As a collaborator, Gray was spectacularly petty. It is not clear whether he ever paid Carter in full or indeed at all. He certainly never shared royalties. He instructed the printers to reduce the size of Carter’s name on the title page and to remove a reference to his medical qualifications, to make him look like a journeyman illustrator. Only Gray’s name appeared on the spine, which is why it became known as Gray’s Anatomy rather than Gray and Carter’s, as it really should have.

  The book was an immediate success, but Gray did not much get to enjoy it. He died in 1861 of smallpox just three years after publication. He was only thirty-four. Carter did somewhat better. In the year of the book’s publication, he moved to India, where he became professor of anatomy and physiology (and later principal) at Grant Medical College. He spent thirty years in India before retiring to Scarborough on the Yorkshire coast. He died in 1897 of tuberculosis two weeks before his sixty-sixth birthday.

  II

  WE ASK A lot of our bodily architecture. The skeleton has to be rigid and yet pliant. We must stand firm but also bend and twist. “We are both floppy and rigid,” as Ben Ollivere says. Your knees have to lock into position when you stand but then immediately unlock and bend up to 140 degrees to let us sit and kneel and move about, and we must do all this with a certain grace and fluidity day after day for decades. 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.

  It is usually said that we have 206 bones, but the actual number can vary a bit between people. About one person in every eight has an extra, thirteenth pair of ribs, while people with Down’s syndrome frequently have a pair missing. So 206 is, for many, an approximate number, and it doesn’t include the (mostly) tiny sesamoid bones that are scattered through all of us in our tendons, primarily in the hands and feet. (“Sesamoid” means “like a sesame seed,” which is largely an apt description but not always. The kneecap, or patella, is also a sesamoid bone, though hardly sesame-like.)

  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 there is an urgent need for bones to be in one place rather than another, but because that’s just where evolution left them.

  Our bones do a lot more than keep us from collapsing. As well as providing support, they protect our interiors, manufacture blood cells, store chemicals, 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 help to 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 protein in the body—40 percent of all your proteins are collagens—and it is very adaptable. Collagen makes the white of the eye but also the transparent cornea. In muscle it forms fibers that behave just like rope in that they are strong when stretched but collapse when pushed together. That’s great for muscle but wouldn’t be so useful in your teeth. So when permanent stiffness is needed, collagen often twins with a mineral called hydroxyapatite, which is strong when compressed and thus allows the body to create good solid structures like bones and teeth.

  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. “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 will see an intricate array of productive cells just as in any other living thing. Because of the way they are constructed, bones are, to an extraordinary degree, both strong and light.

  “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. “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.”

  Even more remarkably, bone will grow back and fill a void. “You can take up to thirty centimeters of bone out of a leg, and with an external frame and a kind of stretcher you can have it grow back,” Ben says. “Nothing else in the body will do that.” Bone, in short, is amazingly dynamic.

  * * *

  —

  The skeleton is, of course, only a part of the vital infrastructure that keeps you upright and mobile. You also need lots of muscle and a judicious assortment of tendons, ligaments, and cartilage. I think it is safe to say that most of us are not completely clear on what exactly some of these do for us or quite what marks the difference between them. So here is a brief rundown.

  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 palm up. Make a fist and a ridge will form on the underside of your wrist. That’s a tendon.

  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 bulk of you, no matter how modestly built you are, is muscle. You have more than six hundred muscles altogether. We tend to notice our muscles only when they ache, but of course they are constantly at our service in a thousand unappreciated ways—puckering our lips, blinking our eyelids, moving food through the digestive tract. It takes one hundred muscles just to get us to stand up. You need a dozen to move your eyes over the words you are reading now. The simplest movement of the hand—a twitch of the thumb, say—can involve ten muscles. Many of our muscles we don’t even think of as muscles—our tongue and heart, for instance. 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 less if you are a proportionately similar woman, and just keeping that mass of muscle uses up 40 percent of your energy allowance when you are at rest, and much more when you are active. 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 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.

  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. Nor, surprisingly, do we all possess the same component parts in our hands. About 14 percent of us lack a muscle called the palmaris longus, which helps to keep the palm tensed. It is rarely missing from top-ranked athletes who need a strong grip to perform, but is otherwise quite dispensable. In fact, the tendon ends of the muscle are sufficiently unneeded that they are frequently used by surgeons when making tendon grafts.

  It is often noted that we have opposable thumbs (by which is meant that they can touch the other fingers, giving the capacity for a good grip) as if this 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 the heart of human civilization. Take them away and our greatest collective achievement might be maneuvering ants out of their nests with sticks.

  * * *

  —

  “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.”

  Considering their importance, we have been surprisingly relaxed about naming the digits. Ask most people 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 of what we know about the comparative strengths of the hand and wrist comes from a series of improbable experiments undertaken by a French physician, Pierre Barbet, in the 1930s. Barbet was a surgeon at the Paris Saint-Joseph Hospital who became obsessed with the physical challenges and limitations of human crucifixion. To test how well humans would remain in place on a cross, he nailed real human corpses to wooden crosses using different types of nails driven through different parts of the hands and wrists. He 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 other disproportionately bony outposts, the feet, receive a lot less praise and attention when it comes to discussing the things that make us special, but in fact the feet are pretty marvelous, too. The foot has to be three different things: shock absorber, platform, and pushing organ. With every step you take—and in the course of a lifetime you will take probably something in the region of 200 million of them—you execute those three functions in that order. The foot’s curved shape, like that of the Roman arch, is immensely strong, but it’s also pliant, lending a springy rebound to every step. The combination of arch and springiness gives the foot a recoil mechanism that helps to make our walking rhythmic and bouncy and efficient in comparison with the more lumbering movements of other apes. The average human walks at a pace of about 4.25 feet per second, or 120 steps per minute, though obviously this varies a great deal depending on age, height, urgency, and much else.

  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 are compromises between strength and mobility. 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. If we had stronger scaffolding, we couldn’t be as nimble. The 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 in modern humans than in the back. Becoming upright put extra pressure on the cartilage disks that support and cushion the spine, in consequence of which they sometimes become displaced or herniated in what is popularly known as a slipped disk. Between 1 and 3 percent of adults have slipped disks. Back pain is the most common of chronic complaints as we age. An estimated 60 percent of adults have taken at least a week off work at some time with back pain.

  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 worn out in your life, and you begin to appreciate just how durable your cartilage is.

  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 many people, the 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 the weight of the body. This 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 pain but left the person with a permanently stiffened leg.

  Surgical relief was always short-lived because every synthetic material tried would soon wear down until the bones were grinding painfully again. In some cases, the plastics used in hip replacements squeaked so loudly when people walked that they were embarrassed to go out. Then a dogged orthopedic surgeon in Manchester, England, named John Charnley devoted himself heroically to finding materials and devising methods that would solve all the problems. Essentially, he realized that wear was greatly reduced if the femur was replaced with a stainless steel head and the socket—acetabulum, to use the formal name—was lined with plastic. Almost no one has heard of Charnley outside orthopedic circles (where he is venerated), but few people have brought relief to greater numbers of sufferers than he did.

  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 and anatomist Sir Astley Cooper liked to quip, “We enter the world through the pelvis and leave it through the hip.”