Listing's law: all eye rotations are such that they are rotations around an axis orthogonal to the primary gaze direction. Mathematically, this means the space of all eye rotations LIST is a subset of the special orthogonal group SO(3).
The geodesic equations in the space LIST (parameterized by the angles and of a spherical coordinate system) are
A. D. Polpitiya, W. P. Dayawansa, C. F. Martin, and B. K. Ghosh , "Geometry and Control of Human Eye Movements," IEEE Transactions on Automatic Control, 52(2), 2007 pp. 170–180.