The orbicularis oris muscle is the sphincter muscle around the mouth. It is a complex of muscles in the lips that encircle the mouth. This muscle acts to shape and control the size of the mouth opening and is important for creating the lip positions and movements during speech. As an expressive muscle, four relatively distinct movements can be produced by it, a pressing together, a tightening and thinning, a rolling inwards between the teeth, and a thrusting outwards.
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