Jason Maxon, age 12, of Hutchinson, Kan., for his question:
HOW MANY TEETH DOES A PERSON HAVE?
Teeth are hard, calcified structures attached to the upper and lower jaws and are used primarily for chewing. In humans, one set of 20 teeth, commonly called baby teeth, is produced for use during early jaw development, and a second set of 32 larger permanent teeth grow as the jaw matures.
Baby teeth are more properly called deciduous teeth. As a result of the growth and enlargement of the jaw, the roots of these deciduous teeth separate, allowing space for the larger permanent teeth to develop between them.
The pressure of developing permanent teeth causes the tissues of the jaw to resorb, or suck up, the roots of the corresponding deciduous teeth, leaving only the crowns. As each permanent tooth erupts, it dislodges the crown of the deciduous tooth.
Human teeth also serve major functions other than chewing. The teeth are directly involved in the process of speech, acting as a brace against which the tongue presses in the formation of certain sounds.
The teeth also affect the dimensions and the expression of the face, the appearance of which may be unpleasantly modified by the loss of neighboring teeth or by any irregularity in tooth growth or coloring.
Human teeth consist of an external portion, called the crown, and a root that is embedded within the jaw. The outer layer of the crown is composed of calcified tissue known as enamel, which is the hardest substance in the body.
Inside the enamel is the dentin, a bonelike substance extending from the inner surface of the enamel into the jaw to form the root. Covering the dentin of the root is a thin layer of hard tissue called cementum. The roots are held in place by elastic fibers that constitute the periodontal membrane.
The 20 primary or deciduous teeth begin to form about seven months before birth. A baby seems to have no teeth when he is born, but hidden in his gums are the seeds or buds of all his deciduous teeth, as well as those of four permanent molars.
As the infant grows, the teeth begin to form around the buds. Deposits of calcium from food, particularly milk, gradually build and harden to form the body and coverings of the teeth. The deposits develop in layers or rings, much like the layers that form the rings of a tree trunk.
The first baby teeth to push through the gums are the incisors. These are the cutting, biting or shearing teeth. They grow in the front center of the jaws.
Then, in order, come the first molars, the canines and the second molars.
The first of the 32 permanent teeth to appear are the first, or 6 year molars. Then come the incisors, the canines, the bicuspids and the second and third molars.