The Gracillimus Orbitis Muscle

The gracillimus orbitis muscle is an anomalous, accessory, or supernumerary extraocular muscle that, although is rarely seen clinically in the human orbit, has been identified in 5% to 14% of dissected cadaver orbits. It arises from the medial surface of the levator near its origin and runs forward between the levator and superior oblique muscles resembling one of the other extraocular muscles. More anteriorly, it becomes thinner, less well defined, and mostly fibrous. Its major insertion is into the fascia surrounding the trochlea, while other fibers may travel to the supratrochlear artery, intermuscular septum, the levator or medial rectus muscle, and the fascia surrounding the superior ophthalmic vein. The function of this anomalous muscle remains unknown in most cases. Knowledge of its presence is important as the oculoplastic/orbital surgeon may encounter it during an eyelid or orbital procedure.