The Body: A Guide for Occupants

by Bill Bryson

also triggered by extreme joy or quiet rapture or intense pride or almost any other potent emotional state.*3 Producing tears involves an extraordinary

Altogether you produce about five to ten ounces of tears a day. The tears drain away through holes known as puncta on the little fleshy knob (known as the papilla lacrimalis) in the corner of each eye beside the nose. When you cry emotionally, the puncta cannot drain the fluid fast enough, so it overflows your eyes and runs down your cheeks. The iris is what gives

the words of Daniel McNeill, and these patterns are unique to each of us, which is why iris recognition devices are now increasingly used to identify us at security

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. People who

we still have just three kinds of color receptors compared with four for birds, fish, and reptiles. It’s a humbling fact, but virtually all nonmammalian creatures live in a visually richer world than we do. On the other hand, we make

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

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.

The tiny quiverings of the eardrum are passed on to the three smallest bones in the body, collectively known as ossicles and individually known as the malleus, incus, and stapes (or hammer, anvil, and stirrup, because of their very vague resemblances to those objects). The ossicles are perfect demonstrations of how evolution is so often a matter of make-do. They were jawbones in our ancient ancestors and only gradually migrated to new positions in our inner ear. For much of their history, those three bones had nothing to do with hearing. The ossicles

help protect us from the damage of really loud noises, we have something called an acoustic reflex, in which a muscle jerks the stapes away from the cochlea, essentially breaking the circuit, whenever a brutally intense sound is perceived, and it maintains that posture for some seconds afterward, which is why we are often deafened after an explosion. Unfortunately, the process is not perfect. Like any reflex, it is quick but not instantaneous, and it takes about a third of a second for the muscle to contract, by which point a lot of damage can

There isn’t any particular reason for this. Stereocilia grow back perfectly well in birds. They just don’t do it in us. The high-frequency ones are at the front and the low-frequency ones farther in. This means that all sound waves, high and low, pass over the high-frequency cilia, and this heavier traffic means they wear out more quickly.

Volume doubles about every 6 decibels, which means that a 96-decibel noise is not just a bit louder than a 90-decibel noise but twice as loud. The pain threshold for noise is about 120 decibels, and noises above 150 decibels can burst the eardrum. For purposes of comparison, a quiet place like a library or the countryside is about 30 decibels, snoring is 60 to 80 decibels, a really loud nearby thunderclap is 120

vestibular system does everything that a gyroscope does on an airplane, but in an extremely miniaturized form. Inside the vestibular channels is a gel that acts a little like the bubbles in a carpenter’s level, in 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

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. Another part of

interesting and important curiosity of our sense of smell is that it is the only one of the five basic senses not mediated by the hypothalamus. When we smell something, the information, for reasons unknown, goes straight to the olfactory cortex, which is nestled close to the hippocampus, where memories are shaped, and it is thought by some neuroscientists that that may explain why certain odors are so powerfully evocative of memories

He reached into his desk and pulled out a vial, which he uncapped and passed to me to sniff. I could smell nothing at all. “It’s a hormone called androsterone,” Beauchamp explained. “About a third of people, like you, can’t smell it. One-third smell something like urine, and one-third smell sandalwood.” His smile broadened. “If you have three people who cannot even agree on

One of the early symptoms of Alzheimer’s is smell loss. Ninety percent of people who lose smell through head injury never get the sense back; a smaller proportion, about 70 percent, who lose smell through infections

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. —

Take the tonsils. We are all familiar with them, but how many of us know quite what they do? In fact, nobody knows quite what they do. They are the two fleshy hummocks that stand sentinel on either side of the throat at the back. (Confusingly, in the nineteenth century they were often called amygdalae, even though that name was already applied to structures in the brain.) Adenoids are similar but lurk out of sight within the nasal cavity.

