Physiologists have established, by carefully-prepared statistics, that the average period of menstruation for women who began to menstruate early is thirty-three years, while those who commenced late have an average of only twenty-seven years. When a woman is forty-five years of age, we may, however, as a rule, look for the change of life to set in; if she goes beyond this age, she may be taken as the exception. The cavity of the womb is the principal source from which the blood comes; while the ovaries and tubes are also greatly congested with blood, the amount that comes from them must be very small. The blood that comes from the womb is not different from blood coming from any other source; the changes and peculiarity of the menstrual show are due to its passage through the vagina, where it becomes contaminated with vaginal secretions. It is supplied from the blood-vessels of the womb, oozing through the mucous membrane of that organ, just as in a case of nosebleed. The entire womb is more or less swollen, and more especially the mucous lining, so that it corresponds, in many instances, to an inflammatory process, and for that very reason, a sudden check of the menstrual flow will often result in a regular subacute or acute inflammation of the womb. If the discharge of blood from the uterus is in small quantities and a gradual, steady flow, it becomes so altered by the secretions in its passage through the vagina that it does not coagulate, but when it is poured out more rapidly or in larger quantities, the menstrual blood coagulates or congeals in the same manner as if it were derived from other sources. If a woman becomes pregnant, the menses as a rule are suspended during the child-bearing period, and usually remain absent after the child is born so long as the woman nurses the child. I have known one woman who menstruated during her entire pregnancy, and another who had had eight children and never menstruated in her life, yet she was, and always had been, in perfectly good health: thus we see, that there is no rule without exceptions. An essential part of the menstrual function is that in which the ova or female germ cells ripen and are expelled from the ovary. The menstruation is only a reflex or side issue, to a more important part that is going on in the female generative system; this is termed _ovulation_, or the ripening and expulsion of the human egg from the tissues of the ovaries. In the physiological process that operates in the economy of nature for reproduction, the _ovaries_ are the principal organs. The other organs are simply accessory, and indeed many of the lower animals have no other organs than the ovaries for the perpetuation of their species. In the human female, the ovaries consist of a tough fibrous tissue, between whose meshes are little cysts, which are called _Graafian vesicles_, and these little vesicles serve as the nests in which the ova or little germ cells mature.