A slip, for once not of the tongue or pen but the foot. Then it was Christmas night in the emergency unit and Boxing Day in the operating theatre. Finally, a bed in a ward, a drip in the arm and a big hunk of plaster where 24 hours before had been a cute new leather-soled flat shoe, made in Morocco.

A man who has spent an hour and a half inserting plates and pins around and into your broken bones looks down, smiling. Things went well, he says. He asks, "Can you describe the pain?"

It began with two sharp percussive strikes on the snare drum, a bilious swell from the woodwind section and a hundred violins bowing sul ponticello. Now it is a discordant symphony, full of jarring notes, piano e forte, percussive twangs and pings, syncopation, non-Western scales and instruments made of bin lids and cacti. Occasionally, an instrumentalist you haven't noticed before rises to perform a solo and you are left agape at the display of bravura. Johnny Rotten sings in the chorus. The pauses between movements are of awkward length: just as the audience thinks the symphony is finished and starts to applaud the conclusion, it starts again, allegro.

It is an earthquake, a roar, a rumble and an upheaval. The room pitches, the lamp sways, the needle jumps on the record. The earth trembles, tears itself asunder and thrashes about. Buildings fall. Someone is screaming. That someone might even be you.

It is a metallic taste with a faint stench, like blood and offal. It is the food of China's Guizhou Province, full of sour flavours and burning chillies, but lacking that cuisine's charm: there are no fried dumplings stuffed full of black sesame paste to add sweetness, there is no joy in the sharpness. It is chicken gone off, a forgotten orange covered in blue mould, the science experiment at the back of the fridge from which you've carelessly taken a bite. It has a texture and an odour all its own. If you are going to eat pain, you need to have at hand - there is no way to say this nicely - a vomit bag.

It is a monstrously hyperactive child rocketing around your home, crashing into vases, knocking over piles of CDs, picking up glasses and throwing them, tearing the stuffing out of the pillows, driving the blade of scissors into the veneer of your grandmother's sideboard, honing in on the most precious and fragile things you own and then breaking them, screeching and whingeing and keeping you on edge, lest he work out how to do something even worse. Your normally ordered home is in chaos. The damage is mounting. He's now playing with matches. His parents think he is adorable. They don't look like leaving anytime soon.

It is a Caravaggio, dangerous, moody, bathed in the orange light of candles which burn with the heat of bonfires but black, pitch black underneath. Dirt under its fingernails, rags for clothing and stained teeth. Crucifixions, severed heads, sacrifices, and Bacchus, laughing. A knife behind the canvas, ready to be used at any time.

It is an exercise in plane geometry. Pain draws parallel and intersecting lines, marking out planes and angles, measuring diameters and hypotenuses and then expanding and contracting them. The geometry of pain colonises the body like a maths teacher's blackboard. A point on the heel sends out a ray which passes through the temple, and the chest, now obscurely and darkly congested, is one endpoint for a line which ends at the knee. The geometry occasionally spreads out from the awkward polygon which you have become: when you lift the lid on a container of homemade food, the smell of salmon makes the woman in the bed opposite retch.

It is a small taste of hell trapped in a foot. In the emergency ward and later in the regular ward, you see people with coronaries and head injuries and kidney diseases and cancer. Pain fills their eyes, and they sing with it. When the news comes on the little TV above your bed, another bomb has gone off somewhere, killing and injuring dozens of people; mudslides are crushing villagers in their homes; cars wrap themselves around poles; wars are taking their ‘toll'; psychopaths are raping and torturing and murdering promising young students, gays, fathers of two and old women living alone. You think of your ankle, which has been so professionally and carefully handled and treated and operated on, and yet still produces so much pain. You imagine yourself as one of those people on the news, lying in pieces on some road, hoping someone will notice you. You have always been good at empathising. Now your whole body is tingling with the pain of everyone, everywhere.

It is a special brand of shame. You are not the friend who broke his ankle and was out of hospital the next day (you took five). He was back at work the day after (you took a week). You're not the brave woman, cousin of someone else, who broke her ankle and tore a ligament, and carried on for three days before she even bothered to see a doctor. You, Loser, couldn't even get up from the pavement, and you were trembling violently and going into shock. You fret that friends are going to get bored with your incapacitation. You know there are many people worse off than you and that no one likes a whinger. You don't. So what is this thing that makes any honest answer to the question ‘How are you?' a whinge? You recognise that sometimes it is necessary to hide pain, to pretend to be chirpy and good so that other people, who are themselves feeling fine, feel even better. But you hate the deception. You want people to understand. You want sympathy, flowers, chocolate, visits, cooked meals, attention, care - but you feel ashamed to ask.

It is the whole world. Nothing else exists. It fills all space.

It is a small room, locked from the outside.

"Does it throb, for example?" Gently, the doctor helps you find the correct vocabulary. He asks you to rate it on a scale from one to ten, and the nurse notes the answer in your folder. A few more questions, and more information is exchanged or collected from the metal probe under your tongue and the clip on the end of your finger and the pneumatic bag wrapped around your upper arm. Finally, you are alone again with your amorphous friend.

You reach for the green glowing button hanging off the rail of the bed. A flurry of morphine drifts through your veins like snow, and, for a few precious minutes, a white and glittering blanket settles over everything. Pain is an old black-and-white photo, faded to sepia.