The Body: A Guide for Occupants

by Bill Bryson

Well, you blink fourteen thousand times a day—so much that your eyes are shut for twenty-three minutes of every waking day.

Your lungs, smoothed out, would cover a tennis court, and the airways within them would stretch nearly from coast to coast. The length of all your blood vessels would take you two and a half times around Earth. The most remarkable part of all is your DNA (or deoxyribonucleic acid). You have a meter of it packed into every cell, and so many cells that if you formed all the DNA in your body into a single strand, it would stretch ten billion miles, to beyond Pluto. Think of it: there is enough of you to leave the solar system. You are in the most literal sense cosmic.

Your DNA is simply an instruction manual for making you. A length of DNA is divided into segments called chromosomes and shorter individual units called

genes. The sum of all your genes is the genome.

An interesting thing about touch is that the brain doesn’t just tell you how something feels, but how it ought to feel. That’s why the caress of a lover feels wonderful, but the same touch by a stranger would feel creepy or horrible. It’s also why it is so hard to tickle yourself.

One of the most memorably unexpected events I experienced in the course of doing this book came in a dissection room at the University of Nottingham in England when a professor and surgeon named Ben Ollivere (about whom much more in due course) gently incised and peeled back a sliver of skin about a millimeter thick from the arm of a cadaver. It was so thin as to be translucent. “That,” he said, “is where all your skin color is. That’s all that race is—a sliver of epidermis.” I mentioned this to Nina Jablonski when we met in her office in State College, Pennsylvania, soon afterward. She gave a nod of vigorous assent. “It is extraordinary how such a small facet of our composition is given so much importance,” she said. “People act as if skin color is a determinant of character when all it is is a reaction to sunlight. Biologically, there is actually no such thing as race—nothing in terms of skin color, facial features, hair type, bone structure, or anything else that is a defining quality among peoples. And yet look how many people have been enslaved or hated or lynched or deprived of fundamental rights through history because of the color of their skin.”

melanin. It is one of the oldest molecules in biology and is found throughout the living world. It doesn’t just color skin. It gives birds the color of their feathers, fish the texture and luminescence of their scales, squid the purply blackness of their ink. It is even involved in making fruits go brown. In us, it also colors our hair. Its production slows dramatically as we age, which is why older people’s hair tends to turn gray.

It has been suggested that light skin may be a consequence of human migration and the rise of agriculture. The argument is that hunter-gatherers got a lot of their vitamin D from fish and game and that these inputs fell sharply when people started growing crops, especially as they moved into northern latitudes. It therefore became a great advantage to have lighter skin, to synthesize extra vitamin D.

The red of sunburn is because the tiny blood vessels in the affected areas become engorged with blood, making the skin hot to the touch. The formal name for sunburn is erythema. Pregnant women frequently undergo a darkening of the nipples and areolae, and sometimes of other parts of the body such as the abdomen and face, as a result of increased production of melanin. The process is known as melasma, but its purpose is not understood. The flush we get when angry is a little counterintuitive. When the body is poised for a fight, it mostly diverts blood flow to where it is really needed—namely, the muscles—so why it would send blood to the face, where it confers no obvious physiological benefit, remains a mystery. One possibility suggested by Jablonski is that it helps in some way to mediate blood pressure. Or it could just serve as a signal to an opponent to back off because one is really angry. At all events, the slow evolution of different skin tones worked fine when people stayed in one place or migrated slowly, but nowadays increased mobility means that lots of people end up in places where sun levels and skin tones don’t get along at all.

Sweating is activated by the release of adrenaline, which is why when you are stressed, you break into a sweat.

Unlike the rest of the body, the palms don’t sweat in response to physical exertion or heat, but only from stress. Emotional sweating is what is measured in lie detector tests.

You lose, on average, between fifty and a hundred head hairs every day, and sometimes they don’t grow back. About 60 percent of men are “substantially bald” by the age of fifty.

