364 



HISTOLOGY 



although the time when it should occur may be indicated by slight uterine con- 

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

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

 struation is suggested by conditions in those mammals in which sexual seasons are 

 annual or infrequent. In them a period of congestion, accompanied by uterine changes 

 which are sometimes closely comparable with those of menstruation, precedes sexual 

 intercourse and ovulation. Thus in the bitch ovulation takes place when the external 

 bleeding "is almost or quite over," and this is the time of coitus. Domestication in 



Disintegrating |iA.. / 1 



epithelium. %'.$/ *;<-!; 

 wV&:&>tfi&&.'sm 



Surface epithelium. 



m2RWy-&$?~~ Blood vessel. 



Muscularis. 



FIG. 367- Mucous MEMBRANE OF A VIRGIN UTERUS DURING THE FIRST DAY OF 

 MENSTRUATION. X 30. (Schaper.) 



various animals causes an increased frequency of the congestive cycles, sometimes 

 unaccompanied by ovulation. It is generally accepted that human menstruation 

 may take place without ovulation, and that ovulation may occur between menstrual 

 periods, and also during pregnancy. It may even occur in children before menstrua- 

 tion has begun. Nevertheless ovulation probably occurs usually and normally at the 

 close of menstruation. Coitus is not considered to be a factor in inducing ovulation, 

 but it is said that in the rabbit and ferret, and in pigeons, ovulation may fail to occur 

 in the absence of the male. 



