=Menstruation.=—Menstruation may be defined as a process wherein a bloody fluid is discharged from the uterus at regularly recurring periods between puberty and the menopause, _except_ during pregnancy and lactation. It is a hæmorrhage which in some way is closely associated with ovulation, but it is not known positively which is the precedent of the other, or whether one causes the other. Menstruation is not essential to pregnancy, for pregnancy may occur when the flow is normally absent, as before puberty, after the menopause, or during lactation. Nevertheless, regularity of menstruation is the rule in fertile women and clinicians agree that while conception _may_ occur at any part of the menstrual cycle, it is _most likely_ to happen just before or just after the menstrual flow. The best authorities at present support the theory that ovulation usually occurs soon after the close of the menstrual period. This is confirmed by the similarity of the physical changes that take place in the endometrium during menstruation and after conception. [Illustration: Fig. 12.—Development of the ovary (after Wiedersheim). _A_, an ingrowth of the germinal epithelium, forming a cell-cord, which breaks up into primitive Graafian follicles; _B_, a primitive Graafian follicle, with its contained primitive ovum; _C_, _D_, _E_, later stages in the development of the Graafian follicle. (Crossen.) ] As the period of the flow approaches, the lining membrane of the uterus becomes hyperæmic and swollen with blood, serum, and glandular secretions. The blood vessels are engorged, the glands become longer and more tortuous, little hæmorrhages appear, and the superficial epithelium is thrown off. A large amount of mucus is produced by the increased activity of the glands, and all is discharged into the vagina as a bloody, incoagulable flow with an odor of marigolds. The process continues usually from three to seven days, when the discharge ceases and the endometrium slowly resumes its uncongested state. [Illustration: Fig. 13.—Graafian follicles. One contains two ovules which, if fertilized, will produce twins. If all three ovules are fertilized, triplets will result. (Bumm.)] Meanwhile, the psychic and bodily conditions have not remained unaffected. The nervous system is disturbed, the disposition is irritable and capricious and the head may ache. The woman takes cold easily. She is indisposed to exertion from a sense of languor and malaise. Pain may develop in the back, or cramps in the pelvis, so severe as to keep the woman in bed. Frequently the approach of the period is signalized by skin changes, such as a marked odor or an eruption of acne pustules. The flow usually returns every twenty-eight days, but it may vary within normal limits from twenty-one to thirty days. The flow continues at such intervals regularly from puberty to the menopause (change of life), which occurs between the ages of forty-five and fifty.