The Body: A Guide for Occupants

by Bill Bryson

liver was long thought to be the seat of courage, which is why a cowardly person was deemed “lily-livered.” It was also considered the source of two of

In this context, humor has nothing to do with amusement. It comes from a Latin word for “moisture.” When we talk today of humoring someone or of people being ill-humored, we are not talking about their capacity for laughter, at least not etymologically.

The pancreas is a gland and the spleen is not. The pancreas is essential to life; the spleen is expendable. The pancreas is a jellylike organ, about six inches long, shaped roughly like a banana (and about the same size), tucked behind the stomach in the upper abdomen. As well as insulin, it produces the hormone glucagon, which is also involved in regulating blood sugar, and the digestive enzymes trypsin, lipase, and amylase, which help digest cholesterol and fats. Altogether every day it produces over a quart of pancreatic juice, a pretty prodigious amount for an organ of its size. The pancreas of an animal when cooked for consumption is known

sweetbread (the word is first recorded in English in 1565), but no one has ever worked out why, because there is nothing sweet or bread-like about it. “Pancreas” isn’t recorded in English until late the following decade, so “sweetbread” is actually the older term. The spleen

Medical students learn to remember the principal attributes of the spleen by counting upward in odd numbers; 1, 3, 5, 7, 9, 11. That is because the spleen is 1 x 3 x 5 inches in size, weighs

7 ounces,

lies between the 9th and the 11th ribs—though in fact all those numbers but the last two are merely averages. Just beneath the liver and also closely associated with it is the gallbladder. It is a curious organ in that many animals have gallbladders and many do not. Giraffes, oddly, sometimes have gallbladders and sometimes don’t. In humans, the gallbladder stores bile from the liver and passes it on to the intestines. (“Gall” is an old word for “bile.”) The chemistry can go wrong for a variety of reasons, resulting in gallstones. Gallstones are a common complaint and were traditionally said to be most often found among women who were “fat, fair, fertile,...

was the first anyone realized that humans can survive with just one kidney. It remains something of a mystery even now as to why we have two kidneys. It is splendid to have a backup, of course, but we don’t get two hearts or livers or brains, so why we have a surplus kidney is a happy imponderable. The kidneys are invariably called the workhorses of

process about 190 quarts of water—that is the amount a bath holds up to the overflow level—and 3.3 pounds of salt. They are startlingly small for the amount of work they do, weighing just five ounces each. They are not in the small of the back, as everyone thinks, but higher up, about at the bottom of the rib cage. The right kidney is always lower because it is pressed down upon by the asymmetrical liver. Filtering wastes is their principal function, but they

Eat too much salt and your kidneys filter out the excess from your blood and send it to the bladder so that you can pee it all away. Eat too little and the kidneys take it back from the urine before it leaves

than most other organs, the kidneys lose function as you age. Between the ages of forty and seventy, their filtration capacity drops by about 50 percent. Kidney stones become more common, as do more life-threatening illnesses. The death rate from chronic kidney disease has jumped by more than 70 percent since 1990 in the United States and by even more in some third world countries. Diabetes is the commonest

The urinary bladder is rather like a balloon in that it is designed to swell as we fill it. (In an average-sized man it holds about a British pint, or about six-tenths of a quart; in a woman, rather less.) As we age, the bladder loses elasticity and can’t expand as it once

which is part of the reason old people spend much of their lives scouting for restrooms, according to Sherwin Nuland in How We Die. Until very recently, it was thought that the urine and bladder are normally sterile. Occasionally bacteria might sneak in and give us a urinary

diarist Samuel Pepys in 1658, when he was twenty-five years old. This was two years before Pepys started his diary, so we don’t have a firsthand account of the experience, but he mentioned it frequently and vividly thereafter (including in the diary’s very first entry when he finally started it) and lived in loquacious dread of ever having to undergo anything like it again. It’s not hard to see why. Pepys’s stone was the size of a tennis ball (albeit a seventeenth-century tennis ball, which was slightly smaller than a modern tennis ball, though the distinction could fairly be called academic to anyone carrying one). While four men held Pepys down, the surgeon, Thomas Hollyer, inserted an instrument called an itinerarium up his penis and into the bladder to fix the stone in place. Then he took a scalpel and quickly and deftly—but excruciatingly—cut a three-inch-long incision through the perineum

Peeling back the opening, he gently cut into the exposed and quivering bladder, thrust a pair of duck-billed forceps through the opening, captured the stone, and extracted it. The entire procedure from beginning to end took just fifty seconds but left Pepys bedridden for weeks and traumatized for life.* Hollyer charged Pepys twenty-four shillings for the operation, but it was money well spent. Hollyer was famous not just for his speed but also for the fact that his patients very generally survived. In one year, he performed forty lithotomies and lost not a single subject—an extraordinary achievement. Doctors in

experience is recorded fully and memorably in Claire Tomalin’s esteemed biography, Samuel Pepys: The Unequalled Self.

