Dissections of the bilateral orbits in a 45-year-old female cadaver, who had no 
ocular movement disorders in her lifetime, revealed anomalous muscles linking 
the superior and inferior rectus muscles. The muscles, situated between the 
optic nerve and the lateral rectus muscle, originated from the annulus of Zinn 
and branched off two heads; one inserted into the medial inferior side of the 
superior rectus muscle and the other inserted into the central superior side of 
the inferior rectus muscle. Each insertion was located on a distal site of the 
myoneural junction of each rectus muscle. Histological investigations showed 
that the muscles had a striated muscle structure. No definite nerve insertion 
was observed in the muscles. Although this type of anomalous muscle has been 
reported in a few Caucasian cases, the present study is the first report in an 
Asian person. Anomalous orbital structures, which are a rare cause of 
strabismus, are important in the differential diagnosis of intra-orbital 
space-occupying lesions, rather than the differential diagnosis of strabismus.