MENSTRUATION. 727 



whole fruitful period of a woman's life, and usually cease 

 between the forty-fifth and fiftieth years. 



The several menstrual periods usually occur at intervals 

 of a lunar month, the duration of each being from three 

 to six days. In some women the intervals are as short as 

 three weeks or even less ; while in others they are longer 

 than a month. The periodical return is usually attended 

 by pain in the loins, a sense of fatigue in the lower limbs, 

 and other symptoms, which are different in different indi- 

 viduals. Menstruation does not usually occur in pregnant 

 women, or in those who are suckling ; but instances of 

 its occurrence in both these conditions are by no means 

 rare. 



The menstrual discharge consists of blood effused from 

 the inner surface of the uterus, and mixed with mucus 

 from the uterus, vagina, and external parts of the gene- 

 rative apparatus. Being diluted by this admixture, the 

 menstrual blood coagulates less perfectly than ordinary 

 blood ; and the frequent acidity of the vaginal mucus 

 tends still further to diminish its coagulability. This has 

 led to the erroneous supposition that the menstrual blood 

 contains an unusually small quantity of fibrin, or none at 

 all. The blood-corpuscles exist in it in their natural state : 

 mixed with them may also be found numerous scales of 

 epithelium derived from the mucous passages along which 

 the discharge flows. 



Cot-pus Luteum. 



Immediately before, as well as subsequently to, the rup- 

 ture of a Graafian vesicle, and the escape of its ovum, 

 certain changes ensue in the interior of the vesicle, which 

 result in the production of a yellowish mass, termed a 

 corpus luteum. 



When fully formed the corpus luteum of mammiferous 



