Active vision has the goal of improving visual perception; therefore, the investigation of ocular motion strategies must play an important role in the design of humanoid robot eyes. Listing's law is a basic principle, which characterizes various ocular movements in humans, including saccades and smooth pursuit, and its neural or mechanical origin has been debated for a long time. Recent anatomical advances suggest that motions compatible with Listing's law could be mainly caused by the mechanical structure of the eye plant. In this paper, we present a bioinspired model of the eye plant, and we formally prove that according to the model, the implementation of Listing's law can be actually explained on the base of the geometry of the eye and of its actuation system. The proposed model is characterized by a limited number of geometric parameters, which can be easily used to set the guidelines for the design of humanoid, and possibly tendon-driven, robot eyes. Simulative and experimental tests performed on a robot prototype are eventually presented to perform a quantitative evaluation of the performance of the model, also in comparison with physiological data measured in humans and primates and reported in the literature.