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.
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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.
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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.”
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.