You need the Real Audio player to hear this which you can download for free . If it still doesn't work try this link .

Back in the bad old days before anaesthetics had been invented, surgical operations had to be done very quickly. The world record for a leg amputation is 15 seconds - by Dominique Larrey, who was Napoleon's chief surgeon. In fact, Charles Darwin was so disgusted by the suffering of surgery without anaesthetics, that he gave up a promising career in medicine - and that's how we got the Theory Of Evolution.

We've been using anaesthetics for one-and-a-half centuries, but there's still one great mystery - we have no idea of how anaesthetics work. "Anaesthesia" means "no sensation" - which is really handy if your dentist is drilling in your mouth, or your surgeon is inserting a metal pin in your leg. A general anaesthetic adds "unconsciousness" to the "lack of sensation". The anaesthetists (or anaesthesiologists as the Americans call them), know how many milligrams of what particular drug to give you per kilogram of your body weight - but down at the level of the cells, they have no idea how general anaesthetics work.

We do know a few things. A general anaesthetic is carried in your blood to the nerves in your brain. The nerve cells stop receiving signals (so you don't feel any pain) and they stop sending signals (so you don't thrash around in the middle of delicate surgery).

We also know what properties a chemical needs to make a good general anaesthetic - it has to evaporate easily from a liquid into a gas, and it has to be soluble in fat. These properties covers proper anaesthetics like halothane, or other chemicals like paint stripper and nail polish remover - which sometimes have nasty side effects, like death. Another thing we know is that the better a chemical dissolves in olive oil, the better a general anaesthetic it is - but again, we don't know why.

Anaesthetics go back a long way in history. The Ancients knew of alcohol, opium and hemp. Around 1800, the British chemist Sir Humphry Davey discovered that nitrous oxide could, after it made you silly, also make you unconscious. In 1842, the American surgeon, Crawford Long, used ethyl ether as a general anaesthetic. But Long didn't publish his results. Today, it's William Morton, an American dentist, who is given credit for the discovery that ether could be used as a general anaesthetic after he showed it off in a public demonstration of a tooth extraction in 1846. Within six months, the use of ether as an anaesthetic had spread around the world.

Many people argued against anaesthetics, claiming that they interfered with God's Natural Order. But the opposition faded away in 1853, when Queen Victoria used chloroform as an anaesthetic when she gave birth to her eighth child, Prince Leopold.

While we still don't know how general anaesthetics work, we're pretty sure that it's something to do with the cell membrane of the nerve cell. Every cell in your body is wrapped inside its cell membrane - it acts as a barrier to keep the outside out, and the inside in. The cross-section of the cell membrane looks like a sandwich - with different layers of water-loving molecules, and water-hating molecules. The pressure in between the different layers of the cell membrane is enormous - around 400 atmospheres, or roughly the pressure four kilometres down at the bottom of the Pacific Ocean.

Now we do know that the cell membrane has a whole bunch of channels which will let certain chemicals go in or out - so there are chloride channels, sodium channels, potassium channels and so on. Each channel passes right through the sandwich of the cell membrane, and the enormous pressure tries to close it up - but, each channel is held open by cholesterol and other fats that are arranged into a fairly rigid liquid crystal. As various chemicals flow in or out of channels of the cell membrane, they change the electrical charge on the cell membrane - and that can switch the nerve cell on or off.

And here's where we run into the various theories about how general anaesthetics work. One of the older theories says that anaesthetics can somehow slide into the sandwich of the cell membrane, and interfere with that rigid liquid crystal that holds open the channel. If the shape of the channel changes, then so will the flow rate of various chemicals (sodium, potassium etc.) that go in or out of the cell. This does explain why so many different volatile chemicals (from nail polish to petrol to halothane), will make you go unconscious.

A more modern theory is that anaesthetics work by sticking to little chemicals around the open mouth of the channel. These little chemicals then can open or close the channel, and again, this can then lead to changes in electrical charge on the cell membrane - and hopefully anaesthesia.

Another theory says that anaesthetics alter that enormous 400 atmospheres of pressure inside the sandwich of the cell membrane, so that they can change the shape of the channels - and again, interfere with the electrical charge on the cell membrane.

We really still don't have a good answer yet for how anaesthetics work. But when we do understand how they work, not only will we get better anaesthetics which have fewer side effects, we could possibly even begin to understand what consciousness is - and until then, the experts will just have to sleep on it.