A Natural History of the Senses

by Diane Ackerman

  Neurologists now believe that one can picture the touch receptor as a membrane in which there are a number of tiny holes, or at least potential holes, like a piece of Swiss cheese covered with cellophane. In the resting state the holes are too small or the cellophane too thick for certain ions to pass through. Mechanical deformation opens up these holes. When … currents are … formed … by a strong pressure such as a pinprick, the currents are strong enough to trigger nerve impulses and the intensity of the prick is signaled by the frequency of the impulses, since this is the only way nerve fibers can code intensity.

  Our menagerie of touch receptors also includes saucer-shaped Merkel’s disks, which lie just below the skin surface and respond to continuous, constant pressure (they give a sustained message, a continuous monitoring); various free nerve endings, which aren’t enclosed in capsules, and respond more slowly to touch and pressure; Ruffini endings, located deep below the skin surface, which register constant pressure; temperature sensors; cylindrical heat sensors; and the most familiar, but oddest, touch receptor of all: hair.

  HAIR

  Hair deeply affects people, can transfigure or repulse them. Symbolic of life, hair bolts from our head. Like the earth, it can be harvested, but it will rise again. We can change its color and texture when the mood strikes us, but in time it will return to its original form, just as Nature will in time turn our precisely laid-out cities into a weed-way. Giving one’s lover a lock of hair to wear in a small locket* around his neck used to be a moving and tender gesture, but also a dangerous one, since to spell-casters, magicians, voodoo-ers, and necromancers of all sorts, a tuft of someone’s hair could be used to cast a spell against them. In a variation on this theme, a medieval knight wore a lock of his lady’s pubic hair into battle. Since one of the arch-tenets of courtly love was secrecy, choosing this tiny memento instead of a lock of hair from her head may have been more of a practical choice than a philosophical one, but it still symbolized her life-force, which he was carrying with him. Ancient male leaders wore long flowing tresses as a sign of virility (in fact, “kaiser” and “tsar” both mean “long-haired”). In the biblical story of Samson, the hero’s loss of hair brings on his weakness and downfall, just as it did for the hero Gilgamesh before him. In Europe in more recent times, women who collaborated with the enemy in World War II were humiliated by having their hair cut short. Among some orthodox Jews, a young woman must cut off her hair when she marries, lest her husband find her too attractive and wish to have sex with her out of desire rather than for procreation. Rastafarians regard their dreadlocks as “high-tension cables to heaven.” These days, to shock the bourgeoisie and establish their own identity, as every generation must, many young men and women wear their hair as freeform sculpture, with lacquered spikes, close-cropped patterns that resemble a formal garden maze, and colors borrowed from an aviary or spray-painted alley. The first time a student walked into my classroom wearing a “blue jay,” it did startle me. Royal-blue slabs of hair were brushed and sprayed straight up along the sides of his head, a long jelly roll of white hair fell forward over his eyebrows, and the back was shiny black, brushed straight up and plastered close to the head. I didn’t dislike it, it just seemed like a lot to fuss with each day. I’m sure my grandmother felt that way about my mother’s “beehive,” and I know my mother feels that way about the curly weather system which is my own mane of long thick hair. One’s hairstyle can be the badge of a group, as we’ve always known—look at the military’s crew cut, or the hairstyles worn by some nuns and monks. In the sixties, wearing long hair, especially if you were a man, often fetched a vitriolic outburst from parents, which is why the musical Hair summed up a generation so beautifully. The police, who seemed so clean-cut and cropped then, were succeeded by a generation of police in long sideburns and mustaches. But I remember at the Boston Love-in in 1967, my first year away at college, hearing one young man say to a passing couple who ridiculed his ponytail: “Fuck you and fuck your hairdressers.” I also remember, in the fifties, walking out of my bathroom with my hair sprayed into a huge bubble. “What have you done to your hair?” my father demanded. “I’ve just teased it,” I said. To which he replied: “Teased? You’ve driven it insane.” I wear my curly hair au naturel these days, in a shag cut the French call la coupe sauvage (“the savage cut”), but its volume and faintly erotic unruliness bother my mother’s sense of propriety. To her generation, serious women have serious hairdos that are formal, sprayed, and don’t move. A few weeks ago, she phoned to warn me that professional women aren’t taken seriously if they don’t have a “wet set” (rollers, hair dryer, setting lotion, hair spray). Loose ends on one’s head signal loose ends in one’s life. From this point of view, which has been popular for ages, a woman grows her hair long but keeps it tightly controlled in a bun, under a hat or scarf, or with hair spray, and lets her hair down only in private at night.