Humans produce twenty digestive enzymes, which is a pretty respectable number in the animal world, but bacteria produce ten thousand, or five hundred times as many, according to Christopher Gardner of Stanford University. “Our lives would be vastly less well nourished without them,” he says.

though it should also be noted that 85 percent of our own cells are red blood cells, which aren’t true cells at all, because they don’t have any of the usual machinery of cells (like nuclei and mitochondria), but are really just containers for hemoglobin.

Passionate kissing alone, according to one study, results in the transfer of up to one billion bacteria from one mouth to another, along with about 0.7 milligrams of protein, 0.45 milligrams of salt, 0.7 micrograms of fat, and 0.2 micrograms of “miscellaneous organic compounds” (that is, bits of food).

On the other hand, that is still a lot of ways to be unwell, and together those 1,415 tiny, mindless entities cause one-third of all the deaths on the planet.

Of the hundreds of thousands of viruses reasonably supposed to exist, just 586 species are known to infect mammals, and of these only 263 affect humans.

This is the virus that gives you chicken pox when you are small, but then may sit inert in nerve cells for half a century or more before erupting in that horrid and painful indignity of old age known as shingles. It

The only really reliable way to transfer cold germs is physically by touch.

The story of Fleming’s accidental discovery of penicillin has been told many times, but hardly any two versions are quite the same. The first thorough account of the discovery was not published until 1944, a decade and a half after the events it describes, by which time details were already blurring, but as best as can be said, the story seems to be this: In 1928, while Alexander Fleming was away on a holiday from his job as a medical researcher at St. Mary’s Hospital in London, some spores of mold from the genus Penicillium drifted into his lab and landed on a petri dish that he had left unattended. Thanks to a sequence of chance events—that Fleming hadn’t cleaned up his petri dishes before departing on holiday, that the weather was unusually cool that summer (and thus good for spores), that Fleming remained away long enough for the slow-growing mold to act—he returned to find that the bacterial growth in the petri dish had been conspicuously inhibited.

Even more appallingly, in the United States 80 percent of antibiotics are fed to farm animals, mostly to fatten them. Fruit growers can also use antibiotics to combat bacterial infections in their crops.

For an object of pure wonder, the human brain is extraordinarily unprepossessing. It is, for one thing, 75 to 80 percent water, with the rest split mostly between fat and protein.

The brain is often depicted as a hungry organ. It makes up just 2 percent of our body weight but uses 20 percent of our energy. In newborn infants, it’s no less than 65 percent.

Neurons are not like other cells, which are typically compact and spherical. Neurons are long and stringy, the better to pass on electrical signals from one to another. The main strand of a neuron is called an axon. At its terminal end, it splits into branch-like extensions called dendrites, as many as 400,000 of them. The tiny space between nerve cell endings is called a synapse. Each neuron connects with thousands of other neurons, giving trillions and trillions of connections—as many connections “in a single cubic centimeter of brain tissue as there are stars in the Milky Way,” to quote the neuroscientist David Eagleman.

Beneath the cerebrum, at the very back of the head about where it meets the nape of the neck, is the cerebellum (Latin for “little brain”). Although the cerebellum occupies just 10 percent of the cranial cavity, it has more than half the brain’s neurons. It has a lot of neurons not because it does a great deal of thinking but because it controls balance and complex movements, and that requires an abundance of wiring. At the base of the brain, descending from it rather like an elevator shaft connecting the brain to the spine and the body beyond, is the oldest part of the brain, the brain stem. It is the home of our more basic operations: sleeping, breathing, keeping the heart going. The brain stem doesn’t get a lot of attention in the popular consciousness, but it is so central to our existence that “brain-stem death” is the fundamental measure of deadness in humans in the United Kingdom.

Scattered through the brain rather like nuts in a fruitcake are many smaller structures—hypothalamus, amygdala, hippocampus, telencephalon, septum pellucidum, habenular commissure, entorhinal cortex, and a dozen or so others—which are collectively known as the limbic system (from the Latin limbus, meaning “peripheral”).

