Rictus

(n.) The gape of the mouth, as of birds; -- often restricted to the corners of the mouth.

Thesaurus Entries

bob, bobble, bounce, bump, didder, dither, falter, grimace, jar, jerk, jig, jigget, jiggle, jog, joggle, jolt, jostle, moue, mow, pout, quake, quaver, quiver, shake, shiver, shock, shudder, snarl, tic, tremble, tremor, twitch, twitter, wobble, wry face, wry mouth