Before menstruation, there is a stage of preparation, a stir and twittering of the endocrines, the premenstrual state. Currents of communication flow between the different glands, messages and replies pass to and fro. When these are properly balanced, so that all goes well, the consciousness of the woman will be disturbed by no knowledge of them. In some women abnormal sensations appear, a sense of fullness in the breasts, or of weight in the back or pelvis, or pain in the head. The last is probably due to swelling of the pituitary beyond the capacity of its bony container. In a good many women, nervous and mental phenomena herald the expected menstruation because of a complete upset of the balance between the internal secretions, with resulting disturbance of the nervous system. Irritability, depression, excitability, melancholia, exaltations, restlessness, hysteria, loss of self-control, or even more marked mental aberrations may appear. Following them, and roughly paralleling them, may come various abnormalities of menstruation itself. The character, extent and duration of these furnish us the best clues to the endocrine stability or instability of the particular feminine organism. Menstruation is simply the uterus saying: well, not this time. As the destined ovum within its nest, the follicle, grows, its fluid affects the interstitial cells to send their specific stuff into the blood. There it circulates, hits this gland and that, makes some more active, others less, transforms the chemistry of the cells, and engorges the mucous membranes, most of all those of the nose and of the uterus. It is all to welcome the mature ovum and its possible impregnation, to prepare a site for its landing and settlement, blood and food for its nutrition, safety for its development. But it is not to be. No sperm at hand, or effective enough to penetrate that wandering ovum. Love's labour's lost. All must return to the so-called normal, really the intermenstrual state. The womb must surrender some of that blood, the glands return to their routine, and a sex diastole of the whole organism succeeds. Until again, another follicle swells, another ovum matures, and the premenstrual state of sex high tide cycles back. Seven to ten days before menstruation we know that sex high tide is beginning for that is when the blood pressure goes up. As this rise of blood pressure is probably controlled by the posterior pituitary, we have a clue to the reason for the rhythmic variations in the rate of production of its secretion by the ovary. For, since menstruation is so closely connected with the phases of the moon and the tides, the rhythmicity of the posterior pituitary may be traced to the days when the pineal was an eye at the top of the head, and in direct relation with the pituitary.