874 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



vessels of the mucous membrane rupture and the escaping blood 

 together with the secretion of the uterine glands, and some epithelial 

 debris from the surface constitutes the monthly or menstrual flow. 

 The amount of destruction of the mucous membrane varies a good 

 deal, but it usually involves not only the surface epithelium, but 

 extends into the interglandular tissue. The flow lasts for three or 

 four days, and the amount of blood lost may be as much as 300 c.c. 

 After the cessation of the flow, the mucous membrane repairs itself ; 

 this takes about a fortnight, and then after a brief period, generally 

 not more than a few days, preparation for the next menstrual epoch 

 begins once more. These phenomena are accompanied with a feeling 

 of malaise, and in some cases with more or less pronounced pain. 



Menstruation is absent during pregnancy, and as a rule also 

 during the subsequent period of lactation. 



There is no doubt that this uterine phenomenon is related to 

 ovulation (that is, the discharge of ova from the ripe follicles) ; for 

 menstruation begins at puberty when ovulation starts, and ceases to 

 occur at the menopause when ovulation ceases ; an artificial menopause 

 may be created by removal of the ovaries, and in certain cases it has 

 been claimed that menstruation was resumed after an ovary had been 

 transplanted back again. This looks as if the relationship is not 

 a nervous one. Moreover the monthly periodicity is the same in 

 both processes. The period of sexual activity varies greatly in 

 different animals; in some of the monkeys the monthly period 

 occurs as in the human female ; but in many of the lower animals 

 the times of sexual activity and sexual desire are much more widely 

 separated. In animals such times are known popularly as heat or 

 rut, and technically as the oestrus. It is the pre-oastral period in 

 animals which is homologous with menstruation, while oestrus is the 

 period of sexual desire, when coitus may occur. 



Although there is no doubt of the relationship between 

 ovulation and menstruation, there has been much discussion as to 

 which occurs first. But in animals, where the phenomena of the 

 pre-cestral, oestral, and post-oestral periods can be followed more 

 easily, there is no doubt that ovulation occurs, or the ova reach 

 maturity after or at the very end of the uterine flow ; that is to say, 

 ovulation takes place during oestrus and not during the pre-oestral 

 period, which is the homologue of menstruation. This has been 

 shown more particularly in the case of the dog, the sheep, and 

 the pig. 



Many more or less fantastic theories have in the past been put 

 forward to explain menstruation. There is, however, but little 

 doubt that its object is the preparation of the uterine lining for the 

 reception of an ovum, either because a monthly regeneration main- 

 tains it in a condition of irritability which enables it to respond 



