352 REPRODUCTION 



hypertrophy until the fifteenth day and then atrophy, the 

 animals not being pregnant. 



When pregnancy or pseudo-pregnancy happen metosstrum 

 does not occur. The cycle which takes place under these 

 conditions may therefore be represented thus — 



Pro-oestrum 



CEstrus 



Pregnancy Pseudo-pregnancy 

 Anoestrum 



Changes in the Human Non-pregnant Uterus 



The Menstrual Cycle. — The complete cycle usually 

 occupies twenty-eight days. It is divided into four stages^ — 



1. Stage of Quiescence. — This lasts about twelve days. 



2. The Constructive Stage. — This begins with an increase 

 in the glands and stroma of the mucous coat, accompanied 

 by dilatation of the blood-vessels. Exudation of blood 

 and serum occurs into the tissue-spaces. These changes 

 are associated with a general thickening of the mucosa. 

 The constructive stage lasts about five days. 



3. The Destructive Stage — Menstruation. — This begins 

 with free extravasation of blood into the stroma, due 

 partly to diapedesis, partly to rupture of the capillaries. 

 The blood accumulating under the epithehum breaks 

 through into the lumen, blood and epithehal cells being 

 discharged from the vagina mixed with mucus from the 

 enlarged glands. The average duration of the menstrual 

 flow is four days. This period is accompanied by a general 

 bodily disturbance — lassitude and pains in the back. 



