The Body Highlights

Mar 21

March 21st, 52068

by Bill Bryson

Or what precisely is a thought? It is not something you can capture in a jar or smear on a microscopic slide, and yet a thought is clearly a real and definite thing. Thinking is our most vital and miraculous talent, yet in a profound physiological sense we don’t really know what thinking is.

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As James Le Fanu has put it, “While we have the overwhelming impression that the greenness of the trees and the blueness of the sky are streaming through our eyes as through an open window, yet the particles of light impacting on the retina are colourless, just as the waves of sound impacting on the eardrum are silent and scent molecules have no smell. They are all invisible, weightless, subatomic particles of matter travelling through space.” All the richness of life is created inside your head.

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Fundamentally, however, memories come in two principal varieties: declarative and procedural. Declarative memory is the kind you can put into words—the names of state capitals, your date of birth, how to spell “ophthalmologist,” and everything else you know as fact. Procedural memory describes the things you know and understand but couldn’t so easily put into words—how to swim, drive a car, peel an orange, identify colors.

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Incidentally, the idea that we use only 10 percent of our brains is a myth. No one knows where the idea came from, but it has never been true or close to true. You may not use it all terribly sensibly, but you employ all your brain in one way or another.

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Although most of the growth of the brain occurs in the first two years and is 95 percent completed by the age of ten, the synapses aren’t fully wired until a young person is in his or her mid- to late twenties.

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— For all its marvels, the brain is a curiously undemonstrative organ. The heart pumps, the lungs inflate and deflate, the intestines quietly ripple and gurgle, but the brain just sits pudding-like, giving away nothing. Nothing in its structure outwardly suggests that this is an instrument of higher thinking. As Professor John R. Searle of Berkeley once put it, “If you were designing an organic machine to pump blood you might come up with something like a heart, but if you were designing a machine to produce consciousness, who would think of a hundred billion neurons?”

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As Professor John R. Searle of Berkeley once put it, “If you were designing an organic machine to pump blood you might come up with something like a heart, but if you were designing a machine to produce consciousness, who would think of a hundred billion neurons?”

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Gradually, as it became evident that Freeman and others like him were leaving trails of human wreckage behind them, the procedure fell out of fashion, especially with the development of effective psychoactive drugs. Freeman continued to perform lobotomies well into his seventies before finally retiring in 1967. But the effects that he and others left in their wake lasted for years. I can speak with some experience here. In the early 1970s, I worked for two years at a psychiatric hospital outside London where one ward was occupied in large part by people who had been lobotomized in the 1940s and 1950s. They were, almost without exception, obedient, lifeless shells.*4

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Floaters are a similar phenomenon. They are clumps of microscopic fibers in the jellylike vitreous humor of your eye, which cast a shadow on the retina. Floaters are a common occurrence as you get older, and are generally harmless, though they can indicate a retinal tear. The technical name for them, if you wish to impress someone, is muscae volitantes, or “hovering flies.”

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We are the only creatures that cry from feeling, as far as we can tell.

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In addition, all the nerve fibers leave the eye via a single channel at the back, resulting in a blind spot about fifteen degrees off center in our field of vision. The optic nerve is fairly hefty—it is about the thickness of a pencil—which is quite a lot of visual space to lose. You can experience this blind spot by means of a simple trick. First, close your left eye and stare straight ahead with the other. Now hold up one finger from your right hand as far from your face as you can. Slowly move the finger through your field of vision while steadfastly staring straight ahead. At some point, rather miraculously, the finger will disappear. Congratulations. You have found your blind spot. You don’t normally experience the blind spot, because your brain continually fills in the void for you. The process is called perceptual interpolation. The blind spot, it’s worth noting, is much more than just a spot; it’s a substantial portion of your central field of vision. That’s quite remarkable—that a significant part of everything you “see” is actually imagined. Victorian naturalists sometimes cited this as additional proof of God’s beneficence, without evidently pausing to wonder why He had given us a faulty eye to begin with. HEARING HEARING IS ANOTHER seriously underrated miracle. Imagine being given three tiny bones, some wisps of muscle and ligament, a delicate membrane, and some nerve cells, and from them trying to fashion a device that can capture with more or less perfect fidelity the complete panoply of auditory experience—intimate whispers, the lushness of symphonies, the soothing patter of rain on leaves, the drip of a tap in another room. When you place a set of $800 headphones over your ears and marvel at the rich, exquisite sound, bear in mind that all that that expensive technology is doing is conveying to you a reasonable approximation of the auditory experience that your ears give you for nothing.

