Yes. The mouthpiece of a bit lies on the tongue and the bars of the mouth, and the various rings and shanks of a bit lie in contact with the lips. The tongue, gums and lips are all richly supplied with sensory nerves. The bit is lying directly above the terminal branches of the sensory nerve to the mouth. This is the lower jaw (mandibular) branch of the fifth cranial nerve. Because the parent nerve has three branches it is known as the Trigeminal nerve. The other two branches supply the eye (ophthalmic branch) and the upper jaw, nasal cavity and muzzle (maxillary branch). In a stallion or gelding, the bit is also lying over the root of the canine tooth (tush). Left to its own devices, a horse is most fastidious about what it puts in its mouth.

In the past, I have not been able to put forward any convincing explanation for the cause of headshaking in the horse, and even less have I been able to suggest any satisfactory treatment. But, from studying the improving effect of the Bitless Bridle on the behavior of the horse, I now recognize that the bit is the major cause of headshaking. I conclude that headshaking is a common sign of trigeminal neuralgia (tic douloureux) brought on by persistent pressure of the bit. In man, the fifth cranial nerve is often the seat of neuralgia. In the horse, local pain in the lower jaw (sensed by the mandibular branch of this nerve) and referred pain in the upper jaw and eye (transmitted by the maxillary and ophthalmic branches of this same nerve) explains all the signs of the headshaking syndrome. Many 'headshakers' show great anxiety, immediately after exercise, with the need to rub their faces on anything handy. I believe this and other signs such a sneezing, snorting and sensitivity to bright light to be a manifestation of facial pain. Some horses stop headshaking immediately the bit is removed, others may need more time for the neuralgia to subside.