A guided tour of the body you live in

Hold up your hand and open and close it slowly. Watch the tendons move under the skin on the back of your hand, like strings being pulled. Now press two fingers gently into the soft hollow beside your windpipe and wait. There it is: a small, steady tap, tap, tap.

You have just seen and felt two different parts of a machine you have lived inside your whole life and have almost certainly never been shown properly. This lesson is that tour. Not how it all works together — that is the next topic, Body systems — but something simpler and stranger: what is in there, and exactly where.

By the end of this layer you will be able to point at your own chest and say, with confidence, my heart is behind that, about the size of my fist, tucked slightly to the left. You will know why you have 206 bones but your baby cousin has about 300. You will know which of your two lungs is smaller, and why. And you will know the name of the smallest bone in your body, which is hiding inside your ear and is roughly the size of a grain of rice.

Everything here is factual and calm. There are no photographs and nothing gruesome — just an honest map of a remarkable piece of engineering that you happen to be walking around in.

Start as small as you can go and build upwards. The body has 5 levels of organisation, and once you see them, everything else falls into place.

A cell is the smallest living unit. On its own it is invisible: you would need a microscope to see one. Cells of the same kind, grouped together and doing one job, make a tissue. Several tissues working together as one structure make an organ. Organs that share a task make an organ system. All the systems together make you — an organism.

This is not a list to memorise. It is a ladder you can climb in either direction. Point at any part of yourself and you can ask: which organ is this? which tissues is it made of? which cells make up those tissues?

How many cells? The honest answer is nobody has counted them, because you cannot count something that small in something that big. But scientists have estimated it carefully, organ by organ: work out the size of a typical cell of each kind, work out the volume of each organ, and add everything up.

The best-known estimate, published in 2013, gives about 37.2 trillion cells for an adult body. Written out, that is 37,200,000,000,000. In the Indian system that is about 37.2 lakh crore cells.

Numbers that size stop meaning anything, so here is a handle. If you counted your own cells at one per second, never sleeping, never stopping, it would take you about 1,179,000 years — more than a million years, which is far longer than humans have existed.

If you tell a friend "the pain is on the left", whose left do you mean? Yours, or theirs as they look at you? Doctors solved this a long time ago with a rule that never changes: left and right always belong to the body being described, never to the person looking at it.

So on a diagram of a body facing you, the heart is drawn on your right side of the page, because it is on its left. That single rule prevents an enormous amount of confusion, and it is worth getting into your head now.

Without bones you would be a puddle. The skeleton does three big things at once: it holds you up, it protects the softest organs, and it gives muscles something firm to pull against so you can move.

An adult human has 206 bones. That number is worth remembering, because it is the answer to a question people ask all their lives — and because, surprisingly, you were not born with it.

It helps to split the skeleton in two.

The axial skeleton is the central column: the skull, the spine, the ribs and the breastbone. It is the part that protects — brain, spinal cord, heart and lungs all live inside it. Counting carefully: 22 skull bones, 6 tiny bones inside the ears, 1 hyoid bone in the throat, 26 bones in the spine, 24 ribs and 1 breastbone, which comes to 80 bones.

The appendicular skeleton is everything hanging off it: the shoulders, arms and hands, and the hips, legs and feet. That is 126 bones. It is the part that reaches and walks.

And 80 + 126 = 206. The famous number is not a magic fact; it is a sum you can check yourself.

Stand sideways in front of a mirror and look at your back. It is not a straight pole. A human spine has 4 gentle curves, alternating like a very shallow letter S: the neck curves forward, the chest region curves backward, the lower back curves forward again, and the fused base curves backward.

Those curves are not a defect. A straight column transmits every footstep straight up into your skull. A curved one flexes slightly at each step and soaks up the shock, the way a bent knee absorbs a jump better than a locked one. Babies are born with a single backward curve; the neck curve appears when a baby learns to lift its head, and the lower-back curve appears when it learns to stand. Your spine's shape was built by the things you learned to do.

A skeleton made of 206 separate bones would be useless if they were all locked together — and equally useless if they could all move any way at all. What you actually have is a careful mixture, chosen joint by joint for the job that joint has to do.