  Most people have about 100,000 hair follicles on their head, and lose between fifty and a hundred hairs a day through normal combing, brushing, or fussing. Each hair grows for only about two to six years, at about five or six inches a year, and then its follicle rests for a few months, the hair falls out, and is eventually replaced by a new hair. So when you see a beautiful head of hair, you’re looking at hairs in many different stages in a complex system of growth, death, and renewal. Fifteen percent of it is resting at any one time, the other 85 percent growing; many dozens of hairs are all set to die tomorrow, and deep in the follicles new hairs are budding.

  Hair has a tough outer coating called the cuticle, and a soft interior called the cortex. People with coarse hair have larger follicles, and also a thin outer coat (10 percent of the hair) with a large inner cortex (90 percent). People with fine hair have smaller follicles, and almost the same amount of cuticle (40 percent) as cortex (60 percent). If the follicle cells grow in an even pattern, the hair will be straight; if they grow irregularly, the hair will be curly. Lice have a hard time attaching to thick hair, which is why black schoolchildren don’t succumb to epidemics of head lice as often as their white classmates. Besides being sexy to most people, head hair protects the brain from the sun’s heat and ultraviolet rays, helps to insulate the skull, softens impact, and constantly monitors the world only a hair’s breadth away from our body, that circle of danger and romance we allow few people to enter.

  Of course, hairs grow in many places around the body, even on the toes and inside the nose and ears. The Chinese, the American Indian, and some other peoples have very little hair on their face and body; those of Mediterranean descent can be so woolly and thickly haired they seem only a step away from our ape-man ancestors. Bald men are sexy men; they go bald from a high level of testosterone in the blood, which is why you don’t see bald castrati or eunuchs. Men with thick mats of hair on their shoulders and backs used to scare me. A word like “carnivore” would form in my mind when I passed them on beaches. Women tend to be smoother-fleshed than men, so it makes sense that we would shave our legs and apply lotions to accentuate the gender difference. But despite efforts to remove hair from our bodies, quite a lot remains on the arms, faces, and heads of women, and the chest, arms, and legs of men, to do what it was intended to do.

  Hair is special to mammals, although reptiles do form scales, which are related. Each hair grows from the papilla, a wad of tissue at the base of a follicle, where there is a nerve ending, and there may be a group of other nerve endings nearby. The average body has about five million hairs. Because hairy skin is thinner, it’s more sensitive than smooth skin. One hair can be easily triggered: If something presses it or tugs at it, if its tip is touched, if the skin around it is pressed, the hair vibrates and sparks a nerve. Down is the most sensitive hair of all and only has to move 0.00004 of an inch to make a nerve fire. Still, it can’t be firing all the time, or the body would go into sensory overload. There is an infinitesimally small realm in which nothing at all seems to be happening, a desert of sensation. Then the merest breeze starts to bl
ow, nothing like a real disturbance. When it grows just strong enough to reach an electrical threshold, it fires an impulse to the nervous system. Hairs make wonderful organs of touch. “Breeze,” our brain says without much fanfare, as a few hairs on our forearms lift imperceptibly. If a dust mote or insect brushes an eyelash, we know at once and blink to protect the eye. Though hairs can take shapes as various as down or antennae, some especially useful ones are vibrissae—the stiff hairs cats have as whiskers—which adorn many mammals, including whales and porpoises. A cat without its whiskers bumps into things at night, and can get its head caught in tight spaces. As we can. If we ever get a say-so in evolution, one of the things I’d vote for is whiskerlike feelers to keep us from bumping into furniture, friends, or raccoons in the dark.

  THE INNER CLIMATE

  Some people meditate, or practice the Zen of archery. I begin each summer morning by strolling around the raised beds in my garden, where twenty-five tea and floribunda rosebushes, twenty-eight lavender and yellow day lilies, a dozen or so shade-loving plants such as hostas and monkshood, and a brilliant range of perennials and annuals flourish. It’s not unusual to spend half an hour choosing a sprig of baby’s breath, a pink lupine, one stem of bluebell-shaped campanella (whose stem oozes white sap—almost always a sign of poison), one orange-red rose called “Bing Crosby,” one stem of red-and-white bleeding hearts, a bright yellow coriopsis, a huge fuchsia dahlia, a red-and-white, daisy-shaped miniature dahlia, and a flamboyantly speckled red-and-yellow Pavonia tigridia which looks like an iris that married a day lily and went to a fiesta (its name means “tiger-faced peacock,” which is wonderful enough, but I’ve always called it a “Mexican hat dance” instead). Because I don’t know what will have opened during the night or early morning, some days it’s a little like discovering an emerald in your soup. Then I spend half an hour or so indoors, arranging the day’s petaled baubles in a glass dish full of clear marbles, driven no doubt by laws of balance, shape, and color, but working with a calm obsessiveness that allows nothing so rude as thought to intrude.

