by Guy Murchie
While smell is a minor sense to most humans, a little training can develop it amazingly, and in former ages physicians seem to have relied on it for diagnosis, their medical treatises repeating such traditional olfactory observations as that the plague smells of honey, scarlet fever of hot bread, measles like fresh-plucked feathers, insanity like mice or deer. But a modern master specialist in odors, called a perfumer or flavorist depending on whether he concentrates on cosmetics or food, may be able (with an array of vials and sampling blotters) to identify something like 10,000 different odors.
It is in the realm of love, however, that the subtleties of smell really come into their own, particularly in the animal kingdom, where it is rarely love at first sight but much more generally love at first smell. A case in point is the seductive long-range scent of certain female flies and moths who may be smelled as far as seven miles downwind if one accepts the evidence of the upwind flight of marked male silkworm moths known to have arrived from that distance. Their type of perfume (C16H30O) in fact is believed to be the most potent substance known to physiology and one caged female pine sawfly exuded such a lecherous lure that more than 11,000 male sawflies turned up pining at her doorstep. But don't assume that male animals are unable or unwilling to put out love emanations of their own. For it is true also that the short-range perfumes some moth swains whiff out of glands under their wings to be flutter-fanned off special lacy hairs toward their sweethearts have such a universal appeal that even half-smuff humans notice them and have identified their flavors as similar to lemon, pineapple, chocolate, musk and several of the most fragrant flowers.
While most people do not seem to consider it very masculine nowadays for human males to entice females with perfume, this very tactic was widely approved as fair practice in ancient Greece, Rome, much of southern Asia, and it even caught on among the affluent in Europe and presumably America in the eighteenth and early nineteenth centunes, when, for example, Napoleon at Fontainebleau is said to have acquired the habit of garnishing his person with eau de cologne at the rate of two bottles a day. If some similar practice should regenerate itself in the West this century, even more significant may be the unpublicized tendency among modern and percipient young men to notice the natural body odors by which undeodonized and unperfumed persons of both sexes can ingratiate each other, by which ash blondes have been thought to exude the subtle aura of ambergris while brunettes have more often wafted to mind sandalwood, almonds or violets - all lovers of course being subject to the natural law that alkalis heighten sexual odors while acids diminish them.
Theories as to what an odor actually is, or how a nose knows a rose from a lily, have been speculated about for millenniums. They range from the idea that smelling is a natural biospectroscopic analysis of infrared heat rays that have passed through (and been patterned by) odorous vapors in the nose to the now widely accepted lock-and-key hypothesis that the smell sense is a biochemical process, specifically a recording of keylike molecules as they fit into various kinds of hospitable keyhole receptacles built into the olfactory center. The latter notion is actually very ancient, having been surmised by Empedokles, who said in 450 B.C. that "smell comes from very tiny particles drawn in along with the breath," and later by Lucretius, who wrote regarding the distinctive flavors of smells that there are "corresponding differences in the shapes of their component atoms. These in turn entail differences in the chinks and channels - the pores, as we call them - in all parts of the body... In some species these are naturally smaller, in others larger; in some triangular, in others square; while many are round, others are of various polygonal shapes ...
Lucretius of course had no way of proving his precocious ideas, nor did the dozens of other smell pioneers with rival theories until Linus Pauling announced the discovery in 1946 that "a molecule the same shape as a camphor molecule will smell like camphor even though it may be quite unrelated to camphor chemically." And three years later R. W. Moncrieff reformulated the whole lock-and-key concept in modern terms, followed by John Amoore, who called it the steric theory and, with the help of colleagues, pretty well established it by demonstrating that odors, like colors, can be sorted into a few primary ones of which all others are mixtures. The primary smells, it turned out, are seven in number: camphoric, musky, floral, minty, ethereal, pungent and putrid, each of them produced by various molecules approximating a distinctive shape or having a definite electric charge, and each smellable only when it is received in the right one of seven different kinds of complementary cavities distributed among seven corresponding areas in the molecular walls of the olfactory nerve cells.
