In the anthropoid apes, Hartmann remarks that several observers have recorded periodic menstruation in the chimpanzee, with flushing and enlargement of the external parts, and protrusion of the external lips, which are not usually visible, while there is often excessive enlargement and reddening of these parts and of the posterior callosities during sexual excitement. Very little, however, appears to be definitely known regarding any form of menstruation in the higher apes. M. Deniker, who has made a special study of the anthropoid apes, informs me that he has so far been unable to make definite observations regarding the existence of menstruation. Moll remarks that he received information regarding such a phenomenon in the orang-utan. A pair of orang-utans was kept in the Berlin Zoölogical Gardens some years ago, and the female was stated to have at intervals a menstrual flow resembling that of women, and during this period to refrain from sexual congress, which was otherwise usually exercised at regular intervals, at least every two or three days; Moll adds, however, that, while his informant is a reliable man, the length of time that has elapsed may have led him to make mistakes in details. Keith, in a paper read before the Zoölogical Society of London, has described menstruation in a chimpanzee; it occurred every twenty-third or twenty-fourth day, and lasted for three days; the discharge was profuse, and first appeared in about the ninth or tenth year.[94] What is menstruation? It is easy to describe it, by its obvious symptoms, as a monthly discharge of blood from the uterus, but nearly as much as that was known in the infancy of the world. When we seek to probe more intimately into the nature of menstruation we are still baffled, not merely as regards its cause, but even as regards its precise mechanism. "The primary cause of menstruation remains unexplained"; "the cause of menstruation remains as obscure as ever"; so conclude two of the most thorough and cautious investigators into this subject.[95] It is, however, widely accepted that the main cause of menstruation is a rhythmic contraction of the uterus,--the result of a disappointed preparation for impregnation,--a kind of miniature childbirth. This seems to be the most reasonable view of menstruation; i.e., as an abortion of a decidua. Burdach (according to Beard) was the first who described menstruation as an abortive parturition. "The hypothesis," Marshall and Jolly conclude, "that the entire pro-oestrous process is of the nature of a preparation for the lodgment of the ovum is in accordance with the facts."[96] Fortunately, since we are here primarily concerned with its psychological aspects, the precise biological cause and physiological nature of menstruation do not greatly concern us.