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
Zooelogical 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 Zooelogical 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