Millions of years of biological trial and error produced the modern human hand, with each generation of our ancestors along the way including some individuals born with hands that differed from the norm for the species. The differences, or variations, made these individuals better or worse equipped to cope with their environment’s challenges, like predators and hostile terrain, or to take advantage of its opportunities, like new food sources requiring novel mining techniques.
As the phenomenon of natural selection would have it, individuals lucky enough to be born with advantageous hand variations were more likely to survive until they could reproduce and transmit the happy traits to their offspring. Individuals born without such variations—or worse, with variations that made it harder for them to hunt, gather, and escape the neighborhood bad guys—were less likely to survive and reproduce, so their suboptimal hand anatomy died with them.
Over time the advantageous variations became the norm, and the hand we know and love today—praised by Aristotle as the “instrument of instruments”—is evolution’s best bet (to date) at what we need in a hand to survive until we can do our part to propagate the species.
So what are the winning anatomical characteristics? Starting from the inside out, our hands boast a remarkably flexible frame constructed of twenty-seven bones (more than any other part of the body, edging the foot out by just one) arranged into five slender digits. Its flexibility comes from the number and variety of joints connecting these bones, which allow the digits and wrist to hinge and rotate in such a way that we can open our hands wide, and reach for and curl our hands around objects of many different sizes and shapes.
Interwoven with the bones, a large number of collaborating structures share an incredibly small space. These include over thirty muscles, attached to the bones of the hand by their thin, sinewy ends, called tendons; a complex network of blood vessels, permeating every tissue of the hand and providing the energy required for movement; and peripheral nerves, which, as you may recall from chapter 2, form a feedback loop with the central nervous system.
All this is neatly kitted out in a layer of skin, whose properties and benefits vary depending on where it lies on the frame of the hand and go well beyond physicians’ old adage that it merely “keeps blood in, rain out.” For starters, skin on the palm of the hand is some of the thickest on the body—second only to that on the sole of the foot—which helps protect it against mechanical stress. It’s also “quilted” to facilitate gripping—that is, lined with fat to increase our comfort when holding angular objects, and tethered tightly to the bone at regular intervals (finger joints, lines across the palm) to contain the fat in cushions so that objects can’t “ride” the fat across the whole of the palm.
If you have little considered this helpful characteristic, try this brief experiment (Go ahead! I do it all the time as I write, like a cookbook writer testing their recipes. People stare but never say anything . . . or almost never): Place your fingertips firmly on the skin anywhere on your palm, then try to push the skin back and forth. Notice how little it moves? Now try the same thing with the skin of your forearm. See how far your fingers ride back and forth on the more mobile skin? You couldn’t reliably hold or maneuver objects if they could travel that much in the grip of your hands.
Then there’s the texture of the skin. Papillary ridges, the thickened bands of skin that form our whorling fingerprints, actually cover the entire palm of the hand, as well as the soles of the feet. They aid grasping by acting as tread on a tire, displacing liquids on wet objects so the skin can connect directly with them, instead of hydroplaning around. They also house ducts that secrete sweat, a little of which lubricates the skin and helps it adhere to a surface (like when you lick your fingertips to turn the page of a book); a lot of which might make you drop things. I rued this cruel fact as a young violinist when my hands sweated before recitals, plaguing me with fears of a fumbled bow or botched arpeggios, until I began playing and the music absorbed me.
Perhaps the most thoughtful innovation in the hand’s gripping tool kit is the skin’s ability to form calluses where repetitive chafing against objects might otherwise cause pain or dysfunction. I appreciated this characteristic, too, as a rower in college. Only, back then, the calluses never seemed to form quickly enough at the beginning of the season, so we winced through initial practices, pulling with raw palms on splintery wooden oars as our skin took its sweet time thickening up.
Flipping to the back of the hand, looser skin than that of the palm can stretch over a clenching fist or retract in folds around the knuckles. Fingernails protect and reinforce our pulpy, vulnerable fingertips and come in handy for scratching itches, peeling Scotch tape off gifts, and other mundane, picky endeavors.
Together, these structural characteristics not only give hands their nearly infinite utility, they also count among the most distinguishing aspects of our unique identities, to which we as individuals, and those around us, relate and even gravitate.
My mother’s hands used to intrigue me, back when I was a child standing just about eye level with the kitchen counter, seeing even more of them in a day than I probably did of her face. I watched them intently, noticing the carefully shaped, medium-length oval nails she tended with a worn, burgundy leather manicure kit she’d received as a gift in college from her mother. As she worked her way around the kitchen preparing meals, I followed the sparkling trajectory of her modest diamond engagement ring, and visually traced the mint-green latticework of her prominent veins. When I got the chance—at church, for instance, where, in the inconvenient absence of toys, Mom indulged most any activity that kept us kids quiet—I liked to stroke her veins’ velvety twists and turns, and compress them to see where the blood went when its path was obstructed.