There is a lot of other stuff in there, too—threaded blood vessels and nerves and tendons, and lots and lots of intestines, all of it just kind of tipped in, as if this poor, anonymous, former person had had to pack himself

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,

well-known case was that of the celebrated Irish giant Charles Byrne (1761–83). At seven feet seven inches, Byrne was the

Your bones are by no means evenly distributed. You have fifty-two in your feet alone, double the number in your spine. The hands and feet together have more than half the bones in the body. Where you have lots of bones isn’t necessarily because

transmit sound (in the middle ear), and even possibly bolster our memory and buoy our spirits thanks to the recently discovered hormone osteocalcin. Until the early years of the twenty-first century, no one knew that bones produced hormones at all, but then a geneticist at Columbia University Medical Center, Gerard Karsenty, realized that osteocalcin, which is produced in bones, not only is a hormone but seems to be involved in a large number of important regulatory activities across the body, from helping to manage glucose levels to boosting male fertility to influencing our moods and keeping our

memory in working order. Apart from anything else, it could

explain the long-standing mystery of how regular exercise helps to stave off Alzheimer’s disease: exercise builds stronger bones and stronger bones produce more osteocalcin. Typically about 70 percent of a bone is inorganic material and 30 percent organic. The most fundamental element of bone is collagen. It is the most abundant pro...

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. “The bone in a professional tennis player’s serving arm may be 30 percent thicker than in his other arm,” Margy Pratten told me, and cited Rafael Nadal as an example. Look at bone through a microscope and you

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. When people speak of sinew, they are referring to tendons. If you want to see a tendon, it is easy to do so. Turn your hand

Tendons are strong, and generally it takes a lot of force to tear them, but they also have very little blood supply and therefore take a long time to heal. That at least is better than cartilage, which has no blood supply at all and therefore almost no capacity to heal. But the

Anatomists categorize them by what they do. Flexor muscles close joints, and extensor muscles open them; levators lift, and depressors lower; abductors move body parts away, and adductors draw them back; sphincters contract. Altogether you are about 40 percent muscle if you are a reasonably slender man, slightly

a proportionately similar woman, and just keeping that mass of muscle uses up 40 percent of your energy allowance

Because muscle is so expensive to maintain, we sacrifice muscle tone really quickly when we are not using it. Studies by NASA have shown that astronauts even on short missions—from five to eleven days—lose up to 20 percent of muscle mass. (They lose bone density, too.) All of these th...

The hand is a marvelous creation without question, but not all its parts are equal. If you curl your fingers into a fist, then try to straighten them out one at a time, you will find that the first two pop up obediently enough but the ring finger doesn’t seem to want to straighten out at all. Its position on the hand means that it can’t really contribute much to fine movement and so it has less in the way of discriminating musculature.

were a uniquely human attribute. In fact, most primates have opposable thumbs. Ours are just more pliant and mobile. What we do have in our thumbs are three small but resplendently named muscles not found in any other animals, including chimps: the extensor pollicis brevis, the flexor pollicis longus, and the first volar interosseous of Henle.* Working together, they allow us to grasp and manipulate tools with sureness and delicacy. You might never have heard of them, but these three small muscles are at

human civilization. Take them away and our greatest collective achievement might be maneuvering ants out of their nests with sticks. —

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.” Considering

how many fingers we have and they will say ten. Then ask them which is their first finger and nearly all will unfurl an index finger, thus overlooking the neighboring thumb and relegating it to a separate status. Ask them then to name the next finger along and they will call it the middle finger—but it can only be in the middle if there are five fingers, not four. In the end, even most dictionaries can’t decide whether we have eight fingers or ten. Most define fingers as “one of the five terminal members of the hand, or one of the four other than the thumb.” Because of the uncertainty, even doctors do not number fingers, because there is no agreement on which is finger number one. Doctors use the usual Latinate technical terms for most parts of the hand except, oddly, the fingers, which they call thumb, index, long, ring, and little. A good deal

discovered that nails driven through the palm of the hand—the method traditionally depicted in paintings—would not support the weight of a body. The hands would literally tear apart. But if the nails were driven through the wrists, the body would stay in position indefinitely, thus proving that the wrists are much more robust than the hands. And by such means does human knowledge creep forward.

