The Body: A Guide for Occupants

by Bill Bryson

limbic system

The hypothalamus is curiously unimposing. Though only about the size of a peanut and weighing barely a tenth of an ounce, it controls much of the most important chemistry of the body. It regulates sexual function, controls hunger and thirst, monitors blood sugar and salts, decides when you need to sleep.

yet in a profound physiological sense we don’t really know what thinking is.

Consider how we see—or, to put it slightly more accurately, how the brain tells us what we see.

it continuously forecasts what the world will be like a fifth of a second from now, and that is what it gives us as the present. That means that we never see the world as it is at this very instant, but rather as it will be a fraction of a moment in the future.

It is a strange, nonintuitive fact of existence that photons of light have no color, sound waves no sound, olfactory molecules no odors.

yet the particles of light impacting on the retina are colourless, just as the waves of sound impacting on the eardrum are silent and scent molecules have no smell.

All the richness of life is created inside your head.

Consider a bar of soap. Has it ever struck you that soap lather is always white no matter what color the soap is? That isn’t because the soap somehow changes color when it is moistened and rubbed. Molecularly, it’s exactly as it was before. It’s just that the foam reflects light in a different way.

That is because color isn’t a fixed reality but a perception.

Fundamentally, however, memories come in two principal varieties: declarative and procedural.

Incidentally, the idea that we use only 10 percent of our brains is a myth. No one knows where the idea came from, but it has never been true or close to true. You may not use it all terribly sensibly, but you employ all your brain in one way or another.

There is quite a lot of disagreement over whether the brain can make new neurons.

On the plus side, the brain is able to compensate for quite severe loss of mass.

“If you were designing an organic machine to pump blood you might come up with something like a heart, but if you were designing a machine to produce consciousness, who would think of a hundred billion neurons?”

Epilepsy isn’t really a single disease but a collection of symptoms that can range from a brief lapse of awareness to prolonged convulsions, all caused by misfiring neurons in the brain.

A decapitated head will still have some oxygenated blood in it, so loss of consciousness may not be instantaneous.

Estimates of how long the brain can keep working range from two seconds to seven,

The person who finally put the study of the human head on something like a sound scientific foundation was none other than the great Charles Darwin.

One theory is that eyebrows are there to keep sweat out of the eyes, but what the eyebrows do really well is convey feelings.

eyelashes subtly change airflow around the eye, helping to waft away motes of dust and other tiny particulates from landing there,

our external nose and intricate sinuses evolved to help with breathing efficiency and with keeping us from becoming overheated on long runs.

the real purpose of most of our facial features is to help us interpret the world through our senses.

for it is built back to front. The rods and cones that detect light are at the rear, but the blood vessels that keep it oxygenated are in front of them.

You might have had the experience of looking at a clear blue sky on a sunny day and seeing little white sparks popping in and out of existence, like the briefest of shooting stars. What you are seeing, amazingly enough, is your own white blood cells, moving through a capillary in front of the retina.

Scheerer’s blue field entoptic phenomena

blue sky sprites.

They are especially visible against a bright blue sky simply because of the way the eye absorbs different wavelengths of light.

“hovering flies.”

essentially it is a camera.

The front part—the lens and cornea—captures passing images and projects them onto the back wall of the eye—the retina—where photoreceptors convert them into electrical signals that are passed on to the brain via the optic nerve.

If there is one part of your visual anatomy that deserves a moment’s tha...

The part of the eye that has the most photoreceptors—that really does the seeing—is called the fovea

Tears not only keep our eyelids gliding smoothly but also even out tiny imperfections on the eyeball surface, making focused vision possible.

Tears come in three varieties: basal, reflex, and emotional.

We are the only creatures that cry from feeling, as far as we can tell.

“a riot of spots, wedges, and spokes,”

People who are “color-blind” normally lack one of the three types of cones, so they don’t see all the colors, just some of them.

we still have just three kinds of color receptors compared with four for birds, fish, and reptiles.

It’s a humbling fact, but virtually all nonmammalian creatures live in a visually richer world than we do.

You don’t normally experience the blind spot, because your brain continually fills in the void for you. The process is called perceptual interpolation. The blind spot, it’s worth noting, is much more than just a spot; it’s a substantial portion of your central field of vision. That’s quite remarkable—that a significant part of everything you “see” is actually imagined.

When you place a set of $800 headphones over your ears and marvel at the rich, exquisite sound, bear in mind that all that that expensive technology is doing is conveying to you a reasonable approximation of the auditory experience that your ears give you for nothing.

the fleshy whorls of our outer ears do a surprisingly good job of capturing passing sounds—and, more than that, of stereoscopically working out where they come from and whether they demand attention. That is why you can not only hear someone across the room speak your name at a cocktail party but turn your head and identify the speaker with uncanny accuracy.

The tiny quiverings of the eardrum are passed on to the three smallest bones in the body,

The ossicles are perfect demonstrations of how evolution is so often a matter of make-do. They were jawbones in our ancient ancestors and only gradually migrated to new positions in our inner ear. For much of their history, those three bones had nothing to do with hearing.

“If we could hear quieter sounds still, we would live in a world of continuous noise, because the omnipresent random motion of air molecules would be audible. Our hearing really could not get any better.”

To help protect us from the damage of really loud noises, we have something called an acoustic reflex, in which a muscle jerks the stapes away from the cochlea, essentially breaking the circuit, whenever a brutally intense sound is perceived, and it maintains that posture for some seconds afterward, which is why we are often deafened after an explosion.

The decibel is logarithmic, which means that its units of increment are not mathematical in the everyday sense of the term but increase by orders of magnitude.

So the sum of two 10-decibel sounds is not 20 decibels but 13 decibels. Volume doubles about every 6 decibels, which means that a 96-decibel noise is not just a bit louder than a 90-decibel noise but twice as loud.

The pain threshold for noise is about 120 decibels, and noises above 150 decibel...