At times, in order that coitus may be effective, some other position than the natural one is indispensable. Such a necessity has been recognized even by theologians, by whom any divergence from nature in this matter has usually been regarded as sinful. For instance, in the work of _Craisson_, _De Rebus Venereis ad Usum Confessariorum_, we read: “_Situs naturalis est ut mulier sit succuba et vir incubus, hic enim modus aptior est effusionis seminis virilis et receptioni in vas femineum ad prolem procreandum. Unde si coitus aliter fiat, nempe sedendo, stando, de latere, vel praepostere (more pecudum), vel si vir sit succubus et mulier incuba, innaturalis est.... Sed tamen minime peccant conjuges si ex justa causa situm mutent, nempe ob aegritudinem, vel viri pinquetudinem, vel ob periculum abortus; quandoque ait St. Thomas, sine peccato esse potest quando dispositio corporis alium modum non patitur._” In certain pathological states, as for the prevention of sterility, an abnormal posture during coitus may advantageously be recommended, in order to favour the entrance of the semen into the cervical canal, and to allow the semen to stay longer in the vagina before it flows out. An old and often efficacious means for this purpose is the performance of coitus with the woman in the knee-elbow posture. In order to favour the entrance of the semen into the deeper portion of the genital tract, _Hegar_ and _Kaltenbach_ recommend that after coitus the woman should remain for some time in the knee-elbow posture, while the man from time to time gently presses up the anterior abdominal wall, and then abruptly relaxes the pressure.—In the _Talmud_, coitus was regarded as unfruitful if performed when the woman was in the erect posture. _Casper_ reports the case of a woman with severe scoliosis, who had long remained sterile, and who only conceived (and was subsequently happily delivered) after performing coitus in the abdominal decubitus. _Guéneau de Mussy_ suggests the following, very characteristic, method of ensuring fertilization, one which also certainly dates from great antiquity: “_Sed haud illicitum mihi visum est, si post diversa tentamina diutius uxor infecunda manserit, ipsum maritum digitum post coitum in vaginam immittere, et ita receptum semen uteri osteo admovere. Et cum ostiolo uteri haeret, ut in pervium canalem spermatozoidum motibus faventibus, prodeat, sperare non absurdum._” _Eustache_ reports a case, the wife of a physician, in which this manoeuvre was effective in ensuring conception.