MENSTRUATION. 461 



strual constructive process goes on without the stimulus afforded 

 by the presence of an ovum. 



As regards the actual changes in the uterus itself, the 

 resemblance between the two cases is so great that it seems neces- 

 sary to suppose that their significance is the same ; and it must, 

 therefore, be concluded that the human uterus periodically pre- 

 pares itself, by the formation of a decidual lining, for the reception 

 of an ovum ; the process occurring at monthly intervals through- 

 out the child-bearing period, and quite irrespectively of the 

 presence or arrival of a fertilised ovum. 



The second or destructive stage, constituting the act of 

 menstruation in the ordinary sense of the term, is much more 

 difficult to explain. At first sight it appears to consist simply 

 in a rapid, and somewhat violent, undoing of the work accom- 

 plished in the preceding stage. 



If, however, it is compared with the changes that take place 

 in the rabbit's uterus during gestation, it is found that the 

 human uterus at the end of the constructive period of menstrua- 

 tion has reached a stage corresponding to that of a rabbit's 

 uterus at the end of the seventh or beginning of the eighth day 

 of pregnancy, when the blastodermic vesicle is still lying freely 

 within the uterus, but is just about to acquire its attachment. 



In the rabbit this attachment is effected, early on the eighth 

 day, by fusion of the wall of the blastodermic vesicle with the 

 epithelium of the modified and hypertrophied placental lobes of 

 the uterus (Fig. 169). This fusion is immediately followed, or 

 rather is accompanied, by degenerative changes in the uterine 

 mucous membrane opposite the area of attachment, which 

 rapidly lead to absorption of the uterine epithelium, and of the 

 mouths and necks of the uterine glands. 



Similar changes occur during the formation of the human 

 placenta, and will be described in the concluding section of this 

 chapter ; and inasmuch as the portion of the wall of the uterus 

 which is concerned in the changes is the same in menstruation 

 and in pregnancy, the menstrual discharge may be viewed, not 

 merely as a destructive process, but as corresponding in a modi- 

 fied form to the rapid absorption of the same parts which occurs 

 normally during pregnancy. 



The constructive stage of menstruation, and, as just seen, 

 the destructive stage as well, may be regarded as phases in the 