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When loss of balance is prolonged or severe, the brain doesn’t know quite what to make of it and interprets it as poisoning. That is why loss of balance so generally results in nausea.

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Even at the simplest level, results are often wildly counterintuitive. If you combine the fruity odor of ethyl isobutyrate with the caramel-like allure of ethyl maltol and the violet scent of allyl alpha-ionone, you get pineapple, which smells wholly unlike its three principal inputs. Still other chemicals have very different structures but produce the same smell, and no one knows why that happens either. The smell of burned almonds can be produced by seventy-five different chemical combinations that have nothing in common beyond how the human nose perceives them. Because of the complexities, we are still very much at the beginning of an understanding of it all. The smell of licorice, for instance, was decoded only in 2016. Many, many other common odors are still to be deciphered.

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I mention all this here to make the point, if it needed making, that the mouth is a place of peril. We choke to death more easily than any other mammal. Indeed, it can reasonably be said that we are built to choke, which is clearly an odd attribute to go through life with—with or without a coin in your trachea.

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As far as taste goes, our tongue can only identify the familiar basics of sweet, salty, sour, bitter, and umami (a Japanese word meaning “savory” or “meaty”).

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synthetic umami, in the form universally known today as monosodium glutamate, or MSG.

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MSG has had a hard time of it in the West since 1968 when The New England Journal of Medicine published a letter—not an article or a study, but simply a letter—from a doctor noting that he sometimes felt vaguely unwell after eating in Chinese restaurants and wondered if it was the MSG added to the food that was responsible. The headline on the letter was “Chinese-Restaurant Syndrome,” and from this small beginning it became fixed in many people’s minds that MSG was a kind of toxin. In fact, it isn’t. It appears naturally in lots of foods, like tomatoes, and has never been found to have deleterious effects on anybody when eaten in normal quantities. According to Ole G. Mouritsen and Klavs Styrbaek in their fascinating study, Umami: Unlocking the Secrets of the Fifth Taste, “MSG is the food additive that has been subjected to the most thorough scrutiny of all time,” and no scientist has ever found any reason to condemn it, yet its reputation in the West as a source of headaches and low-grade malaise now appears to be undimmed and permanent.

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Now consider the fact that none of those flavors or aromas actually exist. All that is really going in your mouth is texture and chemicals. It is your brain that reads these scentless, flavorless molecules and vivifies them for your pleasure. Your brownie is sheet music. It is your brain that makes it a symphony. As with so much else, you experience the world that your brain allows you to experience.

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It has been calculated (and goodness knows how, it must be said) that during the course of a lifetime the heart does an amount of work sufficient to lift a one-ton object 150 miles into the air.

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The two numbers in a blood pressure reading—let’s say 120/80, or “120 over 80” when spoken—simply measure the highest and lowest pressures your blood vessels experience with each heartbeat. The first, higher number is the systolic pressure; the second, the diastolic. The numbers specifically measure how many millimeters of mercury is pushed up a calibrated tube.

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ne in three Americans dies of heart disease and more than 1.5 million suffer a heart attack or stroke each year.”

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Type 1 tends to be inherited; type 2 is usually a consequence of lifestyle. But it’s not quite as simple as that. Although type 2 is unequivocally associated with unhealthy living, it also tends to run in families, suggesting a genetic component.

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how comfortable and well nourished one was in the womb.

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Insulin is a hormone. Hormones are the bicycle couriers of the body, delivering chemical messages all around the teeming metropolis that is you. They are defined as any substance that is produced in one part of the body and causes an action somewhere else, but beyond that they are not easy to characterize. They come in different sizes, have different chemistries, go to different places, have different effects when they get there. Some are proteins, some are steroids, some are from a group called amines. They are linked by their purpose, not their chemistry. Our understanding of them is far from complete, and much of what we do know is surprisingly recent.

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“You can’t mistake death,” Ben says to me. “Live people look alive—and even more so on the inside than on the surface. When you open them up in surgery, the organs throb and glisten. They are clearly living things. But in death they lose that.”

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“The wrist is just a thing of beauty,” he goes on. “Everything has to go through there—muscles, nerves, blood vessels, everything—and yet it has to be completely mobile at the same time. Think of all the things your wrist has to do—take a lid off a jam jar, wave good-bye, turn a key in a lock, change a lightbulb. It’s a magnificent piece of engineering.”

