The interval between one menstruation and the next (the period that elapses, that is to say, between the commencement of one period and the commencement of the next) is in the great majority of cases twenty-eight days. The recurrence in many women is extraordinarily exact, not merely as regards the day, but even as regards the hour of the day. The twenty-eight-day type of menstruation is found in about 70 per cent. of the cases; in the remainder, the thirty-day type is most frequent, and next to that the twenty-one-day type. The periodicity of menstruation in any individual may however be very irregular. The quantity of blood lost during menstruation varies within wide limits; according to approximate estimates the usual loss at a single period is from 90 to 240 grammes (about 3 to 8 fluid ounces). The following summary statement is made by _Krieger_ regarding the quantity lost in different social circumstances and in various nationalities: The amount of blood lost and the duration of the flow are less in strong, healthy women, leading an occupied, active, and regular life, especially in countrywomen and in women who are poor and chaste, than it is in delicate, weakly women, leading a sedentary life, whose diet is abundant and stimulating, and who are accustomed to an ultra-luxurious and enervating existence. In nuns, for example, the quantity of the menstrual discharge gradually declines; shortly after their entrance into the cloister, various irregularities are apt to occur, but ultimately the flow becomes exceedingly scanty and lasts for a single day only. Climate also has a great influence, for in hot countries women usually menstruate very abundantly, whilst in cold countries the flow is scanty, and often appears only in the warmer months of the year. Of the Lapp and Samoyede women this was already reported by _Linnæus_ and _Virey_. _Tilt_ further relates that Eskimo women menstruate only during the summer months, and even then scantily. In southern France, according to _Courty_, the quantity varies from 120 to 240 grammes (about 4 to 8 ounces); but it may rise to 300, 350, and even to 500 grammes (about 10, 12, and 16½ fluid ounces). In the tropics, severe menorrhagia is said to be common; and the fact was already known to _Blumbenbach_, that women of European descent born in the tropics not infrequently succumb to hæmorrhage during childbirth. _L. Mayer_ has endeavored to determine the relations between the quantity and the quality of the discharge, and distinguishes the regular composition, when a considerable quantity of dark-tinted, fluid blood is passed, from the irregular composition, when a small quantity of blood, usually pale in color, is passed, or an excessive quantity of dark blood, often coagulated, or a discharge of varying composition. Of 4,542 women questioned by _Mayer_ in regard to this matter, there were: