The stiffness of the eye and the extraocular muscles of the conscious rabbit was measured and compared with similar measurements made on human subjects. The extraocular muscles of the rabbit developed less than 0.5 g at optimal isometric length when the contralateral eye was in 'primary position'. Deviations of the eye of the rabbit from primary position were accomplished by an increase in force of the agonist muscle with only a nominal decrease in force in the antagonist. With all muscles attached to the globe, the stiffness of the eye for displacements of up to 35° from primary position was 0.11±0.03 g/deg. The stiffness of the human eye to lateral rotations was 1.09±0.24 g/deg. Hence, identical disturbances in force would cause deviations in the position of the eye of the rabbit which are nearly an order of magnitude greater than those of man. The stiffness of the extraocular muscles of the rabbit was sensitive to levels of muscle activation produced either by indirect electrical stimulation or by the vestibuloocular reflex. The implications of these findings for the development of eye position control in animals with good binocular vision are discussed.
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