Altogether, fifty muscles can be called into play just to get a piece of food from your lips to your stomach, and they must snap to attention in exactly the right order to ensure that whatever you dispatch into the alimentary system doesn’t go down the wrong way and end up lodged in an airway, like Brunel’s coin. The complexity of human swallowing is largely because our larynx is low in the throat compared with other primates. To accommodate our upright posture when we became bipedal, our necks became longer and straighter and moved to a more central position beneath the skull rather than

By chance, these changes gave us greater aptitude for speech but also the danger of “tracheal obstruction,” in the words of Daniel Lieberman. The problem is that, we send our air and food down the same tunnel. Only a small structure called the epiglottis, a kind of trapdoor for the throat, stands between us and catastrophe. The epiglottis opens when we breathe and closes when we swallow, sending food in one direction and air in another, but occasionally it errs and the results are sometimes dire.

about five thousand people in the United States and some two hundred in Britain choke to death on food each year—which is odd because those figures, adjusted for population, indicate that Americans are five times more likely to asphyxiate while eating than Britons. Even allowing for the gusto with which my fellow Americans chow down, that seems unlikely. It is more probable

and in the process he built up an extraordinary collection of 2,374 imprudently ingested items. Today the Chevalier Jackson Foreign Body Collection is housed in a cabinet in the basement of the Mütter Museum of the College of Physicians of Philadelphia. Each object is fastidiously cataloged by age and sex of the swallower; type of object; whether it lodged in the trachea, larynx, esophagus, bronchus, stomach, pleural cavity, or elsewhere; whether it proved fatal or not; and by what means it was removed. It is presumed to be the world’s largest assemblage of the extraordinary

recently it was discovered that saliva also contains a powerful painkiller called opiorphin. It is six times more potent than morphine, though we have it only in very small doses, which is why you are not perennially high or indeed notably pain-free when you bite your cheek

Among these are amylase and ptyalin, which begin to break down sugars in carbohydrates while they are still in our mouths. Chew a starchy food like bread or potato for a bit longer than normal and you will soon notice a sweetness. Unfortunately for us, bacteria in our mouths like that sweetness, too; they devour the

produce very little saliva while we sleep, which is why microbes can proliferate then and give you a foul mouth to wake to. It is also why brushing your teeth at bedtime is a good idea because it reduces the number of bacteria you go to sleep with. If you’ve ever wondered why no one wants to kiss you first thing in the morning, it is possibly because your exhalations may contain up to 150 different chemical compounds, not all of them as fresh and minty as we might hope. Among the common chemicals that help to create morning mouth are methyl mercaptan (which smells very like old cabbage), hydrogen sulfide (like rotten eggs), dimethyl sulfide (slimy seaweed),

The outside of your tooth is the enamel. It is the hardest substance in the human body, but forms just a thin layer and can’t be replaced if it is damaged. That’s why you have to go to the dentist for cavities. Under the enamel is a much thicker layer of another mineralized

been called “ready-made fossils.” When all the rest of you has turned to dust or dissolved away, the last physical trace of your existence on Earth may be a fossilized molar.

Still, when you consider how well you can demolish, say, an ice cube (try doing that with your fists and see how far you get) and how little space the five muscles of the jaw occupy, you can appreciate that human chomping is pretty capable. —

Altogether we have about ten thousand taste buds, mostly distributed around the tongue, except in the very middle, where there are none at all. Additional taste

are found in the roof of the mouth and lower down the throat, which is said to be why some medicines taste bitter as they go down. As well as the mouth, the body has taste receptors in the gut and throat (to help identify spoiled or toxic substances), but they don’t connect to the brain in the same way as the taste receptors on your tongue, and for good reason. You don’t want to taste what your stomach is tasting. Taste receptors have also been found in the heart, the lungs, and even the testicles. No one knows quite what they are doing there. They also send signals to the pancreas to adjust insulin output, and it may be connected to that.

difficulty with fugu is that by the time the ill effects become evident, it is much too late to do anything about it. The same is true of all kinds of other substances, from belladonna, or deadly nightshade, to a wide range of fungi. In 2008, in a widely

Nicholas Evans and

members of his family became deathly ill on holiday in Scotland when they mistook a deadly mushroom, Cortinarius speciosissimus, for its benign and delicious cousin cèpe. The effects were horrific—Evans needed a kidney transplant, and all memb...

