Reference for Eruvin 53:7

אמר אביי ואיתימא רבי ירמיה אף אנן נמי תנינא עוד כלל אחר אמרו כל שנישא על גבי הזב טמא וכל שהזב נישא עליו טהור חוץ מן הראוי למשכב ומושב והאדם ותו ליכא והא איכא מרכב

Is there not in fact [that which is suitable for] riding upon? ( What is one to understand by that which is 'suitable for riding upon'? If [it is that on] which [the zab] sat, then [it may be retorted] is it not exactly in the same category as a seat?<span class="x" onmousemove="('comment',' Which was specifically excluded.');">¹⁸</span> - It is this that we mean: Is there not the upper part of a saddle<span class="x" onmousemove="('comment',' Which the rider uses as a handle.');">¹⁹</span>

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