Beyond their visual appeal, though, what entranced me most about Mom’s hands was the speed, assurance, and flair with which they moved. I was filled with awe and questions watching them perform the most mundane tasks, like dialing a rotary phone (How does her finger always know exactly where to go?), shaking a thermometer to bring the mercury down (How come it doesn’t fly out of her hands?!), paring a peeled orange from top to bottom into perfect, juicy, fan-shaped sections (Why doesn’t she cut herself?!). Then there was her quirky practice of hesitating with a pen over the spot of paper where she intended to write, and scratching out a couple of tiny “air letters” with its tip before committing her thoughts to ink, and the repertoire of animated gestures she used (and still does) to dramatize her conversations with grown-ups and children alike.
Best of all was how she used those perfectly shaped fingernails to lull me to sleep when childish, and sometimes not so childish, worries made me restless. Lightly tracing patterns on my back and combing through my hair with them, she’d tell me stories that always began “When I was a little girl . . .” and assure me that “things will look better in the morning” as I drifted into a sound sleep from which I generally awoke feeling better indeed. So there’s a truth that even a five-year-old knows: More than their impressive construction, it’s what hands do that makes them essential.
Hand function has four core components: movement, including both the shape hands can make and their range of motion, plus sensation, strength, and dexterity. Hand movements are typically classified as either prehensile or nonprehensile. Primates, including us humans, differ from most other mammals in that we bring our food to our mouths to eat, instead of bringing our mouths to our food. Some other mammals, like raccoons, squirrels, and the mean little gerbils my family kept when I was a kid, bring food to their mouths, too. But whereas they must grasp it between their two paws, like a basketball player about to make a pass, we can wrap the fingers and thumb of just one hand around a hunk of edibles, grasp it to our palm, and bring it to our mouths. This is an example of prehensile movement, and we owe it to our famously opposable thumbs, which can both hinge and rotate, and therefore swing out and across the palm to meet curling fingertips “head-on.”
While we employ prehensility to hold objects of an infinite variety of shapes, the grips involved mainly boil down to two: the power grip and the precision grip. While not entirely distinct, they differ in important ways. A power grip encloses its object between the palm and full length of the fingers, with thumb in opposition to buttress them or “lock” the object in place, facilitating application of force to the task at hand, whereas a precision grip encloses its object between the tips of opposing thumb and fingers, without contacting the palm, facilitating delicate manipulation.
Examples of both abound here in my local pub in London, where I write today: the barman pulling a pint of lukewarm London Pride ale (power grip); a woman tucking into a traditional Sunday roast, using her knife to push slabs of beef and Yorkshire pudding onto the back of her fork (precision grip); a server loosening the stubborn lid on a screw-top jar of cocktail olives (power grip), then spinning it off (precision grip).
Exhibiting nonprehensile movement, whereby the hand acts on an object without holding it, we have the barman now punching keys on the cash register to ring up that ale, a server pushing tables together to accommodate a large party coming to watch “the football” (which will always be “soccer” to me) on TV, and a dad dabbing a bit of dinner off his toddler’s face with a napkin-wrapped index finger while his partner scrolls through content on an iPad.
Before movement, the nervous system crackles with activity. As discussed in chapter 2, nerves are actually bundles of axons, the stemlike part of a neuron. The axons in peripheral nerves stretch all the way from their cell bodies in, or near, the spinal cord to the specific patch of skin, muscle, or organ the neuron serves. Some peripheral neurons convey movement commands from the central nervous system to muscles throughout the body (motor neurons), and others convey data to the central nervous system about the body’s internal and external environment (sensory neurons).
The sensory nervous system is meticulously structured to do justice to our infinitely sensual world. Different types of sensory data—about the form, texture, and temperature of external stimuli and their mechanical impact on our bodies, about how our skin stretches and hearts pumps, and more—are captured by axon endings tailored to their unique tasks. For instance, axon endings called Merkel disks register the curves, edges, and textures of objects we come into contact with, and axon endings called Meissner corpuscles register the microslips of objects across our skin. When we hold an object, this data helps the central nervous system automatically adjust our hand position and grip strength so that we don’t drop or crush it. Also, most sensory neurons convey just one type of data to the central nervous system, with their transmission speed and frequency optimized to do so.
