influence becomes negligible. It is like attempting to
insert a key into a door which has no lock.

It is among the specimens of normality of the brain cells that we may
look for our examples of endocrine mental deficiency. Included are all
sorts of examples of feeble-mindedness varying from the moron to the
imbecile and idiot, arrested brain life. The cretin is the classic
type of mental deficiency due to endocrine insufficiency, curable or
improvable by the proper handling.

Insanity, degeneration of the normal brain life, may be caused by an
upset of the endocrine balance. Among the commonest manifestations
of insanity are excitements and depressions, apathies and manias,
hallucinations, delusions and obsessions, all of which are
reproducible under known conditions of internal secretion excess or
failure. Alternating states of mania and depression are caused in some
instances by extreme hyperthyroidism. The critical periods of life,
when a profound revolution is overturning the endocrine equilibrium,
puberty, pregnancy, and the menopause, are the periods of most
frequent occurrence of insanity, when mental instability reveals
endocrine instability (Dementia praecox, pregnancy psychosis,
menopause neurosis). Actual insanity need not be the only
manifestation. By far the greater number of mental disturbances due
to aberrations of the internal secretions never see an asylum or a
doctor. They live more or less close to the borderline of insanity as
persons who have spells, eccentricities and peculiarities, hysteria,
tics or just "nervousness."

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
o