Interdigitating muscle fibers throughout orbicularis oris inferior: preliminary observations

Observations of serial sections from two cadaveric specimens of human lower lip are presented. The complex structure of the muscles of the lower lip region includes several fascicles of fibers that are parallel to the vermilion border and other muscle fibers that interdigitate the parallel structures. Two classes of interdigitating muscle fibers are described: The majority originate inferior to the mucosal-gingival border and have a primarily vertical course. The others originate at the hairy epidermis inferior to the vermilion, course diagonally through the horizontal fibers, and insert near the mucosal-vermilion border.