CHAPTER VI 



THE MENSTRUAL CYCLE 



SURELY the process of menstruation is one of the 

 strangest things in all Nature. An important organ — 

 the uterus — serving an indispensable function, is over- 

 taken at regular intervals by a destructive change in the 

 structure of its lining, part of which undergoes dissolution 

 with hemorrhage, and must be reorganized in every monthly 

 cycle. The loss of blood from organic tissues, everywhere else 

 in the animal kingdom a sign of injury, even of danger, is in 

 this one organ the evidence of healthy function. To make the 

 puzzle greater, menstruation is by no means general in the 

 animal kingdom, or even among the mammals. It occurs, 

 indeed, only in the human race, in the anthropoid apes 

 (having been observed in chimpanzees and in the gibbons), in 

 the baboons, and in the Old World monkeys ; in short, in a 

 closely related group of primates, one little portion only of 

 the great class of Mammalia. No other animals, in forest, 

 plain, or sea, hiding in dens or grazing the fields, undergo in 

 the course of their cycles any such phase of hemorrhage. It is 

 a paradox indeed that this curious phenomenon of periodic 

 breakdown, seemingly an imperfection, a physiological flaw, is 

 characteristic solely of the females of those very animals we 

 are pleased to think the highest of earth's creatures (Ap- 

 pendix II, note 9). 



The periodicity of menstruation. In human females, men- 

 struation recurs at intervals of about 4 weeks. There is a 

 common impression that the cycles are normally quite regular, 

 but any woman who will keep an accurate calendar of her 

 cycles will find a surprising variability. 



A recent statistical analysis of thousands of records^ shows 

 in fact that the commonest average cycle length (the "mode" 



1 Leslie B. Arey, "The degree of normal menstrual irregularity." 

 American Journal of Obstetrics and Gynecology, vol. 37, pp. 12-29, 1939. 



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