Zavim, Chapter 4, Mishnah 2. This chapter is built around the tumah of heseit, the tumah that a zav transmits by moving something. A zav does not have to touch an object at all: if he shifts it, shakes it or displaces it, whatever rests upon it is drawn into his tumah as though he had handled it directly. Our mishnah takes that principle into a very practical situation: a zav bumps or leans against something, and a loaf of terumah that was lying on top of it tumbles to the ground. Is the terumah tamei or tahor?
Everything turns on one question: what actually made the loaf fall? If the object the zav struck was wobbly and unstable, so that his knock genuinely moved it, then the fall of the loaf is his doing, and the terumah becomes tamei through heseit. If, however, the object was solid and firmly anchored, and his blow merely sent a tremor through it without displacing it, the zav has not moved anything. The loaf fell from the vibration alone, and vibration is not heseit. The terumah stays tahor.
The list of solid objects
The mishnah now catalogues objects that fall into the second category, all of them heavy, fixed, or firmly set in place:
"Hikish al hamarish": he knocked against a beam, a thick load bearing timber.
"V'al hamalben": a rafter frame, and by the same logic a doorframe, which is set into the structure of the building itself.
"V'al hatzinor": a drainpipe or water spout, attached and fastened to the wall.
"Af al pi she'asuya bachavalim": and even where such a thing is held in place not by nails but by ropes. The mishnah's point is that the manner of fastening does not matter; what matters is that the object is genuinely secured.
"Al hatanur": an oven, a massive, immovable installation.
"V'al hayam": the receptacle set beneath a mill to catch the flour as it is ground, a large and weighty vessel.
"V'al ha'itztaruvul": the wooden support of a hand mill.
"V'al chamor shel reichayim shel yad": the lower bed of a hand mill, again a solid, substantial piece.
"V'al se'ah shel reichayim shel zeisim": the measuring receptacle of an olive press, into which the oil ran and in which it was measured out, a large and stable fixture.
Rebbi Yosi's addition
A dissenting voice now appears. "Rebbi Yosi omer: af al koras habalanim": to the objects already enumerated, Rebbi Yosi would append the bathhouse keeper's plank, the board upon which the attendant sat while tending the bathhouse. He holds that it too is anchored firmly enough to be classified together with the beam, the oven and the mill.
In practice we do not rule this way. The plank of the bathhouse attendant is left out of the list, and Rebbi Yosi's opinion is not adopted as the halacha.
The ruling
Regarding everything the mishnah itself listed, the din is that the terumah is tahor. A loaf of terumah resting on a beam, an oven, a millstone bed or an olive press measure, which fell when a zav struck the object, remains fit to eat. These objects were not moved by his blow. They are anchored and heavy, and the knock produced nothing more than a shudder. Since there was no heseit, the zav has transmitted nothing, and the terumah keeps its taharah.