The 24-year-old Evans was injured when he careered head first into Wales full back Lee Byrne after 35 minutes and had to undergo surgery on his neck last night.
Evans lay motionless on the ground for more than five minutes as the Scottish medical team attended him. Paradmedics gave him oxygen before he was carried off on a stretcher and taken to University Hospital of Wales, Cardiff.
His brother, fellow Glasgow Warriors star Max, had already replaced his brother on the right wing for 13 minutes in the first half after Thom had received a cut. In that time Max struck to score to score Scotland's second try to give them a 15-3 lead over a shell-shocked home side.
"Thom has sustained damage to his neck that has required surgery. He is moving his arms and legs and we are hoping for a speedy recovery. There will be a further bulletin in due course," said Scotland's team doctor James Robson.
Evans' parents, and brother Max, joined him at hospital, where Scotland's full back Chris Paterson was also taken after leaving the field on the day he made his 100th Test appearance with an abdominal injury. Wing Rory Lamont also left the Millennium Stadium on crutches after injuring his right leg.
"Chris sustained damage to his right kidney during a collision in the first-half. On admission to hospital it became evident that the damage would require a period of observation and, in particular, bed rest in order to aid his recuperation," added Robson.
Paterson had taken his long sequence of successful goal kicks to 35 in the 6 Nations with his eighth minute conversion of back row man John Barclay's storming try and the first of two drop goals from Man of the Match Dan Parks came before the Max Evans try.
At that stage the Scots were on course to break a run of away games in the tournament that had seen them win only once in their last 17 games on the road.
All Wales could offer in return were three Stephen Jones penalties as the game ended 18-9 in the Scots' favour at the interval. Parks extended the lead to 11 points with a penalty in the first minute of the second half before Wales hit the comeback trail.
A Byrne try after 55 minutes gave the home fans hope, but then Parks struck again with a massive drop goal 10 minutes later from his own 10 metre line.
With a 10 point cushion, and only 15 minutes left to go, it looked as though Chris Cusiter's side would hold on, but then Irish referee George Clancy sent replacement hooker Scott Lawson to the sin bin in the 73rd minute and yellow carded replacement full back Phil Godman in the 79th.
No sooner had Lawson departed than Cardiff Blues wing Leigh Halfpenny broke clear for a try which Jones converted to cut the gap to three points with four minutes left on the clock.
When Godman was sent to the sin bin for an alleged trip on Byrne as the Welsh full back followed up his chip into the Scottish 22 it left home skipper Ryan Jones with a difficult decision.
Should he take a scrum in the Scottish 22 and go for the try to win the game against 13 men, or take the three points to level the scores and try to score off the final play when the Scots restarted?
In the end, he took the points and Wales received Mike Blair's kick-off, went through the phases and a minute later worked Ospreys star Williams over at the pots for his 49th try for his country. Jones kicked the conversion and Wales had won one of the most dramatic 6 Nations matches of all time with 81 minutes, 25 seconds on the clock.