In 1889, Scottish physiologist John McWilliam published in the British Medical Journal the first systematic description of ventricular fibrillation as the mechanism of sudden cardiac death, and experimentally demonstrated that weak electric currents could induce it in mammalian hearts. His work established that fibrillation — not simple cardiac arrest — was the underlying cause of sudden death by electricity. Prévost and Batelli explicitly cited McWilliam in 1899 as the starting point for their experiments on VF reversal. His work remained little recognized for decades; clinical cardiology took over half a century to translate this finding into therapy.