One of the most important, and less explored aspects of the horse are his mouth and the elements placed within it (pelham and snaffle) which aid management and control.
Very few are familiar with the anatomy of the horse’s mouth, and less of us know about the functions the pelham and snaffle play on the sensitive areas of the horse. Everything is tested via trial and error, waiting for the horse to get used to it so we can manage its movements. Also, the damages caused to the mouth from these elements are rarely repaired; while physical damage heals over time, psychological damage lasts forever.
The anatomy of the horse’s mouth is unique in possessing a mandible (lower jaw) which is narrower than the maxilla (upper jaw). The tongue therefore (the most sensitive part of the horse after the cornea) rests on the lower jaw, where the following of two things can happen: a) the bottom of the mouth is too narrow (closed jaw), or b) the tongue is too big, in which case it hangs out of the side of the jaw.
By closing the horse’s mouth and lifting the upper lip while lowering the bottom, we can see that there is a gap or space where we can place something within the mouth, be it a pelham or a snaffle. This space is generally about 16 millimetres wide.
Normally, when we place something in their mouth, the horse experiences discomfort, not being accustomed to such an intrusion at first. The first thing the horse does is open its mouth. Once in motion, the horse begins to shake its head, biting the pelham or passing its tongue over it every once in while, and revealing the tongue from the corners of its mouth.
A drop noseband is usually used to counteract the reaction of the horse; by using this we apply pressure on the lower part of the mouth. This will place pressure at some point on the neck and temporomandibular joint (TMJ).
Applying pressure on the bar will cause the horse to react by lifting its head; alternatively, if the pressure is on the tongue, the horse will subsequently lower the head.
Running cheek gags, commonly used in polo, create a nut cracker effect, compressing the tongue and acting on the bar in a tangential manner. The jointed pelham, if it is short, acts like a snaffle; however, if its central part is long and wide, it applies pressure to the tongue’s upper surface, lowering the head.
The flat, articulated central part of the jointed pelham applies the least amount of pressure; it is the least aggressive due to its larger area of contact. This concept goes for any element which is inserted into the horses mouth: the thicker it is, the less severe its action will be.
http://www.pololine.com/?lang=1&sec=9¬e=8849¬e=8849&pag=0&anoB=2016