Reference for Niddah 43:17

(ויקרא טו, לב) זאת תורת הזב ואשר תצא ממנו שכבת זרע מה זיבה סותרת אף שכבת זרע נמי סותר

becomes unclean only when the quantity of semen emitted suffices to close up the orifice of the membrum.<span class="x" onmousemove="('comment',' Since the splinter used is inevitably smaller than the orifice, the quantity of semen extracted by it must obviously be less than the prescribed minimum. ');">¹⁶</span>

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