These advances have expanded the known boundaries of human survival from cold.

They have also clarified the mechanisms — the protective effects of cold on oxygen-deprived tissue, the window during which the dying process is reversible, the conditions under which rewarming can succeed without triggering fatal arrhythmias.

What they have not done is make Jean Hilliard’s case entirely routine.

Six hours at -22 degrees Fahrenheit, with the degree of peripheral freezing that made needle penetration impossible, and recovery without major organ damage or loss of digits: this remains a case at the extreme end of the hypothermia survival spectrum.

Gordon Giesbrecht, a physiologist at the University of Manitoba who has become one of the world’s leading researchers on human physiological responses to cold — he goes by the nickname “Professor Popsicle” — has commented on cases like Jean Hilliard’s in the context of his broader work on hypothermia.