The menstrual psychosis most commonly makes its appearance shortly before the flow, becomes less severe with the establishment of the flow, and disappears when the flow ceases; in other cases, the psychosis appears toward the end of menstruation, and speedily passes away; or, again, in amenorrhoeic cases, the attacks of mental disorder replace the proper menstrual flow, and become less severe or disappear entirely as soon as the flow is regularly re-established. The commonest forms of these menstrual psychoses are, melancholia, mania, irresistible impulses, acute amentia, in rare cases alternating insanity (_folie circulaire_) in which the periods of alternation assume the menstrual rhythm. The duration of these psychoses is usually short, from a few days up to a fortnight; there may be only a single attack, or there may be a number of attacks presenting precisely similar characters. The consciousness may be more or less disturbed. _Von Krafft-Ebing_ points out, as a very dangerous peculiarity of the menstrual psychoses, that the fact that the morbid process has once occurred in connection with menstruation furnishes in itself a sufficient reason for the recurrence of such attacks, which are dependent on constantly repeated functional changes in the brain closely analogous to those that occur in epilepsy. When the menstrual insanity recurs frequently, it gradually becomes less acute in its characters and more protracted in its course; the lucid intervals are less clearly indicated and shorter in duration; and thus in course of time the mental disorder may be transformed into chronic imbecility—a transformation liable to occur in all forms of periodic psychosis. In such cases we must always assume the existence of a certain lack of resisting power on the part of the organism, especially of the nervous system, which amounts to a congenital predisposition. During the period of full menstrual activity, the favorable soil for the cultivation of such disorders is usually furnished by anomalies of menstruation, by difficult labor and its consequences, severe losses of blood, prolonged lactation, physical over-exertion, and mental shock and stress. In the development under the influence of menstruation of such periodic acute mental disorders, we may observe various gradations, as for instance short, syncope-like cataleptic seizures, states of hallucinatory confusion lasting several hours or several days, disordered consciousness, and even severe mania.