Keilim, Chapter 11, Mishnah 7. This chapter is occupied with metal utensils and with the special rules that govern them, and our mishnah applies those rules to a musical instrument. The instrument in question is built from more than one material: its main body is the horn of an animal, while the piece a person places against his lips and the wide flared end from which the sound emerges were sometimes fashioned out of a different substance altogether, such as metal. The Mishnah works through the parts one at a time, and then asks what happens once all the parts are fitted together into a single working instrument.
The Mishnah opens with the horn itself: "Keren agulah, teme'ah," a horn played as a musical instrument, when it is bent, can contract tumah. Now a horn is made of bone, and bone belongs to that class of materials whose utensils are susceptible only if they are able to contain something. A bent horn does in fact have a hollow at the place where it turns, and one can put small objects there or draw a bit of liquid with it. Because it functions as a container, it falls under the rules of tumah.
"Ufeshutah tehorah," and a straight horn is tahor. It has no curve that forms a receptacle, and utensils made of bone, as explained, are not susceptible to tumah unless they can contain something. A flat, straight bone instrument therefore stands outside the laws of tumah entirely.
The Mishnah proceeds to the piece a person sets against his lips: "Im hayetah metzupah shelah shel matechet, teme'ah." Should that piece have been fashioned out of metal, it is susceptible to tumah, and it remains so even after it has been taken off the horn and is sitting by itself. Here metal parts company with bone: a metal article contracts tumah even when it is perfectly flat and contains nothing whatsoever. But why should a detached lip-piece count as a utensil standing on its own? Because it fills a role that can be pointed to as its own: without it the horn cannot be sounded. True, it accomplishes that role only in partnership with the horn, and it is ordinarily found fixed to it; nevertheless the task belongs to it, and that suffices to give it the status of a utensil apart from the horn.
Then comes the wide end, the bell: "Hakav shelah," the broad section out of which the sound comes. If that bell was made of metal, "Rabbi Tarfon metamei," Rabbi Tarfon holds that it is susceptible to tumah even when it is detached from the horn, because in his view it too is a utensil in its own right. "Vachachamim metaharin," and the Chachamim declare it tahor, because to their mind the bell is secondary to the horn. It is simply part of the horn itself, not a separate article, and something that is not a utensil of its own cannot receive tumah.
The dispute turns on how we read the bell's function. Rabbi Tarfon says that the bell does have a use it can call its own, to bring the sound out, and even though that use is realized only together with the horn, it is still a purpose that belongs to the bell. The Chachamim answer that the bell has no independent function to speak of; it is fully absorbed into the horn and counts as one thing with it.
The Chachamim nonetheless concede that once the bell has been set onto the horn, it is drawn into the laws of tumah along with the rest. "Uvish'at chiburan, hakol tamei," while they stand joined, all of it is susceptible to tumah. Once the three pieces, the part one blows into, the shaft between, and the flared end, have been assembled into a single horn, then even where metal appears in only one of them, everyone agrees that the instrument as a whole can become tamei. The rule at work is that whatever is fastened to something susceptible to tumah is pulled along and becomes susceptible together with it. The component of metal contracts tumah by its own right, and once it has been fixed in place, the remainder of the horn is bound by tumah no less than it is.
The Mishnah rounds off with a further example of the very same principle: an article put together out of distinct pieces, all of which become susceptible once they are fitted into one whole. Imagine a menorah of metal with its several components: the stems, the little vessels that hold oil and wick, and the stand it rests upon. "Kayotze bo kenei menorah tehorin," along these lines, the stems of a menorah are tahor, for they have no standing of their own. "Haperach vehabasis teme'in," the perach, referring here to the oil holders, together with the basis, upon which the menorah is set, are susceptible to tumah, since each of them contains something. "Uvish'at chiburan, hakol tamei," yet while they are all fastened together, the menorah in its entirety can contract tumah, stems and all, precisely as we found with the horn and its metal lip-piece.