The central nervous system then blends the peripheral nervous system data streams and other sensory input, such as visual and audio data, and translates it into an account—of which we may or may not be conscious—of what’s going on with our bodies, so that we can respond appropriately. The sensory experiences we recognize as pain, vibration, temperature, and texture count among our conscious perceptions, as do a few others to which most of us have given little, if any, thought. One of these is proprioception, the ability to sense the position of our body in space—in other words, knowing versus seeing that our arm is outstretched to hail a taxi (in New York) or a bus (in London)—which the brain cleverly deduces from the tension in our skin, muscles, and other internal structures.
Another is localization of stimuli, the ability to pinpoint the precise position on the body where we have come into contact with external stimuli. When I know without looking that I’ve just stabbed the pad of my left thumb with the lead of a mechanical pencil while blindly rummaging through my purse in search of house keys, it’s localization delivering the bad news. Precisely how the neural activity resulting from such tactile stimulation leads to our conscious experience of it is, so far, as mysterious to neuroscientists as the emergence of any other conscious state, including our emotions.
Hands are sensory SUVs, guzzling up massive amounts of tactile data—data so voluminous and crucial to human survival and thriving that it claims a huge share of the brain’s sensory processing capacity, relative to that claimed by other body parts, even the much larger arms and legs. Receptor-dense fingertips and a profusion of axon endings throughout the skin of the palm are key enablers of the guzzling. And remember papillary ridges? They contribute, too, increasing the hand’s surface area, and thus receptors’ exposure to external stimuli—even more so when we sweat, causing the papillary ridges to swell.
These unique aspects of the hand help explain why we can discriminate so finely between, and take pleasure in, so many tactile sensations at our fingertips. Consider the differences in feel between furry dog tummy and silky toddler hair, bubble wrap and corrugated cardboard, a handful of raw carrot sticks and a handful of raw hamburger meat. We struggle to comprehend sensations (or sounds or colors, for that matter) described in words. Yet we comprehend them instantly with our hands, and particularly the fingertips, and with a richness and subtlety only they can convey. If you need proof, try another experiment: Draw your fingertips across your cheek a few times and think about what they’re telling you. Is your skin smooth or bumpy? Moist or dry? Taut or lax? Now draw the inside of your wrist across your cheek. Much less tactile information, right?
Rounding out the core components of hand function, strength enables us to apply force against resistance; and dexterity—the product of movement, sensation, strength, and the brain’s capacity to plan—is the hand’s ability to effect small, precise movements, like those required to fasten a button or write with a pen.
Put all this complex, purpose-built structure and functionality together and we have an extraordinary machine in (and on) our hands. How does the brain drive it? To illustrate, let’s say I want to drop a physical issue of my guilty pleasure, Hello! magazine, onto the recycling pile. My brain issues a “trash that twaddle” command to my spinal cord in a way that determines the most efficient activation of the peripheral motor nerves serving the muscles involved. The spinal cord relays the command to the nerves, the nerves activate the muscles, and the muscles obediently contract, tugging on their terminal tendons. The tendons tug on the bones in the hand to which they are affixed, and the bone frame assumes the position required to drop the magazine.
Ahead of the mechanics, however, the brain must “decide” to move the body. And as exemplified by the threat-defense program explored in chapter 2, even the most apparently simple maneuver—drop a magazine, call 911, grab a dish towel—is the product of an astonishing amount of brain processing, processing informed not only by our sensing of the “here and now,” but also to various degrees by our evolutionary programming, life experience, knowledge, imagination, goals, physical ability, and any other ready inputs the brain deems relevant.
Apparently, then, the only “say” the hand has in its own deployment is the contribution of sensory data. So is it just a puppet on strings pulled by the brain, dutifully playing the bit part our know-it-all gray matter writes for it in our pursuit of health and happiness?
Far from it. Indeed, anthropologists and neuroscientists widely believe that the hand helped create the modern brain. As the hand evolved and allowed our ancestors to gain greater control over their environment (using found objects as tools for hunting and gathering, for instance), their brains needed to develop the intellectual horsepower necessary to navigate through the more complex and demanding situations the evolving hand presented. Simultaneously, the brain’s increasing capability led to more sophisticated and successful strategies for survival, from toolmaking to building and living within a collaborative society.
So interdependent are hand and brain, in fact, that we cannot explore the realm, method, and influence of one without exploring those of the other. And delving into all of that, we soon encounter tricky, even uncomfortable, questions about ourselves—like why we perceive, feel, believe, and do what we do. Consider these questions in the context of a traumatic injury, and what began as a seemingly straightforward study of the hand becomes a life-changing endeavor.