[The modiolus. Comparative anatomy, embryological and physiological review, surgical importance].

Monkeys, imitators of man's gesture never shared smile with him and their facial expressions only result in grimaces. Study of animal anatomy may explain why subtle expressions of the lips are a more man's special gift than laughter. These fine movements are related to a subcutaneous muscular structure termed modiolus, which is the meeting point of muscles involved in the motion of mouth and comparative anatomy shows this structure to be better defined in Man than in other primates. Embryologically, muscles that form the modiolus derive from a cellular concentration in the region of the second branchial arch. It is then logical to see these muscles converge to the corner of the mouth to form the modiolus, as they emerge from a common area. Functioning of the modiolus is complex, as its position may change many times in a minute to allow the orbicularis oris muscle to take any position and do any movement as if it was inserted on bone. With its complex physiology, the modiolus is of critical importance in the subtle expressions of the lips and must be preserved or reconstructed when performing repair of full-thickness tissue defects of lips.