1. A violent check of a horse by drawing or twitching the reins suddenly and with one pull.
2. A firm pressure of the bow on the strings of a violin, which crowds them down so two or three may be sounded at once.
3. The involuntary jerking movement in the act of swallowing.
4. A quick movement of both eyes in the same direction.Barbara Cassin, A Dictionary of Eye Terminology, Fourth Edition, 2001

   Even at the cinema watching that awful Going my Way, the day they met, he saw every white straying of her ungauntleted hands, could feel in his skin each saccade of her olive, her amber, her coffee-colored eyes. He’s wasted gallons of paint thinner striking his faithful Zippo, its charred wick, virility giving way to thrift, rationed down to a little stub, the blue flame sparking about the edges in the dark, the many kinds of dark, just to see what’s happening with her face. Each new flame, a new face. —Thomas Pynchon, Gravity's Rainbow

A scrimmage in a Border Station—
A canter down some dark defile—
Two thousand pounds of education
Drops to a ten-rupee jezail—
The Crammer's boast, the Squadron's pride,
Shot like a rabbit in a ride!

No proposition Euclid wrote
No formulae the text-books know,
Will turn the bullet from your coat,
Or ward the tulwar's downward blow.
Strike hard who cares—shoot straight who can—
The odds are on the cheaper man. —Rudyard Kipling, Arithmetic on the Frontier, 1886Full poem available here.