teeth hard, calcified structures embedded in the bone of the jaws of vertebrates that perform the primary function of mastication. Humans and most other mammals have a temporary set of teeth, the deciduous, or milk, teeth; in humans, they usually erupt between the 6th and 24th months. These number 20 in all: 2 central incisors, 2 lateral incisors, 2 canines, and 4 premolars in each jaw. At about six years of age, the preliminary teeth begin to be shed as the permanent set replaces them. The last of the permanent teeth (wisdom teeth) may not appear until the 25th year, and in some persons do not erupt at all. The permanent teeth generally number 32 in all: 4 incisors, 2 canines, 4 bicuspids, and 4 (or 6, if wisdom teeth develop) molars in each jaw. Human canines are the smallest found in any mammal.
Among all mammals, the tooth consists of a crown, the portion visible in the mouth, and one or more roots embedded in a gum socket. The portion of the gum surrounding the root, known as the periodontal membrane, cushions the tooth in its bony socket. The jawbone serves as a firm anchor for the root. The center of the crown is filled with soft, pulpy tissue containing blood vessels and nerves; this tissue extends to the tip of the root by means of a canal. Surrounding the pulp and making up the greater bulk of the tooth is a hard, bony substance, dentin. The root portion has an overlayer of cementum, while the crown portion has an additional layer of enamel, the hardest substance in the body. Most nonmammalian vertebrates do not have the outer layer of enamel on their teeth, but instead have a substance known as vitrodentine, similar to dentine, though much harder.
Proper diet is necessary for the development and maintenance of sound teeth, especially sufficient calcium, phosphorus, and vitamins D and C. The most common disorder that affects the teeth is dental caries (tooth decay). A widely accepted explanation of the process of tooth decay is that salivary bacteria convert carbohydrate particles in the mouth into lactic acid, which attacks the enamel, dentin, and, if left untreated, the pulp of the teeth. Regular cleansing and semiannual dental examinations (see dentistry ) are important in preventing dental caries and gum disorders. Fluoridation of public water supplies and use of fluoride toothpastes also help prevent caries. In the study of fossil remains done in paleontology and physical anthropology, teeth are the most frequently found remains, a testament to their high mineral content and resistance to deterioration over time. See dentition .