The basal ganglia, for instance, play an important part in movement, language, and thought, but it is only when they degenerate and lead to Parkinson’s disease that they normally attract attention to themselves. Despite their obscurity and modest dimensions, the structures of the limbic system have a fundamental role in our happiness by controlling and regulating basic processes like memory, appetite, emotions, drowsiness and alertness, and the processing of sensory information.

The most important component of the limbic system is a little powerhouse called the hypothalamus, which isn’t really a structure at all but just a bundle of neural cells. The name describes not what it does but where it is: under the thalamus. (The thalamus, meaning “inner chamber,” is a kind of relay station for sensory information and is an important part of the brain—there isn’t any part of the brain that isn’t important, obviously—but is not a component of the limbic system.) The hypothalamus is curiously unimposing. Though only about the size of a peanut and weighing barely a tenth of an ounce, it controls much of the most important chemistry of the body. It regulates sexual function, controls hunger and thirst, monitors blood sugar and salts, decides when you need to sleep. It may even play a part in how slowly or rapidly we age. A large measure of your success or failure as a human being is dependent on this tiny thing in the middle of your head. The

To help us deal better with this fractional lag, the brain does a truly extraordinary thing: it continuously forecasts what the world will be like a fifth of a second from now, and that is what it gives us as the present. That means that we never see the world as it is at this very instant, but rather as it will be a fraction of a moment in the future. We spend our whole lives, in other words, living in a world that doesn’t quite exist yet.

The nucleus accumbens, a region of the forebrain associated with pleasure, grows to its largest size in one’s teenage years. At the same time, the body produces more dopamine, the neurotransmitter that conveys pleasure, than it ever will again. That is why the sensations you feel as a teenager are more intense than at any other time of life. But it also means that seeking pleasure is an occupational hazard for teenagers. The leading cause of deaths among teenagers is accidents—and the leading cause of accidents is simply being with other teenagers. When more than one teenager is in a car, for instance, the risk of an accident multiplies by 400 percent.

One of the great (and, it must be said, most written about) events in early neuroscience occurred in 1848 in rural Vermont when a young railroad builder named Phineas Gage was packing dynamite into a rock and it exploded prematurely, shooting a two-foot tamping rod through his left cheek and out the top of his head before it clattered back to Earth about fifty feet away. The rod removed a perfect core of brain about an inch in diameter. Miraculously, Gage survived and appears not even to have lost consciousness, though he did lose his left eye and his personality was forever transformed.

Previously happy-go-lucky and popular, he was now moody, argumentative, and given to profane outbursts. He was just “no longer Gage,” as one old friend reported sadly. As often happens to people with frontal lobe damage, he had no insight into his condition and didn’t understand that he had changed. Unable to settle, he drifted from New England to South America and on to San Francisco, where he died aged thirty-six after falling prey to seizures.

In the United States, a doctor named Walter Jackson Freeman heard of Moniz’s procedure and became his most enthusiastic acolyte. Over a period of almost forty years, Freeman traveled the country performing lobotomies on almost anyone brought before him. On one tour, he lobotomized 225 people in twelve days. Some of his patients were as young as four years old. He operated on people with phobias, on drunks picked up off the street, on people convicted of homosexual acts—on anyone, in short, with almost any kind of perceived mental or social aberration. Freeman’s method was so swift and brutal that it made other doctors recoil. He inserted a standard household ice pick into the brain through the eye socket, tapping it through the skull bone with a hammer, then wriggled it vigorously to sever neural connections. Here is his breezy description of the procedure in a letter to his son:

What is perhaps most remarkable is that Freeman was a psychiatrist with no surgical certification, a fact that horrified many other physicians. About two-thirds of Freeman’s subjects received no benefit from the procedure or were worse off. Two percent died. His most notorious failure was Rosemary Kennedy, sister of the future president. In 1941, she was twenty-three years old, a vivacious and attractive girl but headstrong and with a tendency to mood swings. She also had some learning difficulties, though these seem not to have been nearly as severe and disabling as has sometimes been reported. Her father, exasperated by her willfulness, had her lobotomized by Freeman without consulting his wife. The lobotomy essentially destroyed Rosemary. She spent the next sixty-four years in care homes, incontinent, unable to speak, and bereft of personality. Her loving mother did not visit her for twenty years.

