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Terms & Anatomy

Terms & Anatomy

Kinds of teeth

In the adult mouth, there are up to 32 permanent teeth, 16 in each jaw. Young children have only 20 primary teeth, which are replaced by the permanent teeth. The main function of your teeth is to break up food into fragments small enough for you to swallow and digest. Human beings are omnivores—that is, they eat both animal and vegetable food. Human teeth thus have two basic shapes.

The six front teeth in each jaw have single, sharp edges, like knives. The edges of the four incisors are straight, for cutting off pieces of food. The edges of the two canines come to a point, for tearing food as well. Each of these teeth has a single root.

The ten back teeth in each jaw—five on each side—have wide chewing surfaces, to grind food. Each chewing surface contains two or more low mounds, called cusps, separated by hollows and grooves. On each side of the jaw are two premolars (sometimes called bicuspids) and up to three molars. The premolars are smaller than the molars, and have one or two roots. The upper molars usually have three roots; the lower molars have two or three. The additional roots brace these teeth against the heaviest pressures of chewing.

The parts of your teeth most susceptible to decay are the chewing surfaces of the back teeth, surfaces where adjacent teeth meet, and surfaces nearest the gumline.

Mandible: The mandible is the lower jawbone—a single, arch-shaped bone. It is the densest and strongest bone in your skull. Like other bones, it is fairly flexible in childhood and becomes more rigid during adolescence. When your mouth opens and closes, only the mandible moves.

Palatal bones: Like other bones of the cranium, the two bones of the palate are joined together by a thin line of connective tissue called a suture. During infancy and childhood, the suture is flexible and easily stretched, but it becomes stronger and more rigid in adolescence. This change has important implications for orthodontic treatment.

Occlusion: When your jaws close, the teeth meet in occlusion, or bite. A proper occlusion is preferred for more effective chewing. Your upper front teeth should overlap the lowers, and the biting edges of the lower teeth should lightly touch the inner surfaces of the uppers. Your back teeth, the premolars and molars, should meet evenly. The chewing surfaces of the lower teeth should be slightly inside the uppers, so that their outer cusps mesh with the central grooves of the uppers. Ideally, the lower molars and premolars should also be positioned slightly forward of the corresponding upper teeth.

Crown: One of two principal parts of each tooth. The crown is the part that’s visible above the gum, and contains the hard biting surfaces. It has an outer layer of enamel, which protects it from wear and decay. Within these outer layers is the main structure of the tooth, the dentin, which contains numerous fine channels called tubules. Inside the tubules are threadlike extensions of living cells called odontoblasts.

Root: One of two principal parts of each tooth. The root is the part normally located below the gum; it rests in a socket in the bone of the jaw.

Pulp: At the core of each tooth is a channel that extends from mid-crown to root. It contains soft tissue called pulp. The outer layer of the pulp is made up of the odontoblasts, or cells, that protrude into the dentin tubules. The rest is composed largely of nerves and blood vessels. The blood vessels carry nourishment to the tooth, and the nerves are sensitive to pressure, heat, cold, electricity, and certain chemicals.

Cementum: Cementum is a bonelike material that protects the root of your teeth.

Plaque: Plaque is a thin film that develops naturally on the teeth. It is formed by colonizing bacteria and must be kept from adhering to your teeth for more than 24 hours to maintain optimal dental health.