[Anatomy and physiology of the muscles and median frenum of the upper lip. Initial results of selective electromyography].

Usually when reference is made to the Orbicularis oris muscle of the upper lip only the horizontal bands of muscle are mentioned. This muscle also comprises oblique fibres (Orbicularis externus) that are of very great importance in lip movements. The balance of the lip derives from the activity of the two types of muscle fibers. The lip can thus perform, at the same time, a movement of compression of the underlying structures and projection movements whereby these structures are freed. These different modes of action are well demonstrated by selective electromyography. There is also, in the middle of the upper lip, a medial cellulo-fibrous septum forming a prolongation of the septum and penetrating into the median interincisive suture and ending at the frenulum of the upper lip. This medial septum seems to play a very important physiological role in the growth of the premaxilla.