Execution by the cross was used for nearly 1,000 years. The Romans took it over from the Phoenicians and perfected it as a sophisticated, almost scientifically calibrated, technique that produced the maximum of pain and a death struggle of adjustable duration: they could decide how much they wanted it to hurt and how long it would take you to die. It was the standard penalty for serious crimes such as desertion, murder and high treason. Only full Roman citizens were exempt from it.

So common was crucifixion - and so feared a way of death that the evangelists did not waste words to describe it in detail. Everyone in the Roman empire knew all too well how it was done and what it involved.

This is why it took centuries for the cross to become the prime devotional symbol of Christianity, long after the first Christian emperor, Constantine the Great, abolished crucifixion in AD 337. It was such a horrible form of death that it was only as the truth blurred and memories faded that it could replace the fish as the Christian emblem - just as it would never enter anyone's head today to wear a silver model of an electric chair or a gold gas chamber as conventional mark of piety.

WHAT REALLY HAPPENED?
Although the crucifixion story has been known all over the world for centuries, hardly anyone has a clear understanding of what crucifixion entailed or what was the actual medical cause of Christ's death on the cross. They have only a vague image that it was painful. And in many details, what passes for common knowledge is fundamentally wrong. For example:

• The cross used for Christ's execution was a low one, shaped like the letter T and known as the crux commissa. It did not have a head-piece, as usually depicted in churches and around people's necks. Although the now-familiar form of cross existed, known as the crux capita, it was impractical and seldom used.

• Christ did not carry the whole cross, only the cross-piece or patibulum. This was made of cypress wood and weighed 75-125 lb. about as heavy as a bag of cement. Being forced to carry this the 700 yards of narrow, winding streets from Pilate's praetorium to the execution ground at Golgotha was an extra torture for an already flogged - almost flayed - man.

• The nails securing Christ's arms to the cross were driven through his wrists, not his palms. The palms could not withstand the weight of the body; the hands would tear through longways. Instead, the eight-inch nails were driven precisely into the space between the wristbones. These were dislocated, but not shattered. That in itself is painful enough, as anyone who has even sprained a wrist will know - but it was not all.

An important nerve, the median, crosses the wristjoint. The square-edged nails almost always came into contact with the nerve, stretching it over the sharp sides of the nail like the strings over the bridge of a musical instrument. This caused such severe cramp in the thumb that it bent across the palm so violently that the thumbnail embedded itself in the flesh.

The next step was to hoist the vertical stem, or stipes. Then the knees were bent until the sole of one foot could be pressed flat against the stipes, and an eight-inch nail was driven through it, precisely in the middle between the second and third metatarsal bones. As soon as the nail emerged through the sole, the other leg was bent into position so that the same nail could be hammered through the second foot and into the wood. The victim was then left to hang from the three nails. Blood loss was slight, but the pain was unbearable. The death struggle has begun.

A body suspended by the wrists will sag downwards, pulled by gravity. This produces enormous tension in the muscles of the arms, shoulders chest wall. The ribs are drawn upwards so that the chest is fixed in position as if the victim has just drawn a large breath - but cannot breathe out. The condemned man begins to stifle.

The severely strained arm, shoulder and chest muscles develop agonizing cramp. The metabolic rate is raised, but the oxygen supply is reduced.

One result of this is the production of large amounts of lactic acid in the bloodstream, leading to what is known as "metabolic acidosis", often seen in athletes driven to exhaustion and severe cramp. This is aggravated by the difficulty in breathing and in ridding the body of carbon dioxide, leading to "respiratory acidosis". Unrelieved, the victim finally dies of suffocation. This can occur within half an hour.

So swift a death did not satisfy the Romans. This is why they nailed the feet too. The condemned man could buy time by pushing himself up on the nails in his feet, stretching his legs and so raising the body to relieve the chest and arms. This allowed his to breathe better - for a while. But perching with a full weight of the body on a square nail driven through the middle bones of the feet brings intolerable pain. The victim soon lets his knees sag until once more he is hanging from the wrists, with the median nerves again strung over the nail shafts. The cycle is repeated to the limit of endurance.

There were endless "refinements". The torture could be prolonged by using ropes instead of nails, reducing the pain but lengthening the struggle. A sort of seat could be fixed to the vertical stem of the cross, allowing further temporary respite. When this was used, the death throes could be made to spin out for two or even three days.

The executioners could shorten the ordeal too, hastening death by smashing the legs, thus making it impossible for the dying man to push himself up to breathe. No wonder classical Latin writers such as Cicero and Seneca described crucifixion as the most abominable method of execution of all.

This, then, was how Christ died: he had to endure the torturing choice between suffocation and rending pain. The slow and deadly cycle followed its inexorable course for six full hours.

On arrival at Golgotha, the military offered the men awaiting execution - Christ and the two thieves who were to die alongside him - wine containing myrrh and gall. This was probably considered a stupefying and pain-dulling mixture and can be likened to the last cigarette before facing a firing squad.

His clothes were removed, (¹centuries later, out of reverence for the Saviour, artists portrayed a cloth around his waist; 'Now there stood by the cross of Jesus his mother.' John 19:25), he was forced to lie with his raw shoulders on the crosspiece. The nails were driven through his wrists and he was hoisted on the patibulum onto the stipes. Moments later, his feet were nailed to the vertical.

Around noon, the New Testament says, "there came a darkness over the whole land". At this point Christ's death struggle was nearing its climax: sweat ran down his body as his temperature soared. The muscles were in perpetual cramp.

Excessive sweating brought severe thirst. Blood loss and oedema caused by flogging reduced the circulation volume, blood pressure fell and the heart pounded faster. The severely acidotic condition of the blood combined with the excessive loss of salt through sweat, was barely compatible with life. The heart began to fail and the lungs filled with fluid. The beginnings of the death rattle croaked in each failing, painful breath as his heart began to give out.

Finally, the dying man called out "I thirst." One of the soldiers lifted a sponge soaked in posca, a soldier's brew of wine, vinegar, water and beaten eggs. When Jesus had drunk from it, he cried : "It is finished." Then, in the words of St. John, "he bowed his head and gave up his spirit."

Bob Smalhout is professor of anaesthiology at University Hospital, Utrecht, Holland, and a prominent Dutch Bible scholar.