It is quite a fantastic discovery in its way, this molecular revelation in osmics (the science of smell), and full of poetic analogies of love and the intimate courtships and matings of animals and flowers. The camphoric molecules (primary odor no. 1) smell something like mothballs, are shaped roughly round and fit into congenial basinlike hollows in the nerve cells. Musky molecules (no. 2) are styled to go into a microscopic lady's flat, oval hatbox, the kind in which she would keep her broad-brimmed spring hat. Floral molecules (3) carry the scent of a rose garden and the appropriate design of a gardener's wheelbarrow, deep in front and wide at the back. The minty molecules (4) are contoured for a romantic nook such as a cozy two-seated box at the opera where, between the acts, one might savor a mint with one's sweetheart. Ethereal molecules (5) carry the heady aroma of a more mature passion with soporific overtones and the lines of a long flatbottomed barge like Cleopatra's on the Nile. The last two primary molecules (6 and 7), pungent and putrid, are small enough to fit into almost any olfactory aperture, the reception of the first, with the acrid tang of ants and vinegar, governed by its positive electrical charge, and of the other, with the stink of rotten eggs and dung, by its opposite negative charge.
SHAPES OF THE SEVEN PRIMARY MOLECULES OF SMELL
The stench of mercaptan, a garlicky sulfide (RSH), said to be "the worst odor ever compounded" and normally noticeable down to a dilution of a millionth of an ounce in sixty cubic feet of air, is presumably included in the putrid classification. In any case, any substance, to be smellable, must be both volatile and soluble - that is, warm enough so its molecules will actively zoom through the air around it in vast numbers and chemically adaptable enough so that, after they enter the nose, they can dissolve and penetrate the wet and lipid layers that coat the olfactory nerve cells. When the receptacles there are filled with enough odor molecules to block the acceptance of more, however, the sensation of smelling has to diminish while a kind of smuffness often called "smell fatigue" sets in, at least for the scents involved, and this naturally confirms the lock-and-key hypothesis.
Another discovery was that certain alcohols and aldehydes have an extraordinary power to amplify smells. Presumably this is why they are present in so many successful perfumes and may be attributable to their molecules being composed of from four to eight carbon atoms in a chain with side branches that provoke extra contacts with receptor sites, thus intensifying and determining the specific odor. Basic mammalian sex lures, on the other hand, are apt to have heavy ring-shaped molecules: musk a 15-bead necklace of carbon atoms fringed with hydrogen and oxygen. civet a similar 17-bead necklace. But a smell molecule for general alarm purposes that other species can respond to is usually simpler, unspecific and small. All the evidence in fact suggests that every known smell is somehow spelled out by its molecule's shape and defined just precisely enough by it so that a pair of molecule twins, for example, identical in all respects except that one is the mirror image of the other, almost invariably have distinct and individual odors. Indeed the molecule called carvone with mirrored right- and left-handed configurations smells in its "right" form like spearmint and in its "left" like caraway, the amino acid leucine when right is sweet but when left bitter, and another handed molecule named limonene smells in its right version like an orange and in its left like a lemon. Which could mean that, gauche though it may seem, a lemon is in genetic es
sence only a left-handed orange.
Here we are touching on the symmetry and relativity of smells, which may relate to such curiosities as the fact that a derivative of indigo known as indole (C8H7N) smells like raw sewage yet, if you dilute it, it immediately gentles down to the genial scent of narcissus! On the other hand, if you sniff cedarwood for several seconds, its odor may tone down into that of violets, and bitter almonds should turn toward tar, while, if you add the odor of India rubber to cedarwood, it can cancel it so you smell nothing - even as balsam is canceled by beeswax.
This of course brings us back to the phenomenon of camouflage, which applies to smell about as much as to sight and sound. being found throughout nature, embodied in the chipping sparrow who exasperates the bird dogs of Texas by imitating the smell of a quail, or in the prowling rattlesnake who deludes his rodent prey by assuming the dank aroma of a cucumber. If these are olfactory lies, it is because smell too is a kind of language. The rattler, for one, has at least four "words" in his smell vocabulary. Besides his ambush scent of cucumber, he switches to a terrifying effluvium when in combat, exudes a socially somnific savor at hibernation time and wafts a love perfume when looking for a mate.
