Menstruation is a function of the womb and in all probability is brought about through the influence of the ovaries. The bleeding, popularly regarded as the entire menstrual process, is, in fact, indicative of only one of its stages; the others give rise to no symptoms whatever. What the stages in the menstrual process are, what relation they bear to each other, and what the significance of the whole process is, are problems that have been solved with the aid of the microscope. In this way the mucous membrane lining the womb has been studied both at the time of the periods and in the interval between them, and we have learned that it is constantly undergoing changes intended to facilitate the reception and the maintenance of an embryo. Anticipating these duties the mucous membrane receives a more abundant supply of blood; it also increases in thickness and all the structures which enter into its composition become more active. Unless conception takes place these preparations, which represent the most important phase in the menstrual process, are without value; and therefore failure to conceive means that the mucous membrane will return to the same condition as existed before the preparations were begun. The congestion is relieved by rupture of the smallest blood vessels, and there follow other retrogressive steps which completely restore the various structures to their former state. Then there is a pause, though it is not long, until preparatory changes are again initiated, or, as we say, another Menstrual Cycle is begun. Each cycle lasts twenty-eight days, and includes four stages, namely, a stage of preparation, of bleeding, of restoration, and of rest. Although pregnancy may become established at any time during the interval between the periods of bleeding, it is more likely to be established just before a period is expected or shortly after it has ceased. Furthermore, whenever conception does take place, the preliminary preparations for the reception of the embryo are followed by much more elaborate arrangements for its protection and nutrition. Under these circumstances the hemorrhagic discharge does not appear. Were there no other condition to bring about the cessation of menstruation, the diagnosis of pregnancy would be greatly simplified. But any one can appreciate the fact that diseases of the womb may interfere with the menstrual process. Menstruation is influenced, also, by the ovaries. As a result of age, for example, the ovaries undergo changes which invariably bring about the permanent cessation of menstruation, called the menopause. This event occurs prematurely if both the ovaries are removed by operation. In view of these facts it is not surprising that sometimes ovarian disorders abolish menstruation. An impoverished state of the blood, or nervous shock and strain, or constitutional debility may also interrupt the regular appearance of the menstrual discharge.