Abstract:
The authors describe a supernumerary muscle in each orbit of an elderly male subject. There appear to be no previous reports of this muscle; most reports of anomalies of extraocular muscles describe hypoplasia or aplasia. Thirty-five formalin-fixed cadavers assigned to medical students for dissection were studied. The orbits were dissected by a superior approach which involved removal of the orbital plate of the frontal bone and the superior orbital margin. A supernumerary extraocular muscle was seen in each orbit of one cadaver, located between the superior oblique and levator palpebrae superioris muscles. It originated on the inferior surface of the lesser wing of sphenoid bone and was inserted into the skin of the medial one-third of the upper eyelid. It was innervated by a branch from the superior division of the oculomotor nerve. The insertion of the muscle into the upper eyelid produced a crease running obliquely upwards and medially, from the junction of the medial one-third and lateral two-thirds of the lid margin, towards the medial part of the superior orbital fold. The authors suggest the name levator palpebrae superioris accessorius for this muscle in view of its topography and action as tested in the cadaver. The significance of the findings is discussed and the literature on the development of the muscles supplied by the oculomotor nerve is reviewed
