Thus, _Christopher Martin_ maintains that a special menstrual centre exists in the lumbar portion of the spinal cord, the impulses from which proceed to the uterus by way of the splanchnic plexus, the ovarian plexus, or perhaps by both. Similar views are held by _Lawson Tait_, _Collins_, and _Johnstone_, who severally maintain that the ovaries are no more concerned in the production of menstruation than any other organ of the body—the liver, for instance. They direct attention to the periodicity that occurs in the functional activity of various other organs, in respiratory and cardiac activity, for instance, both of which undergo rhythmical changes as a result of nervous influences. The cessation of menstruation after oöphorectomy they attribute, not to the cessation of ovulation, but to the division of the nerves which run across the broad ligaments of the uterus and upon which menstruation depends. Heat and rut in animals have a different significance from menstruation. The latter process is induced by civilization and by the adoption of the upright posture. But, taking all this into consideration, we must hold fast to the fundamental principles, that ovulation occurs at that period of life, and only at that period, during which menstruation proceeds regularly; that ovulation begins when externally and in the whole development of the girl the signs of sexual maturity manifest themselves; and that ovulation ceases at the climacteric, when menstruation also ceases. We must regard as rare exceptions to this rule cases in which ovulation begins before the menarche and persists after the menopause. A physiological interruption of menstruation occurs during pregnancy and lactation; it seems improbable, however, that during this interval ovulation also is in abeyance. It is established by anatomical investigations that ovulation and menstruation commonly occur in association; but that menstruation sometimes, though rarely, occurs in the absence of ovulation; and, finally, that intermenstrual ovulation is also a rare occurrence. In the majority of cases, either just before or just after the commencement of the menstrual flow, rupture of a graafian follicle occurs. After complete oöphorectomy, menstruation ceases; it is only when functionally active portions of ovarian tissue have been left behind, that menstruation continues to occur. In the absence of the ovaries, the menstrual function is in abeyance; hence, for the performance of that function, the presence of ripening ovarian follicles and of other follicles capable of ripening later, is an indispensable requisite.