As organs of sense, man's nose and tongue are obviously less important to him than his eyes and ears, but the priority is reversed in the case of most animals, in whom the chemical senses are their main medium of communication. In social creatures, for instance, messages are commonly delivered in the "code" of pheromones or external hormones such as musk that circulate not inside the body but socially between the members of a family, a colony or a species. Pheromones are definitely a factor in population control for, when soldier termites secrete substances that inhibit the endocrine glands of the younger termite generation, these larvae are unable to grow up and reproduce. And, on the other hand, when the adult males of migratory locusts secrete a volatile liquid from their skins that is passed to the nymphs, these young females mature much faster, provoking in a few weeks the dramatic outbursts of huge foraging plagues of locusts.
Something similar may happen to lemmings, who also have catastrophic population explosions now and then and, with most species of the mouse family, the seductive odor of a friendly male mouse is known to initiate and synchronize the estrous cycles of female mice. Yet these same mice have a remarkably effective population regulator in that, if a strange male mouse approaches a newly impregnated female (as must occur when conditions get crowded), the merest whiff of him is usually enough to suppress the flow of her prolactin and end her pregnancy!
TASTE
Taste is basically like smell except that it is less sensitive, requiring about 25,000 times as many molecules to elicit a sensation because it deals primarily with molecules in solid and liquid form rather than gaseous. And the primary tastes are but four in number: sweet, bitter, salty and sour; each key taste molecule having its own lock receptacle in what is called a taste bud on the human tongue, palate or throat. This means that each primary taste is tastable only in its own location: sweet at the tip of the tongue, bitter at the back, salty on the sides around the tip and in the throat, sour on the sides of the tongue farther back. Like smell, taste is very ancient and fundamental, and I feel no doubt that, when we know more about molecular structure, we will see geometric reasons why applesauce tastes good with pork, mint with lamb, cranberries with turkey, ketchup with baked beans and so on.
In the meantime the study of taste is increasingly bewildering. A couple of sample findings of research in the chemical senses indicate that the flavor of a common brand of coffee is a synthesis of about four hundred compounds (most of which are smelled more than tasted) while the formula for a different and more sophisticated artificial flavor "requires as many as 20,000 separate pieces of information." And this no doubt throws light on why man has been able to invent cameras, phonographs and associated techniques to record, amplify and transmit sights and sounds but has not yet devised any comparable method of recording, amplifying or transmitting a single smell or taste.
Relativity is another factor that makes smell and taste hard to comprehend. If, for example, you put on one side of your tongue a salt solution too dilute to taste noticeably salty but then add a little sugar on the other side, you will instantly begin to taste the salt along with the sugar. The opposite happens when you start with dilute, tasteless sugar on one side and add salt on the other. Furthermore, to most humans, a salt solution will begin to taste sweet when diluted down to .03 parts per million, particularly if it is cool, the amount of dilution needed to make this happen being roughly proportional to the temperature. On the other hand, although Epsom salts taste salty on the front of your tongue, they turn bitter when pushed back to the hind buds. And if you try many kinds of chemically graded salt, you will notice them tasting progressively bitterer as you get to salts of heavier molecular weight. The taste of salt of course is largely electrical (due to ionization of its constitutent atoms) and every sort of electric current has its own flavor: a gentle direct current savoring subtly sour when the positive terminal touches the tongue but "like burnt soap" when the flow is reversed, while an alternating current that smacks of astringent sourness at 50 cycles turns steadily more and more bitter as it escalates toward 1000 cycles. Such relativity is not merely mental but rather an objective part of the natural and paradoxically complex simplicity of the chemical senses which, unlike senses that become electrical only in the final transmission of messages to the brain, may function electrically all the way from their first contact with whatever they perceive.