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

So Scoville came up with something called the Scoville Organoleptic Test, which was a scientific method for measuring the hotness of any pepper. It is still the standard used today. A bell pepper will have a Scoville rating of between 50 and 100.

Capsaicin has been reported to lower blood pressure, fight inflammation, and reduce susceptibility to cancer, among quite a lot else of benefit to the average human. In a study reported in the British Medical Journal, Chinese adults who ate a lot of

Incidentally, we have pain detectors not only in the mouth but also in the eyes, anus, and vagina, which is why spicy foods can cause discomfort there. —

the West, umami is still a rather exotic concept. It is actually a comparatively recent term even in Japan, though the taste has been known for centuries. It comes from a popular fish stock called dashi, which is made from seaweed and dried fish scales, and when added to other foods makes them even more delicious and imparts an ineffable but distinctive flavor. In the early twentieth century, a Tokyo chemist named Kikunae Ikeda determined to identify the source of the flavor and to try to synthesize it. In 1909, he published a brief paper in a Tokyo journal, identifying the source of the flavor as the chemical glutamate, an amino acid. He dubbed the flavor umami, meaning “essence of deliciousness.” Ikeda’s discovery attracted virtually no attention

But in Japan, Ikeda became celebrated, not as a scientist so much, but rather as a co-founder of a great company, Ajinomoto, created to exploit his patent for making synthetic umami, in the form universally known today as monosodium glutamate, or MSG. Today Ajinomoto is a behemoth, making about one-third of all the world’s MSG. MSG

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. The tongue and its taste buds give us just the basic

What we appreciate when we eat is flavor, which is taste plus smell.*3 Smell is said to account for at least 70 percent of flavor, and maybe even as much as 90 percent. We appreciate this intuitively without often thinking about it. If someone hands you a pot of yogurt and says, “Is this strawberry?” your response will normally be to

easy way to experience the limitations of your taste buds is to close your eyes, pinch shut your nostrils, and eat a flavored jelly bean collected blindly from a bowl. You will instantly apprehend its sweetness, but you almost certainly won’t be able to identify its flavor. But open your eyes and nostrils and its fruity specificity becomes immediately and redolently apparent. Even sound materially influences how delicious we find food. People who are played a range of crunching sounds through headphones while sampling potato chips from various bowls will always rate the crunchier, noisier chips as fresher and tastier, even though all the chips are the same. Many tests have been done to demonstrate how easily we are fooled with respect to flavor. In a blind taste test at the University of Bordeaux, students in the faculty of enology were given two glasses of wine, one red and one white. The wines were actually identical except that one had been made a rich red with an odorless and flavorless additive. The students without exception

a bite and savor the velvety smoothness, the rich heady waft of chocolate that fills your head. 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

Stuttering is one of the cruelest and least understood of everyday maladies. It affects 1 percent of adults and 4 percent of children. For reasons unknown, 80 percent

sufferers are male. The victims have included a great many distinguished figures—Aristotle, Virgil, Charles Darwin, Lewis Carroll, Winston Churchill (when young), Henry James, John Updike, Marilyn Monroe, and King George VI of Great Britain, who was sympathetically portrayed

No one knows what provokes it or why different sufferers stumble over different letters or words in different positions in a sentence. It is more common among left-handers than right-handers, especially those who have been made to write right-handed. For many, the stammering miraculously ceases when they sing the words or speak another language or talk to th...

perhaps worth noting that in 2011 a researcher at the Karolinska Institute in Stockholm noticed

that people who had

tonsils removed while young had a 44 percent greater chance of having a heart attack in later life. Of course, the two events may only be coincidentally related, but in the absence of conclusive evidence it suggests that it might be prudent to leave the tonsils alone. The same study also found that people who kept their appendixes had a 33 percent reduced chance of a heart attack ...