This process goes on until the whole of the internal stroma becomes tense and brilliantly injected with blood.... In all essential points the menstruation or pro-estrum of the human female is identical with that of monkeys.... Estrus is possible only after the changes due to pro-estrum have taken place in the uterus. A wave of disturbance, at first evident in the external generative organs, extends to the uterus, and after the various phases of pro-estrum have been gone through in that organ, and the excitement there is subsiding, it would seem as if the external organs gain renewed stimulus, and it is then that estrus takes place.... In all animals which have been investigated coition is not allowed by the female until some time after the swelling and congestion of the vulva and surrounding tissue are first demonstrated, and in those animals which suffer from a considerable discharge of blood the main portion of that discharge, if not the whole of it, will be evacuated before sexual intercourse is allowed." (W. Heape, "The 'Sexual Season' of Mammals," _Quarterly Journal of Microscopical Science_, vol. xliv, Part I, 1900. Estrus has since been fully discussed in Marshall's _Physiology of Reproduction_.) This description clearly brings out the fundamentally vascular character of the process I have termed "tumescence"; it must be added, however, that in man the nervous elements in the process tend to become more conspicuous, and more or less obliterate these primitive limitations of sexual desire. (See "Sexual Periodicity" in the first volume of these _Studies_.) Moll subsequently restated his position with reference to my somewhat different analysis of the sexual impulse, still maintaining his original view ("Analyse des Geschlechtstriebes," _Medizinische Klinik_, Nos. 12 and 13, 1905; also _Geschlecht und Gesellschaft_, vol. ii, Nos. 9 and 10). Numa Praetorius (_Jahrbuch für Sexeuelle Zwischenstufen_, 1904, p. 592) accepts contrectation, tumescence, and detumescence as all being stages in the same process, contrectation, which he defines as the sexual craving for a definite individual, coming first. Robert Müller (_Sexualbiologie_, 1907, p. 37) criticises Moll much in the same sense as I have done and considers that contrectation and detumescence cannot be separated, but are two expressions of the same impulse; so also Max Katte, "Die Präliminarien des Geschlechtsaktes," _Zeitschrift für Sexualwissenschaft_, Oct., 1908, and G. Saint-Paul, _L'Homosexualité et les Types Homosexuels_, 1910, p. 390.