This is not to say that there aren't real differences of taste between individuals, for we all know such differences exist. Ordinary sugar, for example, is tasteless to a small percentage of children while saccharin tastes bitter to a few yet sweet to their brothers and sisters. And there is a synthetic chemical called PTC (for phenyl-thio-carbamide) that tastes intensely bitter to an average of two people out of three all over the world but utterly tasteless to the third person. And the common food preservative sodium benzoate tastes like almost anything or nothing, depending on who tries it. But there is such conclusive evidence that these phenomena are objective that a chemist named Arthur L. Fox has formulated a genetic theory of taste on his finding that 26 percent of people consider PTC bitter but sodium benzoate salty and like almost every kind of food, while 17 percent with different taste genes register both these chemicals as bitter and dislike most sorts of food.
Indeed, of the four primary tastes, bitterness turns out to be the easiest for a human to detect, perhaps because it signals danger in the form of poison. But there always seem to be a few unlucky people who just can't taste anything at all, and them I would call "smumb." They aren't necessarily the same ones as those who are smuff and their deficiency is more serious - but fortunately medical researchers have discovered that most of them, even those who've been stone smumb for years, can be cured within a few days by taking small doses of the trace metal zinc.
Animals naturally vary in sensitivity to taste, some insects going so far as to walk on their "tongues" and to taste with their feet. Other creatures, notably fish, may be trillions of times more sensitive than man with taste buds so densely distributed over their body surfaces that they literally swim in a sapid sea. There is the uncanny account of a coho salmon raised in a California hatchery who, when a year old, was dumped into a strange stream several miles away and allowed to migrate with his fellows down to the ocean. But at spawning time the next year he appeared back in his original tank, having followed the familiar flavor into his home stream, threaded a particular culvert under U.S. Highway 101 which enabled him to enter the hatchery's flume and storm sewer, from which he finally wriggled up a four-inch drainpipe past 900 elbows, climactically knocking off its wire cap and even leaping over the screen that surrounded the drain!
THE SENSES OF HUNGER AND THIRST
Close cousins of taste of course are the twin senses of hunger and thirst. Hunger has long been known to be turn
ed on by rhythmic contractions of the empty stomach and, more recently, by a decline in the sugar content of the blood. Indeed a transfusion of low-sugar blood from a starving dog into a well-fed one will make the latter hungry for the same reason that high-sugar blood from a satiated dog will ease the pangs of a hungry one, Yet hunger can hardly be as simple as this. In fact certain researchers have recently found that the ratio of ions in the brain may regulate hunger and specifically that rats who have eaten to satiation can be induced to resume eating voraciously by injections of calcium ions in the cerebrum. Others predict that, when we fully understand it, hunger will turn out to involve a "hunger hormone," conveyable, if not normally from organism to organism, perhaps in some degree from organ to organ through lymph and blood.
Thirst is about equally mysterious but obviously different from hunger and much more compelling, at least to a water-dependent creature like man, as is proven by the fact that a man can live more than a year without food but, to the best of my knowledge, seventeen days is his world record without water. This record was made in 1821 when a prominent Frenchman named Antonio Viterbi committed suicide by refusing to drink, but of course he may have taken in a significant amount of moisture in whatever he ate. Doctors now say he probably would have survived if he had accepted water on the fourteenth or fifteenth day, but by the sixteenth it was almost certainly too late. There are cases on record of castaways deprived of fresh water for two weeks who were rescued just in time and managed to survive. Presumably all these sufferers were in humid environments, did not sweat and kept evaporation from their bodies to a minimum.
Something to consider also is that the thirst sensation is less influenced by the total amount of water in the body than by the amount of water relative to certain solids, particularly salts. And this accounts for the classic equation of thirst that assures the bartender he will sell in the end more than ten times as much in drinks as the cost of all the "free" salted pretzels, popcorn and potato chips he "gives" away to his customers to bolster their thirst. It also relates to the discovery in 1952 that a fraction of a drop of a salt solution injected into the hypothalamus at the base of a goat's brain will immediately make the animal thirsty, which, in combination with later evidence, seems to have pretty well proved the site of thirst to be the hypothalamus. Strange as it may seem, drinking beyond a certain quantity of water hour after hour increases rather than decreases thirst because the body loses salt in urine and salt deficiency produces thirst, a kind of thirst, however, that can be relieved only by salt.