Ocular torsion and the function of the vertical extraocular muscles.

The vertical corneal meridia are not kept perpendicular to the horizon in human and nonhuman primates when the head or body is tilted, i.e., compensatory counter-rolling of the eyes does not occur. The slight torsional displacement of the vertical corneal meridia noted by many observers may be the result of rotation around an axis or to translation of the globe. The neurologic and structural systems that control the actions of the vertical muscles in human and nonhuman primates do not appear to provide a mechanism for wheel-rotation of the eyes around the pupillary axis. Ocular torsion is not a normal function of the vertical extraocular muscles. Their function is probably the reverse, i.e., the inhibition or prevention of ocular torsion and the stabilization of the eyes when the head or body inclines. Torsional displacement of a vertical corneal meridian occurs only when there is an abnormal muscle imbalance. Wheel-like movements (cycloduction) around the pupillary axis or visual line do not occur. Torsional displacement of a vertical corneal meridian occurs only with a simultaneous vertical movement. The vertical rectus and the oblique muscles in man work together to produce vertical ocular movements regardless of head position of body posture while maintaining the vertical corneal meridia parallel to the sagittal plane of the head. The vestibular apparatus may be responsible for distributing innervation among these muscles, enabling them to function in this manner.