In men, as well as in women, erection and orgasm, or even ejaculation, may be induced by irritation of various other regions of the skin and mucous membrane. These “erogenous” zones in woman are, while she is a virgin, the clitoris, and, after defloration, the vagina and cervix uteri. In woman the nipple particularly seems to possess this quality. Titillatio hujus regionis plays an important part in the _ars erotica_. In his “Topographical Anatomy,” 1865, Bd. i, p. 552, Hyrtl cites Val. Hildebrandt, who observed a peculiar anomaly of the sexual instinct in a girl, which he called _suctusstupratio_. She had her mammæ sucked by her lover, and finally, by gradually drawing on her nipples, she became able to suck them herself,—an act that gave her most intense pleasure. Hyrtl also calls attention to the fact that cows sometimes suck the milk from their own udders. L. Brunn (_Zeitg. f. Literatur_, etc., d. Hamburg. Correspondent, 1889, Nr. 21), in an interesting article on “Sensuality and Love of Kin,” points out how zealously the nursing mother gives herself to nursing the babe, “for love of the weak, undeveloped, helpless being.” It is easy to assume that, by the side of the ethical motives, the fact that the sucking may be attended by feelings of physical pleasure plays a part. The remark of Brunn, which is correct in itself, but one-sided, that, according to Houzeau’s experience, among the majority of animals it is only during the time of nursing that the relations between mother and offspring are close, and thereafter indifferent, also speaks in favor of this assumption. Bastian found the same thing (blunting of the feeling for the offspring after weaning) among savages. Under pathological conditions, as is shown by Chambard, among others, in his thesis for the doctorate, other portions of the body (in hysterical persons) about the mammæ and genitals may attain the significance of “erogenous” zones. In man, physiologically, the only “erogenous” zone is the glans penis, and, perhaps, the skin of the external genitals. Under pathological conditions the anus may become an “erogenous” area. Thus anal auto-masturbation, which seems to be only too frequent, and passive pederasty would be explained. (Comp. Gamier, “Anomalies sexuelles,” Paris, p. 514; F. Moll, “Conträre Sexualempfindung,” p. 163.) The psycho-physiological process comprehended in the idea of sexual instinct is composed of (1) concepts awakened centrally or peripherally; (2) the pleasurable feelings associated with them. The longing for sexual satisfaction (libido sexualis) arises from them. This desire grows stronger constantly, in proportion as the excitation of the cerebral sphere accentuates the feeling of pleasure by appropriate concepts and activity of the imagination; and the pleasurable sensations are increased to lustful feeling by excitation of the erection centre and the consequent hyperæmia of the genitals (entrance of liquor prostaticus into the urethra, etc.).