8 THE GENITAL GLANDS 



brane of the uterus becomes greatly thickened and 

 engorged every month, and haemorrhages take place 

 into it which carry away parts of the superficial 

 layers. We are as far as ever from understanding 

 the real value of its occurrence. According to Blair 

 Bell, a large quantity of calcium salts accumulate 

 in the blood, which menstruation removes, menstrual 

 blood being very rich in calcium. 



There is no doubt that menstruation is determined 

 by an internal secretion from the ovaries, and when 

 these are both removed it almost invariably ceases. 



Marshall and Heape have shown that the process 

 is by no means peculiar to the human subject. In 

 a great variety of animals, such as deer, dogs, sheep, 

 and monkeys there is a regular cycle of changes 

 leading up to the cestrum or rut, and after great 

 overgrowth of the mucous membrane of the uterus 

 there is a mucous and often bloodstained discharge 

 followed by a brief period of fertihty. 



Ovulation. — ^The rupture of the Graafian folhcle 

 and shedding out of the ovum is called ovulation. 

 It has been much debated whether the time of 

 ovulation coincides with that of menstruation in the 

 human subject. In the animals above described no 

 doubt this is true, and the age-limits of fertility and 

 of menstruation are approximately the same. Never- 

 theless the relationship cannot be exact, because 

 pregnancy has occurred before the first menstruation, 

 and observations on the ovaries during abdominal 

 operations at various times in the menstrual cycle 

 show that although ovulation commonly takes place 

 at about the same time as menstruation, this is by 



