previous resistance to the flow being now removed, the blood pours freely into the cavernous spaces of the penis, and distends these to the uttermost. In this manner erection is effected, rendering possible the insertion of the penis into the genital passage of the female; with the culmination of the sexual act, the semen is ejaculated, the muscles of the prostate and the membranous portion of the urethra together with the ischiocavernosus and bulbocavernosus muscles, all acting strongly and simultaneously. By the contraction of the muscular apparatus just described, the penis is constricted in the neighborhood of the pubic symphysis, and this further hinders the outflow of the blood from the corpora cavernosa, increasing the intensity of the state of erection of the penis. Should the relaxation of the corpora cavernosa, dependent upon the stimulation of the nervi erigentes, be incomplete, it is not possible for sufficient blood to pass into the cavernous spaces to exercise considerable pressure upon the efferent veins, and thus complete erection fails to occur. If, again, the contraction of the muscular apparatus at the root of the penis is insufficiently vigorous, complete erection likewise fails to occur; the organ becomes semi-erect only, or erect for a period too short to permit of the completion of intercourse. Since, physiologically speaking, conception is the purpose with which copulation is effected, the ejaculation of the semen must be regarded as the principal object of that act; now in normal conditions, ejaculation takes place only when the penis is fully erect. Associated with the erection of the corpora cavernosa is a swelling of the caput gallinaginis, whereby the orifices of the ejaculatory ducts are directed forwards toward the membranous portion of the urethra, and at the same time the backward passage to the bladder is cut off. By this mechanism, the urethra, which usually serves as the canal for the outflow of urine, is made for the time being solely subservient to the purposes of the sexual act. That the outlet from the bladder is obstructed by the swollen caput gallinaginis when the penis is erect, is shown by the familiar fact that a man whose penis is erect cannot pass water, although the way is freely open for the ejaculation of the semen.