It is not until we reach the monkeys that this character of the flow becomes well marked. Monthly sanguineous discharges have been observed among many monkeys. In the seventeenth century various observers in many parts of the world—Bohnius, Peyer, Helbigius, Van der Wiel, and others—noted menstruation in monkeys.[[89]] Buffon observed it among various monkeys as well as in the orang-utan. J. G. St. Hilaire and Cuvier, many years ago, declared that menstruation exists among a variety of monkeys and lower apes. Rengger described a vaginal discharge in a species of cebus in Paraguay, while Raciborski observed in the Jardin des Plantes that the menstrual hæmorrhage in guenons was so abundant that the floor of the cage was covered by it to a considerable extent; the same variety of monkey was observed at Surinam, by Hill, a surgeon in the Dutch army, who noted an abundant sanguineous flow occurring at every new moon, and lasting about three days, the animal at this time also showing signs of sexual excitement.[[90]]
The macaque and the baboon appear to be the non-human animals, in which menstruation has been most carefully observed. In the former, besides the flow, Bland Sutton remarks that "all the naked or pale-colored parts of the body, such as the face, neck, and ischial regions, assume a lively pink color; in some cases, it is a vivid red."[[91]] The flow is slight, but the coloring lasts several days, and in warm weather the labia are much swollen.
Heape[[92]] has most fully and carefully described menstruation in monkeys. He found at Calcutta that the Macacus cynomolgus menstruated regularly on the 20th of December, 20th of January, and about the 20th of February. The Cynocephalus porcaria and the Semnopithecus entellus both menstruated each month for about four days. In the Macaci rhesus and cynomolgus at menstruation "the nipples and vulva become swollen and deeply congested, and the skin of the buttocks swollen, tense, and of a brilliant-red or even purple color. The abdominal wall also, for a short space upward, and the inside of the thighs, sometimes as far down as the heel, and the under surface of the tail for half its length or more, are all colored a vivid red, while the skin of the face, especially about the eyes, is flushed or blotched with red." In late gestation the coloring is still more vivid. Something similar is to be seen in the males also.
Distant, who kept a female baboon for some time, has recorded the dates of menstruation during a year. He found that nine periods occurred during the year. The average length between the periods was nearly six weeks, but they occurred more frequently in the late autumn and the winter than in the summer.[[93]]
It is an interesting fact, Heape noted, that, notwithstanding menstruation, the seasonal influence, or rut, still persisted in the monkeys he investigated.
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-œstrous 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.
There is, however, one point which of late years has been definitely determined, and which should not be passed without mention: the relation of menstruation to ovulation. It was once supposed that the maturation of an ovule in the ovaries was the necessary accompaniment, and even cause, of menstruation. We now know that ovulation proceeds throughout the whole of life, even before birth, and during gestation,[[97]] and that removal of the ovaries by no means necessarily involves a cessation of menstruation. It has been shown that regular and even excessive menstruation may take place in the congenital absence of a trace of ovaries or Fallopian tubes.[[98]] On the other hand, a rudimentary state of the uterus, and a complete absence of menstruation, may exist with well-developed ovaries and normal ovulation.[[99]] We must regard the uterus as to some extent an independent organ, and menstruation as a process which arose, no doubt, with the object, teleologically speaking, of cooperating more effectively with ovulation, but has become largely independent.[[100]]