tooth shell: see Mollusca.

tooth: see teeth.

or tooth decay

Localized disease that causes decay and cavities in teeth. It begins at the tooth's surface and may penetrate the dentin and the pulp cavity. Microorganisms in the mouth are believed to consume sugars and produce acids that eat away at tooth enamel. The dentin's protein structure is then destroyed by enzymes. Diet, general health, structural tooth defects, and heredity affect the risk of having caries. Prevention involves avoiding excessive sweets, brushing and flossing the teeth, and having regular dental care. Treatment includes restoration of teeth with cavities. Fluoridation of water can reduce the occurrence of caries by as much as 65percnt.

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Any of the hard structures in the mouth used for biting and chewing and in speech. Each consists of a crown above the gum and one or more roots below it, embedded in the jaw. Its inner pulp contains the blood and nerve supply for the bonelike dentin, covered in the crown by enamel, the hardest tissue in the body. Twenty primary (baby) teeth come in by age 2 1/2 and fall out between ages 5 and 13 to be replaced by 32 permanent teeth. The incisors, in front, are shaped mostly for biting, the pointed canines for tearing, and the premolars and molars for grinding food. The teeth are subject to caries (decay), caused by acid from bacteria in plaque, a yellowish film that builds up on teeth. Misalignment of teeth between the upper and lower jaws can grind down the teeth and cause problems in chewing. Elsewhere, it is a cosmetic problem. Both can be treated with braces. Seealso dentistry.

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