Eruvin, chapter ten, mishnah thirteen. This mishnah continues the subject of which activities are permitted in the Temple on Shabbos and which are forbidden. This time we deal with the rabbinic prohibition of healing on Shabbos - doing things that involve curing - and specifically with a retiyah, a bandage containing medication, which ordinarily a person may not apply on Shabbos.
"Machzirin retiyah bamikdash":
A kohen who had a wound on his hand with a healing bandage on it from before Shabbos, and who needed to remove the bandage in order to perform the service - since it forms a barrier between his hand and the vessel, and he may hold the vessel he serves with in the Temple only with his bare hand - is permitted to return that bandage to its place after he has completed the service on Shabbos. The reason: if he were not permitted to return it, there would be a concern that he would refrain from performing the service in the Temple to begin with, because he would not be able to replace the bandage when he finished.
"Aval bamedinah" - outside the Temple this is not permitted, even though the bandage was on the wound before Shabbos and fell off. The usual decree still applies, lest he come to grind medicinal ingredients, or because of some other forbidden labor involved in the process of healing on Shabbos.
"Im mitechilah - kan v'kan asur" - if the bandage was not on him at all, and he now wishes to apply it on Shabbos, it is forbidden not only outside the Temple but even in the Temple. For although we have a principle, "ein issur shevus bamikdash" - that rabbinic prohibitions do not apply in the Temple - that is only when the act is required for the sake of the service. In this case, since the bandage was not on him before Shabbos, applying it is not for the sake of the service; the permission was granted specifically where he needed to remove it in order to perform the service.
The Gemara clarifies that the entire prohibition against returning a medicated bandage on Shabbos outside the Temple was stated specifically where it actually fell to the ground; but if he placed it on a vessel or the like, he may return it to the wound.
"Koshrin nima bamikdash":
The mishnah moves to another case, connected to the musical instruments that were needed in the Temple. The Levites played instruments at the time the offerings were brought, and what is the law if one of the strings of the harp snapped on Shabbos? To this the mishnah says that one may tie, on Shabbos, a string of a musical instrument that tore on Shabbos. The halachah is that preparations for a mitzvah which could not have been done before Shabbos are permitted; and just as slaughtering the offering was permitted on Shabbos, so too repairing the instrument was permitted, since the music played at the time of the offerings is part of the mitzvah of the Temple.
The commentators disagree about the scope of this permission:
Some understand the mishnah as speaking even of a Torah-level labor, even a permanent knot, which one is permitted to tie there.
Some understand that it is not a permanent knot but a temporary one, so that no Torah-level prohibition is involved.
We will understand the matter simply, as Rashi does, that an ordinary knot is meant. Even so, Rashi holds that if an ordinary knot is not needed, he should make a bow, so that he not come to a Torah-level prohibition.
All this is in the Temple, but outside the Temple, of course, it is not permitted. "Im mitechilah - kan v'kan asur" - if he comes to tie the string for the first time, that is, it was not there from before Shabbos, it is forbidden even in the Temple, for this is not a labor that was impossible to do before Shabbos; and only those labors that could not have been done before Shabbos are permitted.
"Hachotchin yabelet bamikdash":
A kohen who has a wart that prevents him from performing the service, because it makes him blemished and disqualifies him from serving, may cut it off on Shabbos, but he must do so with a change, in an unusual manner, such as with his hand or with his teeth. The Gemara notes that with a dry wart, one that has completely dried out, he may even use an instrument. But outside the Temple this is not permitted even in an unusual manner: the permission was granted in the Temple alone, for the sake of being able to perform the service, and even though it is done in an unusual manner, outside the Temple there is a decree lest he come to do it in the usual manner.
All this is when he cuts it with his hand; but if he comes to use an instrument to cut it, "kan v'kan asur" - it is forbidden even in the Temple, because using an instrument is considered a full-fledged labor, and it is therefore forbidden both outside the Temple and in the Temple.
In summary: in this mishnah we learned three laws that were permitted in the Temple for the sake of the service and forbidden outside it: returning a bandage that was removed because it formed a barrier, tying a string that snapped on a musical instrument, and cutting off a wart that disqualifies the kohen. In all three the same principle recurs: the permission depends on the act being required for the service and on its having been impossible to do before Shabbos - and therefore "im mitechilah," and likewise cutting a wart with an instrument, "kan v'kan asur."