Dr. Conrad Murray -- Help, I'm Dying in Jail!!!

Dr. Conrad Murray is pulling a Floyd Mayweather Jr. -- pleading with a judge to spring him from L.A. County Jail because it's taking an extreme physical toll on his body ... and if he doesn't get help soon, Murray believes he'll DIE behind bars.

But here's the crazy thing -- unlike Mayweather, Murray's not asking for house arrest (he knows that will never happen) ... Murray's asking to be sent to PRISON, where he believes he'll get better care.

Murray's lawyer tells TMZ, he wants to serve out his time in the big house because he's being treated like filth in L.A. County jail -- getting fresh air once a month, and fresh underwear once a WEEK.

As a result of the alleged deprivation, we're told Murray's health is failing -- his hair, nails, and skin are gnarly ... and he's been suffering from a constant headache over the last few weeks.

Murray says he's never gotten headaches in the past -- and is worried it might be a brain tumor.

We're told Murray asked to see a doctor, but got shuffled around and has received no medical attention.

Murray told his attorney, Valerie Wass, "I may not make it out of here alive. This is a very dangerous place. I'm in here dying. The system is intent on killing me."