Our feet were designed to grasp, which is why you have a lot of bones in them. They were not designed to support a lot of weight, which is one reason they ache at the end of a long day of standing or walking. As Jeremy Taylor points out in Body by Darwin, ostriches have eliminated this problem by fusing the bones of their feet and ankle, but then ostriches have had 250 million years to adjust to upright walking, roughly forty times as long as we have had. All bodies

price we pay for being able to scamper and sprint is, for many, backache and knee pain in later life—or indeed not so late in life. Such is the pressure on the spine from our upright posture that pathological changes can be detected “as early as the eighteenth year,” as Peter Medawar noted. The problem, of course, is that we come from a long line of beings whose skeletons were designed to take our weight on four legs. We will look at the benefits and consequences of this massive change to our anatomy more closely in the next chapter, but for the moment it’s enough to bear in mind that becoming upright meant a wholesale redistribution of our weight load, and with that has come a lot of pain that we would not otherwise have suffered. Nowhere is this more uncomfortably evident

Our lower limb joints are also highly vulnerable. Every year in the United States, surgeons perform over 800,000 joint replacements, principally of hips and knees, mostly from wear and tear on the cartilage lining the joints. It is pretty impressive that cartilage lasts as well as it does, especially when you consider that it cannot repair or replenish itself. Think of how many pairs of shoes you have

Because cartilage isn’t nourished by blood, the best thing you can do to maintain it is to move around a lot, to keep the cartilage bathed in its own synovial fluid. The worst thing you can do is to pack on a lot of extra body weight. Try walking around all day with a couple of bowling balls tied to your belt and see if you don’t feel it in your hips and knees at the end of the day. Well, that’s essentially what you are doing already all day every day if you are twenty-five or thirty pounds overweight. It’s little wonder that so many of us end up undergoing corrective surgery as the years catch up with us. For

most problematic part of their infrastructure is their hips. Hips wear out because they have to do two incompatible things: they must provide mobility for the lower limbs, and they must support

exerts a lot of frictional pressure on the cartilage on both the head of the femur and the hip socket into which it fits. So instead of swiveling smoothly, the two can start to grind painfully, like a pestle in a mortar. Well into the 1950s, there wasn’t much medical science could do to relieve the problem. Complications from hip surgery were so great that the usual procedure was to “fuse” the hip, an operation that relieved the...

Our bones lose mass at a rate of about 1 percent a year from late middle age onward, which is of course why elderly people and broken bones are so unhappily synonymous. Broken hips are especially challenging for the elderly. About 40 percent of people over seventy-five who break their hips are no longer able to care for themselves. For many, it is a kind of last straw. Ten percent die within thirty days, and nearly 30 percent die within twelve months. As the British surgeon

Some authorities think bipedalism is at least as important a defining characteristic of what it is to be human as our high-functioning brain. Many theories have been proposed as to why our distant ancestors dropped out of trees and adopted an upright posture—to free their hands to carry babies and other objects; to gain a better line of sight across open ground; to be better able to throw projectiles—but the one certainty is that walking on two legs came at a price. Moving about in the open made

By balancing on just two supports, we exist in permanent defiance of gravity. As toddlers amusingly demonstrate, walking is essentially a matter of hurling the body forward and letting the legs run to catch up. A pedestrian in motion has one foot or the other off the ground for as much as 90 percent of the time, and thus engages in constant unconscious adjustments of balance. In addition, our center of gravity is high—just above our waists—which adds to our innate tippiness.

noted earlier, our necks became longer and straighter and joined the skull more or less centrally rather than toward the rear as in other apes. We have

You may think your legs drop straight down from your waist—they do in apes—but in fact the femur angles inward as it descends from pelvis to knee. This has the effect of moving our lower legs closer together, giving us a much smoother, more graceful gait. No ape can be trained to walk like a human. They are compelled by their bone structure to waddle, and to do so in a most inefficient way. A chimpanzee uses four times as much energy to move around at ground level

allowed us to thrive as a species. It is the nuchal ligament, and it has just one job: to hold the head steady when running. And running—serious, dogged, long-distance running—is the one thing we do superlatively well. We are not the speediest of creatures, as anyone who has ever chased a dog or cat

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. That our ancestors could also organize themselves into hunting parties, to harry quarry from different

Despite these deficiencies, our ancient forebears were successfully hunting large animals 1.9 million years ago. They were able to do this because of an additional trick in the Homo armamentarium: throwing. Throwing required us to change our bodies in three crucial ways. We

shoulder joint in humans is not a snug ball and socket, as in our hips, but a more loose and open arrangement. This allows the shoulder to be limber and to rotate freely—exactly what’s needed for forceful throwing—but it also means that we dislocate our shoulders easily. We throw with our whole bodies. Try throwing an object forcefully while standing still and you can hardly do it. A good throw involves a forward step, a brisk rotation of waist and torso, a long backward stretch of the arm at the shoulder, and a powerful hurl. When executed well, a human can throw an object with considerable accuracy at speeds easily in excess of ninety miles an hour, as professional baseball pitchers repeatedly demonstrate.