I know the hymen is a membrane that covers the external opening of the vagina when a girl is virgin...
But my doubt is "How does the blood flow out of the body during menstruation when the hymen is present..doesnt the hymen block the path..?"
I know its quite silly because virgin girls do bleed... but if you can please clear my doubt..
Thanks..!
OK... I got its perforated... but some times blood clots and mscle tissue also comes out rt.. so how does that happen..
Its really unclear to me..!

Yeah even if your a virgin, it doesn't block the flow, its like a dog door? If you see it like that.lol.

From Wikipedia --

Most common forms of the hymen:
* crescent-shaped, crescentic, or posterior rim: no hymenal tissue at the 12 o'clock position; narrow band of tissue starts at 1 or 2 o'clock going clockwise, is at its widest around 6 o'clock, and tapers off at 10 or 11 o'clock
* annular, or circumferential: the hymen forms a ring around the vaginal opening; especially common in newborns
* redundant; sometimes sleeve-like: folds in on itself, which sometimes causes it to protrude; most common in infancy and at/following puberty due to estrogen levels; can be combined with other type such as "annular and redundant"

Less common forms:
* fimbriated, or denticular: an irregular edge to the hymenal orifice; more likely at an age when estrogen is present
* septate: the hymen has one or more bands extending across the opening
* cribriform, or microperforate: the hymen stretches completely across the vaginal opening, but is perforated with several holes
* labial, or vertical: hymen has an opening from the 12 to the 6 o'clock positions and can look similar to a third set of vulvar lips
* imperforate: hymen completely covers vaginal orifice; will require minor surgery if it has not corrected itself by puberty to allow menstrual fluids to escape

The easiest way to explain this is to compare it to a window screen. The window screen lets air in the house, but keeps bugs out.

The hymen works much in the same way, it is a semi-permeable membrane.