Figure 1.

The ‘Mellor pen test.” This simulates bit pressure applied to the gums of the interdental space of the horse. Gums are exquisitely sensitive to painful stimuli, including compression. Rein tension transferred to the bit in contact with the gums of the interdental space causes pain. (A) Position 1: Hold the pen in front of your mouth; (B) Position 2: Open your mouth, place the pen where the upper and lower lips meet on each side, and then push the pen towards the back of your throat. No gum contact, no significant pain; (C) Position 3a: Roll your bottom lip down and locate the pen on your gum, below your central incisors; (D) Position 3b: Now release your lip and with both hands holding the pen, apply compressive pressure to your gum, carefully increasing the pressure in steps from very low until the pain is too intense to continue. How much compression-induced pain could you stand?