  While making a bouquet one morning, I noticed an odd thing about how we perceive temperature. Next to some cutlery soaking in a pan of hot water in the sink was one bowl of cold water and one bowl of warmish water. I put one hand in the cold, one hand in the hot. Then I put both in the warm water and, to my surprise, they gave me contradictory signals. All they were perceiving was the movement of temperature, not hot or cold per se. I’ve also noticed that, for some reason, objects of equal weight feel heavier if they’re cold than if they’re warm. There’s no simple answer for this phenomenon. Maybe the heat receptors are more specialized, whereas the cold receptors register pressure, too.

  Most of the cold receptors lie in the face, especially on the tip of the nose, the eyelids, lips, and forehead, and the genitals are sensitive to cold, as well. It’s our outer shell that seems to fear cold most, acting as a sentry on perpetual watch. Receptors for warmth lie deeper in the skin, and there are fewer of them. Not surprisingly, the tongue is more sensitive to heat than many other areas of the body. If hot soup can pass the tongue test, it probably won’t burn the throat and stomach. Unlike other touch information, temperature reports tell the brain of changes as well as highs and lows, and there are frequent updates. My mother used to urge me to put an ice cube on my wrist when I was too hot. This excites the cold receptors into overreacting and firing furiously. Remove the ice cube, and the wrist stays cold for quite a while. It doesn’t seem like much of a poultice, but your skin only has to be warmed by three or four degrees to make you feel truly warm, only lowered by one or two degrees to make you feel decidedly cold. Then your body starts to correct things and you rub your hands together, shiver, or stick your hands under your armpits to warm up. You drink iced drinks or take a cold shower or go for a swim to cool down. On a brutally hot and humid summer day, one on which the sun feels as if it’s been dipped in lye, the air is so thick it’s drinkable, and your body feels like freshly melted lead, all I have to do is get into a swimming pool and stand up to my neck in cold water, ice down the brain stem, to feel rejuvenated. Why should aspirin be able to lower a fever, but not affect a normal temperature? Because it inhibits the release of the body’s own pyrogen, a substance that causes fever. There are still many mysteries about the body’s ability to regulate its temperature. We wake up cooler than when we go to sleep, but why should we be at our lowest temperature at about 4:00 A.M.?

  Suppose we cooled the body from the inside out? In hypothermic surgery, the blood is chilled and recirculated, which reduces body temperature to about seventy-seven degrees. Science-fiction stories often involve an astronaut whose body temperature has been lowered, sleeping in suspended animation like a naked bear in a glass den. Walt Disney’s family swears it isn’t true, but a popular folk myth for some time now has it that Walt arranged to be frozen when he died and is lying in a magic kingdom of ice, awaiting his rebirth. Trans Time, Inc., a member of the American Cryogenics Society, does freeze people right after death, promising to bring them back to life in a later era, when the mysteries of death are scrutable and the carnage of their diseases reversible. Movies like Ice Man play with the idea of someone being frozen for decades or centuries, then awakening in a new world. What makes it sound so plausible, I suppose, is how familiar the scenario is in religious terms: One dies out of this life to emerge in the next. I don’t think there’s enough evidence that a brain and body could be frozen and defrosted without damage, but proponents of cryogenics argue that one has nothing to lose. Could there be an extreme metabolic reduction instead of freezing? The suspended animation of sci-fi stories? Different tissues have a different freezing profile, don’t they? Wouldn’t that mean that some would be overchilled and others underchilled? How will right-to-lifers (who are already vehemently opposed to freezing sperm, ova, and embryos) and religious zealots feel about thawing people out—what ethical debates and social turmoil will it prompt?

