Accessory lateral rectus orbital geometry in normal and naturally strabismic monkeys.

We conducted anatomic dissections of macaque monkey orbits and made a quantitative assessment of the orbital geometry of the accessory lateral rectus muscle. Our results show that the expected effect of this muscle on rotations of the globe is to produce elevation and abduction. The abducting component could counteract nasal drifts, and thus our findings provide support for the hypothesis that this muscle could render monkeys resistant to the development of esodeviations. Dissections of the orbits from two naturally esotropic monkeys also are consistent with this hypothesis. The accessory lateral rectus muscle was absent in one of them and abnormally small in the other. Humans do not have an accessory lateral rectus muscle, and we speculate that the high prevalence of esodeviations in humans may be related to an evolutionary loss of this muscle system.