Findings of unique small muscle fibers at the superficial portion of the orbicularis oculi in the lateral canthal region of Japanese adult cadavers

This study reports the existence of previously unknown muscle fascicles in Japanese adult cadavers. A bundle of these muscle fascicles diverged from the pretarsal portion of the orbicularis oculi muscle and coursed in a lateral direction superficial to this muscle. When observed with the naked eye, the bundle seemed to originate at the medial canthus and run along or near the edge of the upper eyelid. However, its boundary with the orbicularis oculi muscle was indistinguishable until it crossed superficial to this muscle. Throughout our observations, the thin muscle bundle was identified with high frequency (94%, 49 of 52 individual cadavers), and is thus unlikely to be an artifact. Light microscopy revealed that, in sagittal sections, the thin muscle bundle was located on the superficial side of the lateral portion of the orbicularis oculi muscle, while in horizontal sections, it ran in a superficial plane to the orbicularis oculi muscle in a medial to lateral direction. Despite having some similarity to a muscular raphe, the lateral canthal band, and to one of the previously known inferior muscles of the orbicularis oculi muscle, the results of our anatomical study suggest that the bundle is none of these. Rather, it is a previously unreported muscle that likely contributes to the surface morphology at the lateral canthus.