THE MENSTRUAL CYCLE 



NATURE OF THE MENSTRUAL CYCLE 



Events of the cycle in non-menstruating animals. To get 

 a clear understanding of the process of menstruation, it is 

 necessary to understand first what takes place during the 

 cycle in animals which do not menstruate. This has already 

 been discussed in part and illustrated in previous chapters, 

 and is summarized in the diagram herewith (Fig. 19) which 

 represents the typical or generalized cycle of mammals. If 

 we wish to talk about any one species, we shall have to intro- 

 duce modifications into this scheme, but as it stands it can 

 be used as a basis for understanding them all. In the upper 

 portion, which shows events in the ovary, we see (beginning 

 at the left) the growth and ripening of the follicle. The mo- 

 ment of rupture of the follicle and discharge of the egg gives 

 a convenient point of division, which we may consider as the 

 start of a new cycle. Looking at the third part of the diagram, 

 that indicating sex activity, we see that ovulation occurs 

 during estrus, an arrangement which is adapted to secure 

 fertilization of the egg. Next, the follicle is converted into a 

 corpus luteum. This in turn runs its course, secreting pro- 

 gesterone for about two weeks (in typical species) and then, 

 if the Ggg is not fertilized, the corpus luteum suddenly begins 

 to degenerate and ceases to secrete its hormone. Thereafter, 

 a new crop of follicles begins to develop. In some animals the 

 new cycle follows at once (e.g. the guinea pig, which has a 

 cycle of only 15 or 16 days) ; in others several months may 

 elapse, during which the ovaries are relatively dormant (as 

 in dogs and cats) or a whole year, as in many wild animals. 



Digression about the cycle in general. We come now to the 

 fundamental question of the female reproductive cycle, 

 namely what causes the alternations of structure and function 

 in the ovary. When the cycle was first discussed, in Chapter 

 III, we could deal with it only as an observed phenomenon 

 of natural history, but we are now in a position to consider 



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