About two-thirds of mental deficiency is definitely inherited, about one-third acquired. It is the opinion of a number of psychologists that it is inherited as what the Mendelians call a recessive, that is as a trait which will be overshadowed, if there is admixture of normal mentality, but will crop up by breeding with another mental defective. What we know of the endocrine factors in heredity leads us to suppose that it is the mating of one marked endocrine insufficiency with another that is often responsible for the inherited tendency to feeble-mindedness and insanity. The effect of the hormone system upon the vegetative apparatus may create the more obscure insanities and quasi-insanities. The direct action of the internal secretions upon the brain cells, producing a sort of hair trigger situation within them, may cause the explosive discharges from them which appear as overpowering impulses or uncontrollable conduct. The waves of feeling which precede them are unquestionably endocrine determined. The wave of fear a cat experiences upon seeing a dog is accompanied and indeed preceded by an increase of the amount of adrenalin in the blood. The picture of fright, as observed in a so-called normal person, staring eyes, trembling hands, dry lips and mouth, corresponds to the portrait of the appearance in hyperthyroidism. In persons afflicted with uncontrollable impulses, the inhibiting hormones may not be present in sufficient quantity. Feeble-mindedness, ranging from stupidity to imbecility, may also be a direct effect of insufficient endocrine supply to the brain cells. When there is not enough of the thyroid secretion in the blood, the tissue between the cells in the brain become clogged and thickened, so that a gross barrier to the passage of the nerve impulses is created. We have here an illustration of internal secretion lack actually producing gross changes in the brain. But without a doubt, most endocrine influences upon the brain, at work every minute and second of its life, are the subtle ones of molecular chemistry and atomic energetics. We know that such mental qualities as irritability and stupidity, fatigability, and the power to recover quickly or slowly from fatigue, sexual potency and impotence, apathy and enthusiasm are endocrine qualities. We know also that the thyroid dominant tends to be irritable and excitable, the pituitary deficient to be placid and gentle, the adrenal dominant to be assertive and pugnacious, the thymus-centered to be childish and easy-go-lucky and the gonad deficient to be secretive and shy. This brings us to the relation of the internal secretions to the type of personality as a whole. CHAPTER X THE TYPES OF PERSONALITY THE ENDOCRINE PERSONALITY