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“Cartilage is remarkable, too. It is many times smoother than glass: it has a friction coefficient five times less than ice. Imagine playing ice hockey on a surface so smooth that the skaters went sixteen times as fast. That’s cartilage. But unlike ice, it isn’t brittle. It doesn’t crack under pressure as ice would. And you grow it yourself.

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The most influential anatomical work of the period—and indeed ever since—was Anatomy: Descriptive and Surgical, first published in 1858 in London and known ever since as Gray’s Anatomy, after its author, Henry Gray.

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Think of how jerky and un-lifelike most robots you have ever seen have been—how ploddingly they walk, how tippy they are on stairs or uneven ground, how hopelessly flummoxed they would be in trying to keep up with any three-year-old human at a playground—and you can appreciate what an accomplished creation we are.

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The hands and feet together have more than half the bones in the body.

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We tend to think of our bones as inert bits of scaffolding, but they are living tissue, too. They grow bigger with exercise and use just as muscles do.

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“Bone is stronger than reinforced concrete,” says Ben, “yet light enough to allow us to sprint.” All your bones together will weigh no more than about twenty pounds, yet most can withstand up to a ton of compression.

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“Bone is also the only tissue in the body that doesn’t scar,” Ben adds. “If you break your leg, after it heals you cannot tell where the break was. There’s no practical benefit to that. Bone just seems to want to be perfect.”

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Tendons and ligaments are connective tissues. Tendons connect muscles to bone; ligaments connect bone to bone. Tendons are stretchy; ligaments, less so. Tendons are essentially extensions of muscles.

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All of these things—muscles, bones, tendons, and so on—work together in a deft and splendid choreography. Nowhere is this better demonstrated than in your hands. In each hand you have 29 bones, 17 muscles (plus 18 more that are in the forearm but control the hand), 2 main arteries, 3 major nerves (one of which, the ulnar nerve, is the one you feel in your elbow when you hit your “funny bone”) plus 45 other named nerves, and 123 named ligaments, all of which must coordinate their every action with precision and delicacy. Sir Charles Bell, the great nineteenth-century Scottish surgeon and anatomist, thought the hand the most perfect creation in the body—better even than the eye. He called his classic text The Hand: Its Mechanism and Vital Endowments as Evincing Design, by which he meant that the hand was proof of divine creation.

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“The thumb isn’t just a stubbier shape from the other digits,” Ben Ollivere told me. “It’s actually attached differently. Almost no one ever notices it, but our thumbs are on sideways. The thumbnail faces away from the rest of the fingers. On a computer keyboard you strike the keys with the tips of your fingers but with the side of your thumb. That’s what is meant by an opposable thumb. It means we are really good at grasping. The thumb also rotates well—it swings through quite a wide arc—compared with the fingers.”

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The bulkier an animal is, the more massive its bones must be. So an elephant is 13 percent bone, whereas a tiny shrew needs to devote just 4 percent of itself to skeleton. Humans fall in between at 8.5 percent.

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We are not the speediest of creatures, as anyone who has ever chased a dog or cat or even an escaped hamster will know. The very fastest humans can run about twenty miles an hour, though only for short bursts. But put us up against an antelope or wildebeest on a hot day and allow us to trot after it, and we can run it into the ground. We perspire to keep cool, but quadrupedal mammals lose heat by respiration—by panting. If they can’t stop to collect themselves, they overheat and become helpless. Most large animals can’t run for more than about nine miles before they drop.

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All these changes, in lifestyle and anatomy, happened with exceeding slowness. Fossil evidence suggests that early hominins were walking by about 6 million years ago, but needed an additional 4 million years to acquire the capabilities for endurance running and, with it, persistence hunting. Then a further million and a half years had to pass before they gathered enough cerebral momentum to manufacture tipped spears. That’s a long time to wait for a full set of survival capabilities in a hostile, hungry world. Despite these deficiencies, our ancient forebears were successfully hunting large animals 1.9 million years ago.

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Until recent times, no other animal on Earth was more likely to die in childbirth than a human, and perhaps none even now suffers as much.

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Study after study since then has shown that exercise produces extraordinary benefits. Going for regular walks reduces the risk of heart attack or stroke by 31 percent. An analysis of 655,000 people in 2012 found that being active for just eleven minutes a day after the age of forty yielded 1.8 years of added life expectancy. Being active for an hour or more a day improved life expectancy by 4.2 years.

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“you can’t digest food while you are exercising because the body shunts blood away from the digestive system in order to meet the increased demand to supply oxygen to the muscles.