  Warm-blooded creatures, we overheat easily and then an ancient terror sets in. We moan that we’re being cooked, the way we cook other animals: “I’m roasting,” we say; “I’m burning up”; “It’s like an oven in here.” Now that we’ve lost our heavy body fur we chill fast, so we must wear thick clothing when the temperature plunges. I’ve seen people out walking on a winter’s day wearing layered clothes, wool sweaters, and bulky down coats; they look like freshly made beds on the move. The evolution of warm-blooded animals was an extraordinary breakthrough. It meant that they could keep up their body temperature despite the vagaries of the environment, and could actually migrate. Cold-blooded animals (except butterflies, eels, and sea turtles) can’t migrate much, and some, like rattlesnakes and pit vipers in general, are excellent at heat detection. So are mosquitoes, moths, and other insects (which has led some researchers to conclude that people who are bitten more often than others may be radiating more heat, which makes them prime targets). Although we don’t have such heat-sensing devices built into our bodies, we do create them for military use—heat-seeking missiles that strike like vipers. In recent sci-fi/horror films like Wolfen or Predator, razor-clawed, blood-lusting monsters live in a world beyond our visual range; but we are exquisitely findable by them because they can sense in infrared. The monster appears without warning, disembowels someone, and vanishes. Something about its heat-seeking ability makes it doubly horrifying. It uses one of our loveliest features to destroy us. For millennia we’ve relied on our warm-bloodedness as a life-force; we prize caring, compassionate people by referring to their warmth. And here is a monster homing in on that warmth. Our essence is our undoing says the message of these sensory frightmares.

  Without a thick hair covering to protect us, we have to be vigilant about cold. Although the hands, feet, and other exposed parts of the body seem invaluable, since they register touch so sensitively, when cold hits they become expendable. The hands or feet can freeze, and the body still survive, but if the blood temperature lowers we’re goners.
So the torso responds immediately to changes in temperature, and we sense cold over a wider range of our body than we do heat. Far more women than men claim to have cold hands and feet, which isn’t at all surprising. When the body gets cold, it protects the core organs first (which is why it’s easy to get frostbite in your extremities); in women, it protects the reproductive organs. When your lips turn blue or your toes suffer frostbite, the blood vessels are tightening up and the body is sacrificing the extremities, sending blood to the essential inner section.

  Animals love to lie in the sun and bask. Nothing looks more contented in winter than a black-and-white cocker spaniel sprawling on the living-room carpet in a bright shaft of sunlight. Some creatures, like reptiles or houseflies, do it to regulate their body temperature, and one often sees an American alligator in a Floridian swamp arranging itself in the sun with a voluptuary’s exquisite care: one leg and the tail under the water, the lower back and another leg in the shadow of a bush, the head and back and front legs completely in the sun.… The alligators seem quite finicky about it, but really are governing their thermostats just as we do on a fall afternoon, when we leave on a pullover sweater but take off our hat and gloves. The travel industry relies on human beings’ love of basking, and basking vacations are available to most anywhere. And, though some of us like adventure travel, most prefer to sit in the sun like a rack of spareribs, basting ourselves regularly with sauce, and quietly frying, taking care to turn over so we’ll be cooked on both sides. Why we love to bask isn’t hard to fathom. Evolution, that haute couturier of ingenious patterns, probably designed the sensation so that animals would search for climates conducive to good health. But, when enough becomes too much and an animal overheats, the skin’s smallest capillaries dilate to let the heat escape. A man’s face flushes. A rabbit’s ears flush. All animals perspire in one way or another, and the perspiration evaporates, cooling the body. It’s not the heat, it’s the humidity, we moan on a sultry day when even a cotton shirt seems attached by saliva to one’s back. As the outside air temperature reels close to 98.6 degrees, the body starts to lose track of itself and suffers. But if it’s also humid, which means the air is saturated with water, we still sweat to cool off in the usual way, but nothing happens. The air’s too soggy to allow sweat to evaporate. So one sits on a porch swing in Alabama, listless and sticky, fanning oneself with a flyer from a local construction company that, according to its advertisement, longs to “flash your gutters,” while sipping iced tea flavored with a sprig of fresh peppermint or a leaf of pineapple sage. On the other hand, if an animal gets too cold, most often it raises gooseflesh and shivers—skin muscles contract (to expose a smaller area), and the shaking that follows warms the body. Even though we can’t puff up our fur the way other animals do, either to look big and mean or to keep warm, we have tiny leftover erector pili muscles that cause some of our hairs to stand up when we’re cold or scared. Certain animals have evolved fascinating strategies for keeping warm. Von Buddenbrock reports a German beekeeper who discovered that hives never got very cold: