Question: With Halloween approaching, I'd like to ask a creepy question: Why does a body stiffen up after death from rigor mortis? - A.L., Colorado Springs, Colo.

Answer: Halloween approaches, and little ghouls and goblins will soon be scurrying door-to-door in the early darkness of night in search of a treat. Spirits are in the air, fresh as the smell of autumn's falling leaves.

For centuries upon centuries, the mystery of death and what lies beyond has fascinated man. From a biological point of view, death is a much simpler concept. It's not an event, but a process. This is because the various tissues and organs in the living body die at different rates.

Rigor mortis is due to a complex chemical reaction. During life, our muscles require energy (fuel). They need lots of oxygen; however, during strenuous work or exercise, this is in short supply. Heavy muscle use causes a buildup of lactic acid. It contributes to the feeling of muscle fatigue and "burn." Haven't we all felt that "burn" in our legs with climbing stairs?

During life, the lactic acid quickly dissipates once we rest our muscles. In death, this cannot happen. The breakdown of muscle fuel (glycogen) in death leads to irreversibly high levels of lactic acid. This leads to a complex reaction where the components of muscle fibers fuse together to form a gel. This gel is what makes the body feel stiff in death. Once the muscle is moved, the stiffness is broken and the gel will not re-form.

The stiffness begins at once and becomes complete in 2 to 3 hours, developing faster in the head, neck and arms than in the legs. High metabolic activity in the time just before death, such as running or playing tennis, leads to higher levels of lactic acid and a shorter time for rigor mortis to develop. For those who are muscular, the rigor comes on more slowly and lasts longer. Rigor mortis lasts for 12 to 24 hours, after which decomposition begins.

Question: I understand the tailbone is one of those things from our evolutionary past. What other body parts are also believed to be part of our evolutionary past? - R.M., Jersey City

Answer: The tailbone is the remnant of a tail, something which all mammals have at one point in their development. Although it no longer serves its original function of assisting with balance, it still provides an attachment point for muscles in the low back, pelvis and buttocks.

The appendix is another "vestigial" structure, believed to aid in the digestion of plant material and cellulose fiber. Wisdom teeth are 3rd molars from our evolutionary past which helped our ancient ancestors finely grind down plant material. Ancient ancestors had a larger head than modern man. which could better accommodate a third set of molars; this explains why the jawbone of many folks today cannot accommodate wisdom teeth.

Some folks have extra nipples or breasts, which are a remnant of our ancestors who possessed more than two nipples or breasts.

The formation of goose pimples is a leftover of our ancient past, with its function to raise the body's hair, making one appear larger and scarier to predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. Due to the diminished amount of hair in humans, the reflex formation of goose bumps when cold is also vestigial.

Another vestigial function that has been greatly diminished in modern man is a keen sense of smell - essential for detecting predators and finding food. One further example of remnants from our ancient evolutionary past is "junk DNA," non-coding segments of DNA that appear to have no biological function and do not code for any proteins.

Dr. Mitchell Hecht is a physician specializing in internal medicine. Send questions to him at: "Ask Dr. H," P.O. Box 767787, Atlanta, GA 30076.