36 THE PHYSIOLOGY OF REPRODUCTION 



are capable of copulating at any time, whereas in the females 

 this function is restricted to definite periods. 



The non-breeding season or period of rest in a female Mammal, 

 when the generative organs are quiescent (at least relatively) 

 and the uterus is normal and comparatively anaemic, and the 

 animal shows no disposition to seek out a mate, is called by 

 Heape the Arcestrous period or simply the Ar.ceitrum. This period 

 is generally considerably prolonged, and in many Mammals 

 occupies the greater part of the year. Its close marks the 

 beginning of the sexual season. 



The first part of the sexual season is occupied by the Pro- 

 cestrum. This period is characterised by marked changes in 

 the generative organs, the uterus becoming congested, while 

 in the later stages there is often a flow of blood from the external 

 opening of the vagina. The procestrum is the period often re- 

 ferred to by breeders as the time when an animal is " coming 

 on heat," or " coming in season." 



The next period, CEstrus, or CEstrum (as it is sometimes called), 

 ' ' marks the chmax of the process ; it is the special period of 

 desire in the female ; it is during oestrus, and only at that time, 

 that the female is willing to receive the male, and fruitful coition 

 rendered possible in most, if not in all. Mammals." ^ 



The periods of procestrum and oestrus are commonly referred 

 to together as the " heat " or " brunst " period, and sometimes 

 as the period of " rut," ^ and no attempt is then made to dis- 

 tinguish the time occupied by " coming in season," and the 

 time at which the female is ready to receive the male. This 

 failure to distinguish the two periods (procestrum and oestrus) has 

 led to much confusion, especially in regard to the nature of the 

 relation between " heat " in the lower Mammals and menstrua- 

 tion in the human female. As was first pointed out by Heape, 

 it is the procestrum alone, and not the entire " heat " period 

 which is the physiological homologue of menstruation. This is a 

 point which will be dealt with more fully in the next chapter 

 of this book. 



If conception takes place as a result of coition during oestrus, 



' Heape, loc. cit. 



^ The term "rut" is used by Heape in the case of the male only, the 

 " rutting season," as stated above, being the male sexual season. 



