Reference for Niddah 109:51

שעיר המשתלח יוכיח שגורם טומאה לאחרים והוא עצמו טהור

And it was in connection with this ruling that 'Ulla stated, 'Pentateuchally the human skin is clean if it had been dressed; and what is the reason why they ruled it to be unclean? It is a preventive measure against the possibility that a person might use the skins of his father and mother as spreads'. But does not flesh grow again and yet it is unclean? — Mar son of R. Ashi replied: The place of missing flesh becomes a scar.<span class="x" onmousemove="('comment',' Sc. it does not grow again to its original shape as is the case with hair or nails. ');">⁴¹</span>

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