Friedrich Nietzsche moaned famously that without the protective covering that envelops our bodies we couldn't bear the sight of each other. And yet with it, and with its fibrous appendages (hair and nails), we become objects of each other's desire.

The flesh," Victor Hugo mused, "is the surface of the unknown."

Like the natural hides of other living forms—shells, husks, rinds, etc.—the skin and its extensions function primarily as an outer garment, an exterior structure engineered for a lifetime of rough duty. That they also largely determine our ideas of what it means to be a man or woman, or beautiful, and also our awareness of race, is ironic, since the cells we see—hair, nails, those on the skin's surface—are all dead, their own skins already lost.

Living Envelopes

Like other organs, the flesh and its appendages perform varied functions. Besides protecting us from damage due to physical injury, toxins, sunlight, heat, infection, and excessive water (and also from water loss), it maintains body temperature, detects physical sensation, produces Vitamin D, eliminates some waste materials, improves our grip of objects, and absorbs certain chemical substances. All this takes place within a pliant sheet of organic fabric studded with glands and hair follicles and filigreed with creases, grooves, ridges, and bumps; and ranges in thickness from 1/50 inch in the eyelid to 1/6 inch in the foot sole—approximately the difference between fine angel-hair pasta and fettuccine.

A tough, semiporous laminate, skin is one of the largest structures in the body and consists of 2 distinct layers. The outer coating (epidermis) is a flexible, thin, semitransparent membrane of dead cells firmly dovetailed together (think of the surface of a blister bubble). The underlying base is a feltwork of fibrous and elastic tissue, the key structural element of which is keratin, a protein that also shows up in reptile scales (though not fish scales), bird feathers, claws, hooves, tooth enamel, horns (but not antlers), and hair. Keratin molecules twist around each other and contain sulfur, atoms of which are able to interlock with each other, rigidifying each twist.

Rubbery and elastic in infants, the system hardens and thickens with age, measuring in adulthood up to 2 square meters and weighing 10 to 12 pounds.

The skin's underlying architecture varies according to location and activity, but continual resurfacing occurs throughout the life of the organ. Dead, flattened cells on the topmost layer are constantly worn away—most house dust is skin detritus—and replaced by new cells produced by the division of living cells at the base of the epidermis. The process takes about a month. As skin cells push up and move away from their source of nourishment, they flatten and shrink. As they move up to the "horny layer" at the surface, they lose their nuclei and turn into a lifeless protein—keratin. Keratin in the dead cells creates a tough, continuous fabric, its sulfur bonds too robust to be broken down by normal digestive enzymes. This prevents potentially invasive microbes from getting a purchase.

The inner dermis maintains its structural integrity, a fibrous tissue permeated with blood vessels, nerves, sweat and oil glands, and hair follicles, its deepest layer anchored to underlying tissues. Each square inch of skin houses about 700 sweat glands, 100 sebaceous or oil glands, and 21,000 sensors of heat, pressure, and pain.

Designed for precision handling (try grasping a pin with gloves on), the whorls and arches of the fingertips are well supplied with coiled sweat glands, but no hair or oil glands. A ridged, hairless surface here and on the palms and foot-soles helps grip objects and prevent slipping. Since the swirls and ridges vary between individuals, and since they leave behind sweat patterns (fingerprints) when they come in contact with smooth surfaces, we often can be identified] by what we touch.

Insulating the head while protecting it from harmful radiation from the sun, about 100,000 hairs on the scalp spring from tiny elongated pits in the skin. Living follicle cells in the hair bulb at the bottom of the pit divide to form the hair and push it upward. Like skin cells, as they move upward they fill with keratin and die, making haircutting painless. Oil released into the follicles from sebaceous glands moisten and add flexibility to the hair stalks, and the follicles are so firmly embedded in the scalp that a braided hunk of adult hair can pull an SUV.

When the body overheats, sweat glands release water onto the skin's surface. As the water evaporates, it draws heat away from the body, cooling it down. An inactive person perspires at a rate of 1/12 ounce per hour, while an extremely active one can produce a quart or more of sweat in the same period.

Male and female exteriors, while functionally the same, differ in the production of skin oils and the distribution of hair and body fat. In other words, we deem physical appearance either as "masculine" or "feminine" less on the basis of sex than on specific patterns of development.

Men store body fat chiefly in the abdomen, women in the lower body, which becomes especially apparent in the different ways that we first fill out, then sag, as we age. Meanwhile, steroid hormones called androgens (testosterone is the best known) stimulate the production of sebum, a complex mixture of fats that causes hair to both coarsen and grow in a characteristically "male" configuration—face, chest, and abdomen. Since women also produce androgens, and men produce estrogens (hormones that stimulate "female" sexual characteristics), where, when, and what type of hairs we sprout is a composite of male and female patterns.

Hormones, not gender, are key. Women given testosterone shots develop male-pattern baldness while normal men often develop female-style hair loss.

A tanned skin is something more than respectable, and perhaps olive is a fitter color than white for a man—a denizen of the woods. "The pale white man!" I do not wonder that the African pitied him.

—Henry David Thoreau

The purpose of color in skin and hair supersedes vanity, identity, and other types of camouflage. Acting as a sunblock, the pigment melanin (yellow to red phaeomelanin, and dark-brown to black eumelanin) helps screen ultraviolet radiation, which, if let through, would damage the DNA in underlying tissues.

Tentacled cells called melanocytes stud the base of the epidermis, producing melanin and injecting it up into the layers of keratin-producing cells near the outer skin. Present throughout the body, melanocytes are most concentrated in the sex organs, nipples, armpits, and anal region, supplying extra protection for sensitive regions as well as attention-getting visual accents of use in the mating process.

Hair is produced when actively dividing cells in the follicle ("little bag" in Latin) become increasingly keratinized and die. Root and shaft alike are made of 3 concentric tubes—the outer cuticle, a casing of overlapping shingle-like scales that help keep shafts apart; the middle cortex, which contains pigments in dark hair and air bubbles in white hair; and the inner medulla, a core just a few cells across containing pigments and air pockets.

The shape of the shaft determines the hair's texture—round for straight, oval for wavy, flat for kinky—and the relative amounts of lighter and darker melanin decide whether it'll be blond, red, brown, or black. A tiny strap of involuntary muscle (arrector pili) tugs at the bottom of the root sheath, raising the hair when it contracts to create a barometer-like mechanism for registering cold temperatures, thrills, and terror—"goose pimples."

Guarding the ends of the fingers and toes, nails are hard plates of tightly packed keratinized cells. A hidden matrix of dividing epidermal cells beneath the root (visible as a white crescent at the base of the nail: the lunula) proliferates, pushing the plate forward. The nail bed, the portion of the nail we see, extends past the digit to the free edge, the distal part of the nail that needs to be clipped. Useful for grasping and scratching, nails grow faster in warm weather than cold, and fingernails grow 300 to 400 percent faster than toenails, a favorable ratio (and a relief to the back) since they're easier to clip.

Skin, hair, and nails are mostly protein. Keratin, like all working molecules in cells, is made of long chains of amino acids coiled like phone cords. But it's unique in that its chains contain high levels of sulfur atoms, which, when they come together, create one of the strongest chemical bonds in nature—disulphide linkages. The outer coating of a hair is comprised of dead keratinized cells, bundled lengthwise, their disulphide linkages tying the coils together like the rungs of a ladder.

Keratinized cells on the surface of the skin form tight, interlocking fan-shaped joints, weaving together into a rugged, tarplike fabric that keeps even microbes out and water and other essential substances in. In nails, they overlap like shingles, forging a clear, shell-like casing.

I suppose the body to be nothing but a machine…We see clocks, artificial fountains, mills and other such machines which, although man-made, have the power to move on their own accord…One may compare the nerves of the machine I am describing with the works of these fountains.

