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 Générales de Médecine_, May, 1897; Hegar, _Allgemeine Zeitschrift für 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 Guéniot (_Dictionnaire Encyclopédique des Sciences Médicales_, Art., "Menstruation," p. 694) speak of intermenstrual symptoms, and even actual flow, as occurring in women who are in a perfect state of health, and constituting genuine "_règles surnuméraries_." The condition is, however, said to have been first fully described by Valleix; then, in 18725 by Sir William Priestley; and subsequently by Fehling, Fasbender, Sorel, Halliday Croom, Findley, Addinsell, and others. (See, for instance, "Mittelschmerz," by J. Halliday Croom, _Transactions of Edinburgh Obstetrical Society_, vol. xxi, 1896. Also, Krieger, _Menstruation_, pp. 68-69.) Fliess (_Die Beziehungen zwischen Nase und weiblichen Geschlechts-Organen_, p. 118) goes so far as to assert that an intermenstrual period of menstrual symptoms--which he terms _Nebenmenstruation_--is "a phenomenon well known to most healthy women." Observations are at present too few to allow any definite conclusions, and in some of the cases so far recorded a pathological condition of the sexual organs has been found to exist. Rosner, of Cracow, however, found that only in one case out of twelve was there any disease present (_La Gynécologie_, June, 1905), and Storer, who has met with twenty cases, insists on the remarkable and definite regularity of the manifestations, wholly unlike those of neuralgia (_Boston Medical and Surgical Journal_, April 19, 1900). There is no agreement as to the cause of _Mittelschmerz_. Addinsell attributed it to disease of the Fallopian tubes. This, however, is denied by such competent authorities as Cullingworth and Bland Sutton. Others, like Priestley, and subsequently Marsh (_American Journal of Obstetrics_, July, 1897), have sought to find the explanation in the occurrence of ovulation. This theory is, however, unsupported by facts, and eventually rests on the exploded belief that ovulation is the cause of menstruation. Rosner, following Richelet, vaguely attributes it to the diffused hyperæmia which is generally present. Van de Velde also attributes it to an abnormal fall of vascular tone, causing passive congestion of the pelvic viscera. Others again, like Armand Routh and MacLean, in the course of an interesting discussion on _Mittelschmerz_ at the Obstetric Society of London, on the second day of March, 1898, believe that we may trace here a double menstruation, and would explain the phenomenon by assuming that in certain cases there is an intermenstrual as well as a menstrual cycle. The question is not yet ripe for settlement, though it is fully evident that, looking broadly at the phenomena of rut and menstruation, the main basis of their increasing frequency as we rise toward civilized man is increase of nutrition, heat and sunlight being factors of nutrition. When dealing with civilized man, however, we are probably concerned not merely with general nutrition, but with the nervous direction of that nutrition.