The lips are formed by musculus orbicularis oris . The function of this muscle is narrowing of lips. Their function is lifting of the middle part of the upper lip. We can also merge the function of musculus zygomaticus major and musculus levator angulis ori . 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.