Menstruation was formerly regarded as the exclusive prerogative of woman, but we know now that woman shares the privilege with many animals. All warm-blooded animals that stand or walk erect, without exception, menstruate. The appearance of the discharge is simply due to posture. The process is going on in all quadrupeds, as the sheep, the cow, the dog, the cat, etc., but on account of the position of the uterus (in quadrupeds the fundus of the uterus is situated lower than the neck) the blood is usually retained in that organ, reabsorbed through the lymphatics into the blood, and consumed in the vital process or eliminated through excretory glands. In some animals the discharge consists of mucus. In dogs the writer saw real blood. It is only a play of nature, that in one species the congestion of the genitals manifests itself by a sweating of viscous mucus, in the other by a flow of blood. Even the lower animals shed the same degenerated material. They menstruate through the lymphatics, if for one cause or another impregnation has been prevented. But as a rule, in animals, living in freedom, impregnation always follows the preparation of the uterus, and the phenomenon of menstruation fails to appear. It is only among domesticated animals that the discharge from the genitals is sometimes observed. [J] According to Bischoff, at the moment of the highest excitement the uterus is pressed down into the small pelvis, and the uterine orifice opens and receives the sperma by a kind of suction. Eichstedt claims that the uterus, which is usually flattened in the sagittal direction, assumes a round, pear-shaped form during the excitement and for some time afterwards. In this way a real cavum uteri is produced, where previously only a virtual cavity existed. The vacuity then sucks in the sperma by means of aspiration like a pump. Kisch says: During the orgasm the uterus descends deeper into the pelvis. It is assisted in its descent by the pressure of the abdominal muscles. The muscles of the uterus open then the uterine orifice, and the formerly flat opening becomes round. At the same time the uterine orifices of the tubes also open. Simultaneously the secretion of the cervical glands is expressed, and a suction of a small amount of sperma into the cervix ensues. Rohleder says: The plicae palmatae of the cervix form an obstacle to the penetration of any fluid into the uterus. But during the excitement, this obstacle is overcome by the increased secretion of the cervical glands. At the same time the uterine orifices of the tubes, which are generally closed, open widely through the excitement and almost challenge the entrance of the spermatozoa. Kristeller describes the secretion of a clear transparent mucus in the form of a cord about one to four millimeters thick and one to six centimeters long in the uterus of every mature woman, who never was pregnant. This cord is hanging out of the orificium externum uteri.