Menthol is a trick: you only think it's cool.

Chewing spearmint gum, sniffing a nasal inhaler, puffing on a mentholated cigarette or otherwise encountering a product containing this widely used additive causes its molecules to act on special cold-sensing nerves in the nose and mouth. The nerves fire as if chilled, sending a signal that the brain interprets as "cold."

Menthol, in other words, produces an olfactory illusion.

It is through such discoveries that scientists are moving toward a deeper appreciation of the senses of taste and smell. Though the other senses -- vision, hearing and touch -- have been favored research subjects for centuries, the chemically activated senses have been scientific orphans, receiving sustained, rigorous study only in recent years, much of it at one of the country's most unusual research institutions, the Monell Chemical Senses Center in Philadelphia.

"The people who study vision or hearing have it pretty easy," said Morley R. Kare, Monell's director. "They're just dealing with one kind of stimulus -- the light spectrum or the sound spectrum. Hell, a cup of coffee's got 800 different compounds. How do you make sense out of that?"

The brain does, integrating signals not only from the usual taste and smell nerves but from separate nervous systems also in the mouth and nose.

The nerves that respond to menthol, for example, are extensions of the trigeminal nerve, which evolved as the body's warning system, sensing potentially noxious stimuli such as cold, hot, and pain.

"A lot of the sensations that we ordinarily think of as being part of taste actually come from a different set of nerves," said Wayne L. Silver. "They're the reason menthol tastes cool and chili peppers taste hot."

The sting of ammonia, the eye irritation of cut onions and the fire of Szechuan food, caused by a chemical in peppers called capsaicin, are among the sensations originated by the trigeminal nerve. Trigeminal stimulation typically makes the eyes and nose water, apparently an effort to wash out the irritating (and, in many cases, poisonous) substance.

Menthol's ability to fool the senses is exploited in inhalers and other products for stuffy noses. The menthol does not open the nasal passages, said Barry Green, who studies the substance's effects on perception. The cool sensation merely makes it seem as if more air is passing through the nose.

There is yet a third chemically activated nervous system -- a nose within a nose, as some describe it -- that goes beyond supplying sensations. It can, at least in animals, control behavior.

When a pregnant female mouse, for example, sniffs the urine of a strange male mouse, her body reacts by aborting. Nobody knows exactly how this happens, but it does as late as the 18th day of what would normally be a 21-day gestation period.

The substance acts on a tiny structure within the nose called the vomeronasal organ that many animals, including humans, possess. It has its own set of nerves that connect with a different part of the brain than that receiving ordinary smell signals. If the nerve linking the vomeronasal organ to the brain is cut, the abortion reaction no longer occurs.

Similar nerve-cutting experiments have shown that the vomeronasal organ receives chemical signals that govern a wide variety of social behaviors. Male mice with cut nerves no longer attack intruder males, for example, nor experience a surge in sex hormones when they encounter a female.

Although humans have vomeronasal organs, there is no evidence yet whether or how these organs function. They are most prominent in fetuses and newborns and shrink with age.

Research of this sort is routine at Monell, an independent research center where more than 60 scientists delve into almost every conceivable aspect of taste and smell from the abilities of perfumers to discriminate hundreds of odors to ways of expanding the world's food supply by making unpalatable foods tasty.

The center, which is loosely associated with the University of Pennsylvania, was created in 1968 by the Ambrose Monell Foundation. It is supported by grants from government, industry and foundations.

About all that is known about taste and smell, aside from their effects, is that there are nerve endings in the nose and mouth that fire when certain kinds of molecules come close or, perhaps, stick to "receptor sites" on nerve cells.

There appear to be only a few different types of nerves to respond to different classes of molecules. Scientists are baffled as to how so few types can allow the brain to tell differences among hundreds of odors and flavors.

What scientists can say about perceiving a cup of coffee is little more than the fact that many of those 800 compounds act on various nerve endings and when the brain receives the melange of signals, it integrates them into something that makes sense.

Or, in the case of chemical signals to the vomeronasal organ, the brain uses them as powerful regulators of an animal's social and sexual life.

