The Body: A Guide for Occupants

by Bill Bryson

Extraordinarily, some birds and marine mammals are able to switch off one half of their brain at a time, so that one half remains alert while the other is snoozing.

Why the eyes move during REM sleep is uncertain.

Most men have erections during REM sleep. Women likewise experience increased blood flow to the genitals. No one knows why, but it seems not to be overtly associated with erotic impulses. Typically, a man will be erect for two hours or so a night.

Some part of us, it seems, pays heed to the outside world, even for the heaviest sleepers.

Tests have also shown that people are pretty good at waking themselves at a predetermined time without an alarm clock, which means that some part of the sleeping mind must be tracking the real world outside the skull.

Foster found that our eyes contain a third photoreceptor cell type in addition to the well-known rods and cones. These additional receptors, known as photosensitive retinal ganglion cells, have nothing to do with vision but exist simply to detect brightness—to know when it is daytime and when night.

Your hair grows faster in the summertime, for instance.

As David Bainbridge has neatly put it, “The pineal is not our soul, it is our calendar.”

The more adenosine you accumulate, the drowsier you feel. Caffeine slightly counteracts its effects, which is why a cup of coffee perks you up.

When a teenager struggles to get up in the morning, that isn’t laziness; it’s biology.

Sufferers simply lose the ability to fall asleep and slowly die of exhaustion and multiple organ failure. The disease is always fatal. The destructive agent is a type of corrupted protein called a prion (short for proteinaceous infectious particle).

Another disorder that disrupts sleep is narcolepsy. It is commonly associated with extreme drowsiness at inappropriate times, but many with the condition have as much trouble staying asleep as staying awake.

Babies yawn in the womb. (They hiccup, too.) People in comas yawn.

Nettie Stevens deserves to be better known.

She worked for several years as a teacher and librarian before finally entering Stanford University in 1896 at the advanced age of thirty-five, and she was forty-two and tragically near the end of her short life when she finally earned her PhD.

Taking a position as a junior researcher at Bryn Mawr, she embarked on a blizzard of activity, publishing thirty-eight papers as well as discovering the Y chromosome.

Women (and we are talking here about healthy, fit women) carry about 50 percent more fat on their frames than fit, healthy men. This not only makes the woman more agreeably soft and shapely to suitors but also gives her reserves of fat she can call upon for milk production during times of hardship. Women’s bones wear out sooner, particularly after menopause, so they suffer more breaks in later life. Women get Alzheimer’s twice as often (partly because they also live longer) and experience higher rates of autoimmune diseases. They metabolize alcohol differently, which means they get intoxicated more easily and succumb to alcohol-related diseases like cirrhosis faster than men do.

Women even tend to carry bags differently than men do. It is thought that their wider hips necessitate a less perpendicular carrying angle for their forearm so that their swinging arms aren’t constantly banging against their legs. That’s why women generally carry bags with their palms facing forward (allowing their arms to be slightly splayed) while men carry them with palms facing back. Far more significantly, women and men have heart attacks in quite different ways. A woman suffering a heart attack is more likely to experience abdominal pain and nausea than a man, which makes it more probable that it will be misdiagnosed. In a thousand ways large and small, they are quite different beings.

Men have their own differences. They get Parkinson’s disease more often and commit suicide more, even though they suffer less from clinical depression. They are more vulnerable to infection than females (and not just humans but across nearly all species). That may indicate some hormonal or chromosomal difference that hasn’t yet been determined, or it may simply be that males on the whole lead riskier, more infection-prone lives. Men are also more likely to die from their infections and from physical injury, though again whether that is b...

All this is important because until recently drug trials very often excluded women, largely because it was feared their menstrual cycles could skew results.

The drug phenylpropanolamine was commonly used in over-the-counter medications for colds and coughs for years until it was discovered that it appreciably increased the risk of hemorrhagic stroke in women but not in men. Similarly, an antihistamine called Hismanal and an appetite suppressant called Pondimin were withdrawn after they were shown to pose serious risks to women, but only after the first had been on the market for eleven years and the second for twenty-four. Ambien, a popular sleep medication in America, had its recommended dosage for women cut in half in 2013 when it was found that a high proportion of female users were suffering impaired performance if they tried to drive the next morning. Men did not suffer in anything like the same way.

Women are anatomically different in one other very significant way: they are the sacred keepers of human mitochondria—the vital little powerhouses of our cells.

Sperm pass on none of their mitochondria during conception, so all mitochondrial information is transferred from generation t...

A woman endows all her children with her mitochondria, but only her daughters have the mechanism to pass i...

Over time, the mitochondrial pool for humans has shrunk so much that, almost unbelievably but rather wonderfully, we are all now descended from a single mitochondrial ancestor—a woman who lived in Africa about 200,000 years ago.

Matters specific to women—menstruation above all—were almost totally a mystery to medical science.

