Like most parts of the body, people usually don't give much thought to the teeth unless they're causing problems. Canines, molars, impacted wisdom teeth and cavities keep dentists busy.

Cavities are perhaps the most common infectious disease in children. Residue of milk or sugary drinks feed bacteria that make acid, which eats away at the hard enamel surface, exposing the tender nerve-laden pulp inside each tooth.

Ouch!

At moments like these, people are forcibly reminded that teeth, despite their hard exteriors that apparently have more in common with a stone than cells, are alive.

Let's start from the outside. Tooth enamel is mainly minerals, the closest you'll ever come to having rocks in your head (or your body, unless you're unfortunate enough to have kidney stones). Enamel is made of various calcium compounds, such as calcium phosphate.

While enamel has no cells, it's made by cells called ameloblasts. These ameloblasts secrete enamel-forming proteins that help turn atoms of calcium, phosphorus and other minerals into crystals in the newly forming enamel. The crystals grow in long, parallel lines called prisms. About 95 percent of enamel is mineral.

Just inside the enamel is a substance called dentin, which is somewhat less mineralized than enamel. That means dentin is not as hard as enamel, and even more vulnerable to cavities. Most of the tooth's volume is dentin, enamel is a thin covering.

Pulp is at the center of the tooth. It's a small soft mass of tissue that descends into the root of the tooth. It contains blood vessels, connective tissue and a nerve ending. The nerve senses hot or cold, possibly causing pain when hot or cold liquids pass through microscopic pores in the enamel and reach the nerve.

In advanced tooth decay, the pulp may become damaged or die. The tooth can be salvaged through a root canal. The nerve may be removed, because it is not necessary for the tooth. Damaged pulp may become infected, spreading down to the roots of the tooth where it anchors into the jaw. This causes an extremely painful abscess, which can loosen the tooth so it falls out.

The mummy of Pharaoh Amenhotep III has extensive tooth cavities and abscesses; one of his unanticipated claims to fame. England's Queen Elizabeth I, who was very vain, would have been mortified to learn of the attention given to her notoriously bad teeth, so decayed they were black, and her speech difficult to understand.

Good Queen Bess' rotten choppers were the subject of an article in the Nov. 25, 1989, British Dental Journal by A.S. Hargreaves.

Besides being painful and disfiguring, tooth and periodontal diseases can spread infections to other parts of the body.

- Periodontal disease is connected with respiratory problems, researchers from the University of Buffalo found in a study published in the Journal of Periodontology in January 2001.

- A September 2003 study found a link between periodontal (gum) disease and heart disease. The study was published in the journal Stroke: Journal of the American Heart Association.

- Periodontal disease was linked to complications in pregnancy such as premature birth, in a study released Sept. 20, 2005, by researchers at the University of North Carolina at Chapel Hill and the University of Kentucky.

Regular tooth brushing is a key to avoiding these unpleasant scenarios. However, brushing can be too vigorous and damage the teeth, according to a June 2003 report published in the Journal of Clinical Periodontology by researchers from Newcastle University upon Tyne in Britain, and Britain's Centre for Health Services Research.

According to the research, a brushing time of two minutes with a pressure of 150 grams, about the weight of an orange, is the best for the average person.

And that's the tooth, er, truth.

Contact staff writer Bradley J. Fikes at bfikes@nctimes.com or (760) 739-6641.