Opening and closing the jaw is a relatively simple movement compared with the joint movement that occurs while grinding the teeth during the act of chewing. During grinding, one jaw joint slides forward, while one slides back. This movement can be felt by placing your fingers in front of your ears while making grinding movements with your teeth. If no problems exist, this motion is performed with ease, and quietly. However, audible clicking or popping noises or pain may indicate that the joints are not functioning normally. (See TMJ in Part Two.)

The Teeth
Teeth-they're not just for chewing. Our teeth perform important functions such as helping us to eat and speak, and keeping our facial muscles from sinking in. Teeth are not for opening bottles, holding nails and pins, or biting fingernails or pencils. If mistreated, teeth are subject to breakage, cavities, yellowing, and other forms of degeneration. Caring for your teeth requires that you see the mouth and teeth as important organs of your body.

Teeth are an integral and growing part of the body. The same blood supply that goes to the heart takes oxygen and nutrients to every tooth. And, as anyone who has ever had a toothache knows, the teeth are very much alive with nerves.

The better we understand the structure and development of teeth, the greater our appreciation of them will be. Practicing proper oral hygiene and maintaining good nutritional habits will help keep teeth and gums healthy. To better maintain our teeth, let's begin with a discussion of their structure and development.

Structure
The same structure can be seen in all human teeth, whether they are baby teeth (also known as deciduous or milk teeth) or permanent teeth. The care given to the baby teeth will be reflected by the adult teeth. Preventive care must begin even before the first tooth appears because teeth begin to develop before they erupt. Parents are now aware, for instance, of the damage done to teeth when infants are allowed to sleep with bottles in their mouths. (See Bottle Mouth Syndrome in Part Two.)

As seen in Figure 1.4, there are three main parts of a tooth-the crown, neck, and root. When you look at a tooth, the top part, called the crown, is visible. The root is the part that is imbedded in bone and covered by the gums, unless gum disease exists. The junction between the crown and the roots is called the neck. The parts of a tooth are composed of various materials.

The outer surface of the tooth, the enamel, is a hard layer covering the crown of the tooth and the upper part of the neck. The function of enamel is to resist abrasive wear and protect the tooth from damage and pain. It is the hardest tissue of the body and is composed almost entirely (97 percent) of inorganic salts. Inorganic salts, in general, are mineral constituents of the body and play specific roles in the functions of cells.

Most of the inner bulk of the tooth is made of dentin. Dentin is 67 percent inorganic salts and is not as strong as enamel. Specialized cells in dentin called odontoblasts form new dentin from minerals transported by the blood. Small tubelike structures in dentin transmit pain sensations to the nerves in the pulp, the soft tissue containing the nerve and blood supply of the tooth. When dentin is exposed by thinning or other forms of damage to enamel, the tooth may become sensitive. When tooth decay or a cavity reaches the pulp, treatment of the nerve or loss of the tooth may be inevitable.