| lip sulcus | A furrow between the developing lip and gum. Synonym: labiodental sulcus, lip sulcus, primary labial groove. (05 Mar 2000) |
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| lower lip | The muscular fold bounding the opening of the mouth inferiorly. Synonym: labium inferius oris. (05 Mar 2000) |
| low lip line | The lowest position of the lower lip during the act of smiling or voluntary retraction, the lowest position of the upper lip at rest. (05 Mar 2000) |