I highly recommend the book The Body by Bill Bryson to anyone interested in science, particularly human biology. Bryson presents facts in an engaging and accessible manner, infusing his writing with humour and relevant medical history.
For posterity, this is a collection of notes I highlighted during my read.
Brain
The brain is often depicted as a hungry organ. It makes up just 2 per cent of our body weight, but uses 20 per cent of our energy. In new born infants, it is no less than 65%. That’s partly why babies sleep all the time - their growing brains exhaust them - and why they have a lot of body fat.
Your brain requires only about 400 calories of energy a day - about the same as you get in a blueberry muffin. Try running your laptop for 24 hours on a muffin and see how far you get.
Unlike other parts of the body, the brain burns its 400 calories at a steady rate no matter what you are doing. Hard thinking doesn’t help you slim. In fact, it doesn’t seem to confer any benefit at all. An academic at the University of Cambridge at Irvine named Richard Haier used positron emission tomography scanners to find that the hardest-working brains are usually the least productive. The most-efficient brains, he found, were those that could solve a task quickly and then go into a kind of standby mode.
Neurons are not like other cells, which are typically compact and spherical. Neurons are longand stringy, the better to pass on electrical signals from one to another. The main strand of a neuron is called an axon. At its termial end, it splits into a branchlike extensions called dendrites - as many of 400,000 of them.
At the top, literally and figuratively, is the cerebrum, which fills most of the cranial vault and is the part that we normally think of when we think of the ‘the brain’. The cerebrum, (from the Latin word for ‘brain’) is the seat of all our higher functions. It is divided into two hemispheres, each of which is principally concerned with one side of the body, but, for reasons unknown, the vast majority of wiring is crossed, so that the right side of the cerebrum controls the left side of the body and vice versa. The two hemispheres are connected by a band of fibres called the corpus callosum (meaning ‘tough material’ or literally ‘calloused body’ in Latin).
The exact pattern of grooves and ridges in brains is distinctive to each individual - as distinctive as your fingerprints.
Beneath the cerebrum, at the very back of the head about where it meets the nape of the neck, is the cerebellum (Latin for ‘little brain’). Although the cerebellum occupies just 10 per cent of the cranial cavity, it has more than half the brain’s neurons. It has a lot of neurons not because it does a great deal of thinking but because it controls balance and complex movements, and that requires an abundance of wiring.
The most important component of the limbic system is a little powerhouse called the hypothalamus, which isn’t really a structure at all but just a bundle of neurel cells. The name doesn’t describe what it does but where it is: under the thalamus. (The thalamus, meaning ‘inner chamber’, is a kind of relay station for sensory information, and is an important part of the brain - there isn’t any part of the brain that isn’t important, obviously - but is not a component of the limbic system.) The hypothalamus is curiously unimposing. Thought only about the size of a peanut and weighing barely a tenth of an once (three grams), it controls much of the most important chemistry of the body. It regulates sexual function, controls hunger, and thirst, monitors blood sugars and salts, decides when you need to sleep. It may even play a part in how slowly or rapidly we age. A large measure of your success of failure as a human being is dependent on this tiny thing in the middle of your head.
The hippocampus is central to the laying down of memories. (The name comes from the Greek for ‘seahorse’ because of its supposed resemblance to that creature.)
The amygdala (Greek for ‘almond’) specializes in handling intense and stressful emotions - fear, anger, anxiety, phobias of all types. People whose amygdalae are destroyed are left literally fearless, and often cannot even recognize fear in others. The amygdala grows particularly lively when we are asleep, and thus may account for why our dreams are so often disturbing. Your nightmares may simply be the amygdalae unburdening themselves.
The eyes send a hundred billion signals to the brain every second. But that’s only part of the story. When you ‘see’ something, only about 10 per cent of the information comes from the optic nerve. Other parts of your brain have to deconstruct the signals - recognize faces, interpret movements, identify danger. In other words, the biggest part of seeing isn’t receiving visual images, it’s making sense of them.
💡 For each visual input, it takes a tiny but perceptible amount of time - about 200 milliseconds, or one-fifth of a second - for the information to travel along the optic nerves and into the brain to be processed and interpreted. One-fifth of a second is not a trivial span of time when a rapid response is required - to step back from an ongoing car, say, or to avoid a blow to the head. To help us deal better with this fractional lag, the brain does a truly extraordinary thing: it continuously forecasts what the world will be like one-fifth of a second from now, and that is what it gives us the present. That means that we never see the world as it is at this very instant, but rather as it will be fraction of a moment in the future. We spend our whole lives, in other words, living in a world that doesn’t quite exist yet.
The brain tricks you in a lot of ways for your own good. Sound and light reach you at very different speeds - a phenomenon we experience every time we hear a plane passing overhead and look up to find the sound coming from one part of the sky and a plane moving silently through another. In the more immediate world around you, your brain normally irons out these differences, so that you sense all stimuli as reaching you simultaneously.
💡 The brain manufactures all the components that make up our sense. It is strange, non-intuitive fact of existence that photos of light have no colur, sound waves no sound, olfactory molecules no odour. As British doctor and author James le Fanu has put it, ’while we have the overwhelming impression that the greenness of the trees and the blueness of the sky are streaming through our eyes as through an open window, 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. They are all invisible, weightless, subatomic particles of matter travelling through space’. All the richness of life is created inside your head. What you see is not what is, but what your brain tells you it is, and that’s not the same thing at all. Consider a bar of soap. Has it ever struck you that soap lather is always white no matter what colour the soap is? That isn’t because the soap somehow changes colour 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. You get the same effect with crashing waves on a beach - greenish-blue water, white foam - and lots other phenomena. That is because colour isn’t a fixed reality but a perception.
