Bilateral supernumerary rectus muscles of the orbit.

A bilateral anomaly of the rectus muscles and a unilateral variation of the levator palpebrae muscle were found in the right and left orbits of an 84-year-old man. The anomaly was in the form of a supernumerary rectus muscle lying on a sagittal orientation between the optic nerve and lateral rectus muscle (in the right orbit) and adjacent to the inferior rectus muscle (in the left orbit). In each orbit the anomalous muscle originated occipitally from the common tendinous ring and frontally-near the eyeball-joined the terminal part of the inferior rectus. On its superior margin the anomalous muscle had, in the right orbit, a broad (4 mm), and in the left orbit a slender (2 mm) muscular bridge to the superior rectus muscle. Because of their connection to the superior rectus and their innervation by N.III the accessory orbital muscles are not deemed to be vestiges of the retractor bulbi muscle which, with the exception of the primates, is a typical occurrence in vertebrates and is always innervated by N.VI. Our anomalous orbital muscle must be explained as a supernumerary rectus muscle. A further variation occurred in the right orbit: an isolated medial part or belly of the levator palpebrae muscle (the so-called M. gracillimus). In primates this variation is known as a remnant of the membrana nictitans (third eyelid of the amniotes). Ignorance of anomalies in the orbital muscles may lead to confusion and error in diagnostic identification and surgical exposure.