—Seventeenth-century mathematician and philosopher René Descartes

Descartes meant the comparison. He understood all human action as reflexes: A stimulus excites a sense organ, which sends a message upward to the spinal cord or brain which in turn relays the "excitation" downward to a muscle or gland thus producing activity. He believed nerves were open tubes conveying tiny gusts of pressure throughout the body, much like the water pipes that made Renaissance fountains erupt in elaborate sequences. Descartes located the control center for these bursts of spirit—a kind of master switching station where all the tubes in and out of the brain ultimately converged—in the soul, which he said resided in the bean-sized pineal gland in the middle of the forebrain.

Why this and other theories of animal spiritus persisted as long as they did seems curious, since by Descartes' time anatomists had dissected human brains and nerves and discovered that they were made of fibers, not tubes. Yet it wasn't until the late eighteenth century that physiologists put together recent work on electricity and theories about the nervous system. Anatomists could trace the branching architecture of neural pathways, but the chemical and electrical signals animating them were beyond comprehension.

Not anymore. Neurologists and basic biologists, who see the nervous system as an ultra-high-speed communications network, have identified many of the electrochemical mechanisms at work. These include dozens of large molecules that convey signals within cells and from cell to cell, as well as newly discovered families of proteins (little nose cones) that guide nerve cells to make effective connections and others that hold all 100 billion neurons—the body's wiring—in place. Molecular glue, so to speak. The eminent Danish anatomist of Descartes' era who moaned, "The brain, the masterpiece of creation, is almost unknown to us," would be dazzled, although scientists and philosophers are probably no closer to locating a biological seat for the soul.

The universal dictum that nature builds from the bottom up is nowhere expressed more magnificently than in the development of the human brain. Barely 21/2 weeks after conception, a spoonlike groove appears along the midline of the embryo's outermost layer of cells. At first, the cells running along the groove look no different than those around them, but as the groove deepens, then folds itself over into a cylinder, each new generation becomes progressively thinner, reeling out long filaments. Inside the tube, the rate of production of new cells—250,000 per minute—is staggering.

Within 10 days, the cells begin migrating, streaming over and past each other as if on invisible ladders. Complex and subtle chemical releases choreograph their movements as the nervous system, which eventually must carry signals to every cellular subsystem in the body, constructs itself literally from the inside out. New nerve channels weave themselves headlong into cables that ramify and connect over and over as the embryo becomes a fetus, and then, in the last trimester, undergoes explosive growth in the brain.

From a traffic-engineering standpoint, the migration of cells that will become the thinking center—the cerebral cortex—is a nightmare. "It's as if the entire population of the East Coast decided to move en masse to the West Coast," Yale neuroscientist Dr. Pasko Rakic has said, and "marched through Cleveland, Chicago, and Denver to get there." As the 2 hemispheres of the forebrain sprawl across almost all other brain structures in the seventh month, their surfaces erupt in precise folds—fissures—that in every normal human brain are located in precisely the same position, with identical depth, and with exactly the same connections to other fissures. The wiring map of this superstructure, which involves an estimated 10 trillion connections—some, in molecular terms, are thousands of miles long—would dwarf that of the Internet.

When the French master sculptor Auguste Rodin gave form to the suffering, malaise, and ennui of the nineteenth century in his famous creation "The Thinker," he celebrated a humanity virtually lost in deep thought. But it's the non thinking portions of the nervous system that maintain control of the body, managing the multifaceted business of life so as to afford the upper brain the leisure to anguish over what it means.

Encased in the skull and spine, the tangles of nerves responsible for processing information and directing actions—the central nervous system (CNS)—are supremely capable but ultimately fragile: severed or injured, they don't grow back. By comparison, the 30,000 miles of nerves outside the CNS (the peripheral system) are individually less complex, yet they divide and branch to permeate as many body parts and tissues as possible and regrow lushly when harmed.

Attached along meridians extending the length and breadth of the body, the peripheral nerves are organized by function. Either they relay sensory information, stimulate voluntary muscles, or carry instructions to organs and glands. This last division is further cleaved into 2 distinct branches, the sympathetic or "fight or flight" branch, which in emergencies involuntarily makes you sweat as your heart pounds and your mouth gets dry, and the parasympathetic, which quiets the body and conserves energy when all is finally well.

Fifty years ago, psychotherapist Carl Jung dismissed Freud's theory of sexuality as too mechanistic: "The brain," Jung spat, "is viewed as an appendage of the genital glands." In fact, researchers now understand that in men, the penis is under the complete control of the CNS, both during arousal and at rest, and that many regions of the brain contribute to male sexual response—from the hindbrain center that deepens breathing to areas in the cerebral cortex that concoct erotic fantasies. During arousal, excitatory signals either from the brain or from physical stimulation spur nerves in the penis to release chemicals that signal the smooth muscles of the arteries to relax and fill with blood, resulting in an erection. Switching off the activity of the sympathetic nervous system also enhances erections, which explains why it's practically impossible to become sexually aroused while running from a bear.

In women, the sheer number of excitatory neurons makes for a more direct, unmediated sexual response. Consider the clitoris, which as writer Natalie Angier notes, "is simply a bundle of nerves; 8,000 nerve fibers, to be precise. That's a higher concentration of nerve fibers than is found anywhere else on the body, including the fingertips, lips, and tongue, and it is twice the number in the penis. In a sense, then, a woman's little brain is bigger than a man's." Touché, Dr. Freud.

Neurobiologists have reported structural differences in 2 primary areas of the human brain: the main connecting cable between the hemispheres, and the hypothalamus, center of the primitive brain's feedback system. A cell cluster in the front part of the hypothalamus, thought to help direct sexual behavior, is twice as large in men, with double the number of neurons. Meanwhile, another cluster that acts as a built-in clock—governing circadian rhythms and, in women, ovulation—has a more elongated shape in females, although scientists have yet to determine any corresponding functional distinction.

The oldest brain—the ancient temple, so to speak—consists of the conical structures of the brain stem. Active even in deep sleep, this archaic center controls muscles, balance, breathing, and heart rate, and is often called the "reptilian brain" because, as in snakes and lizards, its behavioral program is obsessive, ritualistic, and paranoid. It governs the most rudimentary elements of survival.

Lovers close their eyes when they kiss because, if they didn't, there would be too many visual distractions to notice and analyze—the sudden close-up of the loved one's eyelashes and hair, the wallpaper, the clock face, the dust motes suspended in a shaft of sunlight. Lovers want to do serious touching, and not be disturbed. So they close their eyes as if asking two cherished relatives to leave the room.

—Diane Ackerman, A Natural History of the Senses

Portals to the outside world, the contact senses are specialized extensions of the central nervous system designed to gather and relay information. The smell of a baby's scalp, the sight of a hawk, the taste of a lover's lips, a needle prick: all involve submicroscopic receptors receiving stimuli at very localized sites and passing electrochemical signals along a high-speed network, evoking a response. Biologically speaking, Ackerman is right: in order to focus, we can only take in and process so much sensory information at one time.

The data we receive is another matter, since in it lies the experience of life. Everything we see, hear, smell, taste, and feel forms our knowledge of the world and of ourselves, triggering cascades of association and emotion. "We are all instruments endowed with feeling and memory," the French philosopher Denis Diderot wrote, in 1769. "Our senses are so many strings that are struck by surrounding objects and that also frequently strike themselves."

Thinning to focus on a distant object, thickening to focus on a near one, the rubbery, convex lens of the eye works the way a camera does, adjusting the distance between the opening and the chemical surface. Clustered at the back of the eyeball—a kind of biological movie screen where light converges and images form—are specialized nerve cells, each packed with up to 100 million molecules of light-sensitive pigment that convert light into electrochemical signals. When a smaller molecule (retinal) within the pigment receives light energy in the form of a "packet," it twists its backbone, launching a chain of chemical reactions.

