At one time woman was thought to be the only creature which menstruated. But science now tells us that all warm blooded animals which walk erect menstruate. The discharge is chiefly due to the position which in standing upright, throws the large part of the uterus higher than the neck. In animals, such as dogs, cats, etc., the same process goes on, but the position of these animals keeps the large part of the uterus lower than the small part, where the blood is retained and then reabsorbed into the system. This process goes on every four weeks in girls after they reach the age of puberty and continues at regular periods as long as the egg is not fertilized until the reproductive age is over, which is usually between the forty-fifth and fiftieth year. If, however, the egg is fertilized the menstrual flow ceases and this blood supply goes to nourish the new life in the uterus. It does not appear again until after the birth of the child, and usually ceases while the child depends upon the milk from the mammal glands. The age at which this process (menstruation) first takes place in girls differs in individuals. Climate has some effect upon it, for girls in warm or Southern climates mature earlier than in colder places. In this climate the average girl reaches puberty at fourteen years of age. Some have been known to reach it as early as the eleventh and others not until the eighteenth year, all in the same place and yet normal and healthy, which shows there is no reason for anxiety if the girl does not menstruate at fourteen, provided she is developing normally and is in good health. During the first few years after its appearances the periods are likely to be irregular. This is because the sexual organs are not fully developed. Often the period does not occur after the first time for three, five, eight months and sometimes a year. This irregularity continues for two or three years. Cases of girls coming from Europe have been known where the period was perfectly established over there, but after arriving in this climate the menstrual flow did not occur again for a year and over. Usually this irregularity lasts only a few months, and when once it has become regular, there should be no worry over its arrival a day or two earlier or later. The length of time the period lasts differs in women also. The average length of time is four or five days, yet there are women in which it lasts fully a week, and others but a few hours. The length of time should not be of as much concern as the amount of discharge which is expelled each time. It is, of course, difficult to estimate this, but physicians claim that more than three protectives in twenty-four hours should not be used. In all women the flow is most profuse during the first two days.