridation of water, many people keep their secondary teeth for life.
The part of the tooth that protrudes from the gum is called the crown; the part beneath the gumline is called the root. The crown is covered with a protective layer of enamel, the hardest substance in the body. Enamel is composed of rod-shaped calcium salt crystals and cannot renew itself. As a result, the enamel may eventually become worn down from abrasion, or it may be damaged by acid present in food and drinks or produced by bacteria in plaque. If the enamel is worn through, a cavity is formed. This can be repaired with a dental filling.
Beneath the enamel layer lies the dentine, a hard ivory-like substance that surrounds the pulp cavity. This central cavity contains nerves and blood vessels that extend through tiny channels into the dentine, making the dentine sensitive to heat, cold, and pain.
About two-thirds of a tooth is made up of the root, buried below the gumline in a deep socket in the jaw. Each root is attached to the jawbone by a periodontal ligament, which cushions the root in its socket while the teeth are grinding down food.
Teeth have various shapes and sizes to enable them to hold, cut, tear, and chew food efficiently. For example, to enable teeth to grind down food effectively, the contoured chewing surfaces of the upper and lower back teeth meet and are similar in shape. The actions of the teeth are controlled by the upper and lower jaws, which are able to clamp the teeth together with great force – up to 500 kg/sq. cm (7,000 lb/sq. in) – with the help of four powerful sets of muscles.
The bone and periodontal ligaments that support the teeth in the jaw are covered by a layer of protective tissue known as the gums or gingiva. Healthy gums are pink or brown and firm. They form a tight seal around the neck of the tooth, preventing food particles and plaque from invading underlying tissues and the root of the tooth.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.