It is assumed as a matter of course that the brain itself is there, which, to carry out our analogy, means that the crude soil or earth is there. Sufficient quantity and adequate quality of nerve tissue must be regarded as prerequisite. If the brain has been damaged in any way during development or birth, if it has been smashed up in any way, or if it has failed to evolve the minimum number of healthy nerve cells, the endocrine 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."