THE PHENOMENA OF SEXUAL PERIODICITY. I. The Various Physiological and Psychological Rhythms--Menstruation--The Alleged Influence of the Moon--Frequent Suppression of Menstruation among Primitive Races--Mittelschmerz--Possible Tendency to a Future Intermenstrual Cycle--Menstruation among Animals--Menstruating Monkeys and Apes--What is Menstruation--Its Primary Cause Still Obscure--The Relation of Menstruation to Ovulation--The Occasional Absence of Menstruation in Health--The Relation of Menstruation to "Heat"--The Prohibition of Intercourse during Menstruation--The Predominance of Sexual Excitement at and around the Menstrual Period--Its Absence during the Period Frequently Apparent only. Throughout the vegetable and animal worlds the sexual functions are periodic. From the usually annual period of flowering in plants, with its play of sperm-cell and germ-cell and consequent seed-production, through the varying sexual energies of animals, up to the monthly effervescence of the generative organism in woman, seeking not without the shedding of blood for the gratification of its reproductive function, from first to last we find unfailing evidence of the periodicity of sex. At first the sun, and then, as some have thought, the moon, have marked throughout a rhythmic impress on the phenomena of sex. To understand these phenomena we have not only to recognize the bare existence of that periodic fact, but to realize its implications. Rhythm, it is scarcely necessary to remark, is far from characterizing sexual activity alone. It is the character of all biological activity, alike on the physical and the psychic sides. All the organs of the body appear to be in a perpetual process of rhythmic contraction and expansion. The heart is rhythmic, so is the respiration. The spleen is rhythmic, so also the bladder. The uterus constantly undergoes regular rhythmic contractions at brief intervals. The vascular system, down to the smallest capillaries, is acted on by three series of vibrations, and every separate fragment of muscular tissue possesses rhythmic contractility. Growth itself is rhythmic, and, as Malling-Hansen and subsequent observers have found, follows a regular annual course as well as a larger cycle. On the psychic sides attention is rhythmic. We are always irresistibly compelled to impart a rhythm to every succession of sounds, however uniform and monotonous. A familiar example of this is the rhythm we can seldom refrain from hearing in the puffing of an engine. A series of experiments, by Bolton, on thirty subjects showed that the clicks of an electric telephone connected in an induction-apparatus nearly always fell into rhythmic groups, usually of two or four, rarely of three or five, the rhythmic perception being accompanied by a strong impulse to make corresponding muscular movements.[75]