Though the great majority of the retinal cells—about 100 million—are rods, which work best in dim light but cannot distinguish color, several million cones conduct color, but only respond to bright light. Because genes for the red and green color receptors are located on the female sex chromosome, genetic red-green color blindness affects men much more often than women. The difference is purely mathematical: women are red-green colorblind if both their X chromosomes are affected; men if their only X chromosome is defective, which means that the incidence in males is the square of that in females. (In the United States, about 8 percent of men and 0.5 percent of women have some difficulty discerning colors.)

Like the camera it inspired, the eye is a device for taking pictures. Light pours through an adjustable opening, traverses a blackened space, and hits a surface coated with chemicals. When the light hits, the chemicals change in structure, forming a composite image, a projection. Overpowering the other senses, the eyes are “like a mirror.” The Persian anatomist Avicenna wrote in the early eleventh century. “the visible object is like the thing reflected in the mirror.”

The auditory pathway, Ackerman writes, "looks something like a maniacal miniature golf course, with curlicues, branches, roundabouts, relays, levers, hydraulics and feedback loops"; or else "a contraption some ingenious plumber has put together from spare parts." Either way, Rube Goldberg could only marvel in disgust. Rather than using maximum effort to achieve minimal results, as Goldberg's cartoon inventions did, the ear translates sound waves into fluid waves, then electrical impulses, with astonishing efficiency: 99.9 percent of the sound energy transmitted into the tiny snail-like cochlea, which contains the organ of hearing, is conserved. The energy doesn't have to enter through the ear. Thomas Edison, who invented the first phonograph despite being nearly deaf, vetted pianists for early recording sessions by biting on the leg of the piano and listening to them through his teeth.

With so much moving in the world, torrents of vibrations rush at us from all around. The outer ear funnels them into a short tube ending at a fanlike drumhead (the eardrum), which vibrates with them, which in turn relays the vibrations across a trio of hinged bones, the last of which (the smallest bone in the body) looks like a stirrup and taps like a piston-operated telegraph key on a window into the inner ear.

There the waves cross from air to liquid. The cochlea is subdivided into 3 fluid-filled chambers, spiraling in parallel around a bony core. A spiral within a spiral, the organ of Corti (the hearing organ) contains thousands of bristle-coated hair cells arranged in rows. When the fluid vibrates, the hairs move, exciting nerve cells, which send impulses to the brain, which interprets them as sounds.

The power of locomotion is that which contracts and relaxes the muscles whereby the members and joints are moved, extended or flexed. This power reaches the limbs by way of the nerves and there are as many forms of power as there are of movement. Each muscle has its own peculiar purpose and it obeys the decree of the composite sense.

—Eleventh-century Persian anatomist Avicenna

The poetry of human motion lies under the skin, in the exquisite coordination of more than 600 individual pulling devices, a rigging of powerful elastic straps comprising about half the body's weight. Since the devices work one way only—by shortening, then returning to their original length—each muscle is partnered with an "antagonist" that pulls the same bone in the opposite direction. Teams of 30 or more muscles hoisting and stretching together can move, lift, and rotate bones in a group, engineering the body's major movements and postures.

Researchers have understood since the mid-nineteenth century that muscles are chemicomechanical devices: they convert chemical energy into work, like minuscule internal combustion engines. Muscle cells, their structural units, are long rods consisting of bundles of fibers that are themselves ganged "threads" of hair-thin microfibers. In the early 1930s a German chemist, Karl Lohmann, identified the universal fuel that powers them—ATP, adenosine triphosphate, an energy-rich compound. When ATP breaks down in the cell and releases its fuel, these microfibers attach and slide past each other, as if pulling a rope line hand-over-hand. Repeated hundreds of times per second across thousands of filaments in each fiber, then over and over again across the whole fabric of stringy, resilient muscle tissue, this rapid cycling enables muscles to contract and relax.

Three types of muscles start to organize by the end of the first month after conception—heart, gut, and voluntary. Four weeks later, all muscle blocks have appeared, as the heart partitions into chambers, the alimentary canal extends from the mouth to the intestines, and the limb buds emerge. By the middle of the second trimester, the heart recirculates about 25 quarts of blood through the body per day while the skeletal muscles develop strength, coordination, and reflex action—well before newborns will need them.

Although the system of skeletal (voluntary) muscles is fully laid down at birth, it takes several years for mature movement patterns to develop. Take gait, for instance—our manner of walking on 2 legs, which in adults involves shifting the body's weight smoothly over a narrow pivot point with minimal changes in the center of gravity and with what orthopedists call "reciprocal arm swing.” All this must be learned—first by suppressing uncontrolled and chaotic primitive reflexes to bring the musculature firmly under the brain's control, then by many years of practice.

First, children toddle; their stance is wide, they walk on their heels, they're off balance, their hips rotate while their knees remain flexed, their arms are held in "high guard." By age 2, as the brain begins to integrate the movement of many joints and muscles, children's pelvises tilt less, 80 percent of them have begun to swing their arms, and the amount of time spent supported by a single leg increases to about one third. By age 3, most children have developed an adult pattern of joint angles, but it takes several more years of refinement and bone growth to achieve the same cadence, step length, and velocity as adults.

Muscle tissue generally appears either "striped" or "unstriped" (smooth), reflecting, in the first case, the ability to be put into action and controlled by the will, and in the second, to be part of an automatic process like digestion. When we blink or swing a sledgehammer, ropelike cylindrical fibers, some up to a foot long, contract, helping to move parts of the body. When we swallow and breathe, sheets of tightly packed fibers rolled into tubes or sacs also contract, reducing the space inside them and moving a visceral process along.

The tissue of the heart, laced with a magnificent wiring system that takes nerve signals to the deepest regions and jolts them into rhythmic pulsing, is a special case. Smaller than those of other striped muscles, cardiac fibers contract and release automatically, about 100,000 times a day.

Men have fifty percent greater muscle mass, based on weight, than women, and muscle structures vary. Smaller-sized muscles are capable of exerting less force, which increases wear and breakage. Studies show, for instance, that women suffer up to 8 times more knee injuries in "pivot" sports, where knees may endure heavy turns, twists, and jerks. Less massive, their knee muscles often don't produce enough stiffness (without being specifically trained) to protect sensitive ligaments behind the kneecap.

...your bones,
round rulers, round nudgers, round poles,
numb nubkins, the sword of sugar.
I feel the skull, Mr. Skeleton, living its
own life in its own skin.

—Anne Sexton

Twice as tough as granite for withstanding compression forces, 4 times more resilient than concrete in standing up to stretching, about 5 times as light as steel. As D'Arcy Thompson observed, bone is an architect's dream, a building material so malleable that it can be hammered into any shape, so versatile that when it's assembled into a light and durable framework it can execute and withstand complex mechanical movements, and so strong that it gives shape to and stiffens the whole human form without buckling. Not simply exquisite, as all great architecture must be, the edifice of the human skeleton is a perfect diagram of the lines of stress, tension, and compression involved in bearing the loaded structure—us—through a century or more of activity.

In the first stage of erecting a building or bridge, an initial frame is assembled of hot-rolled, carbon steel rods (rebars). Cement is then poured around them, forming a tight union to create a structure that can withstand rocking motions while maintaining strength. Without the steel rods, the cement would be too brittle, and would fracture easily; without the cement, the rebars would be too weak to support the overall load, and would bend.

So it is with bone. At 6 weeks after conception, rods of collagen, tightly wound chains of long protein molecules, become the body's incipient rebars, laying down a model for the full skeleton. Within 2 months, minerals from the blood crystallize and surround the rods, hardening like cement, although the bones still aren't connected at the joints. At birth, the bones have ossified enough to support the body, but it will take another year or more before complex joint mechanisms tie them all together to deliver enough strength and flexibility to permit, say, toddling.

