[E] Many authorities (among others Rohleder, Berliner Klinik, 1909, Heft 257) claim that the libido, or the pleasurable sensations, originate at the orifices of the ejaculatory ducts during the passage of the thick semen through these narrow openings. This hypothesis will not stand a critical analysis. People who began to masturbate in early childhood relate that for years they experienced orgasm without noticing the least trace of an ejaculation. One day, usually between the fourteenth and sixteenth year, they were surprised by the first ejaculation. Still there was no material change in the quality of the libido during the orgasm accompanied by ejaculation from that experienced during the previous orgasms without ejaculations. This tends to show that the libido experienced at the sexual paroxysm has its cause not in the removal of the material congestion but in the relief from the nervous tension. The orgastic paroxysm has its analogy in the epileptic crisis. The nervous tension in the epileptic patient is gradually increased until one day an explosion ensues, and the epileptic attack removes, so to say, the accumulated nervous energy. The patient is then relieved for a certain time. An analogous relief from the nervous tension is effected during the orgastic crisis, and the cause of the libido in men and in women is this discharge of nervous energy. The ejaculation of the few drops of semen or of Kristeller’s plug is only an accidental incident, which may contribute to the well-being of the individual, just as the discharge from bladder and rectum, but which is not the cause of orgastic libido. [F] This monthly periodicity is attributed by some authorities to cosmic influences (the new moon and full moon). But since other vital processes also show a certain undulatory increase and decrease in strength (e. c. the systole and diastole of the heart), it is more probable that the periodicity of ovulation is due to an undulatory motion, where the length of the wave is about four weeks. [G] According to Pflüger (Veit’s Handb. der Gyn. 1908, Vol. 3, p. 25) the pressure of the periodically growing Graafian follicle causes a continual irritation of the ovarian nerves, which, by reflex action, is the cause of the great congestion of the genitals. When the stimulus has reached a certain height, the congestion leads, on the one side, to the rupture of the follicle, on the other hand, to the menstrual change of the uterine mucosa. This theory has been abandoned, and the modern trend of opinion is to attribute the general congestion to chemical substances, thus returning to Brown-Sequard teachings that the periodical congestion is caused by a chemical substance which, produced within the ovary, enters the blood circulation.