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.