[Microscopic anatomy and muscle architecture of the commissure of the lips in the newborn infant and the adult].

The muscular structure of the commissura of the buccal orifice in humans confers on it a special significance. It consists of two layers each apparently having a well-defined function. Deep layer. Quite distinct at the embryonic stage, it develops rapidly in the new-born. It consists of the internal orbicular muscle, or Poirier's sphincter oris, and the two buccinator muscles. This layer, of functional importance for digestion, is formed by a central sphincter surrounded by two cavitary muscles. In the new-born, it forms a real suction apparatus. From this point of view, the internal orbicular and the two buccinator muscles are the equivalent respectively of the circular and longitudinal fibres of the muscular digestive tunic. This layer is more cavitary and sphincteral than platysmal. Superficial layer. Platysmal, muscular it is dependent on the cephalic superficial fascia, it is mimetic. This layer is made up of platysmal muscles converging on the commissura. The labial muscular network of the platysmal muscles forms the external orbicular. This layer, which is rudimentary in the new-born, develops during growth alongside relationships (mimic-phonation). It appears complete in adults. Facial superposition and commissural fusion of the deep and superficial layers confer a dual function of the buccal orifice: --digestive by means of the deep sphinctero-cavitary layer, --mimetic by means of the superficial platysmal layer. Wilst growth is taking place, the digestive layer in fact assumes mimetic function and the platysmal layer may intervene at will in the functioning of the digestive sphincter. The commissura labiorum is the anatomical site at which the cutaneous and digestive coats converge. Briefly, the buccal orifice is a digestive sphincter complete with mimetic function. Has not the face become a cortical organ in the course of evolution?