The book contains the works of the French physiologist P. Regnard, based on the materials of lectures he gave at the Sorbonne.  The essays analyze various forms of mental epidemics that took place from the 15th to the 19th centuries. The author gives a detailed description of the phenomena under study and proves the existence of a connection between the historical era and the form that this or that epidemic takes - witchcraft, morphinism or delusions of grandeur.