The structural matrix of bone—a tight, interactive mix of protein and minerals—makes it a better building material than alloys and composites, but the true brilliance of its design is that it lives. The skeleton, like any living system, breaks down and renews itself continually. As the body grows to adulthood, it adapts its shape and proportions to match the demands of maturation. When bones break, they mend themselves. Growing outward from the middle of the shaft, the long bones that give the body its adult contours continue to grow until the age of 17 to 21.

Brilliantly engineered to distribute force, the living skeleton not only bears the body's load and enables movement but also stores minerals, protects internal organs, and, in its spongy interiors, houses the main bloodworks. Grayish in color, this flexible armature pivots along a central axis (the spine), which bends and turns and is attached to rings of bones (girdles) that connect it to the limbs. Arms and legs share the same fundamental structure—1 long bone, 2 shorter ones, and a clawlike assembly of yet smaller ones, all hinged by freely moveable joints.

Parts of a Whole

Hamlet contemplated Yorick's skull as a window into life's deepest mysteries. A sound choice. The most complex part of the skeletal frame, the skull gives shape to the head and face, protects the brain, and houses the special sense organs. It's comprised of 22 separate bones—21 of which are butted and unit-welded together with fibrous joints so adhesive and durable that they function as 1 block; the other one hinged, allowing the lower jaw to drop. Air-filled spaces (sinuses) in some of the bones surrounding the nasal cavity lighten the skull's weight and act as an echo chamber, adding resonance to the voice.

A bone's shape dictates what it does and vice versa. Long bones raise and lower as levers, short bones bridge spaces, flat bones shield and protect, irregular bones like the vertebrae and ilium (1 of the bones in each half of the pelvis) serve as customized flanks, flanges, collars, and crowns.

Sesame seed-shaped bones embedded in soft tissue (like the patella) cap vulnerable areas. The structural matrix of each bone—how cells align throughout the tissue—is arranged to transmit compression and tension forces optimally within its particular shape.

The size of the female skull is, on average, 4/5 that of the male, and a woman's facial skeleton is generally less angular than a man's; less square in the chin, less severe in the jaw, more convex in the forehead. In "gender-confirming facial surgery" —the remodeling of the face's bony architecture in people who've had sex changes—particular attention is paid to the area of the orbits, the pyramid-shaped cavities that house and protect the eyeballs. These appear higher, more rounded, and relatively large in the female skull, providing what one study on gender and expression calls "the soft and 'sincere' look of the feminine face.”

Women have narrower shoulders and their arms are usually shorter, meaning they have less leverage in throwing. Similarly, their hips are wider (the result of a more open pelvis, for childbearing), which increases the angle between the pelvis and thigh bone and makes it harder for them to raise their knees as high as men, or push on the ground with as much force, while running. On the other hand, lower hips and narrower shoulders create a lower center of gravity, making women's bodies more stable.

Exemplifying the idea of body-as-machine are the synovial joints—the long-lasting, lubricated bearings between bones that allow them to slide by each other in passing. Bones like those in the shoulder, where a spherical head fits into a bowl-shaped cavity, allow movement in 2 planes plus some twisting (which, in combination, allow us to scratch our own backs). Ellipsoidal (saddle) joints, like those at the base of the thumb, fit 2 spoon-shaped bone ends snugly together to form a pivot (making it possible, at the piano, to scoot the thumbs under the hands). With both concave and convex areas, the bones can rock back and forth and side to side, but have limited rotation.

Like, say, car transmissions and door hinges, synovial joints are designed to ease the transfer of mechanical forces while cutting down on friction and resisting wear. Encapsulated in its own flexible bag, each such joint contains a waterproof cavity in which smooth bone ends are cushioned by a shock absorber of pearl-smooth elastic material and "oiled" by the thick, slippery fluid (synovia) for which it's named.

As late as 1620, a learned Scottish physician could lecture his students, "Bone is...generated out of semen, fat and earth by the power of heat and the innate spirit."

It wasn't until 70 years later, when the English physiologist Clopton Havers first described the microscopic structure of bones, that their principle organizing features were identified.

The underlying structure of bone consists of living cells and protein fibers wrapped around layers of hard mineral salts. Long bones like the femur consist of concentric rings of these mixtures, ranging in texture from corallike to compact or woodlike, to rock-hard—a pattern of cylinders packed within cylinders laced with wormholes. Surrounding the fat storehouse of the inner marrow, the spongy, innermost cylinder is a matrix of rigid struts, which take some of the load that the bone has to bear while leaving space for production of red blood cells.

Havers discovered that the middle, compact layer is made up of concentric bony tubes bundled like logs on a truck, each one laced with a system of canals containing blood vessels. Special holes, or pits, allow enough space for individual bone cells to reside within, where they secrete bone salts needed for growth and repair. The cylinder's outer layer, called the cement line, attaches to the enamel-hard exterior shell, which is irrigated by blood vessels and innervated by neurons that can signal pain.

Though the heart is, mechanically speaking, a pump, and the circulatory system a network of pipes through which liquid tissue flows, physiologists and poets since Aristotle have marveled most at the system's motive (and emotive) power—its heat. The heart contracts 70 to 80 times each minute (about 100,000 times a day) and expands on its own steam, delivering life-sustaining oxygen and nutrients through a system of tubes that, laid end to end, would circle the world 3 to 4 times. Over the course of a 70-year lifetime it will beat 2.5 billion times, pumping approximately 1 million barrels of blood. Such an engine is a dynamo: "the hearthstone and source of innate heat by which the animal is governed," ancient Greek physician Galen called it.

It is also the organ along with the brain and senses, most sensitized to (and expressive of) love and beauty, fear and courage. When the Tin Woodman in The Wizard of Oz wants "to register emotion, jealousy, devotion, and really feel the part," and to "be kind'a human," he craves a heart. A woman's heart, 25 percent smaller on average than a man's, beats 5 to 8 more times per minute, meaning most women tire sooner than men. Women's hearts also generally last longer. "Who shall measure the heat and violence of the poet's heart when caught and tangled in a woman's body?" Virginia Woolf asked.

Before birth, the cardiovascular system has no use for the lungs. Gasses are exchanged via the mother, and blood returning from the body to the right atrium (top chamber) is shunted either to the left atrium through a temporary opening called the foramen ovale, an oval-shaped window, or into the thicker-walled right ventricle (bottom chamber). With each pulse, both sides pump blood directly to the body, in parallel, fueling its prodigious growth. Constant high pressure in the deflated lungs stanches any backwash.

In all plumbing systems, liquid rushes from areas of higher to lower pressure, and when our lungs first inflate with air at birth, the heart's mechanics shift. "Pulmonary resistance" drops sharply and more blood gushes from the right atrium into the right ventricle, which pumps it into the lungs through the pulmonary veins. Blood from the lungs then flows into the left atrium, increasing the pressure there.

Altogether, these changes press the tough, elastic wall between the atria to seal the "oval window" shut, thereby completing the separation of the apparatus into 2 halves, left and right.

Within days, the major duct diverting blood away from the lungs from the aorta also closes, and the mature heart—in fact 2 pumps in 1—begins to beat.

An elegant union of function and form, the heart and blood vessels are designed not just for circulation but recirculation—a more complex and dynamic affair. Though continual, the cycle can be said to start when the heart relaxes. The atria fill with blood; valves close to guard against backflow to the lungs and body. Then the atria contract, pushing blood into the ventricles.

The original valves remain closed, but valves between the upper and lower chambers are pushed open. In the third phase, the ventricles contract, sending blood to the lungs and around the body. The surge of blood reopens the first pair of valves while the second pair closes, resetting the apparatus for the next cycle.

Sexual differences occur chiefly with chronic diseases, long thought of primarily as afflicting men but lately understood to affect many women. Coronary heart disease, the leading cause of death in the United States for both sexes, results when plaques of cholesterol and other cellular materials in the inner lining of the arteries choke off blood flow to the heart muscle—causing cells, and eventually the organ itself, to die. Women develop symptoms, on average, 10 to 20 years later than men, yet for reasons that are unclear, tend to die sooner after diagnosis.

