Reference for Bekhorot 74:51

וחכמים אומרים

For we have learnt: A spinal column and a skull which have shrunk [do not cause uncleanness].<span class="x" onmousemove="('comment',' The spinal column and the skull cause levitical uncleanness, rendering any object unclean under their shelter, like the greater number of the limbs of a dead body or the greater part of a dead body. If however, they are not complete, they do not cause this uncleanness.');">¹⁹</span>

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