CHAP, i.] FEMALE ORGANS. 1113 



elements, and the blood vessels become enlarged, more or less 

 irregular spaces filled with blood being developed close under 

 the surface. In this way a modified superficial portion of the 

 mucous membrane becomes differentiated from the underlying 

 portion ; and from this modified portion, which may be compared 

 to the decidua of pregnancy of which we shall presently speak, 

 a certain amount of haemorrhage into the cavity of the uterus 

 early takes place. But the blood thus escaping does not form 

 the main menstrual flow. The growth of the modified mucous 

 membrane is immediately followed by its rapid degeneration, 

 and the part so degenerated is cast off, laying bare the deeper, 

 less-changed layers of the mucous membrane, from the torn and 

 open irregular blood spaces of which a more copious flow of 

 blood takes place. It is this freer escape of blood which, mixed 

 with the detritus, or even with conspicuous pieces of the shed 

 membrane and containing many cells resembling leucocytes, 

 constitute the menstrual flow; as the haemorrhage diminishes, 

 these constituents other than actual blood become more promi- 

 nent, and the discharge becomes less and less coloured. The 

 degeneration and sheading is in turn followed by new growth 

 from the deeper parts of the mucous membrane left behind, 

 whereby the normal mucous membrane is restored in its entirety. 

 The amount of change which takes place probably differs in 

 different individuals ; in some cases possibly the amount of 

 proliferation and subsequent degeneration is relatively slight, 

 the haemorrhage being more comparable to that from any con- 

 gested mucous membrane, such as nasal haemorrhage; in other 

 cases again, according to some observers, the whole thickness 

 of the mucous membrane ^nay be removed, the muscular coat 

 beneath being laid bare. The blood as it passes through the 

 vagina becomes somewhat altered, probably by the influence 

 of the other constituents of the discharge, and when scanty 

 coagulates but slightly ; when the flow however is considerable, 

 distinct clots may make their appearance. 



