Darius Jones, age 9, of Winston‑Salem, N.C., for his question:
WHAT ARE TEETH MADE OF?
Each person grows two sets of teeth. The first is called the primary set which appears early in childhood. This set is replaced by permanent teeth that begin to appear when a child is about six years old. The first primary teeth to appear are the two central incisors on the lower jaw and they are followed almost immediately by two on the upper jaw.
Teeth are the bonelike structures in your upper and lower jaws that are used to chew food and form some of the sounds of speech.
When a baby is born, hidden in his gums are the seeds or buds of all his primary teeth as well as those of four permanent molars.
AS the child grows, the teeth start to form around the buds. Deposits of calcium from food, and especially milk, gradually build and harden to form the body and coverings of the teeth.
The calcium deposits of the teeth develop in layers or rings, much as the layers that form the rings of a tree trunk.
Teeth are composed largely of calcium and phosphorous. Rich supplies of these elements can be found in green leafy vegetables as well as milk.
Four different kinds of tissue make up a tooth itself: enamel, dentin, dental pulp and cementum.
Enamel which covers the crown of each tooth is the hardest material in the entire body. The dentin is an ivorylike substance that lies beneath the enamel and extends down into the root. It makes up the body of the tooth.
Next comes the dental pulp which fills the central portion of a tooth. Here you'll find the tooth's tissues, nerves and blood vessels.
The cementum is a bonelike protective material that covers the root.
The tooth's periodontal membrane covers the cementum and also the socket in the jaw. This material cushions the tooth and holds it in place.
The socket in the jaw where the root fits is called the alveolus.
This is bone material and is actually part of the jaw. A tooth's roots fit firmly into the alveolus.
Incisors, the front teeth, worklike scissors or knives to bite off pieces of food. Next back are the canines which are strong, pointed teeth. These work much like the sharp tines of a fork to tear food into small pieces.
Behind the canines are the bicuspids which work much like nutcrackers.
They crush food to a coarse, grainy mass. In the back are the molars which shred and grind food.