A joint is simply a place where two bones meet. Some joints are built to move a lot, some a little, and some not at all.

Bones cannot move themselves. Every movement you have ever made — blinking, running, whispering, swallowing — was made by a muscle shortening.

You have about 600 skeletal muscles, and together they make up roughly 40% of your body's mass. That is more than the skeleton and all the internal organs put together.

One skeletal muscle deserves its own paragraph, because it is the one most people have never heard of and everyone is using right now.

The diaphragm is a dome of muscle stretched right across the inside of your body, just under the lungs. It is the floor of the chest and the ceiling of the belly. When it contracts it flattens downwards, which makes the chest bigger, and air flows in. When it relaxes it springs back up into its dome, and air flows out.

It is unusual because it works both ways: it runs by itself while you sleep, and you can also take over and control it deliberately. Put a hand flat on your belly just below the ribs and take a slow breath in. The gentle push outwards against your hand is your diaphragm flattening. A hiccup is that same muscle twitching suddenly out of turn.

Now for the tour itself. For each organ, three things: what it is, where it sits, and how big it is. Sizes matter more than people expect. Once you know that a heart is the size of your fist and a brain is about 1.4 kg, the inside of a body stops being a vague mystery and becomes a place with a layout.

The masses below are for a typical adult. Yours will be smaller — everything scales with the body it is in.

Two "largest organ" facts get muddled all the time, so keep them apart:

• The largest organ in the body is the skin. It covers 1.5 to 2.0 square metres in an adult and weighs around 4.0 kg with its fatty under-layer. People forget skin is an organ at all, because it looks like packaging. It is not: it keeps water in, keeps germs out, senses touch and temperature, and helps control how hot you are.
• The largest organ inside the body is the liver, at about 1.5 kg. It is a dark red wedge under the right ribs, and it does hundreds of different jobs — storing energy, making substances the blood needs, and cleaning things out of the blood. You have one liver and it has no spare.

Five sense organs bring the outside world in. Each one is really the same trick done five ways: something out there — light, vibrating air, a chemical, a touch — is turned into a signal a nerve can carry to the brain.

Stand in front of a mirror and draw an imaginary line down the middle of yourself. Two eyes, two ears, two arms, two legs, two nostrils, two lungs, two kidneys. Humans are bilaterally symmetric: our outsides come in matching left and right halves. Almost every animal that moves head-first is built this way, because a body that travels forwards needs the same equipment on both sides.

Then look inside, and the tidy symmetry falls apart.

Everything so far has been about a typical body. Yours is a real one, and the best way to make anatomy stick is to measure it.

Every measurement below is safe, needs nothing but a measuring tape and a clock, and involves no comparing, no judging and no numbers anyone has to share if they would rather not. That last rule is not optional. Bodies vary enormously and every single one of the numbers you collect is normal.

One last thing about your own body: it is not finished.

• Bones are still fusing. Near the ends of your long bones are growth plates — bands of cartilage where new bone is added. They are why you are getting taller. They close in the late teens and early twenties, and when they do, you stop growing.
• Teeth are being replaced. You grew 20 milk teeth as a small child and are steadily swapping them for 32 adult teeth — 12 more, because an adult jaw is bigger and has room for extra grinding teeth at the back.
• Your proportions are changing. A newborn's head is about a quarter of its whole length — roughly 4 head-lengths tall. An adult is about 8. Nobody's head shrinks; the rest of the body simply grows much faster than the head does, which is why children look the way children look and why artists count in heads when they draw people.

Fair question. Nobody can see inside a living person just by looking. So how is any of this known?

For most of history there was only one way: studying bodies after death, with the permission and care that such a thing demands. In India, the Sushruta Samhita — a surgical text traditionally dated to around 600 BCE — describes detailed study of the body along with about 121 surgical instruments and operations including the rebuilding of a damaged nose. In Europe, the Flemish anatomist Andreas Vesalius published a book in 1543 based on what he saw with his own eyes, and corrected hundreds of errors that had been copied from ancient texts for over a thousand years.

Then, in 1895, everything changed.