THE MENSTRUAL CYCLE 



here, however, a cycle in which the eggs are not fertilized. In 

 such a case they are transported through the oviduct to the 

 uterus, where about 8 or 9 days after they first left the ovary 

 they go to pieces and disappear. The corpus luteum holds on 

 until the 14th or 15th day, then degenerates and ceases to 

 deliver progesterone to the blood stream. The endometrium 

 is thus deprived of its hormonal support. The changes induced 

 by progesterone disappear in the course of a few days. The 

 blood flow through the uterus diminishes, the lining becomes 

 thinner, the cells of its surface epithelium and glands diminish 

 in number and height, and the glands resume the simpler form 

 that characterizes the interval and follicular phase of the 

 cycle. Generally speaking, the steps of this reversion are 

 gradual ; it is spread over several days, and gives no outward 

 sign to let us know it is in progress. 



In our diagram (Fig. 19) the whole sequence of changes 

 in the lining of the uterus is illustrated by the middle portion, 

 which is a conventionalized representation of the glands in 

 their successive phases. 



The cycle in menstruating animals. The cycle of the men- 

 struating animals and the human species is fundamentally 

 similar to that of other animals. Two important difl*erences, 

 however, exist. In the first place there is not a sharply defined 

 phase of sexual receptivity like the estrus of other mammals. 

 Although cyclic fluctuations of sex activity occur in some of 

 the apes and monkeys, this is by no means as well defined as 

 in most other animals, and in the human female sex desire is 

 obviously much more influenced by all sorts of moods, social 

 situations, domestic ups-and-downs, and the like, than by any 

 tendency to cyclic alternation. Mating may occur on any day 

 of the cycle. There is no outward sign, like the estrous be- 

 havior of lower mammals, to indicate the time of ripening of 

 the ovarian follicle and its ^gg. 



It is interesting to speculate about the effect of this sup- 

 pression of estrous rhythm upon human life and the progress 



{ us } 



