The eye has both internal and external muscles. The internal muscles control focusing and pupil size. The external muscles direct the eyes to the point of interest and keep the retinal image in constant slight motion.
Any muscle system is subject to fatigue when it is used continuously without periods of rest.
As eye muscles are not unique, they too show evidence of fatigue when visual tasks require added effort and concentration. And, they too recover their optimal levels of function after rest. The external muscles of the eyes must work together to keep both eyes centered on the point of interest. Such eye movements are responses to complex neural stimuli, such as from the vestibule of the ear, affecting a sense of balance. In general, such movements are precise and relatively fast, and are not subject to fatigue.