In terms of surviving another disease of the heart—congestive heart failure, in which the heart can't pump enough blood to other organs as blood backs up in the veins—women have the edge, living twice as long as their male counterparts, according to recent studies.

The complex structures of the circulatory system, which not only transports oxygen, nutrients, and waste but also helps regulate the body's water content, temperature, and pH balance, reflect regional differences based on function.

Since the brain uses about one fifth of the oxygen absorbed into the lungs, and brain tissue is at risk of permanent damage after just 5 minutes of blood deprivation, 2 pairs of large vessels—one running through the vertebrae, the other arising from the chest—ensure a generous, constant supply of blood.

In the lower digestive tract, deoxygenated blood from the stomach, spleen, intestines, and pancreas drain into the liver, where nutrients are absorbed and stored and toxins are removed, then returns to the heart and lungs through a large central vein; an independent vascular network (the portal system) with its own smooth-muscle—driven locomotion is required.

D. H. Lawrence called sex "pure blood consciousness ... the consciousness of the night." In fact, blood flow to the reproductive organs is vital, involving intricate arrangements of vessels, capillary networks, and, especially, communications between veins and arteries.

In women, periodic disruptions and changes in the body's status quo require vascular structures that can adapt. Chief among these are specialized corkscrew-shaped (spiral) arteries that provide blood to the endometrium, the lining of the uterus. During pregnancy, they flood the placenta with oxygen and nutrients; during menstruation, blood leaks out of their bases, killing endometrial tissue and forcing it to shed. As in men, blood flow is also critical for erotic pleasure, with blood rushing to expand the fibromuscular tube of the vagina, and the mechanism for clitoral engorgement being essentially the same as for erectile tissue in the penis.

In his influential 1932 book, The Wisdom of the Body, Harvard physiologist Walter Cannon coined the term homeostasis, from the Greek words for "same" and "steady." A homeostatic system—be it the earth's environment, the world oil market, a car and driver, a society, a person, or a cell—maintains its structure and activities by means of internal control devices. That is, self-regulation. Homeostasis explains how our bodies keep a constant temperature despite 100-degree swings in the weather, how a baby's sucking stimulates a breast to release milk, how we all aren't diabetic, disabled by too much or too little blood sugar. It explains how we can survive in an ever-changing world. "Not being able to control events, I control myself," the Renaissance essayist Michel de Montaigne wrote, "and I adapt myself to them, if they do not adapt themselves to me."

Like the autonomic nervous system, which regulates bodily activity through circular feedback loops (another term of Cannon's; his "wisdom" in a nutshell), the far-flung hormone-producing glands of the endocrine system control the body's toweringly complex chemical enterprise through an elaborate hierarchy of signals and counter-signals—a cyclical system of stimulus, monitoring, and response. Unlike the nervous system, the endocrine mechanisms (and the information they carry) are chemicals, which are transported through the blood, thereby requiring a more diffuse architecture.

Hormones, from the Greek word for "excite" or "spur on," are body chemicals that arouse and dictate the behavior of other body chemicals—molecular dominatrixes. Some of their activity is targeted and some is broadcast (some are like phone calls across town, others like spam mail on the Web), which influences the size, location, and organization of the tissues that produce them.

In addition to homeostasis, hormones also control reproduction, metabolism, behavior, growth, and development—a staggering number of chemical processes each second. Secretory glands grow larger and more productive with age; except, notably, the thymus, a soft, flattened, pinkish-gray organ perched illustriously in front of the heart in infants.

The seat of the immune system, the thymus takes up white cells from the blood and using hormones that it produces, converts them into T cells ("T" for thymus), sentinels that locate, identify, and destroy invaders. The thymus at birth is the size of a fist, in children the size of an orange. But once the immune system is set up around puberty, it shuts down and shrinks significantly. In 2002, Australian immunologists began attempts to "regrow" the gland back to full size, in order to churn out new T cells in patients whose immunities had been destroyed.

Establishing a flow chart of the body's chemical messaging system has been likened to memorizing the schedules of all the trains, buses, and planes serving New York City. Ten or so major glands and a host of minor ones produce more than 200 hormones and hormonelike substances, each with a specific list of target tissues or "stops," many of them other endocrine organs. As the molecules cascade through the blood, their timetables are maintained by overlapping traffic control systems, each with an array of built-in sensors, monitoring devices, and feedback loops.

The primary centers are located in the head and neck, where they are closely allied with the central nervous system. At the helm is the hypothalamus, the so-called "King of Glands," which produces some hormones but most notably connects endocrine function to the brain. A few inches away, the thyroid gland, a blood rich, butterfly-shaped organ in the front of the throat, governs metabolism, including heart rate and energy use, by releasing the hormone thyroxine. When the hypothalamus, which monitors thyroxine concentrations in the blood, senses low levels, it discharges a chemical signal to the neighboring pituitary gland, which in turn releases thyroid stimulating hormone (TSH), which spurs the production of more thyroxine. The parathyroids are 4 small, paired glands that work with the thyroid, drawing calcium from the bones, where it's stored, into the blood for use in activating muscles.

Males, as Natalie Angier writes, were "the hapless recipients of the first experiments in endocrinology...Men were castrated to render them trustworthy." The ancients observed that gelding the testicles made men more manageable and less libidinous, and for centuries promising, preadolescent sopranos were castrated to prevent their vocal cords from thickening, thereby lowering their pitch. Halting the flood of androgens (hormones that spur male sex characteristics) before it started, parents helped produce in their sons a magnificent (and androgynous, and potentially lucrative) vocal instrument: man-sized lungs bellowing through what was essentially a woman's voice box.

Sex differences due to hormonal and physical changes peak at puberty, but distinctions arise before that. In both boys and girls, starting before age 8, rising levels of androgens produced in the adrenal glands may set off premature biochemical surges. More common in girls, these include the precocious sprouting of pubic hair, glandular odors, and acne. With the onset of sex hormone secretion by the gonads, sexual development and reproductive function begin. Although men and women both produce male and female hormones, the effects tend to be gender- specific. Increased testosterone levels have been connected with aggression and social dominance, while fluctuating estrogen levels correlate with mood swings.

In women, the primary source of chemical messengers that sexualize the body—causing pubic hair to grow, fat to gather on the breasts and hips ("gynoid" distribution), the pelvis to widen, and menstruation to start—are the almond-shaped ovaries. These hormones (estrogen steroids) help govern the reproductive cycle. For instance, estradiol, the principal female hormone of the childbearing years, causes the lining of the vagina to become "cornified"—its cells lengthen into shapes resembling ears of corn-during the part of the cycle when the egg is prepared for ovulation. This matures and thickens the vaginal lining, priming it for sexual contact.

Running the body, like the maintenance of any homeostatic system, involves regular surveillance and precisely timed and aimed chemical releases. An archipelago of outlying detachments—outposts—infuses the blood with key hormones that control specific daily, even hourly, functions. Tucked away under the brain, the pineal gland secretes melatonin, which regulates body rhythms such as sleeping and waking. The pea-sized parathyroids produce parathormone, helping to adjust blood calcium levels. The thymus secretes hormones which regulate T cell maturation, proliferation and function; cell clusters in the pancreas, the islets of Langerhans, make insulin and glucagons which regulate blood glucose; and the kidneys secrete both erythropoietin, which stimulates the production of red blood cells, and renin, which helps control blood pressure. Since all of these products (except T cells) can be synthesized in the lab, many of the diseases caused by their absence are now controlled by replacement therapy.

Designers of computer protection software (read: security geeks) exalt the immune system. It's their image of fulfillment, the focus of their longing. Foreign code in the form of a virus or worm; unanticipated code in the form of a Trojan Horse (a name also used for microbial hijackers like HIV); faulty bits of code—bugs; unauthorized users; corrupted data—like virtual pathogens, digital enemies threaten constantly from within and without. Programmers design counter-code to block and destroy them. Computer security systems are called "architectures for self-healing" and are increasingly modeled according to the concepts and principles behind our own inherent capacity to block disease.

