Kelim, Chapter Eleven, Mishnah Five
1) The scorpion [-shaped] bit of a bridle is susceptible to impurity, but the cheek-pieces are clean.
a) Rabbi Eliezer says that the cheek-pieces are susceptible to impurity.
b) But the sages says that the scorpion-bit alone is susceptible to impurity,
2) When they are joined together it is all susceptible to impurity.
Section one: The two pieces of equipment mentioned here are used in bridling an animal. According to the Rambam, the scorpion-shaped bit is susceptible because it has its own name, whereas the cheek-pieces do not.
Rabbi Eliezer holds that since the cheek-pieces are made for people to use and they are part of the bit, they too are susceptible.
Section two: The sages agree that if the cheek-pieces are attached to the bridle, they too are susceptible to impurity.