Memory storage is idiosyncratic and strangely disjointed. The mind breaks each memory into its components parts - names, faces, locations, contexts, how a thing feels to the touch, even whether it is living or dead - and sends the parts to different places, then calls them back and reassembles them the whole is needed again. A single fleeting thought or recollection can fire up a million or more neurons scattered across the brain. Moreover, these fragments of the memory move around over time, migrating from one part of the cortex to another, for reasons entirely unknown. It’s no wonder we get details muddled.
The upshot is that memory is not a fixed and permanent record, like a document in a filing cabinet. It is something much more hazy and mutable. As Elizabeth Loftus told an interviewer in 2013: ‘It’s a little more like Wikipedia page. You can go in there and change it, and so can other people.’ Another extraordinary example of imaginary memories occurred in an experiment at an unidentified university in Canada where sixty volunteer students were confronted with the accusation that during adolescence they had committed a crime involving theft or assault for which they had been arrested. None of this had actually happened, but after three sessions with a kindly but manipulative interviewer, 70 per cent of the volunteers confessed to these imaginary incidents, often adding vivid incriminating details - entirely imaginary but sincerely believed.
Short-term memory is really short - no more than half a minute or so for things like addresses and phone numbers. (If you can still remember something after half a minute, it is no longer technically a short-term memory. It’s long-term.)
What is certainly true is that we retain a great deal more than we can easily summon to mind. You may not recollect much of a neighbourhood you lived in when you were small, but if you went back and walked around it you were small, but if you went back and walked around it you would almost certainly remember very particular details you hadn’t thought about for years. With sufficient time and prompting, we would probably all be astonished by how much we have stored away inside us.
What is surely the most striking feature of the brain is that all its higher processes - thinking, seeing, hearing, and so on - happen right at the surface, in the 4mm-thick sheath of the cerebral cortex. The person who first mapped this area was the German neurologist Korbinian Brodmann (2868-1918). Brodmann was one the most brilliant and least appreciated of modern neuroscientists. In 1909, while working at a research institute in Berlin, he painstakingly identified forty-seven distinct regions of the cerebral cortex, which have been every since as Brodmann areas. ‘Rarely in the history of neuroscience has a single illustration been as influential,‘.
The area that Brodmann mapped, the cerebral cortex, is the brain’s celebrated grey matter. Beneath it is the much greater volume of white matter, which is so called because the neurons are sheathed in a pale fatty insulator called myelin, which greatly accelerates the speed at which signals are transmitted. Both white matter and grey matter are misleadingly named. Grey matter isn’t terribly grey in life, but has pinkish blush. It only becomes conspicuously grey in the absence of blood flow and with the addition of preservatives.
💡 Incidentally, the idea that we use only 10 per cent 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.
The brain takes a long time to form completely. The wiring in a teenager’s brain is only about 80 per cent completed (which may not come as a great surprise to the parents of teenagers 😂). No one knows where the idea came from, but it has never been true or close to true. You may not use it at all terribly sensibly, but you employ all your brain in one way or another.
An interesting thing about touch is that the brain doesn’t just tell you how something feels, but how it ought to feel. That’s why the caress of a lover feels wonderful, but the same touch by a stranger would feel creepy or horrible. It’s also why it is so hard to tickle yourself.
The great paradox of the brain is that everything you know about the world is provided to you by an organ that has itself never seen that world. The brain exists in silence and darkness, like a dungeoned prisoner. It has no pain receptors, literally no feelings. It has never felt warm sunshine or a soft breeze. To your brain, the world is just a stream of electrical pulses, like taps of Morse code. And out of this bare and neutral information it creates for you—quite literally creates—a vibrant, three-dimensional, sensually engaging universe. Your brain is you. Everything else is just plumbing and scaffolding.
Microbes
Make no mistake. This is a planet of microbes. We are here at their pleasure. They don’t need us at all. We’d be dead in a day without them
Thin people have more gut microbes than fat people; having hungry microbes may at least partly account for their thinness.
Interesting Facts
Most of us have experienced that abrupt feeling of falling while asleep known as a hypnic or myoclonic jerk. No one knows why we have this sensation. One theory is that it goes back to the days when we slept in trees and had to take care not to fall off. The jerk may be a kind of fire drill. That may seem far-fetched, but it is a curious fact, when you think about it, that no matter how profoundly unconscious we get, or how restless, we almost never fall out of bed, even unfamiliar beds in hotels and the like. We may be dead to the world, but some sentry within us keeps track of where the bed’s edge is and won’t let us roll over it (except in unusually drunk or fevered circumstances). Some part of us, it seems, pays heed to the outside world, even for the heaviest sleepers.
In breathing, as in everything in life, the numbers are staggering – indeed fantastical. Every time you breathe, you exhale some 25 sextillion (that’s 2.5 × 1022) molecules of oxygen – so many that with a day’s breathing you will in all likelihood inhale at least one molecule from the breaths of every person who has ever lived.1 And every person who lives from now until the sun burns out will from time to time breathe in a bit of you. At the atomic level, we are in a sense eternal.
Immune System
Every day, it has been estimated, between one and five of your cells turn cancerous, and your immune system captures and kills them.
An increase of only a degree or so in body temperature has been shown to slow the replication rate of viruses by a factor of two hundred—an astonishing increase in self-defense from only a very modest rise in warmth.
(Note: This page is a work in progress. I’m currently revisiting the book and updating my highlights. Content will be added incrementally as I review.)