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1. Kusim are believed when they say that they buried or did not bury a stillborn in a certain area.
2. Kusim are also believed with regard to whether an animal is a firstborn.
3. Kusim are believed even to mark gravesites, even though doing so is a Rabbinic law.
4. The Mishnah discusses the law of blood found on various areas of a woman's body.
5. The Mishnah discusses the law of blood found on various areas of a woman's clothing.
A BIT MORE
1. The Gemara explains that Kusim are believed with regard to [almost all] Torah laws if they themselves are clearly acting based on what they tell us. (For example, a Kohen Kusi who brings Terumah to that area is believed to say that there are no stillborns buried there.) However, we must see that they are practicing based on their information, as they do not concern themselves with causing others to sin (Lifnei Iver).
2. If we see them shearing and working the animal, we know that it is not a firstborn animal.
3. Since this concept is explicitly mentioned in Yechezkel (39:15), the Kusim are careful to mark gravesites. Accordingly, any area they do not mark is presumed to be free of graves.
4. For example, if blood is found on the insides of her legs or thighs (where, if she would stand straight with her legs touching each other, this part of her body would touch the other side of her body), the blood is deemed Tamei. If it is found on the outer sides of her legs or thighs (parts that do not touch the other side), it is deemed Tahor (since it is presumed to have come from a different source).
5. If the blood is found above the belt it is presumed Tahor, while if it is below the belt it is presumed Tamei. If it is the lower part of a sleeve that is sometimes dropped below the belt (figuratively) while she is wearing it, it is Tamei.
Index to Revach for Maseches Kerisus