The Body: A Guide for Occupants

by Bill Bryson

we all indulge in ‘microexpressions’ – flashes of emotion, no more than a quarter of a second in duration, that betray our true inner feelings regardless of what our mor...

All the many species of hominids that preceded us had prominent brow ridges, but we Homo sapiens gave them up in favour of our small, active eyebrows.13 It’s not easy to say why. One theory is that eyebrows are there to keep sweat out of the eyes, but what the eyebrows do really well is convey feelings. Think how many messages you can send with a single arched

Most mysterious of all is the chin. The chin is unique to humans and no one knows why we have one. It doesn’t seem to confer any structural benefit to the head, so it may be simply that we find a good chin dashing.

the real purpose of most of our facial features is to help us interpret the world through our senses. It’s curious that we always speak of our five senses because we have way more than that. We have a sense of balance, of acceleration and deceleration, of where we are in space (what is known as proprioception), of time passing, of appetite. Altogether (and depending on how you count them) we have as many as thirty-three systems within us that let us know where we are and how we are doing.

marvelled at the intricacy of the eye that they often cited it as proof of intelligent design. It was an odd choice because the eye is really rather the reverse – literally so, 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. There are vessels and nerve fibres and other incidental detritus all over, and your eye has to see through all this. Normally, your brain edits out any interference, but it doesn’t always succeed.

You may 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.17 Because white blood cells are big (compared to red blood cells) they sometimes get stuck briefly in the narrow capillaries, and that is what you are seeing. The technical name for these disturbances is Scheerer’s blue field entoptic phenomena (named for a German ophthalmologist of the early twentieth century, Richard Scheerer), though they are more commonly and poetically known as blue-sky sprites.

cornea. This modest, dome-shaped goggle not only protects the eye from worldly assaults, but actually does two-thirds of the eyeball’s focusing. The lens, which gets all the credit in the popular mind, does only about a third of the focusing.20 The cornea could hardly be less imposing. If you were to pop it out and lay it on the tip of your finger (where it would fit very comfortably) it wouldn’t seem much at all. But on closer examination, as with almost every part of the body, it is a wonder of complexity. It has five layers – epithelium, Bowman’s membrane, stroma, Descemet’s membrane and endothelium – laminated into a space just slightly over half a millimetre thick. In order to be transparent, it has a very modest blood supply – indeed, practically none.

most photoreceptors – that really does the seeing – is called the fovea (from a Latin word for ‘shallow pit’; the fovea inhabits a slight depression).

Tears not only keep our eyelids gliding smoothly, but also even out tiny imperfections on the eyeball surface, making focused vision possible.21 They also contain antimicrobial chemicals, which successfully keep most pathogens at bay. Tears come in three varieties: basal, reflex and emotional. Basal are the functional ones that provide lubrication. Reflex tears are those that emerge when the eye is irritated by smoke or sliced onions or similar. And emotional tears are of course self-evident, but they are also unique. We are the only creatures that cry from feeling, as far as we can tell. Why we do so is another of life’s many mysteries. We get no physiological benefit from erupting in tears. It is also a little odd surely that this act signifying powerful sadness is also triggered by extreme joy or quiet rapture or intense pride or almost any other potent emotional state.

The fact is that odours and flavours are created entirely inside our heads. Think of something delicious – a moist, gooey, warm chocolate brownie fresh from the oven, say. Take a bite and savour the velvety smoothness, the rich heady waft of chocolate that fills your head. Now consider the fact that none of those flavours or aromas actually exist. All that is really going in your mouth is texture and chemicals. It is your brain that reads these scentless, flavourless molecules and vivifies them for your pleasure. Your brownie is sheet music. It is your brain that makes it a symphony. As with so much else, you experience the world that your brain allows you to experience.

Despite the vast differences in heart rates, nearly all mammals have about 800 million heartbeats in them if they live an average life. The exception is humans. We pass 800 million heartbeats after twenty-five years, and just keep on going for another fifty years and 1.6 billion heartbeats or so.

A typical mammal uses about thirty times as much energy in a day as a typical reptile, which means that we must eat every day what a crocodile needs in a month.3

ATP, which may be the most important thing in your body you have never heard of. Every molecule of ATP is like a tiny battery in that it stores up energy and then releases it to power all the activities required by your cells – and indeed by all cells, in plants as well as animals. The chemistry involved is magnificently complex.

we are the size we are because that is about the only size we can be.

you are stressed or exhausted, you are much more likely to suffer an infection, for instance.

as a woman ages, the number and quality of her eggs diminishes, and that can be a problem for those who postpone motherhood to the later stages of their productive years, which is exactly what is happening throughout the developed world.

the age of thirty-five a woman’s stock of eggs is 95 per cent exhausted and those that remain are more liable to produce faults or surprises – like multiple births, for instance.

Once women pass thirty, they are much more likely to have twins. The one certainty of procreation is that the older both parties get, the more difficulty they are likely to have conceiving, and the more problems they may encounter if they do conceive.

In many ways, we feel the pain we expect to feel.

the invention of agriculture. Jared Diamond has called it ‘a catastrophe from which we have never recovered’.13 Perversely, farming didn’t bring improved diets but almost everywhere poorer ones. Focusing on a narrower range of staple foods meant most people suffered at least some dietary deficiencies, without necessarily being aware of it. Moreover, living in proximity with domesticated animals meant that their diseases became our diseases.

medical care is actually making things worse by treating the symptoms of mismatch diseases so effectively that we ‘unwittingly perpetuate their causes’.

Because they are so seemingly normal, the body sometimes fails to detect them and doesn’t invoke an inflammatory response as it would with a foreign agent. That means that most cancers in their early stages are painless and invisible. It is only when tumours grow big enough to press on nerves or form a lump that we become aware that something is wrong.

In 2011, an interesting milestone in human history was passed. For the first time, more people globally died from non-communicable diseases like heart failure, stroke and diabetes than from all infectious diseases combined.1 We live in an age in which we are killed, more often than not, by lifestyle.