They lift the corner of mouth and they move it aside. Musculus risorius moves the corner of mouth aside. Musculus depressor anguli oris, musculus depressor labii inferioris are used for the movement of the lower lip. Musculus depressor anguli oris moves the corner of mouth downwards and aside. Musculus depressor labii inferioris moves the middle part of the lower lip downwards.
Musculus masseter moves the lower jawbone upwards and downwards.
Two points of the vector determine places, where the muscle is attached on the lip and on the skull. The first point is mobile and we call them control points. The second point is immovable. The movement of the muscles is implemented as extending or reducing of a distance between points of the vector. This reduction or extension performed by movement of control point.