[La pierre de la folie. The stone of madness].

During the 16th and 17th centuries the expression "to have a stone in the head" was synonymous with peculiar behaviour or mental symptoms. Many quacksalvers earned their living by treating such patients with a scalp incision, pretending to remove a stone. Some of the great painters from this period have used the subjects as models for their works of art. A series of paintings of this kind have been traced in different museums, catalogues and publications, and are described, followed by a short survey of popular beliefs and superstitions connected with stones.