MENSTRUATION. 



299 



tractions and also by those which cause the delivery of the child. Thus the 

 duration of pregnancy is described as ten menstrual cycles. The significance 

 of menstruation is still obscure. In mammals generally, a period of congestion 

 accompanied by uterine changes which are sometimes closely comparable with 

 those of menstruation, precedes sexual intercourse and ovulation. Ovulation 

 ordinarily occurs at that time, independently of coitus. (In the rabbit and 

 ferret, also in pigeons, ovulation may fail to occur in the absence of the male.) 

 In the bitch ovulation takes place when the external bleeding "is almost or quite 



Disintegrating .^fJ&, _.? - 

 epithelium. Y.-.-.-.Y.?;' .r:.v'-V - : <*"*. &?>& 



Blood vessel, ^'.\t^f. 



J|8b- Superficial epithelium. 



Blood vessel.-- 



Muscularis. 



FIG. 342. Mucous MEMBRANE OF A VIRGIN UTERUS DURING THE FIRST DAY 

 OF MENSTRUATION. X 30. (Schaper.) 



over," and this is the time of coitus. Domestication in various animals causes 

 an increased frequency of the congestive cycles, sometimes unaccompanied by 

 ovulation. It is generally accepted that human ovulation is independent of 

 coitus and to some extent of menstruation. The spermatozoa of rabbits retain 

 their activity and are capable of fertilizing the ovum for about ten days, and 

 it is perhaps true that if human ovulation takes place within some such period 

 after coitus, fertilization may occur. The ovum is said to take four days in the 

 rabbit and eight or ten in the bitch to pass through the tube to the uterus. The 



