Shabbos, Chapter 6, Mishnah 10. This mishnah continues the subject of the previous mishnah, dealing with going out into a public domain carrying items whose purpose is to serve as an amulet or a kind of charm.
The three things one may go out with:
"Beitzas chargol" - the chargol is one of the locust species, and its egg is an extremely small item. People would use it as a remedy for ear pain, and apparently they would place it in the ear.
"Shen shel chuldah" - the tooth of a fox. For someone who suffered from insomnia, they would place on him the tooth of a dead fox; and for someone who suffered from excessive sleep, they would place on him the tooth of a live fox.
"Masmer min hatzaluv" - a nail taken from someone who had been crucified. They would hang it around a person's neck as a remedy for an infection or for swelling that had developed from a wound, and also for various types of fever, and it may be used even on Shabbos.
The dispute between Rabbi Meir and the Sages:
All of these were permitted "mishum refuah" - when they are done for a medical purpose, and these are "the words of Rabbi Meir." However, "vachachamim omrim: af bachol assur, mishum darchei ha'Emori" - in their view one may not go out with these items, and indeed one may not use them at all, even on weekdays.
What are 'darchei ha'Emori'?
This term parallels the prohibition of "uvechukoseihem lo seileichu" - "and you shall not follow their statutes." The verse teaches that a person must not follow the idolatrous practices of the nations, and anything for which no logical reason can be understood is liable to fall into this category.
The scope of the permission - an established remedy:
The Gemara establishes, however, that something which has become established as a remedy, as an accepted method used for medical purposes, carries no concern of darchei ha'Emori. And the Rambam explains: as long as the physicians say that the item is effective, it does not fall under the heading of darchei ha'Emori, and therefore it is permitted.
In summary: In this mishnah we learned of three things one may go out with into a public domain for medical purposes - a chargol egg, a fox's tooth, and a nail from a crucified person - according to Rabbi Meir; while the Sages forbid them even on weekdays because of darchei ha'Emori. We clarified the meaning of 'darchei ha'Emori' as the prohibition of "uvechukoseihem lo seileichu," and the principle that something established as a remedy, whose effectiveness physicians attest to, is not included in darchei ha'Emori and is permitted for use.