The proposed book is devoted to the experimental study of fatigue during mental and physical work, the mechanisms of overexertion of cortical processes during mental activity. Particular attention in the work is given to taking into account the speed, quantity and strength of the formation of temporary connections in the cerebral cortex in people "of various ages (from 8 to 80 years) with various types of mental activity. The author in each specific case sought to find out the reason for the decline in mental performance and the location of the functioning link, which is out of order earlier than others.