THE MENSTRUAL CYCLE 



cycle from the first day of menstruation. Ovulation most 

 commonly takes place about the 12th to the 16th day, 

 although in individual cases it may be earlier or later than 

 this. The corpus luteum is active for about 13 or 14 days, 

 and therefore its degeneration brings on menstruation again 

 25 to 30 days after onset of the last period. 



The "safe period.** Let us digress again for a moment, to 

 discuss, in passing, an interesting and important deduction 

 that follows from the schedule of the human cycle, as shown 

 in the diagram, Fig. 22. There is evidence from many species 

 of animals that the eggs can be fertilized only while in the 

 oviduct, during the first two or three days after their dis- 

 charge from the ovary. We know also that the sperm cells 

 cannot survive more than a few days in the female repro- 

 ductive tract. It follows that the only part of the human 

 cycle during which fertilization of the egg can occur is 

 the few days following ovulation. Since, however, there is 

 no way of ascertaining the date of ovulation and it may vary 

 by several days, we shall for the sake of caution estimate 

 the presumably fertile period as a few days longer each 

 way, say from the 8th to the 20th day of the cycle, counting 

 from the first day of the menstrual period. All the rest of the 

 cycle, i.e. from about the 20th day to the 8th of the next 

 cycle, will be a period of sterility, during which mating will 

 not result in pregnancy. This is the theoretical basis of the 

 so-called "safe period" method of birth control. If all women 

 had regular cycles and things never happened out of turn, 

 it would no doubt be a fully effective method, but irregularity 



Plate XXII. The uterus of the Rhesus monkey during menstruation. A^ first 

 day of flow; at bl., small collections of blood in the lining of the uterus. B, 

 third day; note loss of surface tissues of lining and disappearance of progesta- 

 tional pattern of glands. C, anovulatory menstruation, first day. Note, in com- 

 parison with A, that there is no progestational change of the glands. Magnified 

 10 times. Ay B, Corner collection (nos. 39, 22) ; C, courtesy of G. W. Bartelmez 

 (B. 128). 



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