Through training, the horse learns to respond to sensations in his mouth. A feeling that begins on the tongue results in how the horse places his feet. The bit in a bridle helps you direct the horse's orientation, speed, and attitude. The horse has a strong jaw and pliable mouth. To evade bit pressure, he can try to move his jaw in a circle, or reposition his strong, muscular tongue and mobile lips.
Most bits act primarily on the tongue.
The most critical location for pain in the performance horse is at the very base of the tongue, primarily over the third molars. When the horse flexes at the poll, there's no way that he can keep his tongue from moving against those back teeth. A restricted tongue affects more than the horse's mouth. Joyce Harman, DVM, Washington, Va., explained how the tongue is attached to the hyoid bones, lying between the mandibles. Like half of a rectangle, it goes straight down the mandible, across the bottom, and up the other side.