A woman who menstruates is impure with niddah impurity and is forbidden to have sexual relations with her husband. At what point of the flow from the womb is the woman considered impure? The early sages (Tanaaim) ruled that it is when the blood reaches the “outer chamber.”
The scholars clarified where exactly this “outer chamber” is. One of the scholars, Reish Lakish, said that the part of the sexual organ that can be seen when a young girl sits down is what is called the outer chamber. The sage R’ Jonathan asked: This is considered an exposed area and should not be considered the outer chamber. In his opinion, the outer chamber is within her vagina, up to the point where the flesh thins. The scholars asked: Is the area where the flesh thins considered the outer chamber and impure, or an inner chamber and pure? Answer: The area where the flesh thins in her vagina is considered the outer chamber and yet is considered pure. Other sages defined the outer chamber as being that place to which the male sexual organ can reach, what they called “the place where the attendant threshes.” (From this Halachic discourse we rule that a woman must check her possible purity with a cloth inserted into her sexual organ as far as the male sexual organ reaches, though not all organs are equal: “not all appendages are equal.” A medium-sized organ should be used for estimation.)
(Babylonian Talmud, Tractate Niddah 41b)