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As James Hamblin put it in The Atlantic, “You can’t undo sitting.” In fact, people with sedentary occupations and sedentary lifestyles—which is to say, most of us—can easily sit for fourteen or fifteen hours a day, and thus be completely and unhealthily immobile for all but a tiny part of their existence.

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One area where animals are curiously—almost eerily—uniform is with the number of heartbeats they have in a lifetime. Despite the vast differences in heart rates, nearly all animals have about 800 million heartbeats in them if they live an average life. The exception is humans. We pass 800 million heartbeats after twenty-five years, and just keep on going for another fifty years and 1.6 billion heartbeats or so.

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An increase of only a degree or so in body temperature has been shown to slow the replication rate of viruses by a factor of two hundred—an astonishing increase in self-defense from only a very modest rise in warmth.

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Body size has a great deal to do with how we are affected by gravity. It will not have escaped your notice that a small bug that falls off a tabletop will land unharmed and continue on its way unperturbed. That is because its small size (strictly, its surface area-to-volume ratio) means that it is scarcely affected by gravity. What is less well known is that the same thing applies, albeit on a different scale, to small humans. A child half your height who falls and strikes her head will experience only one thirty-second the force of impact that a grown person would feel, which is part of the reason that children so often seem to be mercifully indestructible.

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Even with the advantage of clothing, shelter, and boundless ingenuity, humans can manage to live on only about 12 percent of Earth’s land area and just 4 percent of the total surface area if you include the seas. It is a sobering thought that 96 percent of our planet is off-limits to us.

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The existence of Unit 731 was a well-guarded secret, by Japanese and American officials alike, and would have remained unknown to the wider world forever except that in 1984 a student from Keio University in Tokyo came across a box of incriminating documents in a secondhand bookshop and brought them to the attention of others.

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If you are stressed or exhausted, you are much more likely to suffer an infection, for instance.

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Unlike red blood cells, white blood cells can leave the circulatory system to pass through surrounding tissues, like an army patrol searching through jungle.

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Equally bewildering is that autoimmune diseases are grossly sexist. Women are twice as likely as men to get multiple sclerosis, ten times more likely to get lupus, fifty times more likely to suffer a thyroid condition known as Hashimoto’s thyroiditis. Altogether, 80 percent of all autoimmune diseases occur in women.

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I am in the habit of going to sea whenever I begin to grow hazy about the eyes, and begin to be over conscious of my lungs. —HERMAN MELVILLE, MOBY-DICK

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“Sinuses are strange,” Ben Ollivere of the University of Nottingham and Queen’s Medical Centre told me one day. “They are just cavernous spaces in your head. You would have room for a good deal more gray matter if you didn’t have to devote so much of your head to the sinuses.” The space isn’t a complete void, but rather is riddled with a complex network of bones, which are thought to help with breathing efficiency, though no one can say quite how.

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Interestingly, the discomfort you feel when you hold your breath for too long is caused not by the depletion of oxygen but by a buildup of carbon dioxide. That’s why the first thing you do when you stop holding your breath is blow out.

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Globally, asthma is more common among boys than girls before puberty, but more common in girls than boys after puberty. It is more common in blacks than whites (generally but not everywhere) and in city people than rural people. In children, it is closely associated with both being obese and being underweight; obese children get it more often, but underweight children get it worse. The highest rate in the world is in the U.K., where 30 percent of children have shown asthma symptoms. The lowest rates are in China, Greece, Georgia, Romania, and Russia, with just 3 percent. All the English-speaking nations of the world have high rates, as do those of Latin America.

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There is no cure, though in 75 percent of young people asthma resolves itself by the time they reach early adulthood. No one knows how or why that happens either, or why it doesn’t happen for the unfortunate minority. Indeed, where asthma is concerned, no one knows much of anything.

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“You probably think asthma is caused by dust mites or cats or chemicals or cigarette smoke or air pollution,” says Neil Pearce, professor of epidemiology and biostatistics at the London School of Hygiene and Tropical Medicine. “I have spent thirty years studying asthma, and the main thing I have achieved is to show that almost none of the things people think cause asthma actually do. They can provoke attacks if you have asthma already, but they don’t cause it. We have very little idea what the primary causes are. We can do nothing to prevent it.”

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A hiccup is a sudden spasmodic contraction of the diaphragm, which essentially startles the larynx into closing abruptly, making the famous hic sound. No one knows why they happen.

