42 INTRODUCTION TO SEXUAL PHYSIOLOGY 



as with the mare, cow, ewe, and sow (polyoestrous animals in 

 Heape's terminology), or there may be only one oestrus to the 

 sexual season, as with the bitch (monoestrous animals). 



A simple oestrous cycle in such an animal as the dog has 

 been divided into the following periods : — 



Anoestrum (period of rest). 



Prooestrum (period of growth, congestion, and uterine haemor- 

 rhage). 



GEstrus (period of desire). 

 Pregnancy or pseudo-pregnancy. 



The complete cycle in the monoestrous dog lasts about six 

 months, there being two sexual seasons and two " heat " or 

 oestrous periods in one year, these occurring typically in spring 

 and autumn. 



The anoestrum. lasts about three months, when the generative 

 organs are in a state of quiescence. The Graafian follicles in the 

 ovary probably undergo a slow process of growth and ripening 

 throughout the interval, but they do not become conspicuous 

 upon the ovarian surface until the approach of a new heat period. 

 The uterus is relatively anaemic and the glands inactive. The 

 mammary glands are also inactive unless lactation is in progress 

 after a recent pregnancy. 



The prooestrum, or period of " coming on heat," is marked by 

 an increased activity on the part of all the generative organs. 

 The ovarian follicles come to protrude from the surface. The 

 uterus undergoes some amount of growth and becomes much 

 vascularised, and the glands become active. Blood corpuscles 

 are then extravasated within the mucous membrane and tend 

 to congregate below the uterine epithelium. The latter ruptures 

 in certain places, and blood is poured out into the cavity and 

 passes down to the vagina and out to the exterior. Bleeding 

 at the vulva may go on for a week or more. The entire pro- 

 oestrum lasts for ten to fourteen days. The uterine changes in- 

 volved, which usher in the period of desire or " heat " proper, 

 have been regarded as of the nature of a preparation for the 

 attachment of the fertilised ovum. 



CEstrus is the period of desire, and coition is usually restricted 

 to this time. Ovulation, or the discharge of the ova, takes place 

 during this period, and the ova become mature in the way 