As recently as 1956, it was illegal in seventeen U.S. states for epileptics to marry; in eighteen states, epileptics could be involuntarily sterilized.

Anton-Babinski syndrome, for instance, is a condition in which people are blind but refuse to believe it. In Riddoch syndrome, victims cannot see objects unless they are in motion. Capgras syndrome is a condition in which sufferers become convinced that those close to them are impostors. In Klüver-Bucy syndrome, the victims develop an urge to eat and fornicate indiscriminately (to the understandable dismay of loved ones). Perhaps the most bizarre of all is Cotard delusion, in which the sufferer believes he is dead and cannot be convinced otherwise.

This instinct on the part of European authorities to prove all other races inferior was widespread, if not universal. In England, in 1866 the eminent physician John Langdon Haydon Down (1828–96) first described the condition that we now know as Down’s syndrome in a paper called “Observations on an Ethnic Classification of Idiots,” but he referred to it as “Mongolism” and its victims as “Mongoloid idiots” in the belief that they were suffering an innate regression to an inferior, Asiatic type. Down believed, and no one seems to have doubted him, that idiocy and ethnicity were conjoined qualities. He also listed “Malay” and “Negroid” as regressive types.

You might have had the experience of looking at a clear blue sky on a sunny day and seeing little white sparks popping in and out of existence, like the briefest of shooting stars. What you are seeing, amazingly enough, is your own white blood cells, moving through a capillary in front of the retina. Because white blood cells are big (compared with red blood cells), they sometimes get stuck briefly in the narrow capillaries, and that is what you are seeing. The technical name for these disturbances is Scheerer’s blue field entoptic phenomena (named for a German ophthalmologist of the early twentieth century, Richard Scheerer), though they are more commonly and poetically known as blue sky sprites. They are especially visible against a bright blue sky simply because of the way the eye absorbs different wavelengths of light.

Our eyes contain two types of photoreceptors for vision—rods, which help us see in dim conditions but provide no color, and cones, which work when the light is bright and divide the world up into three colors: blue, green, and red.

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 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.

The ear is also responsible for keeping you balanced thanks to a tiny but ingenious collection of semicircular ducts and two tiny associated sacs called otolith organs, which together are called the vestibular system.

that the gel’s movements from side to side or up and down tell the brain in which direction we are traveling (which is how you can sense whether you are going up or down in an elevator even in the absence of visual clues).

The reason we feel dizzy when we jump from a merry-go-round is that the gel keeps moving even though the head has stopped, so the body is temporarily disoriented.

That gel thickens as we age and doesn’t slosh around as well, which is one reason why the elderly are often not so steady on their feet (and why they especially shouldn’t jump from moving objects). When loss of balance is prolonged or severe, the brain doesn’t know quite what to make of it and interpr...

Under the enamel is a much thicker layer of another mineralized tissue called dentin, which can renew itself. At the center of it all is the fleshy pulp with nerves and blood supply.

They come in three different shapes: circumvallate (or rounded), fungiform (mushroom shaped), and foliate (leaf shaped). They are among the most regenerative of all cells in the body and are replaced every ten days.

We have about ten thousand taste receptors, but we actually have more pain and other somatosensory receptors than taste receptors in our mouths. Because they exist side by side on the tongue, we sometimes mix them up. When you describe a chili as hot, you are being more literal than you might suppose. Your brain interprets it as being actually burned. As Joshua Tewksbury of the University of Colorado has put it, “Chilies innervate the same neurons that you activate when you touch a 335-degree burner. Essentially, our brain is telling us that we have got our tongue on the stove.” In the same way, menthol is perceived as being cool even in the heated smoke of a cigarette.

The active ingredient in all chili peppers is a chemical called capsaicin. When you ingest capsaicin, the body releases endorphins—it’s not at all clear why—and that provides us with a literally warm glow of pleasure. As with any warmth, however, it can quickly grow uncomfortable and then intolerable.

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.