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bers of the first set, the primary teeth (deciduous teeth), usually erupt through the gums (gingiva) at regular intervals between the ages of six months and two to four years. The ten primary teeth in each jaw are located from the midline toward the sides in the following sequence: central incisor, lateral incisor, cuspid (canine), first molar, and second molar.

The primary teeth are usually shed in the same order they appeared, after their roots are resorbed. Then, the secondary (permanent) teeth push the primary teeth out of their sockets. This secondary set consists of thirty-two teeth—sixteen in each jaw—and they are arranged from the midline as follows: central incisor, lateral incisor, cuspid (canine), first bicuspid (premolar), second bicuspid (premolar), first molar, second molar, and third molar (fig. 17.9). Table 17.2 summarizes the types and numbers of primary and secondary teeth.

The permanent teeth usually begin to erupt at six years, but the set may not be completed until the third molars appear between seventeen and twenty-five years. Sometimes these third molars, which are also called wisdom teeth, become wedged in abnormal positions within the jaws and fail to erupt. Such teeth are said to be impacted, and must be removed to alleviate pain.

The teeth break food into smaller pieces, which begins mechanical digestion. Chewing increases the surface area of the food particles, enabling digestive enzymes to interact more effectively with nutrient molecules.

Different teeth are adapted to handle food in different ways. The incisors are chisel-shaped, and their sharp edges bite off large pieces of food. The cuspids are cone-shaped, and they grasp and tear food. The bicuspids and molars have somewhat flattened surfaces and are specialized for grinding food particles.

Each tooth consists of two main portions—the crown, which projects beyond the gum, and the root, which is anchored to the alveolar process of the jaw. The region where these portions meet is called the neck of the tooth. Glossy, white enamel covers the crown. Enamel mainly consists of calcium salts and is the hardest substance in the body. Unfortunately, if abrasive action or injury damages enamel, it is not replaced. Enamel also tends to wear away with age.

The bulk of a tooth beneath the enamel is composed of a living cellular tissue called dentin, a substance much like bone, but somewhat harder. Dentin, in turn, surrounds the tooth's central cavity (pulp cavity), which contains a combination of blood vessels, nerves, and connective tissue called pulp. Blood vessels and nerves reach this cavity through tubular root canals, which extend upward into the root. Tooth loss is most often associated with diseases of the gums (gingivitis) and the dental pulp (endodontitis).

The root is enclosed by a thin layer of bonelike material called cementum, which is surrounded by a periodontal ligament (periodontal membrane). This ligament contains bundles of thick collagenous fibers, which pass

Incisors

Cuspid

Incisors

Cuspid

Bicuspids (premolars)

Molars

Molars

Bicuspids (premolars)

Cuspid

Incisors

Bicuspids (premolars)

Molars

Molars

Bicuspids (premolars)

Cuspid

Incisors

Lateral

First premolar

Second premolar

Lateral

First premolar

Second premolar

Second Premolar

Central Lateral First Second incisor incisor premolar premolar

(b) Cuspid

Figure 17.9

(a) The secondary teeth of the upper and lower jaws. (b) Anterior view of the secondary teeth.

Central Lateral First Second incisor incisor premolar premolar

(b) Cuspid

Figure 17.9

(a) The secondary teeth of the upper and lower jaws. (b) Anterior view of the secondary teeth.

between the cementum and the bone of the alveolar process, firmly attaching the tooth to the jaw. The ligament also contains blood vessels and nerves near the surface of the cementum-covered root (fig. 17.10).

The mouth parts and their functions are summarized in table 17.3. Clinical Application 17.1 describes the effect of bacteria on teeth.

Enamel

Dentin

Pulp cavity

Enamel

Dentin

Pulp cavity

Medical Images Genitourinary

Crown

Gingiva

Alveolar process

Root canal

Periodontal ligament

Cementum

Figure 17.10

A section of a cuspid tooth.

Crown

Gingiva

Alveolar process

Root canal

Periodontal ligament

Cementum

Figure 17.10

A section of a cuspid tooth.

How do primary teeth differ from secondary teeth?

How are types of teeth adapted to provide specialized functions?

Describe the structure of a tooth.

Explain how a tooth is attached to the bone of the jaw.

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