The problem solved by the human immune system is the problem of determining "self" from "nonself." This "immunological conscience," as Sir Peter Medawar calls it, involves sophisticated notions of identity and protection—at cellular and molecular levels. Critical functions include being able to form a stable definition of what self is; prevent or detect, then eliminate, dangerous foreign activity (infection); recall previous infection; recognize new infection; and protect the immune system itself from attack.

Like baubles studding a pale brocade spanning the body—none larger than a fist, most smaller than a grape—the organs and glands of the immune and lymphatic systems do all this and more. Microsoft can only dream.

As a "security force" trained to identify and destroy chemical and biological attackers, the fetal immune system is fully mustered and positioned, but it remains to be programmed—that is, active immunity is developed, based on the body's ability to recognize threats against it. At birth, a mix of generically designed lymphocytes and immune proteins (antibodies) "downloaded" from the mother's blood temporarily enables newborns to resist harmful agents they haven't encountered before.

Ultimately, though, recognition is key.

Like alien computer code, antigens (protein markers on the surface of cells) distinguish pathogens, cancer cells, and other disease agents as "nonself." While nonspecific defenses provide protection against all pathogens, the immune system marshals attacks against specific pathogens and remembers them, filing their antigen structures in a kind of database so that if they attack again, the body can immediately respond by making antibodies. Though the system is exquisitely sensitive to differences, mistakes are sometimes made. Autoimmune diseases, often compared with friendly fire, result when our own cell antigens are wrongly identified as "nonself" and the system erroneously attacks them.

When an airborne microbe enters the nose

A reinforced shield girds the body perimeter: an outer barrier made of external membranes (intact skin and mucosa) that prevent the entry of microorganisms, backed up by a rapid-response capacity—inflammation. Imagine a computer, online, protected by a firewall and other security devices from the intrusion of countless new viruses and worms, with a back-up system in which Pac-Man-like bits of code burst out of nowhere to storm, block, and destroy anything that gets through.

Nicked while shaving, the skin is breached. Circulating in the underlying blood vessels are white blood cells that migrate to the site of the injury and release chemical signals that trigger inflammation, which occurs as bacteria-eating cells (neutrophils) arrive to inhibit opportunistic invaders. Chemical signals also call out short-lived white blood cells—monocytes—that mature on site into long-lived, hydralike macrophages, which have the ability to recognize and ingest all foreign antigens, and do the mopping up.

When the skin is injured an inflammatory response is triggered to rid the body of harmful invaders. To begin this response, mast cells and basophils secrete chemical signals that promote blood flow to the area.

Basophils, mast cells, and red blood cells are joined within one hour of the injury by bacteria-eating cells called phagocytes, which include neutrophils and monocytes.

Closer magnification of the area shows phagocytes beginning to ingest pathogens and cellular debris.

Monocytes, having matured into macrophages (meaning "big-eaters"), continue to destroy pathogens and cellular debris by ingesting them, while the inflammation process begins to subside.

We breathe, therefore we are.

Not just at birth, when tiny and at times sodden lungs are ignited by their first scorching gulp of air and cry out. Nor only at the end, when we exhale for the last time and expire. Nor because, evolutionarily, the first eel-like creatures that dragged themselves onto land eons ago are our ancestors.

Each of the body's trillions of cells needs oxygen to function and produce exhaust. While the body itself isn't a blimp, and has no capacity for storing gasses, it must draw them in and release them...continuously...rhythmically. The relentless exchange of gas molecules within cells in the production of physical energy-in essence, combustion—requires an involuntary apparatus of powerful intakes, vents, pumps, and gauges, attached to the bloodstream through the heart.

"Tis her breathing that/Perfumes the chamber thus," Shakespeare wrote. That respiration, a gasworks, may also sweeten human encounters is one of life's more pleasurable enhancements, like a tender voice, sensual gaze, or tasty skin.

Muscular and compact, the respiratory system consists of branching tubes that perform 2 functions—one concerned with getting enormous volumes of air in and out of the body; and the other concentrated on getting oxygen into, carbon dioxide and water out of, the blood. How it does this in the space of 1 to 2 square feet of body cavity is a marvel of engineering and efficiency.

The core of the structure is the bronchial tree, an inverted system of airways in which the "trunk" is the windpipe and the "branches" are the subdividing passages that permeate the lungs. These branches split and split again until they are so numerous and so thin at their membranous tips that gas molecules cross into the blood through a network of capillaries which, laid end to end, would measure more than 1,000 miles. The accumulated physical force required to compress so much filtration into such a tight space is prodigious, as demonstrated by the highest recorded "sneeze speed"—about 100 miles per hour.

Because men on average are larger than women, their vocal cords, as seen here from above, are thicker and their vocal tract longer, thus producing both lower tones and resonances: men tend to have deeper voices. But what about the significant size overlap between women and men? Why are the voices of large women higher than those of similarly sized men?

Part of the reason appears to be chemical—male vocal cords have more receptors for male sexual hormones. According to linguist James Fidelholtz, however, the prevailing reason may be social—men jut out their jaws, intentionally dropping their voice registers, while women often speak "smilingly," stretching their lips to shorten their vocal tracts.

The Adam's apple is a bulge of the thyroid cartilage protruding under the skin. Its main purpose is to shield the larynx. A secondary sexual trait like body hair, its growth is spurred by testosterone flowing through men's, and to a lesser extent, women's bodies.

The amount of air that can be moved in and out of a person's lungs—so called vital capacity (VC)—is affected by age, height, and gender. Younger and taller people have larger VCs, while men exhibit more VC than women because their lungs are bigger, thus helping to explain why men grunt and bellow and find it easier to be blowhards.

Snaking smoothly from lips to lower bowel, the digestive tract reverses the normal course of evolution, breaking down complex biological structures into simpler ones. In 2001, its twisting architecture of tubes, valves, and reservoirs inspired a Los Angeles-based mixed-media sculptor named Tim Hawkinson to build a giant self-playing reed organ. By inflating 12 organically shaped clear balloons, some 20 feet high, with pressurized air through a central ductwork of polyethylene tubing, Hawkinson created an instrument that looked—and could blast, toot, and growl—like a gargantuan digestive system. His Uberorgan, as he called it, was effectively the world's biggest bagpipe, a football field from end to end.

Our own gastrointestinal tract is 26 to 30 feet long, and though at times embarrassing, similarly expressive. The core is a long, looping tube with a lubricated lining—the alimentary canal—operated by muscle power. Festooned along its length are a varied group of task-oriented "accessory" organs, from teeth and tongue to gallbladder and pancreas. Think of a pulsing disassembly line where successive precision machines cut, crush, whisk, sort, convert, and discard waves of complex nutrients in a high-throughput process for extracting fuel molecules one at a time.

It's estimated that the average person eats more than 20 tons of food in a lifetime. Recent obesity studies show a higher prevalence in women, who because they are generally smaller and less muscular than men, burn calories at a slower rate, and whose eating behaviors and energy storage needs are understood as advantages in bearing and raising children.

Top to bottom, the digestive systems of men and women consist of all the same structures, in the same proportions. There are a few exceptions: the jaw and teeth of the male are somewhat larger, adapting perhaps to the role of provider; and the organs of the lower intestinal tract of the female settle a bit lower and wider due to the shape of the female pelvis. Yet with eating- and drinking-related chemical processes, sex matters often decisively. For instance, females appear to empty both solids and liquids more slowly than males; male livers secrete twice as much of certain chemicals that transport many toxic molecules, including drugs, out of cells; and females produce less of the gastric enzyme that breaks down alcohol in the stomach. After drinking the same amount, women have higher blood-alcohol content than men, even after allowing for size differences. Men and women differ as well in how their bodies metabolize drugs, resulting in different therapeutic responses and long-term side effects.