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In the beginning, vitamins were named in more or less strict alphabetical order—A, B, C, D, and so on—but then the system began to fall apart. Vitamin B was discovered to be not one vitamin but several, and these were renamed B1, B2, B3, and so on up to B12. Then it was decided that the B vitamins weren’t so diverse after all, so some were eliminated and others reclassified, so that today we are left with six semi-sequential B vitamins: B1, B2, B3, B5, B6, and B12.

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In simplest terms, a protein is a chain of amino acids.

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Carbohydrates and fats are the principal fuel reserves of the body, but they are stored and used in different ways. When the body needs fuel, it tends to burn up the available carbohydrates and store any spare fat. The main point to bear in mind—and you are no doubt well aware of it each time you take your shirt off—is that the human body likes to hold on to its fat.

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The conviction that we should all drink eight glasses of water a day is the most enduring of dietary misunderstandings. The idea has been traced to a 1945 paper from the U.S. Food and Nutrition Board, which noted that that was the amount that the average person consumed in a day. “What happened,” Dr. Stanley Goldfarb of the University of Pennsylvania told the BBC radio program More or Less in 2017, “was that people sort of confused the idea that this was the required intake. And the other confusion that occurred was then people said that it is not so much that you should take in eight ounces eight times a day, but that you should consume that in addition to whatever fluid you consume in association with your diet and your meals. And there was never any evidence for that.”

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In the United States, we are left in the bizarre and paradoxical situation that we are essentially the world’s most overfed nation but also one of its most nutritionally deficient ones.

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As we part, he adds an afterthought. “There’s a really easy way to do food shopping in supermarkets,” he says. “Just stick to the outside aisles. The aisles in between are almost entirely filled with processed foods. If you stick to the outside, you will automatically have a healthier diet.”

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Just because you exercise regularly and eat a lot of salad doesn’t mean you have bought yourself a better life span. What you have bought is a better chance of having a better life span.

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Happiness is a good bank account, a good cook and a good digestion. —JEAN-JACQUES ROUSSEAU

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Equally pernicious is salmonella, which has been called “the most ubiquitous pathogen in nature.” According to a USDA study, about a quarter of all chicken pieces sold in stores are contaminated with salmonella.

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researchers from the University of Chicago kept ten rats awake until they died and discovered that it took between eleven and thirty-two days for exhaustion to fatally overcome them. Postmortems showed the rats had no abnormalities that could explain their deaths. Their bodies just gave up on them.

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And yet it is universally assumed that sleep must answer some deep elemental need. As the eminent sleep researcher Allan Rechtschaffen observed many years ago, “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.” Nonetheless, as far as we know, all sleep does is (in the word of another researcher) “make us fit to be awake.”

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Typically, a man will be erect for two hours or so a night.

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Most of us have experienced that abrupt feeling of falling while asleep known as a hypnic or myoclonic jerk.

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That may seem far-fetched, but it is a curious fact, when you think about it, that no matter how profoundly unconscious we get, or how restless, we almost never fall out of bed, even unfamiliar beds in hotels and the like. We may be dead to the world, but some sentry within us keeps track of where the bed’s edge is and won’t let us roll over it (except in unusually drunk or fevered circumstances).

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“What’s really interesting about these third receptors,” Foster told me when we met in his office at Brasenose College, just off the High Street, “is that they function completely independently of sight. As an experiment, we asked a lady who was completely blind—she had lost her rods and cones as a result of a genetic disease—to tell us when she thought the lights in the room were switched on or off. She told us not to be ridiculous because she couldn’t see anything, but we asked her to try anyway. It turned out she was right every time. Even though she had no vision—no way of ‘seeing’ the light—her brain detected it with perfect fidelity at a subliminal level. She was astonished. We all were.”

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Melatonin levels within us rise as evening falls and peak in the middle of the night, so it would seem logical to associate them with drowsiness, but in fact melatonin production also rises at night in nocturnal animals when they are most active, so it is not promoting sleepiness.

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In a famous experiment in 1962, a French scientist named Michel Siffre isolated himself for about eight weeks deep inside a mountain in the Alps. Without daylight, clocks, or other clues to the passage of time, Siffre had to guess when twenty-four hours had elapsed and discovered to his astonishment that when he had calculated thirty-seven days to have passed, it was actually fifty-eight.

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For many people, the principal cause of insomnia is the snoring of a partner. It is a very common problem. About half of us snore at least sometimes. Snoring is the rattling of the soft tissues in the pharynx when one is unconscious and relaxed. The more relaxed, the greater the snoring, which is why drunken people snore particularly robustly. The best way to reduce snoring is to lose weight, sleep on your side, and not drink alcohol before retiring.

