(Mishnah): Chachamim compared the interior of a woman to a Cheder (room), Prozdor (corridor) and Aliyah (second story);

Blood of the Cheder is Tamei.

If [blood] is found in the Prozdor, the Safek (where it came from) is considered Tamei, for the Chazakah is that it came from the Makor (Cheder. More blood comes from there.)

(Gemara): Our Mishnah means that blood of the Cheder is Tamei, and blood of the Aliyah is Tahor.

(Rav Yehudah citing Shmuel): The Cheder is further in than the Prozdor, and the Aliyah is on top of the Cheder, until half of the Prozdor. The opening of the Aliyah is open to the Prozdor.

Question (Rav Nachman bar Rav Yitzchak, to Rav Huna): Does our Mishnah discuss when [blood] is found from the opening of the Aliyah and further in?

Answer (Rav Huna): If it is found from the opening of the Aliyah and further in, Vadai (it is from the Makor! The Mishnah would not say that it is due to Chazakah!)

Rather, the case is, is found from the opening of the Aliyah and further outside.