The normal duration of the flow is from two days to a week. The amount of fluid discharged is from 2 to 9 ounces. Menstruation occurs every twenty-eight days, counting from the beginning of one period to the beginning of another. The menstrual interval is subject to considerable individual variations, which appear to be within the limits of health. It sometimes occurs with regularity every two, three, or five weeks. When it occurs every two weeks, the alternate flows are often but small in amount. The occurrence of, or the attempt at, menstruation every two weeks, in a woman who had previously menstruated monthly, is sometimes a symptom of beginning uterine disease. Menstruation commonly ceases at about the forty-fifth year, when the menopause appears. Most of the disorders of menstruation have already been considered as symptoms of the various lesions of the genital organs that have been described in the previous pages. There are some disorders of menstruation, however, often unaccompanied by discoverable lesions, which now demand consideration. =Amenorrhea.=--Amenorrhea is the absence of menstruation. Failure of the menstrual blood to be discharged from the vagina, such as occurs in cases of atresia, is not necessarily amenorrhea; menstruation may have taken place, though the most marked phenomenon of this process, the discharge of blood, is concealed. The term primary amenorrhea, or _emansio mensium_, is applied to those cases in which menstruation has never appeared. Secondary amenorrhea, or _suppressio mensium_, is applied to those cases in which menstruation has ceased after having once been established. Amenorrhea is due to defective development of the organs of generation; to premature atrophy, such as occurs in superinvolution of the uterus; to lesions, pathological and traumatic; to acute and chronic general diseases; and to psychical disturbances. Menstruation is often absent during the acute diseases, such as typhoid fever, and it may remain suppressed until the general health is fully restored. Amenorrhea may also occur in any chronic debilitating condition. It is common in chlorosis, anemia, phthisis, and malaria. It frequently results from changes of climate and surroundings, and continues until the person becomes adapted to the new environment. It is seen in emigrants from other countries, and in women who move from the country to large cities. It is often caused by overwork, physical and mental, and by insufficient food. It is not uncommon in studious school-girls. Amenorrhea is sometimes due to the excessive general development of fat, even in young woman who are apparently in good general health. Amenorrhea is frequently associated with insanity. It may be caused by fright, grief, or anxiety. The fear of pregnancy after illicit coitus sometimes produces it.