In this act of conjugation between two individuals of the same species, differentiated each from the other by the characteristics of sex, the active, provocative rôle is allotted to the male, the passive, receptive rôle, to the female. The modest and coy reluctance characteristic alike of the maiden and of the wife, promote an increase of sexual excitement in the opposite sex, and this not only in a man of purely sensual character, whose vanity is stimulated by his being the chosen one among many—a circumstance which, in view of the great dependence of the sexual act upon psychical processes and imaginative influences, is by no means devoid of importance. The woman’s coy reluctance must be overcome by means of a tender strategy before she is willing to grant the final possession of her body; and the act of copulation forms at the same time the conclusion of the physical and mental yearnings of the lover, and the commencement of the new-coming being. There is thus a physiological reason for the advice given by the celebrated surgeon, _Ambroise Paré_, that a man, before completing coitus, should employ some of the delicate and sensually stimulating manipulations of the earlier stages of courtship, for, he writes, “aucunes femmes ne sont pas si promptes à ce jeu que les hommes.” The potency for intercourse of the sexually mature man, his capacity for the introduction of the erect penis during the act of copulation, is dependent on the fact that sexual excitement gives rise to a sufficient stimulus which, acting on the erection centre (and presuming that the centre and its afferent and efferent tracts are normal), leads to an increased flow of arterial blood to the penis and a diminished outflow through the veins of that organ, and consequently to its erection. The cerebrum is the organ in which the sensation of libido sexualis, of sexual excitement, has its seat; with this higher centre is connected by means of intercentral nerve tracts a lower, mechanical, reflex centre, situated in the lumbar enlargement of the spinal cord, and presiding over the performance of the act of copulation; it is moreover probable that nerve fibres proceed from the spinal cord direct to the blood vessels of the erectile tissue, by means of which the calibre of these vessels can be lessened or their extensibility diminished. The relation of the erector nerves (nervi erigentes) to the penis is by many physiologists compared to the relation of the vagus nerve to the heart. In the quiescent state the small arteries of the penis and perhaps also the cavernous spaces of that organ are in a state of mean contraction, so that they offer a considerable resistance to the passage of the blood current. When now the nervi erigentes are excited to activity, the hitherto tonically contracted vessels of the penis undergo, according to the school of physiologists just mentioned, relaxation, so that they dilate under the pressure of the blood within their walls, and, the