On the Cavalry Commander (6.8-6.9)

If he takes the bit, of course the bridle should be put on. But if he refuses to open his mouth, the man must hold the bit to his teeth and put the thumb of the left hand in the horse’s jaw. Most horses open the mouth when this is done. If he still resists, the man should squeeze his lip against the tusk; and very few resist when they are treated in this way.

The groom should also be instructed in the following points: first, never to lead the horse on the rein—that gives the horse a hard mouth on one side—and secondly, what is the correct distance from the bit to the jaws. For if it is too high up, it hardens the mouth so that it loses its sensitiveness; and if it lies too low in the mouth, it gives the horse power to take it between his teeth and refuse to obey.