Mesorat%20hashas for Chullin 111:21

כי סליק לנהרדעא שלח להו

Verily, it has been reported in the name of R'Simeon B'Lakish that one may examine [the membrane of the brain against the bite of] a weasel with the finger but not with a nail,<span class="x" onmousemove="('comment',' The test with a nail (or a needle or a straw, cf. infra) is a delicate operation. The bone of the skull must first be removed and the nail must be passed gently over the surface of the membrane of the brain. If anything catches or holds up the nail in its course it indicates a perforation and it is trefah. The danger in this operation is that the man whilst making this test might inadvertently pierce the membrane with the nail, and he would then have to declare the bird trefah, though it was not really trefah, thus occasioning loss unto the Israelite.');">¹²</span>

Explore mesorat%20hashas for Chullin 111:21. In-depth commentary and analysis from classical Jewish sources.