Gisela Epple has found, for example, that marmoset monkeys use odors as a birth-control mechanism. The dominant female in a family group smears tree branches with a substance produced by a special scent gland. The substance prevents other females in the family from ovulating. They mate but never become pregnant. If the phenomenon can be understood better, it might lead to a new method of human birth control.

Androstenone, an aromatic substance produced by males of many mammalian species, including humans, is known to have a dramatic effect on female pigs. It is exuded from the saliva of boars. If a sow in heat smells it, she immediately assumes the mating stance.

This discovery has led to two applications for the substance. One has been overwhelmingly successful. Synthetic versions of androstenone are sold in spray cans as Boarmate and are used by hog breeders to win the cooperation of finicky sows.

The second application is another matter. The Jovan perfume company has put androstenone -- which men secrete in their underarm sweat, saliva, scalp oils and urine -- into a new product for men and women. Called Andron, it was advertised as the first perfume to contain a proven sex attractant.

In fact, nobody knows whether it works on people, and sales have reportedly been so-so. One reason may be that about half the human population, males and females alike, cannot consciously detect it. Most of those who can smell it find it powerfully objectionable, having the odor of stale urine.

Odor blindness of this sort, Monell scientists have found, is common. Almost everybody can't smell something. About 3 percent of the population, for example, cannot detect isovaleric acid, a key component of sweat.

Odor blindness can be dangerous, especially for sewer workers among the 10 to 20 percent of people who cannot smell hydrogen cyanide, the vapor used to execute people in the gas chamber. It forms spontaneously when ammonia and methane, a common product of organic decay, react.

About 33 percent of the population cannot smell cineole, the odor of camphor. The "malty" odor of isobutyraldehyde can be detected by about 64 percent. About 6 percent cannot sense the fishy odor of trimethylamine, which some women secrete in vaginal fluids.

One form of odor blindness gave trouble to Procter & Gamble when it tried to develop a new kind of cake mix. About one of every 10 people who tried the product said it smelled awful. It turned out that the iodine in the salt was reacting with something in the lemon to make iodocreosol, a substance that 90 percent of the population cannot smell. P&G switched to non-iodized salt.

Studies of odor blindness, said Charles J. Wysocki, could lead to a better understanding of the mechanism of olfaction and help answer a longstanding question: Does the nose work like the tongue in discriminating sensations? The dominant theory of taste holds that the tongue registers four primary tastes -- sweet, salty, sour and bitter (Japanese scientists add a fifth taste, umami, the flavor of monosodium glutamate). Complex flavors result from a combination of primary tastes, trigeminal nerve sensations, and odors.

Is the nose, Monell scientists would like to know, sensitive to primary odors?

Gary K. Beauchamp, another Monell researcher, has found that the widespread preference for salty flavors is not the acquired taste that nutritionists have claimed on the basis of his earlier research. Years ago he found that newborns show no preference for salt but that 2-year-olds do. The argument was made that the salt preference was learned, partly as the result of salt in commercially prepared baby food.

"We don't think that any more," Beauchamp said. "When we looked more closely, we found that the salt preference develops naturally around the age of 4 months."

Beauchamp said animal studies show a similar pattern. He speculated that the delayed appearance of a salt preference (in contrast to the sweet preference present at birth) may be linked to the fact that newborn kidneys are not fully developed and cannot yet handle much salt in the diet.

Some Monell findings have no direct consequences for humans.

J. Russell Mason, for example, has found that starlings sniff out plants such as wild carrot that exude natural pesticides and gather fresh sprigs to put in their nests. Starlings do not build nests of their own but take over abandoned nests, which often are infested with lice and bacteria.

"These plants are natural fumigants," Mason said.

The senses of smell and taste are several times as sensitive as the most sophisticated laboratory analytic equipment. Monell scientists assume that sensitivity evolved for a reason.

"I think we're beginning to realize," said Kare, Monell's director, "that the world is full of things that exist in very low concentrations that can have a tremendous effect on organisms. There's been a lot of concern about toxicity of various compounds in trace amounts. A decade from now, we'll be concerned with more subtle effects -- not just toxicity but physiological effects."