Arched illustriously over the small intestine like the edge of a shield, the bowel's job is to reduce, solidify, and eliminate the waste stream coming from the digestive tract—a combination trash compactor and composter. Since most of the volume of human waste is water, the inner lining is coated with water-absorbing cells. About 20 pints of water per day are needed to move food and nutrients through the alimentary canal, but almost all of it is eventually reabsorbed, so that the body doesn't become dehydrated. Meanwhile, muscle bands cinch and gather the colon into a series of pockets that help compact waste before passing it on.

The last stages of digestion are accomplished not by the body but by resident microbes. There are more bacteria in the warm, moist interior of the large intestine than throughout the rest of the body—so many that they form up to a 1-inch coating. Their job is to break down organic matter—manufacturing in the process useful chemicals such as Vitamin K that are absorbed back into the blood—and to produce gas. The final, necessary product of digestion is feces, an amalgam of water, undigested fiber, dead gut cells, living and dead bacteria (50 percent of the fecal mass), and digested bile pigments, which give them their color.

Bowel movements begin involuntarily but end, thanks to the ringlike outer sphincter of the anus, within our control—like swallowing in reverse.

Sex is something I really don't understand too hot...I keep making up these sex rules for myself, and then break them right away.

—Holden Caulfield, The Catcher in the Rye

It's not only teenage virgins who are confounded by sexual reproduction. Comedian Marty Feldman said, "Sex is two and two makes five" and aphorist Mason Cooley describes it as "not imaginary, but it's not quite real either." British author Anne Cumming called sex "a shortcut to everything"; John Dos Passos, "a slot machine"; gay wit Quentin Crisp, "the last refuge of the miserable"; Henry Miller, "one of the nine reasons for reincarnation—the other eight are unimportant." John Updike says, "Sex is like money, only too much is enough," while social critic Camille Paglia responds, "Sex is metaphysical for men, as it is not for women. Women have no problem to solve through sex."

The problem solved through sex could not be more universal or key—the imperative for species and individuals to replicate—yet men and women disagree, often radically, about its meanings. This dispute can be both needed and desirable, since sex, as Peter Ustinov observed, "is a conversation carried out through other means." But the fact that we confer sexuality and the mechanisms of sexual behavior with meaning at all is perhaps what gives them their greatest power and mystery, and invites so many contradictory attempts to have the last word. "Sex is," Gore Vidal summarized tersely. "There is nothing more to be done about it. Sex builds no roads, writes no novels, and sex certainly gives no meaning to anything in life but itself."

The male and female reproductive systems share the same basic 4-part organization: 1) the gonads (ovaries or testes) produce special reproductive cells (eggs or sperm), which carry half the normal chromosomal contingent of other cells in the body and unite during fertilization; gonads also carry steroid hormones essential for reproduction, bodily growth, and development; 2) the genitals (penis in males, clitoris and vulva in females) allow for passage of the sperm from the male into the female; 3) the ducts and accessory glands connect the gonads to the genitals, forming a kind of ecosystem in which final preparations for fertilization are made and the resulting embryo is nurtured; and 4) a 2-tiered feedback network ties the gonads to the hypothalamus and other endocrine structures in order to regulate the secretion of the steroid hormones.

Structurally speaking, the chief, oft-poeticized differences between the sexes stem from the expanded role, in women, of the ducts and accessory organs—the vagina, uterus, and breasts. Built of skin, muscle, and fibrous tissue and lined with a lubricating membrane, the vagina is a 4-to-5-inch tunnel extending from the outer opening to a donut-shaped ring of muscles that guards the hollow, muscular, pear-shaped uterus—an elegant vestibule to the embryo containment area at the heart of the apparatus. Clusters of mammary glands in the breasts are made up of lobes and lobules that secrete milk into 15 to 20 tubelike ducts, each discharging through a tiny separate orifice in the nipple.

In both biological and design terms, the development of the sex organs starts with indifference; that is, males and females are the same. Though genetically already assigned, the gender of a 6-week-old embryo is physiologically indistinguishable, even as primitive sex cords, 2 thick ridges of sexually determined cells, first arise. Approaching the end of the second month, the internal sex organs, including ovaries and testicles, develop. Not until the third month do the vagina and penis start to form, and when they do, the initial protrusion is shaped and molded in such a way that it can produce characteristics familiar to both. This Rorschach-like structure is called the indifferent penis, which makes it sound like another name for a 25-year-old slacker who's just split up with his lover.

Of all the body's cells, the mature ovum alone is spherical. "The shape makes sense," science writer Natalie Angier notes. "A sphere is one of the most stable shapes in nature. If you want to protect your most sacred heirlooms—your genes—bury them in spherical treasure chests."

Six to seven million immature eggs are produced during gestation, with all but 40,000 or so destroyed naturally before puberty. From then on, once a month, a potential crop of 15 to 20 are selected to begin maturation. A wave of hormones released from the pituitary washes over the pods, or follicles, in the ovaries where they're stored.

The selected follicles begin secreting estrogen, the female sex hormone, which awakens and nourishes the undeveloped eggs. At day 10 or so, for reasons not yet understood, a single egg is selected to ripen fully.

Four days later, the brain releases another stimulating hormone and the activated follicle splits open, its egg sailing toward the fallopian tube where fingerlike tendrils sweep it inside. The egg bounces along the wall of the tube to await fertilization, eventually drifting toward the uterus. While the follicles are developing in the ovary, the lining of the uterus is busy creating a highly vascularized, nurturing environment in preparation for egg implantation. If fertilization does not occur, the egg washes away and the uterine lining sloughs off during menstruation and the cycle begins anew.

The penis, novelist William Gass wrote, "is a ridiculous petitioner...It is so easily teased, insulted, betrayed, abandoned; yet it must pretend to be invulnerable, a weapon which confers magical powers on its possessor; consequently this muscleless inchworm must try to swagger through temples and pull apart thighs like the hairiest Sampson, the mightiest ram."

Passion and eroticism aside, male "performance" results mainly from 2 types of physics: hydraulics and bioelectrics. When a male is sexually aroused, the 3 panels of erectile tissue in his penis fill with blood; the organ becomes distended. Once the blood is in place, the drainage vessels become compressed, slowing down blood outflow, prolonging erection. The penis is rafted with excitatory neurons, and when nerve impulses fired by further stimulation produce orgasm and ejaculation, drainage vessels open up again allowing it to return to a flaccid state. As a triple-purpose organ—eliminative, reproductive, copulatory—the human penis is unique among primates in the absence of a penile bone, which could be easily fractured and incapacitated as he stands upright and walks on two legs.

In part, hydraulic action also accounts for female sexual pleasure. Blood fills the capillary net in the vaginal walls, literally squeezing moisture out of the cells around it; with no place else to go, mucus seeps into the vagina, which like the penis is engineered for more than one function (it also plays a role in reproduction). The clitoris, on the other hand, has one purpose only—excitation—the product of extraordinary bioelectrics. Pea-sized, it contains a charged hive of 8,000 nerve endings, twice as many as the much larger penis and the most of any external structure in the body including fingers, lips, and tongue. Freud disastrously underestimated the clitoris as "the only bodily organ which is really regarded as inferior": "the atrophied penis," he called it. In fact, as Angier writes, in terms of sensation, "a woman's little brain is bigger than a man's."

The second-century Greek physician Galen observed of the snug complementarity between the male and female genitals, "Turn outward the woman's, turn inward, so to speak, and fold double the man's, and you will find both the same in every respect." Mirror-image homology, however, is just the physical set-up. The biomechanics of mating are kickstarted by arousal.

As we become sexually excited, our sex organs prepare for coitus through changes in the circulatory and nervous systems. The brain, receiving signals from the genitals, is tantalized, anticipating erotic pleasure. Our hearts hasten, flooding our arteries, while our veins constrict. Blood engorges the erectile tissue of the penis and clitoris as well as the testicles, ovaries, and labia minora—two thin folds of integument that lie just inside the vestibule of the vagina. Muscles tense. Nipples stiffen.

