At the time of puberty, the generative centre or the centre of voluptas begins rapidly to grow and comes into increased activity. The internal messages become then so frequent that they charge the entire body with a considerable amount of nerve energy and cause in the generative organs a certain congestion, stimulation and irritation. This spontaneous tumescence, produced with the growth of the centre of voluptas in the brain, is then the cause of calling to life the impulse of detumescence. The individual has the strong desire to free itself of the material congestion and of the nervous tension. In animals the impulse of detumescence is, with a very few exceptions, identical with the sex-instinct. Most animals have no desire for contrectation. In man the impulse of contrectation is the more important. If the individual practises total abstinence, i. e., if he abstains even from the gratification of the impulse of contrectation, then the state of spontaneous tumescence would return through the internal messages, at certain periods, just as rut periodically appears in the animal. As a rule, however, total abstinence from the gratification of the impulse of contrectation, material or mental, is very rarely or never found in the civilized adult man or woman. Hence the state of tumescence in man is not given the chance to reach its periodicity, but is always produced by the gratification of the impulse of contrectation. _Mechanism of sex-activity._—The mechanism of sex-activity may thus be compared with the charging of a Leyden jar with electricity. The generative organs must first be charged, like the jar, with a certain material turgescence and with nervous energy, in order to evoke the impulse of detumescence. The comparison with the Leyden jar may be moved even a step farther. Just as the charge of the jar with electricity is of a longer duration, compared with the instantaneous discharge at its contact with the earth, so is the charge of the organism with nervous sex-tension usually of longer duration in comparison with the short duration of the discharge.