The regular occurrence of the menstrual period in women is one of the indisputable evidences of a rhythm of functional activities in the human organism. Menstruation is presumably associated with comparable periodic changes in the condition of the ovaries, though there is a difference of opinion as to the exact relations between ovulation and menstruation. Cycles of performance, alterations of rest and activity, are familiar in the daily routine of all persons; but the extent to which more far reaching though normally regular variations or recurrences of physiologic states of unlike character may be found in nature is not easily answered.
The theory that the life processes of women have a demonstrable rhythm which centers about the menstrual period was suggested in 1876 by Dr. Mary Putnam Jacobi.1 Her observations on the pulse, temperature, blood pressure and muscular strength have been repeated and extended by other workers. In general,