Who was the inspiration behind Francis Bacon 's series of paintings that began in 1957 ?
The work is noted for its physical , expressive brush strokes and for being Bacon 's first work to refer to Diego Velázquez 's " Portrait of Innocent X " , a painting that was to haunt the artist ; " Head VI " is the first of Bacon 's " screaming popes " , which total more than 45 individual works .