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Finally, we should say a word about that mysterious but universal harbinger of weariness, the yawn. No one understands why we yawn. Babies yawn in the womb. (They hiccup, too.) People in comas yawn. It is a ubiquitous part of life, and yet what exactly it does for us is unknown.

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As Siddhartha Mukherjee observed in The Gene: An Intimate History, humans don’t actually reproduce at all. Geckos reproduce; we recombine.

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In either case, overwhelmingly they fail. The chances of a successful fertilization from a single randomly timed act of sex have been calculated to be only about 3 percent.

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But if all goes well, within a week the zygote has produced ten or so cells known as pluripotent stem cells. These are the master cells of the body and one of the great miracles of biology. They determine the nature and organization of all the billions of cells that transform a little ball of possibility (known formally as a blastocyst) into a functioning and adorable little human (known as a baby).

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The average cost of childbirth in the United States is about $30,000 for a conventional birth and $50,000 for a Cesarean, about three times the cost for either in the Netherlands. Yet American women are 70 percent more likely to die in childbirth than women in Europe and about three times more likely to suffer a pregnancy-related fatality than women in Britain, Germany, Japan, or the Czech Republic.

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Only belatedly have researchers come to realize that the placenta does much more than just filter wastes and pass on oxygen. It takes an active role in the development of the child: stops toxins from passing from the mother to the fetus, kills parasites and pathogens, distributes hormones, and does everything it can to compensate for maternal deficiencies—if, say, the mother smokes or drinks or stays up too late. It is in a sense a kind of proto-mother for the developing baby. It can’t work miracles if the mother is truly deprived or neglectful, but it can make a difference.

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The person who first identified nociceptors—who can indeed fairly be called patriarch of the central nervous system altogether—was Charles Scott Sherrington (1857–1952), one of the greatest and most inexplicably forgotten British scientists of the modern era.

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Nobody knows quite why placebos work, but they do. In one experiment, people who had just had a wisdom tooth extracted had their faces massaged with an ultrasound device and overwhelmingly reported feeling better. What was interesting was that the treatment worked as well with the machine turned off as on.

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Smallpox is almost certainly the most devastating disease in the history of humankind. It infected nearly everyone who was exposed to it and killed about 30 percent of victims.

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on May 8, 1980, the World Health Organization announced that smallpox had been eradicated from Earth, the first and so far only human disease to be made extinct.

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Today some 40 percent of us will discover we have cancer at some point in our lives. Many, many more will have it without knowing it and will die of something else first.

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In 2000, a landmark paper in the journal Cell listed six attributes in particular that all cancer cells have, namely: They divide without limit. They grow without direction or influence from outside agents like hormones. They engage in angiogenesis, which is to say they trick the body into giving them a blood supply. They disregard any signals to stop growing. They fail to succumb to apoptosis, or programmed cell death. They metastasize,

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Two things can be said with confidence about life expectancy in the world today. One is that it is really helpful to be rich. If you are middle-aged, exceptionally well-off, and from almost any high-income nation, the chances are excellent that you will live into your late eighties. Someone who is otherwise identical to you but poor—exercises as devotedly, sleeps as many hours, eats a similarly healthy diet, but just has less money in the bank—can expect to die between ten and fifteen years sooner. That’s a lot of difference for an equivalent lifestyle, and no one is sure how to account for it.

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Among rich countries, America is at or near the bottom for virtually every measure of medical well-being—for chronic disease, depression, drug abuse, homicide, teenage pregnancies, HIV prevalence.

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There is an old joke in medicine that seems especially apt here: Q. What is the definition of a well person? A. Someone who hasn’t been examined

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But now it seems we have reached a point of diminishing returns. By one calculation, if we found a cure for all cancers tomorrow, it would add just 3.2 years to overall life expectancy. Eliminating every last form of heart disease would add only 5.5 years. That’s because people who die of these things tend to be old already, and if cancer or heart disease doesn’t get them, something else will. Of nothing is that more true than Alzheimer’s disease. Eradicating it altogether, according to the biologist Leonard Hayflick, would add just nineteen days to life expectancy.

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As Jo Marchant notes in her book Cure, Costa Ricans have only about one-fifth the personal wealth of Americans, and have poorer health care, but live longer.

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The bewildering fact is that it is possible to have dementia without having buildups of amyloid and tau, and it is equally possible to have amyloid and tau buildups without having dementia. One study found that about 30 percent of elderly people have substantial beta-amyloid accumulations but no hint of cognitive decline.

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