But whether or not women actually possess a G spot is a matter of continuing, and sometimes heated, debate.

A survey of a thousand men, conducted in conjunction with a campaign called Gynecological Cancer Awareness Month, found that the majority could not reliably define or identify most of a female’s private parts—vulva, clitoris, labia, and so on.

The vulva is the complete genital package—vaginal opening, labia, clitoris, and so on. The fleshy mound above the vulva is called the mons pubis. At the top of the vulva itself is the clitoris (probably from a Greek word for “hillock,” but there are other candidates), which is packed with some eight thousand nerve endings—more per unit of area than any other part of the female anatomy—and exists, as far as can be told, only to give pleasure. Most people, including females, are unaware that the visible part of the clitoris, called the glans, is literally only the tip of it. The rest of the clitoris plunges into the interior and extends down both sides of the vagina for about five inches. Until the early twentieth century, “clitoris” seems generally to have been pronounced “kly-to-rus.”

The vagina (Latin for “scabbard”) is the channel connecting the vulva to the cervix and uterus beyond. The cervix is a doughnut-shaped valve that stands between the vagina and the uterus. “Cervix” in Latin means “neck of the womb,” which is precisely what it is. It serves as a gatekeeper, deciding when to let substances (like sperm) in and when to let others (like blood during menstruation and babies during birth) out. Depending on the size of a man’s organ, the cervix is sometimes hit during sex, which some women find pleasurable and others find uncomfortable or painful.

The uterus is simply a more formal name for the womb, where babies grow. The uterus normally weighs two ounces, but at the end of a pregnancy it may weigh two pounds. Flanking the uterus are the ovaries, where eggs are stored, but they are also where hormones like estrogen and testosterone are produced.

The ovaries are connected to the uterus by Fallopian tubes (prope...

Eggs are usually fertilized in the tube and then pushed outward into the uterus.

Male reproductive anatomy is considerably more straightforward. It consists essentially of three external parts—penis, testicles, and scrotum—with

For the record, however, I will note that the testicles are factories for producing sperm and some hormones; the scrotum is the sac in which they are housed; and the penis is the delivery device for sperm (the active part of semen), as well as outlet for urine.

What seems to the man purely a burst of pleasure really is a kind of rocket launch.

The chances of a successful fertilization from a single randomly timed act of sex have been calculated to be only about 3 percent.

Among the suggested causes have been diet, lifestyle, environmental factors, frequency of ejaculation, and even (seriously) wearing tight underpants, but no one knows.

It is a curious fact that every woman is born with her lifetime’s supply of eggs already inside her.

As she enters her childbearing years, a woman will have about 180,000 eggs primed and ready to go. Why she loses so many eggs along the way and yet enters her childbearing years with vastly more than she will ever need are two of life’s many imponderables.

by the age of thirty-five a woman’s stock of eggs is 95 percent exhausted and those that remain are more liable to produce faults or surprises, like multiple births. Once women pass thirty, they are much more likely to have twins.

In 1847, a medical instructor in Vienna named Ignaz Semmelweis realized that if doctors washed their hands before conducting intimate examinations, the disease all but vanished.

At all events, we now know, most miscarriages and other setbacks in pregnancy are because of problems with the placenta, not the fetus.

The moment of birth, the starting of a new life, really is quite a miracle. In the womb, a fetus’s lungs are filled with amniotic fluid, but with exquisite timing at the moment of birth the fluid drains away, the lungs inflate, and blood from the tiny, freshly beating heart is sent on its first circuit around the body.

If ever there was an event that challenges the concept of intelligent design, it is the act of childbirth.

“Women in labour have pretty much the same pain-relief options as their great grandmothers—namely gas and air, an injection of pethidine (an opioid) or an epidural anaesthetic.”

We are only beginning to understand the importance and nature of a woman’s vaginal microbiome. Babies born by Cesarean section are robbed of this initial wash.

Various studies have found that people born by C-section have substantially increased risks for type 1 diabetes, asthma, celiac disease, and even obesity and an eightfold greater risk of developing allergies.

Other useful microbes are picked up from the mother’s skin. Martin Blaser, a doctor and professor at New York University, suggests that the rush to clean up babies as soon as they are born may actually be depriving them of protective microorganisms.

One of the most extraordinary features of early life is that nursing mothers produce over two hundred kinds of complex sugars—the formal name is oligosaccharides—in their milk that their babies cannot digest because humans lack the necessary enzymes. The oligosaccharides are produced purely for the benefit of the baby’s gut microbes—as bribes, in effect. As well as nurturing symbiotic bacteria, breast milk is full of antibodies. There is some evidence that a nursing mother absorbs a little of her suckling baby’s saliva through her breast ducts and that this is analyzed by her immune system, which adjusts the amount and types of antibodies she supplies to the baby, according to its needs. Isn’t life marvelous?