In the following illustration, structure "a" is the

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At the midline of the upper lip, a fold of connective tissue known as the maxillary labial frenum can be found. A corresponding structure, the mandibular labial frenum, is present at the midline of the lower lip. The upper frenum is usually more pronounced than the lower. Both structures, as well as additional less well-defined frena, serve to attach the lips to the arch and contain only connective tissue. Structure "b" is the mandibular labial frenum. Structure "c" is the mucogingival junction. Structure "d" is the mucobuccal fold.

In the Following Illustration, Structure "A" Is the