Azara noted among the Guaranis of Paraguay that menstruation was not only slight in amount, but the periods were separated by long intervals.  Among the Indians in North America, again, menstruation appears to be scanty.  Thus, Holder, speaking of his experience with the Crow Indians of Montana, says:  “I am quite sure that full-blood Indians in this latitude do not menstruate so freely as white women, not usually exceeding three days."[86] Among the naked women of Tierra del Fuego, it is said that there is often no physical sign of the menses for six months at a time.  These observations are noteworthy, though they clearly indicate, on the whole, that primitiveness in race is a very powerless factor without a cold climate.  On the other hand, again, there is some reason to suppose that in Europe there is a latent tendency in some women for the menstrual cycle to split up further into two cycles, by the appearance of a latent minor climax in the middle of the monthly interval.  I allude to the phenomenon usually called Mittelschmerz, middle period, or intermenstrual pain.

Since the investigations of Goodman, Stephenson, Van Ott, Reinl, Jacobi, and others, it has been generally recognized that menstruation is a continuous process, the flow being merely the climax of a menstrual cycle, a physiological wave which is in constant flux or reflux.  This cycle manifests itself in all a woman’s activities, in metabolism, respiration, temperature, etc., as well as on the nervous and psychic side.  The healthier the woman is, the less conscious is the cyclic return of her life, but the cycle may be traced (as Hegar has found) even before puberty takes place, while Salerni has found that even in amenorrhoea the menstrual cycle still manifests itself in the temperature and respiration. (Rivista Sperimentale di Freniatria, XXX, fasc. 2-3.)

For a summary of the phenomena of the menstrual cycle, see Havelock Ellis, Man and Woman, fourth ed., revised and enlarged, Ch.  XI; “The Functional Periodicity of Women.”  Cf.  Keller, Archives Generales de Medecine, May, 1897; Hegar, Allgemeine Zeitschrift fuer Psychiatrie, 1901, Heft 2 and 3; Helen MacMurchy, Lancet, Oct. 5. 1901; A.E.  Giles, Transactions Obstetrical Society London, vol. xxxix, p. 115, etc.

Mittelschmerz is a condition of pain occurring about the middle of the intermenstrual period, either alone or accompanied by a slight sanguineous discharge, or, more frequently, a non-sanguineous discharge. (In a case described by Van Voornveld, the manifestation was confined to a regularly occurring rise of temperature.) The phenomenon varies, but seems usually to occur about the fourteenth day, and to last two or three days.  Laycock, in 1840 (Nervous Diseases of Women, p. 46), gave instances of women with an intermenstrual period.  Depaul and Gueniot (Dictionnaire Encyclopedique