With regard to a priest who was injured on his finger on Shabbat, he may temporarily wrap it with a reed so that his wound is not visible while he is serving in the Temple. This leniency applies in the Temple, but not in the country, as it also heals the wound, and medical treatment is prohibited on Shabbat due to rabbinic decree.
The term gemi (reed) is, apparently, a general term that refers to the products derived from the papyrus plant – Cyperus papyrus L. This plant was used throughout the ages to produce a variety of manufactured goods. From its hard, outer part, mats were woven; in ancient Egypt and other countries, its inner parts were used for making paper. The soft inner parts also were used for producing strips with which things could be tied, and sometimes – as in our case – for bandages.
The Gemara specifies that only a gemi can be used, and not a small cloth. The small cloth would be a problem either because:
(a) it would be a hatzitza – a separation between the kohen and the utensil that he needs to hold, or else it involves
(b) yitur begadim – an extra item of clothing beyond the basic uniform of the kohen
While the Gemara makes it sound as though either of these could be the problem, there are significant differences between the two. The problem of hatzitza is one of separation; thus, if the cloth is put on the left hand, for example, there would be no problem, since the Temple service was done only with the right hand. Yitur begadim is an independent problem, based on bal tosif – that it is forbidden to add to the commandments of the Torah – so once it is defined as a beged (garment), it would create problems no matter where it was placed on the kohen’s body.