The actual period of the menstrual flow corresponds, in Heape's terminology, to the congestive stage, or pro-œstrum, in female animals; the œstrus, or period of sexual desire, immediately follows the pro-œstrum, and is the direct result of it. See Heape, "The 'Sexual Season' of Mammals," Quarterly Journal of Microscopical Science, 1900, vol. xliv, Part I.
It may be noted that (as Barnes, Oliver, and others have pointed out) there is heightened blood-pressure during menstruation. Haig remarks that he has found a tendency for high pressure to be accompanied by increased sexual appetite (Uric Acid, 6th edition, p. 155).
Sir W. F. Wade, however, remarked, some years ago, in his Ingleby Lectures (Lancet, June 5, 1886): "It is far from exceptional to find that there is an extreme enhancement of concupiscence in the immediate precatamenial period," and adds, "I am satisfied that evidence is obtainable that in some instances, ardor is at its maximum during the actual period, and suspect that cases occur in which it is almost, if not entirely, limited to that time." Long ago, however, the genius of Haller had noted the same fact. More recently, Icard (La Femme, Chapter VI and elsewhere, e.g., p. 125) has brought forward much evidence in confirmation of this view. It may be added that there is considerable significance in the fact that the erotic hallucinations, which are not infrequently experienced by women under the influence of nitrous oxide gas, are more likely to appear at the monthly period than at any other time. (D. W. Buxton, Anesthetics, 1892, p. 61.)