Reference for Niddah 34:13

מן הלול ולפנים ודאי טמא מן הלול ולחוץ ספקו טמא

it is deemed unclean<span class="x" onmousemove="('comment',' Since it is obvious that it came from the chamber. Had it come from the upper chamber it could not in the natural course have made its way backwards to the spot where it was discovered. ');">¹¹</span>

Jastrow

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