II Jan., 1909.] Elements of Amnial Physiology. 29 



THE SEXUAL CYCLE IN THE FEMALE. 



In all female mammals after puberty a definite cycle of changes is 

 ■obser\ed in the genital organs when pregnancy has not occurred. These 

 changes have been named as follows — proce strum or period of preparation, 

 CESTRUS or period of sexual desire, metce strum which occurs if fertiliza- 

 tion is absent and is a period of suljsidence of genital excitement, and 

 the ANCE STRUM or period of rest in which the sexual organs lie fallow. 

 At the end of the anoestrum a new sexual season occurs and the cycle 

 begins again with the prooe.strum. Animals that conform to this type are 

 termed moncestrous. Examples of moncestrous animals are many wild 

 sheep in which the sexual season occurs once a year and the anoestrum or 

 fallow period extends over several months. Another monoestrus animal is 

 the domestic bitch in which however the anoestrum last only a few months 

 and so three or even four cycles can take place in a year. The other type 

 has been termed polycestrous; in this case, in the absence of fertilization, 

 there takes place after the metoestrum instead of a long anoestrum, a very 

 short fallow period of a few days called the dicestrum, then a new cycle 

 beginning with procestrum starts again. After a few of these cycles a true 

 anoestrum appears and lasts until a new sexual season occurs. . As an 

 example of a polyoestrouis animal the mare may be taken. During .a certain 

 jH;rtion of the year she passes, when net pregnant, through a series of cycles 

 separated each from the other by a short dioestrum, then a true anoestrum 

 occurs and ends with the advent of a new sexual season. The polyoestrous 

 condition is shown bv domesticated cattle, sheep, and pigs. An extreme 

 form of polvoestrous rh\ thm is displayed by man, certain monkeys, and 

 some domesticated sheep. Here the anoestrum is absent and the cycles are 

 separated only by short dioestral periods and the female is therefore cap- 

 able of being impregnated throughout the year. But in most polyoestral 

 animals the dioestrcus cycles are only two or three in number. Domesti- 

 cation and change of climate and food have a marked effect on these cycles. 

 Generally stated domestication tends to shorten or even obliterate the 

 anoestrum and thus make the periods of oestrous or desire more frequent 

 though possibly less intense. 



The period of prooestrum has been term.ed the period of preparation, 

 i.e., preparation for a fertilized ovum. The external genitals become 

 swollen and a discharge takes place from the vagina which may be bloody ; 

 there is also some general excitement and in some animals special indica- 

 tions, such as drooping ear in the sow, and blood-shot eye in the rabbit. 

 But it is in the uterus that the most significant changes happen. ^ The 

 lininor membrane thickens and its blood vessels become enormouslv dilated ; 

 some extravasation of blood beneath the surface generally occurs and may, 

 in some animals, be so pronounced as to cause true bleeding into the uterine 

 ■cavity. The innermost layer of the uterus breaks down in part and passes 

 out as a mucous discharge, in some animals mixed with blood. These 

 uterine changes may be regarded as all preparatory for the reception of a 

 fertilized ovum. Immediatelv following the prooestrum is the oestrus or 

 period of desire in which the' male is sought. Its duration is short and 

 rarelv exceeds twentv-four hours. In manv animals the female in oestrus 

 possesses an odour which informs the m^ale of her condition and excites 

 him sexually. The oestrus in some animals, e.g.. sheep, may overlap the 

 prooestrum. to some extent. If the sexual act takes place, and should a 

 fertilized ovum arrive in the uterus from either Fallopian tute, the uterine 

 wall is now in a fit condition for the attachment and nourishment of the 