These effects plateau. In a woman, the outer third of the vagina becomes vasoconstricted, moistening, while the inner two thirds expand slightly and the uterus becomes elevated—all in preparation for receiving sperm. Male pre-ejaculate adds lubrication near the cervix. Not our exertions but our involuntary nervous systems increase breathing and quicken pulses.

Orgasm occurs with a loss of control, a shuddering release, something like a sneeze. In men, this occurs in two stages. As the intensity builds, reflex centers in the spinal cord send impulses to the genitals, prompting the smooth muscles of the testes, epididymides, and vasa deferentia to contract and squeeze sperm into the urethra. It's the filling of the urethra that triggers the muscles encasing the base of the penis to contract and force the semen out. Women's orgasms involve the uterus, outer vagina, and clitoris but not the inner regions of the vagina. Typically harder to stimulate than men, yet capable, once excited, of multiple orgasms, women appreciate gentle, sustained stimulation.

With the exception of one species of monkey, humans are the only animals who can mate while looking at each others' faces. As part of the "bipedal" family of mammals, the sex organs of the male and female are not oriented from the back as is the case for most mammals who naturally mate with the male approaching from behind the female. The female's downward-tipping vagina extends toward the back of her body internally and suggests that copulation face to face is natural and most suitable. With modern technology, such as Magnetic Resonance Imaging (MRI), the internal characteristics of the male and female anatomies can be seen during intercourse.

Upon orgasm, the uterus and cervix repeatedly dip down and up like a plunger drawing the sperm upward into the uterus. Recent research has lead to a new theory which hypothesizes that it is these dense, muscular contractions of the uterus that are responsible for the acceleration of the sperm toward the egg, and not the flagellum of the sperm, as has been the conventional belief.

After fertilization, the second great challenge of reproduction is the joining of 2 lives—embryo and mother. The blastocyst, a parasitic, barely visible dot measuring less than 1/100 of an inch across, is swept along the length of the fallopian tube and tumbles into the uterus. Meanwhile, hormones from the abandoned follicles in the mother's ovaries bathe the uterine lining, making it soft, porous, and absorbent—receptive. As cells along the rim of the blastocyst secreting machete-like enzymes to erode the uterine lining, make contact, they latch on. Rupturing the mother's capillaries, these rim cells fuse into a tough protective membrane (chorion). Burrowing further, the blastocyst becomes awash in the mother's starch-rich blood, and fingerlike structures from inside the chorion reach out and tap the mother's circulation. (Vampire myths may originate in the unconscious recollection of this earliest mother-child encounter.) The ideal site for implantation is located on the back wall of the uterus, near the support and protection of the spine.

A disk-shaped mass of blood-rich tissues soon emerges from the blastula to form an exchange organ (placenta) between the chorion and the uterine wall. Blood vessels from the mother and embryo intertwine in the placenta without actually joining, allowing stale blood to be exchanged for fresh, waste products for nourishment, embryonic hormones for maternal hormones, all while barring the passage of infection and most other harmful agents.

Biomechanically, a woman's reproductive system becomes, at birth, a kind of pump, as the same organs that once coordinated sexual maturation, delicately handled the transfer of male DNA during conception, then for 9 months served as an incubator, combine to expel a writhing baby that has gotten too big.

Gentle uterine contractions at the onset of labor prompt a positive feedback loop with the pituitary, which secretes oxytocin, the so-called "love hormone," inducing even stronger contractions. Another hormone secreted by the ovaries relaxes the cervix, the sphincter at the base of the uterus that, stretched by the baby's head, prepares to widen gradually from a half-centimeter—about the size of a peanut—to 10 centimeters in diameter. Meanwhile the vagina, relaxing and stretching to allow passage, secretes a complex carbohydrate called glycogen, which metabolizes on the spot to lactic acid, an infection-fighting agent.

When the contractions, which are involuntary, come every 2 to 3 minutes and last about a minute, a woman can safely bear down and push hard with her abdominal muscles.

THE UTERUS: During pregnancy, the uterus expands to hold up to 1,000 times its normal capacity and gains weight due to an increase in muscle fiber. Afterward, hormones work to restore it to its original size and shape.

PELVIC CHANGES: To accommodate the passage of the baby through the birth canal, significant changes take place to the mother's skeletal system. The pelvis actually has to open up, like a hinge, from the back, which results in the stretching, and sometimes tearing, of the ligaments and cartilage that joins the pelvis anteriorly (in the front).

BREASTS BEFORE AND AFTER: Mammary glands double in size in anticipation of breast feeding, and the small glands around the nipple may become raised and lumpy and change color. Breasts resist returning to their original state as the mammary glands shrink, mainly because the connective tissue between them has become overly stretched.

The body is a water-in, water-out system, but unlike the lungs—our other chief exchange organ—the urinary apparatus doesn't simply take in, then strip apart vital materials. It also recharges them, adjusting their levels, balancing their presence in the blood. Central and secondary intakes and outflows are separated by an infrastructure of dams, reservoirs, sluices, valves, chemical treatment units, and feedback mechanisms. The scientific term for urination, micturition, means "making water," and urine isn't merely effluent. It's the outflow of some of the body's most advanced fluid and chemical engineering.

Water comprises 60 percent of our adult weight and is concentrated not only in the blood but in tissues such as skeletal muscle that hold moisture. Water distributes heat, transports nutrients and hormones within and between cells, dilutes toxins, and serves as mother liquor for all chemical reactions. Yet we lose 2 quarts or more daily in exhaled breath, sweat, and feces, and if it becomes too salty or acidic, or too sweet, or contains too few red blood cells, we get sick. (Medieval physicians engaged in "urine gazing," inspecting and sometimes tasting urine to diagnose disease.) Mechanically speaking, the challenge of filtering microscopic wastes from 15 gallons of daily recirculating fluid while preserving pressure, volume, and compositional balance has yielded a 3-phase solution: accumulation, storage, and elimination. Men and women mainly differ, as women often lament, in the last phase, when anatomical variations are pronounced.

Layers of relaxed smooth muscle in the bladder wall encapsulate a folded inner lining. As the bladder swells with urine, the muscles stretch, causing the folds to iron out and disappear. Nerve receptors in the wall relay the extent of the stretching to the brain, which signals the desire to relax 2 ringlike sphincters—muscular spigots. The bladder's expandability is prodigious; though it can hold 200 to 300 milliliters (one- third to one-half a pint) before we have the urge to void, it's capable of ballooning to hold twice that much before we absolutely have to.

The work of protecting and regulating the fetal blood supply is outsourced, like back-office functions of Wall Street brokerages that are turned over to firms in Delhi. During pregnancy, fetal blood is cleaned and reconstituted in the placenta, even though unborn kidneys are functional by the fourth month. The mother's kidneys are forced to work overtime while her ballooning uterus presses harder and harder on her bladder, a sponge-like holding tank, held back only by a short muscular tube one-fifth the length of a man's. No wonder she doesn't sleep. (She gets her revenge years later, when the enlarging prostates of the men throttle their urethras, causing similar problems.)

When the baby is born, the mother's kidneys are suddenly relieved of a load (her bladder will take longer to recover) just as the newborn's spring instantly into use. That babies often urinate within seconds after their umbilical cords are cut attests to the speed of the transfer.

Produced in a continuous trickle at a rate of about 1 milliliter per minute, urine is "milked" by two contracting tubes (ureters) that lead to the bladder, a highly stretchy holding tank crammed in the reproductive region. The physical (and social, and psychological) objective behind channeling and controlling urine flow beyond that point is to ensure a voluntary, if not always or convenient, release. "In France one must adapt one's fragrance of the urinal," Gertrude Stein noted, adding Parisians are all wine drinkers and for a gentleman the more restless than for a lady."