Rouen - Nicolas Cocaign, 39,  went on trial on Monday for allegedly killing his prison cellmate and then slicing open his chest to remove and intend to fry up the man's heart with onions on a makeshift grill and devouring it. - but mistakenly eating his lung.

He appeared in court in the northern city of Rouen for killing Thierry Baudry in January 2007.

Cocaign then allegedly sliced open Baudry's chest with a razor blade, removed a rib and pulled out an organ which he believed was the man's heart, but which in fact was a lung.

He is accused of eating part of the lung raw and then frying the rest of it with some onions on a makeshift cooker in the cell in the Rouen prison.

"I wanted to take his soul," Cocaign, who at the time of the crime was in custody awaiting trial for attempted armed robbery, allegedly told an investigating judge probing the case.

What started as a fight between the two men in the cell ended up with Cocaign pummeling his 41-year-old victim with his fists, then suffocating him with a garbage bag and slashing his chest open with a pair of scissors.

"[I was] curious to see what he tasted like," Cocaign told investigators.

Cocaign then broke one of Baudry's ribs and removed his lung, mistaking it for the heart, eating part of it raw before frying up another morsel.

A third cellmate, David Lagrue, witnessed the attack and was so horrified that it is believed to have been the motivation for his November 2009 suicide in another prison, the Daily Mail reported.

Not surprisingly, Cocaign's attorney, Fabien Picchiottino, is pushing for an insanity defense.

"My client should not be in prison, he should be in a mental institution," Picchiottino told the court. "The prison authorities must share the blame for not spotting his fragile mental condition."

A verdict is expected on Thursday