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winter; the appliances and lights function reasonably well; and the drinking water is as safe as the city can make it (we drink straight from the tap). My house makes a lovely metaphor for my brain. A bit of a mess, but it works.
    Of the twelve paired cranial nerves in the human brain, the fifth one, the trigeminal nerve, is the most immediate example of evolution's nonlinear ways, and the most exasperating to medical students trying to keep track of its disparate functions and elaborate pathways. The trigeminal nerve controls some facial muscles and facial sensations. It registers the astringency of ammonia and the heat of jalapenos. We think of those as smells and tastes, but they're really sensations, closer to the sense of touch than to taste or smell. The trigeminal nerve also supplies the withering pain we associate with dental procedures, and the hammer blows of a migraine headache.
    By contrast, the olfactory nerve is a thing of breathtaking clarity. It has one assignment only: to send odor messages from the nose to the brain.
    Olfaction—the process of smelling—is another matter. It is like that building whose pipes and wires have undergone countless renovations. The human sense of smell is a work in progress whose original design has to support the weight of all the cumbersome improvements required for it to retain its value to our species.
    Smelling is not only part of the limbic system—it created the limbic system. The several different species for which it was designed in the beginning depended on smell every minute of every day. Other limbic structures emerged to support this all-important alert system. The fight-or-flight response is regulated in the amygdala, while the hippocampus stores the meanings (perilous or pleasurable?) that odors convey. Deprived of smell, our dog, Mel, would be hunted down and eaten for supper by the nearest coyote; in the safety of our home, his demise would be more protracted but still inevitable without a working nose to remind him to eat. Dogs depend on smell to survive.
    Humans don't. Perhaps there was a time when we did. That time is long gone. University of Chicago evolutionary biologist Yoav Gilad sparked renewed speculation about smell's long-term prospects in humans when he discovered that a certain type of rhesus monkey has color vision (most primates see in black and white). Mother Nature didn't come up with this so the monkey could enjoy the colorized version of
Citizen Kane.
Color vision enhances the monkey's ability to detect food and predators. However, it seems that the improvement required, in effect, handing over some of the rhesus monkey's smell genes in exchange for better sight. Have our human smell genes been going in the same direction?
    Even if they have, smell's intimate link to the limbic system means it carries still-buried clues to how our brains evolved and to how they work now. The debate among psychologists—the few who bother to think about smell—is whether humans depend on the primary sense for emotional homeostasis. Do the myriad pleasures of scent—from the obvious ones connected with food and everyday delights like walking in the woods to the more subtle and mysterious fragrances we associate with sex—help to offset the burden that humans alone carry, the knowledge that we are mortal? Is smell's ability to trick us into losing ourselves in the moment (in pure delight) a cornerstone of human happiness? Without smell to make life worth living, indeed endurable, would our species have lasted this long?

    Perfume designers call the chemicals they blend to make their alluring olfactory compositions notes. Anyone with an introductory chemistry class on her college transcript (that wouldn't be me) knows that recognizable scents are made up of molecules. The scents' formulas are like the ingredients in a recipe, a kind of shorthand that tells how much of which molecules an odor contains. The complex scent of a rose has 1,215 odor molecules; a