Stiffening of Jaw and Tongue - Drooling
Sara Wyche:
Underline muscles of the neck.
"A system of muscles (Longus colli going from vertebrae to vertebrae, my addition) spans the underside of the neck vertebrae, again protection the area of the 1st rib by extending under the thoracic vertebrae into the ribcage. In conjunction with the sternomandibularis which connects the sternum to the bottom jaw, they flex and pull the head towards the chest. However, long muscles also connect the sternum to the bone at the base of the tongue. When the horse's head is tied in (held deep behind the vertical by his own muscles, my addition), he is virtually unable to swallow. This makes the action of the bit rather uncomfortable, and presumably defeats the object of enforcing a particular head carriage on the horse."
Excessive foaming and drooling caused by
tension in the underline muscles.
The drooling that one sees on many deep or forcefully ridden horses comes from this fact. The pull stiffens the tongue, and the effect of the bit, especially when heavy or harsh, makes it worse. The pull on the jaw causes the horse to clench the jaw, stiffening further. A tight noseband can be and is frequently used to hide this. But the fact remains that the horse will use his tongue as an eggbeater in side his mouth, to try to escape the pain and cramp caused by the bit and the sternohyoid. This is why these horses foam and drool profusely. Their open lips sometimes leave strings of saliva in the arena, an certainly over their own frontlegs.
A moist mouth with some white lipstick is a good thing. One often observes it in retrained horses that start to work the right way - one change is that their mouths soften and become wet. But when the wetness is whipped into a stiff foam, the tongue is too active, and the mouth is gaping open, from nervousness, cramp or pain.
For further reading on the muscle function of the neck in the correct telescoping of the neck versus behind the vertical, visit The Signs of Collection - A Source of Misconceptions.