Eye motion strategies in animals and humans have the goal of optimizing visual perception, therefore, the study of eye motions plays an important role in the design of humanoid robot eye systems. Saccades and smooth pursuit in humans and primates are a significant class of ocular motions, which obey the so called Listing's Law, stating that admissible eye's orientations have always zero torsion during motion. In this paper we present a model of the eye plant proving that Listing's Law implementation is strongly related with the geometry of the eye and its actuation system (extraocular muscles). The proposed model has been used to provide the guidelines for the design of a tendon driven humanoid robot eye. Experimental tests, presented in this paper, validate the model by performing a quantitative comparison of the performance of the robot eye with physiological data measured in humans and primates during saccades.