A physiological interruption of menstruation occurs during pregnancy and lactation; it seems improbable, however, that during this interval ovulation also is in abeyance. It is established by anatomical investigations that ovulation and menstruation commonly occur in association; but that menstruation sometimes, though rarely, occurs in the absence of ovulation; and, finally, that intermenstrual ovulation is also a rare occurrence. In the majority of cases, either just before or just after the commencement of the menstrual flow, rupture of a graafian follicle occurs. After complete oöphorectomy, menstruation ceases; it is only when functionally active portions of ovarian tissue have been left behind, that menstruation continues to occur. In the absence of the ovaries, the menstrual function is in abeyance; hence, for the performance of that function, the presence of ripening ovarian follicles and of other follicles capable of ripening later, is an indispensable requisite.
A certain analogy between heat and rut in animals and menstruation in women may, according to the investigations of Bischoff, Hegar, Strassmann, and others, certainly be maintained. Heat or rut is a process occurring in mammals, dependent on the reproductive glands, characterized by an increase in sexual and general excitability, with congestion of the pudendum and the vagina, swelling of the sebaceous glands of the external genitals, and increased secretion; from the vulval cleft there flows a peculiar, strong-smelling mucus, often tinted red from admixture with blood; there is frequent micturition, the uterine glands are swollen, the Fallopian tubes are also swollen, and are soft and erected. A well-developed menstrual bleeding, analogous to that which occurs in the human species, occurs, among the lower animals, only in apes. Maturation of ova precedes the period of heat, and rupture of the graafian follicle occurs during that period.
Heat or rut occurs in animals at certain seasons of the year, which may, according to the species and the mode of life of the animal concerned, be in spring, summer, autumn, or winter. The season of heat or rut has further several periods of heat, each lasting several days, and among domesticated animals, mares, cows, and bitches, succeeding one another at intervals of three or four weeks; in wild animals, rut occurs once only in the year. In animals, sexual intercourse takes place during the time of the menstrual discharge, and during this time also the capacity for conception is increased; in the absence of heat, the genital organs are in a more quiescent condition. In this connection, the experiments on animals made by Strassmann, with a view to determining the influence upon the uterus of rise of pressure in the ovary, are of great interest; these experiments showed that a rise of intra-ovarian pressure, produced by the injection of fluid into the parenchyma of the ovary, led to changes in the endometrium and the external genital organs corresponding to those occurring in an animal on heat.
In the human species, however, in contradistinction to what occurs in the lower animals, there is a certain disinclination, on the part of the male at any rate, to sexual intercourse during menstruation. The human female moreover, notwithstanding the periodicity of her sexual life, is at all times capable of conception; this capability is not confined to any particular part of the intermenstrual period, for conception may occur at any time during that period, and has even been known to result from intercourse during menstruation. This peculiar characteristic of the human reproductive capacity has been regarded as compensatory, furnished by nature in her continual endeavour for the perpetuation of the species, to counteract the restricting influences imposed by civilization on the normal process of reproduction.
Credible observations even exist, indicating that among many primitive peoples, in whom at the time of puberty no social laws hinder the limitless exercise of the reproductive functions, this capacity on the part of woman to conceive at any time has no existence, and that the reproductive capacity of such human beings is, like that of the lower animals, confined to a certain season of the year. Thus, G. Schlesinger reports of the Ainus of the island of Yezo, “A friend of mine in Sapporo believes himself to have observed that the Ainus have a certain definite rutting period, and that in them, as in many of the lower animals, the process of reproducing the species occurs only at a certain season of the year.” An identical statement is current concerning the Indians of Western America.
The mucous membrane of the uterus undergoes during menstruation important changes, and a question much disputed is, whether in the course of menstruation the whole of the uterine mucous membrane is removed, or a part only, whether it is shed in its entire thickness, or is at least deprived of its epithelium. According to the observations made by Leopold on dead bodies, the mucous membrane of the uterus becomes swollen shortly before the commencement of the menstrual discharge, until, partly in consequence of cellular proliferation, partly in consequence of œdematous infiltration, and partly in consequence of enlargement of the lymph-spaces, it attains a thickness of 6 to 7 millimetres (¼ of an inch). The superficial capillaries are notably enlarged, and an effusion of blood-elements continues for several days, without the occurrence of any fatty degeneration in the tissues. The epithelium and the most superficial cell-layers of the mucous membrane are, however, undermined and shed. No complete destruction of the mucous membrane occurs, however, and fatty degeneration forms no part of the menstrual process as such.
Möricke, who examined portions of the uterine mucous membrane removed with the curette during menstruation from living women, found the superficial layers of the mucous membrane to be intact, and he regards the shedding of the epithelium described by other authorities as cadaveric phenomenon. Sinéty, who also found the uterine mucous membrane intact during menstruation, adheres to the same view.
Von Kahlden concludes, as a result of investigations made post mortem, that during menstruation the greater part of the mucous membrane, not the superficial epithelium only, but the stroma itself down to its deepest layers, is shed. According to von Tassenbroek and Mendes le Leon, however, the most superficial layers only are shed during menstruation.
According to Westphalen, whose investigations were made, partly on masses removed by the curette, and partly on freshly extirpated uteri, a sanguineo-serous infiltration of the mucous membrane begins about ten days before menstruation. Great vascular dilatation occurs only just before menstruation. The uterine glands undergo enlargement, and during and immediately after the flow, numerous shed epithelium cells occupy the lumen of the glands. For the rest, however, in the interior of the uterus shortly after menstruation, we find an almost continuous epithelial covering. Some days after menstruation, the proper regeneration of the mucous membrane occurs.
Mandl, who examined totally extirpated uteri, asserts that during menstruation the epithelial covering of the mucous membrane is never completely lost, but that just as little does it remain completely intact. The regeneration of the lost areas of epithelium proceeds even during menstruation.