Repeated irritations are however necessary to provoke the ejaculation of semen. This is finally produced by excitation of a special muscle which compresses the seminal vesicles in a spasmodic manner and ejaculates the semen by the urethra. After ejaculation, the accumulation of blood in the cavernous bodies gradually diminishes and the penis again becomes flaccid. This apparatus is thus very complicated and is put in action by several nervous irritations which may be disturbed in many ways in affections of the nervous system. We may observe here that the nervous centers of erection and ejaculation may be put in action directly by the brain, or indirectly by peripheral irritation of the glans. Those peripheral nerves which provoke sexual excitation are especially the nerves of the glans. This possesses a skin or mucous membrane which is extremely delicate and is protected against external irritation by a fold of skin called the _prepuce_, or foreskin. The prepuce is often too narrow so that it cannot be withdrawn behind the glans. It then forms a pocket in which sebaceous matter, semen, urine, etc., accumulate and decompose. This anomaly, called _phimosis_, does not exist among the Jews owing to circumcision, or the removal of the prepuce in the newly born, which forms part of their religious rites. Hygienic considerations sometimes oblige us to perform this operation in others. The bad habit of masturbation, so common in boys, is often provoked by phimosis, and shows that simple mechanical irritation of the glans, due here to secretions contained in the prepuce, may lead to ejaculation of semen as well as to erection. We have seen above that the male and female germinal glands arise from the same primitive organ in the embryo. If the embryo becomes male, this organ is transformed into the two testicles which descend gradually in the canal of the groin and become placed in the scrotum. If it becomes female, the two sexual glands remain in the abdominal cavity and are transformed into ovaries.