CHAPTER IV.

THE INSANE DIATHESIS.

The _ideal_ human system--The _actual_ human system--Physical and mental differences among persons in health--Periods during which there exist considerable changes in the character of mental action in the case of many persons while in a state of health--Excitement and depression of mental activity--This tendency to unstable activity of the nervous system may be inherited or acquired--Illustrations from the effects of over-exertion, "writer's cramp," chorea--A similar condition of that portion of the brain which is concerned in mental operations may exist--Illustrations--The effects of sudden mental shocks and long-continued mental application-- Recapitulation--The condition which we term the Insane Diathesis is the prime factor in the causation of insanity 53